)E75’ 7 DAY 7o 7W. Geohydrology of Project Gnome Site, Eddy County, New Mexico f - GEOLOGICAL SURVEY PROFESSIONAL PAPER 712—A Prepared in cooperation witn tne U.S. Atomic Energy Commission Geohydrology of Project Gnome Site, Eddy County, New Mexico By JAMES B. COOPER and V. M. GLANZMAN HYDROLOGY OF NUCLEAR TEST SITES GEOLOGICAL SURVEY PROFESSIONAL PAPER 712—A Prepared in cooperation wz'tfl Me U.S. fltomic Energy Commission UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1971 03386 UNITED STATES DEPARTMENT OF THE INTERIOR ROGERS C. B. MORTON, Secretary GEOLOGICAL SURVEY W. A. Radlinski, Acting Director Library of Congress catalog-card No. 70—179645 For sale by the Superintendent of Documents, US. Government Printing Oflice Washington, DC. 20402 - Price $1 (paper cover) Stock Number 2401—1165 CONTENTS Page Page Abstract ___________________________________________ Al Ground water ______________________________________ A8 Introduction_ _ _ _ _ _ _ _; ______________________________ 1 Availability and utilization _______________________ 10 Location and physiographic setting _______________ l Aquifer characteristics at and near project site _____ 10 Methods of investigation ________________________ 2 Transmissivity and storage coeflicient _________ 11 Well-numbering system __________________________ 2 Aquifer yield _______________________________ 11 Geology_ __________________________________________ 2 Test holes at and near project site ________________ 12 Regional setting ________________________________ 2 COUStruCtion ------------------------------- 13 Strati graphy ___________________________________ 4 Testing ------------------------------------ 14 Castile Formation __________________________ 6 Initial investigation of water wells within 15 miles of S _ project site ______________________________________ 14 alado Formation ___________________________ 6 . . . _ Observations at tlme of nuclear explosron ______________ l7 Rustler Format1on """""""""""""" 6 Investigations after the nuclear explosion ______________ 20 Dewey Lake Redbeds ----------------------- 8 Water-level fluctuations _________________________ 20 Triassic formations -------------------------- 8 Effects of atmospheric pressure and earth tides _____ 20 Tertiary formations _______________ '. ........ 8 ConcluSions ________________________________________ 21 Quaternary formations ______________________ 8 Selected references __________________________________ 22 PLATE FIGURE TABLE ILLUSTRATIONS Pale 1. Map showing location of wells and water-level contours for geologic formations in the Project Gnome area, Eddy and Lea Counties, New Mexico _______________________________________________________________ In pocket 1. Map showing location of the Project Gnome site and the area of investigation ______________________________ A3 2. Well-numbering system ______________________________________________________________________________ 4 3. Map showing extent of the Delaware basin in New Mexico, and adjacent structural and geographic features- - __ 4 4. Geologic section between Carlsbad Caverns National Park and the Project Gnome site ______________________ 5 5. Generalized stratigraphic section at Project Gnome site _________________________________________________ 7 6. Map showing general distribution of water-bearing formations in the Project Gnome area ____________________ 9 7. Map showing location of U.S. Geological Survey test holes near the Project Gnome site ______________________ 11 8. Calculated time-drawdown plot of water level in U.S. Geological Survey test hole 1, when pumped continuously, at selected rates, Project Gnome site ______________________________________________________________ 12 9. Hydrographs showing fluctuation of water level in U.S. Geological Survey test hole 1 prior to the nuclear ex- plosion, Project Gnome site ______________________________________________________________________ 15 10. Hydrograph showing fluctuation of water level in U.S. Geological Survey test hole 4 at time of Project Gnome explosion, Project Gnome site ____________________________________________________________________ 20 11. Hydrograph of water level in U.S. Geological Survey test hole 1, 1960—63, Project Gnome site _______________ 21 12. Graphs showing fluctuations of water level in U.S. Geological Survey test hole 1 and inverted fluctuations of atmospheric pressure, January 9—14, 1963, Project Gnome site _______________________________________ 21 TABLES . Page 1. Generalized section of the rocks exposed in the Project Gnome area _______________________________________ A5 2. Records of wells in the Project Gnome area ____________________________________________________________ 16 3. Chemical analyses of water samples from wells in the Project Gnome area _________________________________ 18 4. Radiochemical analyses of well water in the Project Gnome area _________________________________________ 19 5. Water wells Within a 5-mile radius of Project Gnome Site, Eddy County ___________________________________ 19 III HYDROLOGY OF NUCLEAR TEST SITES GEOHYDROLOGY OF PROJECT GNOME SITE, EDDY COUNTY NEW MEXICO By JAMES B. COOPER and V. M. GLANZMAN ABSTRACT On December 10, 1961, a nuclear device was detonated 1,200 feet underground in the massive salt of the Salado Formation in southeastern Eddy County, N. Mex. In support of the public safety program of the U.S. Atomic Energy Commission, the water regimen of about 1,200 square miles, mostly east of the Pecos River and within 15 miles of the project site, was inves- tigated before, at the time of, and after the explosion. Preliminary investigations determined that about 70 wells were in use in the area around the Project Gnome site. The wells range in depth from a few tens of feet to nearly 800 feet and tap water in rocks of Permian, Triassic, Tertiary, and Quaternary ages. Most ground water in the area is saline; however, it is a valuable resource utilized for stock water. Seven test holes drilled at the project site penetrated similar sequences of rocks overlying the massive salt. All aquifers in the Project Gnome area lie above the massive salt of the Salado Formation. The only water at the project site is contained in the Culebra Dolomite Member of the Rustler Formation, a confined aquifer about 30 feet thick. The aquifer is about 500 feet beneath the surface and about 200 feet above the top of the salt. The potentiometric surface for the confined water is 75 feet above the top of the aquifer and slopes 0.2 to 1.0 foot per mile westward. Fluctuations of water levels observed in wells are small. Data from a pumping test on the aquifer at the Project Gnome site indicate that 100 gallons per minute could be pumped continuouSly from a well for as long as 1 year without lowering pumping levels below the top of the aquifer. Pressure changes in the aquifer due to the release of water through drill holes and removal by bailing were easily detected by a recording gage. Water levels in four observation wells at the site were being recorded at the time of the nuclear explosion. The estimated maximum rise of water level immediately following the explo- sion was about 4 feet at a distance of 2,000 feet from ground zero. About 3,200 feet from ground zero, the water level rose about 2.2 feet. then gradually returned to normal level within a period of about 11 hours. Water in the four test holes returned to preexplosion levels 2 days after the explosion. Continued observations of water level in those holes and three additional observation wells indicated no anomalous water-level fluctuations after the nuclear explosion, suggesting that the aquifer probably was not significantly ruptured by the explosion. INTRODUCTION Project Gnome, an underground nuclear experiment, is a part of the US. Atomic Energy Commission Plow- share Program which is directed toward the develop- ment of peaceful industrial, scientific, and civilian uses of nuclear explosions. The Project Gnome nuclear device was detonated December 10, 1961, at 1200 m.s.t., about 25 miles southeast of Carlsbad, N. Mex., in an under- ground chamber approximately 1,200 feet below the earth’s surface in a thick, massive salt formation. The US. Geological Survey, in support of the public safety program of the US. Atomic Energy Commission, made studies and investigations relative to Project Gnome to provide information on the water regimen of the project site and surrounding area before and after the explosion. Primary objectives of the studies were (1) determination of the hazard, if any, of the under— ground nuclear explosion contaminating surface or ground waters, and (2) if such waters were contam- inated, determination of their rate of movement and of the possible maximum concentration levels of contam— inants in them. Preliminary studies of the physical properties of the rocks by other investigators indicated that the explosion would not rupture the beds above those containing the explosion chamber. This report described the hydrology and the general geology of the Project Gnome area and project site and discusses studies of ground-water hydraulics made by the Geological Survey after the nuclear explosion. LOCATION AND PHYSIOGRAPHIC SETTING Ground-water and geologic investigations were made in an area of more than 1,200 square miles in south- eastern Eddy County and southwestern Lea County, N. Mex. The site of Project Gnome is in the approximate center of this area, in sec. 34, T. 23 S., R. 30 E., Eddy A1 A2 County, east of the Pecos River about 25 miles southeast of Carlsbad (fig. 1). Detailed investigations of water wells were made within a 5-mi1e radius of the Project Gnome site. The Project Gnome area is sparsely populated and is utilized for cattle ranching, farming, potash mining, and petroleum production. The climate is semiarid; the average annual precipitation is about 12 inches. Drilled wells are the principal source of water for livestock and domestic uses. I The site is in the Pecos Valley section of the Great Plains physiographic province. The Pecos River flows through the southwestern part of the area and divides the area physiographically into two sections; east of the river, the Mescalero pediment, and west of the river, the alluvial plain north of Malaga and the Gypsum Hills south of Malaga. The land surface east of the river slopes about 30 feet per mile westward and is chiefly a karst surface with numerous depressions ranging in width from a few tens of feet to about 5 miles. Nash Draw, the largest depression, extends southward through the west—central part of the area. ”Within it are several smaller depressions, the largest of which is Laguna Grande de la Sal (Salt Lake). Much of the land surface west of the river is mantled by dune sands, al- though in places on the uplands, caliche is exposed, and older formations crop out elsewhere. METHODS OF INVESTIGATION Reconnaissance fieldwork for this investigation was done in October 1958. Wells were located, water levels measured, and water samples, well logs, and other sub- surface data collected from February through May 1959. Fifteen wells, nine of which were located within a 5-mile radius of the Project Gnome site, were investi- gated during March and April 1959. The wells within the 5-mi1e radius were reinvestigated in December 1961. From July 1960 to June 1961, observations were made in the shaft and tunnel that were being constructed for access to the detonation point. Geological Survey test holes 1, 2, 4, and 5 were drilled near the site to expand knowledge of the water-bearing formations in the immediate vicinity of the site and to provide water—level observation wells during the nu— clear experiment. Geological Survey test holes 4 and 5 were completed in December 1961 just before the detonation. Several reports on specific aspects of the ground water of the Project Gnome area and site have been issued as preliminary or progress reports. These reports, as well as other reports used for background infomation, are listed under “Selected References.” HYDROLOGY OF NUCLEAR TEST SITES WELL-NUMBERING SYSTEM All wells referred to in this report are identified by a location number used by the Geological Survey and the State Engineer for numbering water wells in New Mexico. The location number is a description of the geographic location of the well, based on the system of public land surveys. It indicates the location of the well to the nearest 10-acre tract, if the well can be located that accurately. The location number consists of a series of numbers corresponding to the township, range, sec- tion, and tract within a section, in that order, as illus- trated in figure 2. If a well has not been located closely enough to be placed within a particular section or tract, a zero is used for that part of the number. Letters a, b, and c are added to the number to designate the second, third, and fourth wells in the same 10-acre tract. . GEOLOGY REGIONAL SETTING The Project Gnome site is in the northern part of the Delaware basin (Adams, 1944; King, 1948; and Newell and others, 1953), a deep oval sedimentary basin about 135 miles long and 75 miles wide in southeastern New Mexico and western Texas. This structural basin is gen- erally considered to be the area surrounded by the Capi- tan Limestone (a reef limestone of Late Permian age) (fig. 3). W'ith notable exceptions, the Capitan Lime— stone is buried beneath the present land surface. West and southwest of Carlsbad, it is at the surface and forms La Barrera del Guadalupe and El Capitan Peak. Large caverns—primarily Carlsbad Caverns—were formed in the reef limestone through the solvent action of circulating ground water. Southeastern New Mexico and adjacent parts of Texas were submerged beneath the sea in Permian time, and the pre—Permian rocks were buried beneath an accumulation of sediments. The irregular floor of the sea was characterized by structural basins, platforms, and broad shelves. Fine sand and limestone accumu- lated in the basins; reefs formed on the margins of the shelves and platforms; limestone and sand accumulated immediately behind the landward side of the reefs; and gypsum, anhydrite, other evaporite rocks, and silt and clay accumulated in the shallow waters: of the shelves. Eventually, the reef growth was halted by increasing salinity of the sea water, and evaporite sediments (Cas- tile, Salado, and Rustler Formations) were deposited in the Delaware basin. Evaporite deposition was inter- rupted during two intervals of time, during which the water was less saline and limestone was deposited. To- ward the end of Permian time, deposition of the evapo- , 34° 33° 32° GEOHYDROLOGY 0F PROJECT GNOME SITE, NEW .MEXICO 105° 104' e site (sec. 34, T. 23 8., R. 30 E.) 0 20 40 MILES ; l l I FIGURE 1.—Location of the Project Gnome site and the area of investigation (unshaded). 1 03° A3 A4 HYDROLOGY OF NUCLEAR TEST .S-I'I‘E-S FIGURE 2.——Well-numbering system. rite rocks ceased and deposition of terrestrial red beds (Dewey Lake Redbeds) began. Terrestrial deposition continued during parts of Triassic time. Additional thin deposits of sediments accumulated in Quaternary time. A total of 18,500 feet of sedimentary material, in- cluding pre-Permian sedimentary rocks, was deposited in places in the Delaware basin. A geologic section be- tween Carlsbad Caverns National Park and the Project Gnome site is shOWn in figure 4. STRATIGRAPHY Sedimentary rocks of Permian to Quaternary age crop out in the Project Gnome area. The strata dip gently east and southeast, and older rocks are mantled progressively to the east by younger formations. The oldest exposed rocks crop out at the western margin of the area and belong to the Castile Formation of Per- mian age. Rocks of the Rustler Formation of Permian age crOp out in numerous exposures in the west-central part of the area and in the Gypsum Hills south of Ma- laga. The Dewey Lake Redbeds of Permian age are exposed at several localities and crop out in a narrow and discontinuous north-south belt a few miles east of the Pecos River in the north-central part of the area. Rocks of Triassic age are represented by the Santa Rosa Sandstone in the north—central and southeastern parts of the area. Unconsolidated rocks of Tertiary age crop 3| 0 25 50 MILES L__.__L————«—l FIGURE 3.—Extent of the Delaware basin in New Mexico, and adjacent structural and geographic features. After King (1948) and Stipp and Hagler (1956). out near the eastern margin of the area. Rocks of Qua- ternary age are the Gatuna Formation of Pleisto— cene( ?) age and alluvium, windblown sand, caliche, and playa lake deposits of Holocene age. A generalized sec- tion of the rocks exposed in the Project Gnome area is given in table 1. Regional structural features of the rocks are due to the regional dip of the beds in the subsurface and to GEOHYDROLOGY 0F PROJECT Vales ormatlon Capiian Limestone \ (reel) N) W \ Anhydnte \ \\\\\ Lamar Limestone Member 0' *\ Bell Canyon Formation Anhyame L ‘ and salt Bell Canyon Formation of Delaware Mountain Group FIGURE 4.—Geologic section between Carlsbad Caverns National Park and the Project Gnome site. Modified from Cooper (1960). solution of the evaporite rocks of the Permian System. No deeply buried faults are known. Locally in rocks overlying the salt of the Salado Formation, structural GNOME SITE, NEW MEXICO A5 features are present which may be related to the hy- dration and chemical change of anhydrite to gypsum (Vine, 1960). Nash Draw is a prominent surface ex- pression of evaporite solution. In the southern part of the area, solution of evaporites has created a deep trough which has been subsequently filled with sedi- ments of Tertiary and Quaternary age to a thickness of more than 1,000 feet in some places (Maley and Huff- ington, 1953, p. 539). Excavation of a 1,200-foot vertical access shaft 12 feet in diameter, begun on July 1, 1960, presented an unusual opportunity for inspection and study of the rocks beneath the surface at the Project Gnome site. Formations exposed in the shaft consisted of about 92 feet of sand, sandstone, and conglomerate of Holocene and Pleistocene( ?) age; 202 feet of siltstone of the Dew ey Lake Redbeds of Permian age; 357 feet of gypsum, anhydrite, siltstone, and dolomite of the Rustler For- mation of Permian age; 58 feet of claystone, siltstone, and anhydrite and gypsum breccia (residual material and solution breccia derived from rocks of the Rustler and Salado Formations) ; and 493 feet of salt containing thin layers of claystone, polyhalite, and anhydrite of TABLE 1.—G’eneralized section of the rocks exposed in the Project Gnome area Thick- System Series Group Formation Member (pests) Physical character Water-bearing properties so Windbliown 0400:}: Very fine to coarse reddish-brown sand .......... Yields no water to wells in Project Gnome area. san Plays lake 7 Silt, quartz sand, and gypsum sand ............ May yield small quantities of water in large plays deposits lakes. Holocene Alluvium 0—2001: Silt, sand, gravel, and conglomerate ............. Yields large quantities of water to wells near Pe- Quaternary cos River. Caliche 0-305: Limestone with included sand grains and rock Yields no water to wells in Project Gnome area. fragments. Pleisto- Gatuna 0-200: Clay, silt, sand, gravel, and conglomerate. Red- Yields small guantities oi water to wells in parts cone (‘1) dish orange to gray. of Project nome area. Tertiary Pliocene Ogallala 0-300d: Silt, sand, and gravel ............................ Yields iairly large quantities of water to wells north and east oi Project Gnome area. Upper red 0- Shaie, siltstone, and sandstone. Bad to brown... Yields small quantities of water to wells in some beds 1,0005: localities. Triassic Upper Dockum Santa Rosa 0400i Sandstone, conglomeratic, interbedded with clay- Yields small quantities of water to wells in places Sandstone stone. Red to gray. in eastern part of Project Gnome area. Dewey 0-3503: siltstone, sandy shale,.shale, and sandstone. Red Not known to yield water to wells in Project Lake Red- to reddish orange with greenish-gray reduction Gnome area. beds spots. Forty- 0-805: Gypsum, gray to white. Siltstone, claystone, and May yield water to wells in parts of the Project niner sandstone, reddish-brown with greenish-gray Gnome area. reduction spots. Magenta 0-301: Dolomite, gray to magenta. Anhydrite and sele- Yields small quantities of water to wells in Nash Permian Ochoa Rustler Dolomite nite ___________________________________________ Draw .......................................... Tamarisk 0-120: Gypsum, gray to red. Siltstone and claystone, Mag; yield water to wells in parts of the Project reddish-brown. nome area. Culebrs 10-40:: Dolomite, grayish-white ......................... Principal aquifer at site of Project Gnome. Dolomite Lower 90-1805: Sandstone, claystone, and gypsum. Reddish M537 yield water to wells in parts oi the Project brown to light gray. nome area. Cutile 04,6003: Gypsum and siltstone. Gray to red ............. Not known to yield water to wells in Project Gnome area. 428—472 0 - 71 - 2 A6 the Salado Formation of Permian age (Gard, 1968, p. 5). The top of the massive salt was 709 feet beneath the land surface. A generalized columnar section of rocks beneath the surface at the Project Gnome site is given in figure 5. ‘l The nuclear device was detonated in the Salado For- mation, at a depth of about 1,200 feet, in a chamber at the end of a drift extending about 1,100 feet N. 50° E. from the bottom of the shaft. Thus, the rock cover over the detonation point consisted of about 500 feet of mas- sive salt and about 700 feet of various other types of sedimentary rocks. CASTILE FORMATION The Castile Formation is composed principally of anhydrite and calcite-banded anhydrite. Other con- stituents are salt and minor amounts of limestone, clastics, and other evaporites. The formation is confined to the Delaware basin. To the northwest of the basin it thins and pinches out against the reef of Capitan Lime- stone. To the southeast it thickens to more than 2,100 feet in west-central Ward County, Tex. (Adams, 1944, p. 1604). Extensive outcrops of the Castile Formation are present in the western part of the Delaware basin just southwest of the Project Gnome area, and two small patches of the formation crop out along the west side of the area. The formation, where exposed, contains no salt, and the anhydrite has been hydrated by ground water to gypsum, probably to depths of several hun- dred feet. Water is yielded to stock and domestic wells in the outcrop area of the Castile Formation. No wells are known to obtain water from the Castile within the Pro- ject Gnome area, and it is not known if the formation contains water beneath the project site. SALADO FORMATION The Salado Formation is mostly salt, although it contains extensive deposits of potash minerals which are mined at many localities and refined for potash fer- tilizers. Salt comprises more than 75 percent of the formation except Where the section has been thinned by removal of part of the salt by ground-water solution. The remainder of the formation consists of potassium minerals and minor amounts of sandstone, siltstone, shale, anhydrite, and dolomite. The Salado does not crop out near the Project Gnome site. Circulating ground water reportedly has not been found in the formation in the potash mines or in any of the numerous drill holes scattered throughout the area. Small pockets of entrapped water have, however, occasionally been found during mining. At various HYDROLOGY OF NUCLEAR TEST SITES depths in the formation, drill holes have penetrated pockets of nonflammable gas or air which are often under sufficient pressure to cause geysering or blowouts of drilling fluid to heights of several tens of feet above the land surface. West of the Pecos River most of the salt in the Sa- lado Formation has been removed by solution. In the northwestern part of the Project Gnome area, over the buried reef of Capitan Limestone, the salt is absent, and beneath an area of nearly 100 square miles in the south—central part, the salt is thin or absent (Cooper, 1962, fig. 5). Within Nash Draw, solution has removed, and is presently removing, salt from the uppermost part of the formation. In the Project Gnome area the contact between the Salado Formation and the overlying Rustler Forma- tion is marked at many places by a layer of solution breccia and residual material. This layer consists of broken and collapsed layers of gypsum separated by irregular seams and masses of reddish-brown and gray clay and silt. The material is derived from rocks of both formations. While many geologists designate this residual layer as the leached member of the Salado Formation (Gard, 1968, p. 7), others assign it to the Rustler Formation (Hale and others, 1954, p. 19). This discrepancy re- sults partly because the Rustler Formation to the east is reported to contain some beds of salt, and partly be- cause the leached zone. of the Salado Formation cannot be clearly distinguished from the lower member of the Rustler Formation in either drill cuttings or geophysi- cal logs. Most of the data used in this report to inter- pret subsurface conditions in the Project Gnome area and at the project site were obtained from records of drilled wells. Thus, in this report the top of the upper— most thick salt bed is considered to be the top of the Salado Formation, and the residual layer is considered to be a part of the lower member of the Rustler Formation. RUSTLER FORMATION The Rustler Formation is mostly anhydrite (com- monly altered to gypsum) but includes some gray car- bonaceous sandstone and red siltstone composed of fine quartz grains. Dolomitic limestone and some red and gray shale are also present in the formation. All members of the Rustler Formation crop out in the Project Gnome area. The exposures are in the west- ern part near Nash Draw and southward along both sides of the Pecos River. The structure of the Rustler Formation is mildly un- dulating. Solution and collapse have commonly affected the formation to depths of 200—300 feet, and locally to greater depths. Generally the dolomite members of the GEOHYDROLOGY 0F PROJECT GNOME SITE, NEW MEXICO A7 FORMATION AND MEMBER THICKNESS, IN FEET sand and Gatuna w“ °$ 6 \.’‘~ 9 N .... O 0‘50“ Undifferentiated 0°“ Rustler Bone Sprung 0RDOV|ClAN-SILURIAN-DEVONIAN Leonard UNCONFORMITV PRECAMBRIAN EXPLANATION Cast‘i Ie I IV m Sand and gravel Calcareous anhydrite Undifferentiated I. I ' Sandstone Wolfcamp Shale or siltsume UNCONFORMITV Bell Canyon Limestone Oolites Cherry Canyon Undifferentiated Granitic rocks Guadalupe PENNSYLVANIAN Delaware Mountain UNCONFORMITV Brushy Canyon Undifferentiated Undi'ferentiated MISSISSIPPIAN From 5.—Generalized stratigraphic section at Project Gnome site. A8 formation are less affected by solution than the gypsum member and are present throughout most of the area. At the Project Gnome site the two dolomite members (table 1) are generally separated more than 100 feet by the Tamarisk Member. In some localities in the Nash Draw area, solution and collapse removed all the Tam- arisk between them, and the Magenta Dolomite Mem- ber rests directly upon the Culebra Dolomite Member. The Culebra Dolomite Member has been identified in many hundreds of drill holes. Its lithologic char- acter is different from that of all other rocks in the area, and as a result it is an excellent subsurface marker bed. This member is the principal aquifer at the Proj- ect Gnome site and. in addition, yields water to wells in the western half of the area investigated (fig. 1). DEWEY LAKE REDBEDS The Dewey Lake Redbeds crop out at several places near the Project Gnome site, and thick sections of these beds are exposed about 15 miles north of the site. These red beds are composed of a series of fine sandy to earthy red beds and thin beds of sandstone dotted with green reduction spots and generally irregularly veined with thin secondary selen‘ite fillings. The color and tex- ture of the beds are nearly uniform. The Dewey Lake Redbeds are not known to yield water to wells. Small bodies of perched water possibly could be present Where beds of standstone occur within the formation. TRIASSIC FORMATIONS The Santa Rosa Sandstone of Triassic age, which unconformably overlies the Dewey Lake Redbeds, is present a few miles to the east of the Project Gnome site. In Lea County, a series of red shale, siltstone, and sandstone beds, probably the Chinle Formation of the Dockum Group, overlies the Santa Rosa. The Santa Rosa and beds of sandstone within the overlying red beds yield water to wells. TERTIARY FORMATIONS Northeast and southeast of the Project Gnome site, in Lea County, small outcrops of the Ogallala Forma- tion of late Tertiary (Pliocene) age are present, but the formation does not extend westward as far as the project site. The Ogallala yields water to wells in its outcrop area. QUATERNARY FORMATIONS The Gatuna Formation of Pleistocene( ‘2) age is the oldest formation of Quaternary age in the Project Gnome area. The formation crops out at several places and is present at the project site. The formation is HYDROLOGY OF NUCLEAR TEST SITES composed of poorly consolidated sand, silt, and clay with minor amounts of medium- to coarse-grained sand— stone and pebble conglomerate. Locally, the Gatuna yields small supplies of water to wells. The water is perched in permeable sand lenses of limited areal extent. Caliche, alluvium, playa lake deposits, and wind— blown sand are present at or near the Project Gnome site. The caliche forms a cap over older formations in much of the area investigated. It is mostly covered with windblown sand but crops out at many places. At the project site a layer of friable caliche is present from about 7 to 10 feet beneath the land surface, and minor amounts of caliche are present within the unconsoli- dated sand which overlies the Gatuna Formation. About 7 feet of dune sand is present at the site, and in the immediate vicinity there are, conspicuous dunes several tens of feet thick. Alluvial sand and silt, locally conglomeratic, is pres- ent on some gentle slopes and in valley bottoms in the area investigated. The most extensive deposits are on the west side of the Pecos River north of Malaga. Scattered small patches occur along both sides of the river. Near the river and west of the river, the alluvium yields several hundred gallons per minute to wells. Small playas occur mainly in the vicinity of Nash Draw. The floors of the playas consist of silt, siliceous sand, and gypsum sand deposited in shallow and inter- mittent lakes. Laguna Grande de la Sal is a shallow lake that occupies a part of a. large playa in Nash Draw. The lake has no surface outlet and contains a brine solution that has been concentrated by evaporation. Since 1932, Laguna Grande de la Sal has been used for disposal of waste products from the potash refinery of United States Borax and Chemical Co. near the west side of the lake. GROUND WATER Figure 6 shows locations of water-bearing forma- tions in the Project Gnome area. The Culebra Dolomite Member of the Rustler For- mation is the most widespread aquifer in the Project Gnome area. Yields of water from this aquifer vary considerably from place to place and are dependent upon the size and number of openings and fractures in the dolomite; these openings and fractures in turn are apparently related to the thickness of the overlying formations. In the upper part of Nash Draw the dolo- mite is near the surface and has been subjected to ex- tensive solution weathering. Wells in this area are reported to yield as much as 700 gpm (gallons per min- ute). East of Nash Draw, where the dolomite is cov- ered by several hundred feet of younger rocks, yields of wells are generally not large. Water in the Culebra GEOHYIDROLOGY 01“ PROJECT GNOME SITE, NEW IMEXICO A9 . , A” . INK. INC. RAIL till. I 2.! ‘0‘.” E "W 1930' V. 2| 5. V. 2| 5. 122 s. 11:25. 1.235. 7.233. 32' Is‘ 32- II‘ [24 S. 1.21:. T. 25 5‘ V. :5 s. u- oo' n- 00' 130:. T E X A s ”I: ”2: use. Compiuo u, Jumol new»... We! Enu Imm n" III-mo sum mgn-oy quadvcnqu maps Lea County Isel, Eddy County ISSO O 5 10 M l LES L I I I I I I EXPLANATION \ \ \ \ :\ \\\\\\ Wells obtain Water from Wells obtain water from Wells obtain water from Wells obtain water from rocks of Permian age or sand of Tertiary or Quater. sandstone of Triassic age rocks of Permian age or locally from sand of Pleis- nary age or locally from sand and locally from alluvium tocene(?) age or alluvium gravel of Tertiary or Qua- ternary age /// // ///// ____ Wells obtain water Area of brine aquifer Area boundary from alluvium of Permian age FIGURE 6.—General distribution of water-bearing formations in the Project Gnome area. A510 is under artesian pressure, except beneath the alluvium west of the Pecos River and in its outcrop area, where the water in the member is unconfined. The member ap- parently is recharged in the northwestern part of the area, where it is overlain by saturated sandstones of Triassic age. East of the Pecos River the aquifer dis- charges into Nash Draw or into the alluvium which ad- joins the river. The Santa Rosa Sandstone is the principal aquifer in Lea County, in the eastern part of the Project Gnome area. In places where the Santa Rosa is overlain by red beds, probably the Chinle Formation, wells commonly are completed in beds of sandstone of these younger strata. In the upper part of the Triassic section the wa- ter is probably under water-table conditions or, in places, under perched—water conditions. Water in the Santa Rosa is under artesian pressure where it is cov- ered by younger formations. Wells in the south-central part of the area obtain water from beds of sand, each as much as 50 feet thick and separated by somewhat thicker beds of clay, within the deep depression formed by solution of evaporites of Permian age. Most of these deposits probably are of Quaternary age, but some may be of later Tertiary age. The «total thickness of saturated deposits is about 800 feet. Yields of several hundred gallons per minute are obtained from wells finished in these deposits. The pri~ mary source of recharge to these beds of sand probably is local precipitation that infiltrates from large sink- holes and other surface depressions. The Gatuna Formation occurs erratically and con- tains no continuous saturated zone. Water in the for- mation is from local precipitation and is perched over relatively impermeable beds of the Dewey Lake Red- beds. However, at two locations within the study area, beds of sand and conglomerate in the formation yield water to wells. Wells penetrate less than ‘20 feet of sat- urated material, and continuous yields of only a few gallons per minute can be eXpected. West of the Pecos River and north of Malaga, water is obtained from many wells in the alluvium. Most of the wells yield several hundred gallons per minute and are used for irrigation. Water in the alluvium is under water-table conditions and is hydraulically connected with water in the underlying Rustler Formation. Re— charge to the alluvium west of the river is from precipi- tation and from Pecos River water that enters the al— luvium from leaking canals and irrigated fields. In the Malaga. Bend area and west and north of La- guna Grands de la Sal, alluvium extends as much as 3 miles east of the river. Two wells in sec. 14, T. 23 S., R. 28 E., are reported by the owner to yield from 1,500 to 3,000 gpm each. Recharge to the alluvium east of the HYDROLOGY OF NUCLEAR TEST SITES river is from precipitation, inflow of ground water from older formations to the east, excess irrigation wa- ter, and the river itself. On the uplands east of the river only a few wells are finished in alluvium. In Nash Draw and eastward to near the Project Gnome site, an aquifer (the residual layer of the Rus- tler Formation) overlying the salt contains a brine so— lution. This aquifer discharges into the Pecos River, and its discharge is a major factor in the salt contami- nation of the river water. No water is known to occur in the project area below the top of the salt of the Salado Formation. AVAILABILITY AND UTILIZATION Wells within 15 miles of the Project Gnome site ob- tain water at depths ranging from 30 to 800 feet from various formations. The depth to water below the land surface ranges from a few feet to about 500 feet. Ground water is generally under artesian pressure, and the gen- eral direction of movement is toward the Pecos River. Plate 1 shows the location of wells and the general di- rection of ground-water movement in the geologic for- mations in the Project Gnome area. Ground water in the Project Gnome area east of the Pecos River is used for domestic, stock, industrial, irri— gation, and construction supplies. Most of the wells used for domestic supplies also furnish water for stock. Most ranches also have several stock wells on land used for grazing. The El Paso Natural Gas 00., operates two wells to supply water for its turbine station. Many domestic, stock, and irrigation wells are in use on the west side of the Pecos River. The irrigation wells, most of which are north of Malaga, supply water that is used primarily to supplement the river water used for irrigation. At the time of the nuclear experiment, there were 10 privately owned water wells within a 5—mile radius of the test site. These wells were used primarily for water— ing livestock; however, the water from two wells was also used for human consumption. AQUIFER CHARACTERISTICS AT AND NEAR PROJECT SITE At the Project Gnome access shaft and in the Geologi— cal Survey test holes that were constructed on the site (within sec. 34, T. 23 S., R. 30 E.) , water was found only in the Culebra Dolomite Member of the Rustler Forma- tion (fig. 7). There, it is in fractures in the dolomite and is confined by gypsum and anhydrite beds above and by clay and anhydrite beds below. Average thickness of the aquifer is 30 feet, and the top is at a depth of about 500 feet. Water in the aquifer is under artesian pressure and near the access shaft rises to a level about 75 feet GEOHYDROLOGY 0F PROJECT above the dolomite. Water-level data from the vicinity of the project site (pl. 1) indicate that the potentiomet- ric surface slopes generally westward about 12 to 15 feet per mile. Measurements in test [holes at the project site do not indicate this amount of slope. Differences in altitude of the static water level in test hole 1 and test hole 4 (less than one-half mile apart) are 1 foot or less. The direction of movement of wa-ter'a-t the site is west- ward, although both southwesterward and northwest- ward components of movement occur within short dis- tances. Blocks of dolomite from the Culebra Dolomite Mem- ber were obtained during the excavation of the access shaft. These were analyzed for porosity by the US. Geological Survey. One sample, from a depth of 505 feet, had a total porosity of 14.4 percent and an effective porosity of 7.8 percent. A second sample, from 515 feet, had total porosity of 13.7 percent and effective porosity of 11.1 percent. An average value for effective porosity of these samples appears to be about 10 percent. During excavation of the shaft, small-diameter holes were drilled into the Culebra Dolomite Member, and is r4 r5 I28 R 30E. 0 1/2 1 M I LE L_g;1_l__]\__l EX PLA N ATl ON 01 .7 Drilled before Drilled after explosion explosion Test hole and US. Geological Survey number FIGURE 7.—Location of U.S. Geological Survey test holes near the Project Gnome site. GNOME SITE, NEW MEXICO A1.1 cement and chemical grout were pumped into the forma- tion to reduce water inflow into the shaft. TRANSMISSIVITY AND STORAGE COEFFICIENT The transmissivity, T, quantitatively describes the ability of the aquifer to transmit water. It is expressed as the rate of flow of water, in gallons per day, at the prevailing water temperature, through a vertical strip of the aquifer 1 foot wide extending the full saturated height of the aquifer under a hydraulic gradient of 100 percent. The storage coefficient, S, of an aquifer is the volume of water the aquifer releases from or takes into storage per unit surface area of the aquifer per unit change in the component of head normal to that surface (Ferris and others, 1962, p. 74). , The data obtained during the pumping test of March 16 and 17, 1963, were plotted and interpreted according to the Theis nonequilibrium formula (Ferris and others, 1962, p. 92). Average values of 3,500 gpd per ft (gallons per day per foot) for the transmissivity and 2><10*l5 for the storage coefficient were determined for the Cule- bra Dolomite Member at the Project Gnome site. The transmissivity obtained for this aquifer test is somewhat less than that calculated from a similar test made in 1960 (4,000 gpd per f-t, Cooper, 1962, p. 37). However, these values are of the same order of magni- tude, and the difference is not great enough to necessitate changes in estimates of hydraulic conductivity and the rate of water movement in the aquifer. The rate of movement, or the velocity, V, of water in an aquifer is directly proportional to the slope of the po- tentiometric surface, I , and to the hydraulic conductiv- ity of the materials, If ; it is inversely proportional to the porosity, 9, of the materials. The velocity is ex- pressed by the formula V=Kl/0. A calculation of the average rate of movement of water through the Culebra Dolomite Member in the vicinity of the Project Gnome site, using K= 116.7 gpd per square foot, I =4.2 X 10", and 0=0.10, results in a figure of about one-half foot per day. Because the calculation is based on average hydraulic conductivity and porosity, the velocity may be more or less than 1 foot per day in some zones within the aquifer. AQUIFER YIELD Values obtained for the transmissivity and storage coefficient of the Culebra Dolomite Member are ex- tremely small. These data indicate that a well pumping only 100 gpm will create large drawdowns in the well itself, and that the cone of depression Will extend over a large area. A12 10 2O 30 4o 50 Drawdown. in feet 60 70 80. 1 10 HYDROLOG'Y OF NUCLEAR TEST SITES 450 470 Depth to water, in feet below land surface 500 510 100 1000 TIME, IN DAYS FIGURE 8.—Calculated time-drawdown plot of water level in U.S. Geological Survey test hole 1, when pumped continuously, at selected rates, Project Gnome site. Figure 8 shows the drawdown of water level at se- lected pumping rates that could be expected in a well at the Project Gnome site where aquifer characteristics are similar to those determined in Geological Survey test hole 1. (For a description of the aquifer test made in test hole 1, see section on “Testing.”) These calcula- tions are for a well pumping continuously. If pumping is not continuous and the water level is allowed to re- cover, higher pumping rates would be possible. More- over, the calculations do not take into account the pos- sibility that recharge boundaries might be intercepted during long pumping periods, which would make pos- sible higher pumping rates, or conversely, impermeable boundaries might be intercepted, which would make possible lower pumping rates The maximum expected yields, at various pumping rates of the Culebra Dolomite Member at the Project Gnome site, as shown in figure 8, are for a single pumped well. However, little, if any, additional water could be obtained, over a period of more than a few days, by continuous pumping simultaneously from two or more closely spaced wells. For example, pumping 100 gpm simultaneously from each of two wells spaced as far as 3,700 feet apart would seriously 10wer the water level in both wells after a period of about 10 days. TEST HOLES AT AND NEAR PROJECT SITE The most detailed water-level observations prior to the detonation at the Project Gnome site were made in four test holes drilled specifically to observe water levels and to obtain information on aquifer characteristics Additional data were obtained from observations made in the access shaft and from holes drilled for another purposes. Geologic and hydrologic studies made to evaluate the effect of the nuclear explosion on the aquifer at the proj- ect site have in part been integrated with studies made by other geologists of the US. Geological Survey and other investigative agencies. One such study was made to determine the shock-induced solid-state characteris- tics of the rock types at the site. The two drill holes from which rock cores were obtained for this study were completed as a ground-water test hole. A hole drilled at the site to obtain rock cores for use in seismic studies by GEOHYDROLOGY OF PROJECT GNOME SITE, NEW MEXICO Roland F. Beers, Inc., was also completed as a ground- water test hole. This report contains data on the completion of these three holes as ground-water test holes. Results of the study of the rock cores are not included. Locations of all seven test holes are shown in figure 7. CONSTRUCTION U.S. Geological Survey test holes 1 and 2 were drilled in August and September 1960 for the purpose of test- ing the occurrence of water in rocks above the top of the salt of the Salado Formation. Test hole 1 (993 ft south and 393 ft west of the access shaft) was drilled to a depth of 722 feet. The Culebra Dolomite Member of the Rustler Formation, between the depths of 517 and 549 feet, contributed the only water to the hole. After drilling to 722 feet the hole was completed by plugging it with cement from 566 to 722 feet and perforating the casing opposite the aquifer. The water in the hole rose to about 440 feet below land surface (altitude of 2,985 ft above mean sea level). A water sample was collected from the hole for chemical analysis. In test hole 2 (about 2 miles southwest of the access shaft) the Culebra Dolomite Member was dry, and the only water was in the section of rocks between the Cule- bra and the top of the salt of the Salado Formation. The hole was drilled to a depth of 607 feet and com- pleted by slotting the casing opposite the water-bear- ing formations, from 452 to 582 feet below land surface. The water in the hole rose to about 394 feet below land surface (altitude of 3,008 ft above mean sea level). A water sample was collected from the hole for chemical analysis. Although a site was selected for test hole 3, a hole was not drilled. Test holes 4 and 5 were completed in November and December 1961. These two holes are downgradient from the detonation point, in the general direction of water movement, and were drilled for ob- serving water levels and other hydrologic conditions just before, during, and after the explosion. In test hole 4 (394 ft north and 2,375 ft west of the access shaft), water was found only in the Culebra Dolomite Member. The hole was drilled to a depth of 511 feet, and it penetrated the Culebra Dolomite Mem- ber from 472 to 506 feet. It was completed with casing to the top of the dolomite. Water in the hole rose to about 429 feet below land surface (altitude of 2,984 ft above mean sea level). A water sample was collected from the hole for chemical analysis. In test hole 5 (633 ft north and 3,175 ft west of the access shaft), water was found at depths of about 417 feet, in the Magenta Dolomite Member of the Rustler A13 Formation; at about 500 feet, in the Culebra Dolomite Member; and at about 685 feet, in the residual material of the Rustler and Salado Formations directly above the salt of the Salado Formation. The hole is constructed so that only the water in the lower aquifer, just above the salt, can enter the hole. This brine aquifer is only a few feet thick. Water in the hole rose to about 490 feet below land surface (altitude of 2,948 ft above mean sea level). A water sample was collected from the hole for chemical analysis. Test holes 6 and 7 were drilled during the period from January 31 to March 30, 1962. They are approximately 150 feet and 500 feet, respectively, S. 40° E. from ground zero of the detonation point. Upon completion of drill- ing and coring, the drill holes were used as ground-water test holes to observe water levels and to provide sampling points for water in the aquifer. In test hole 6, drilled to a depth of 1,489 feet, water was found only in the Culebra Dolomite Member. The hole was completed by plugging it with cement from 1,489 to 567.5 feet and by perforating the casing opposite the aquifer, between 498 and 532 feet. Water from the aquifer rose to about 417 feet below the land surface (altitude of 2,983 ft above mean sea level). In test hole 7, drilled to a depth of 1,507 feet, water was found only in the Culebra Dolomite Member. The hole was completed by plugging it with cement from 1,507 to 563 feet and by perforating the casing opposite the aquifer, between 514 and 545 feet. Water in the hole rose to about 420 feet below the land surface (altitude of 2,982 ft above mean sea level). Water samples were collected after several hours of hailing from both test holes 6 and 7. However, the analyses indicate that the water sampled was not repre- sentative of the water in the Culebra Dolomite Member but instead was either brackish water used in the drill- ing of the hole or a mixture of drilling water and water from the aquifer. Test hole 8 (alternately identified as the Carlsbad J oint-Use well) was drilled in October 1962. Upon com- pletion of drilling and coring, the drill hole was used as a ground-water test hole for making water-level observations. Test hole 8 is 125 feet due east of test hole 4, and the two holes are hydraulically connected. In test hole 8, drilled to a depth of 722 feet, water was found only in the Culebra Dolomite Member. The hole was completed by plugging it with cement from 7 22 to 495.5 feet and leaving it uncased opposite the aquifer, be- tween 460 and 494 feet. Water in the hole rose to about 429 feet below the land surface (altitude of 2,981 ft above mean sea level). A water sample was collected from the hole for chemical analysis. A14: TESTING In December 1960, excavation work was stopped in the vertical access shaft at a depth of 460 feet, about 35 feet above the top of the Culebra Dolomite Member, and a water-sealing program was begun in the shaft to shut out water from the aquifer. Holes were drilled into the dolomite from the 460—foot level, and a cement and chem- ical grout mix was injected into the formation under pressure. The release of water and pressure through drill holes from the aquifer during the water-sealing pro- gram in the shaft and during the excavation of the dolomite section was recorded graphically on the record- ing gage installed in Geological Survey test hole 1. Water level in the test hole stood about 442 feet below land surface before the grout injection holes were drilled into the aquifer. On December 12 and 13, 1960, after the initial grout injection, several more holes were drilled into the aquifer, and tests were made by the excavation contractor to determine the amount of water flowing from the individual open holes in the shaft. The discharge of water into the shaft from any one hole during this test was reported not to exceed 30 gpm. The discharge of water from the holes into the shaft caused a total water-level decline of about 1 foot in test hole 1. (See fig. 9A) . By February 10, 1961, the water level in test hole 1 had declined to about 444 feet. (See fig. 98.) On Feb- ruary 10, injection of grout behind the concrete shaft liner, against the Culebra Dolomite Member in the in- terval from 515 to 525 feet, reduced water leakage into the access shaft from about 13 gpm to about 5 gpm. In response to this partial sealing of the aquifer, the water level in the observation well rose about 1 foot. An additional water-level rise of about half a foot was noted in the well on February 14, when the interval from 490 to 515 feet was grouted, and the leakage of water into the access shaft was reduced to minor seep- age. After February 14, the water level in the aquifer gradually rose toward its normal level. Water-level fluctuations were also observed in test hole 1 during bailing from test hole 4. (See fig. 90.) On December 4, while mud was being bailed from test hole 4, the water level in test hole 1 temporarily de- clined about one—quarter of a foot. A water-bailing test in hole 4 on December 5, covering a period of 8 hours at a bailing rate of 20 gpm, caused a water-level decline of about 1 foot in test hole 1. The sensitivity of the aquifer to internal water—pres- sure changes was demonstrated by the fluctuations of water levels in test holes 1 and 4 as a result of the 30 gpm discharge into the access shaft. These data suggest that leakage of water out of, or into, the aquifer near the shot point in amounts of 5 to 15 gpm would be HYDROLOGY OF NUCLEAR TEST SITES noticeable on a recording gage installed in an observa- tion well. On March 16 and 17, 1963, a 24-hour pumping and water—level-recovery test was made in test hole 1. The objective of this test was to make a comparison with the pumping-test data obtained in 1960 dur- ing the drilling of the test hole, and to refine data on the transmissivity and other characteristics of the Cule- bra Dolomite Member. Test hole 1 was pumped at a nearly constant rate of 54 gpm for a period of 24 hours with an electric-drive submersible pump. Many of the water-level drawdown measurements made in the pumped well were very er- ratic due to periodic caving of loose silt down the an- nular space behind the well casing, which temporarily caused clogging of the perforations in the casing op- posite the aquifer. The source of the silt is a zone of weathered gypsum from 500 to 509 feet, about 9 feet above the top of the aquifer. These conditions affected only the pumping rates and the color of the discharged water in the pumped well and were not reflected in the measurements made in the two observation wells. At the time of the 1960 pumping test in test hole 1, no observation wells were available in which draWdown measurements could be made to obtain data on the storage coefficient of the aquifer. During the pumping and recovery test in March 1963, observations of water level were made in test hole 6 (1,960 ft N. 40° E. from the pumped well) and in test hole 8 (2,310 ft N. 53° W. from the pumped well) (fig. 7). At the end of the 24-hour pumping period in test hole 1, the drawdown of water level in test hole 8 was 2.74 feet, and that in test hole 6 was 4.08 feet. The water levels in both observation holes and in the pumped hole returned to near normal during the recovery pe- riod of 24 hours. Data obtained from measurements of drawdown and recovery of water levels in the pumped well and, in particular, in the two observation wells were used to compute the transmissivity and the storage coefficient. These data were then used to compute the amount of water, at various pumping levels, that the aquifer would yield over an extended period of time. (See fig. 8). INITIAL INVESTIGATION OF WATER WELLS WITHIN 15 MILES OF PROJECT SITE During the hydrologic investigations from October 1958 through May 1959, all known wells, used or un— used, on the east side of the Pecos River within 15 miles of the Project Gnome site were visited. At that time, water for domestic use for 12 installations was supplied by wells east of the Pecos River. These instal- lations include several ranch headquarters, single GEOHYDROLOGY 0F PROJECT GNOME SITE, NEW MEXICO l Period of water flow from drill holes Normal static water level DEPTH TO WATER, IN FEET BELOW LAND SURFACE l0 ll l2 DECEMBER 1960 ,A. Fluctuations caused by release of water from aquifer through grout drill holes in shaft. l l Approximately 59pm leaking info shaft Approximately Ing M leaking im‘o shaft m \ Final grout injection back V of concrete liner from ‘- 490 to 5|5 feel Grout injection back of concrete liner from 5|5lo 525 feel BELOW LAND SURFACE ,— LLl LL} LL Z Li LLl l— < 3 O p— I ,— CL LL} 0 l 2 FEBRUARY 1961 B. Fluctuations during final phase of water—sealing program in the Gnome shaft. Normal static water level Boiled mud Boiled about 8 hours at Zngm l DEPTH TO WATER, IN FEET BELOW LAND SURFACE 3 4 DECEMBER 1961 (I. Fluctuations during bailing tests at test hole 4. FIGURE 9.—Fluctuation of water level in US. Geological Survey test hole 1 prior to the nuclear explosion, Project Gnome site. A16 , HYDROLOGY OF NUCLEAR TEST SITES homes, and a group of houses at the Pecos turbine sta- wells for insertion of measuring devices and for inspec- tion of the El Paso Natural Gas Co. , tion of the pumps. Fifteen wells, including nine within To accurately determine the water level, well depth, a 5-mile radius of the test site, were investigated in capacity and condition of the pump, casing size, and this manner during March and April 1959. Specific other aspects of some wells in the proximity of the proj- data on the 67 wells investigated are given in table 2. ect site, pumping equipment was removed from the Water samples were collected to determine the chemi- TABLE 2.—Records of wells in the Project Gnome area Owner or name: The oWner of, or name used for, Well at time of visit. Geologic source: Prl; lower member of the Rustler; Prc; Culebra Dolomite Member Altitude: From topographic maps. of the Rustler; Prm: Magenta Dolomite Member of the Rustler;fir, rocks of Triassic Depths: Reported depths are given to the nearest foot; measured depths are given to age; Tu, undifferentiated rocks of probable Quaternary and Tertiary age; Qg; Ga- the nearest 0.1 foot. P, pumping level. tuna ormation; Qal, uaternary alluvium. Diameter: The diameter of the casing, or the mean diameter of the hole, if uncased. Type of pump, poWer, an use: Pump designations: T, turbine; L, cylinder; N, none. Measuring point: pr, edge of pump base; Ls, land surface; Tal, top of air-line flange; PoWer designations: W, windmills; Ic, internal combustion; N, none; Use: S, stock; Tap. top of access pipe; Tc, top of casing; ch, top of concrete block; Tcm, top of metal D, domestic; I, irrigation; In, industrial; N , none. cover; Tpc, top of pipe clamp; Tpp, top of pump pipe; TWc, top of wood cover. Remarks: Name enclosed in quotation marks is local name of Well. CA, chemical analysis available; L, electric logs available; R, reported information. Altl Water level Measuring point tude Diam- De th Distance Type of Location No. Owner or name above Depth eter be ow Date of above Geologic pump, Remarks sea (feet) (inches) land- measure- Descrip- land- source power, level * Surface ment tion surface and use (feet) datum datum (feet) (feet) 1.29. 25.441 A.V. Pugh ........... 3,320 210 10 180 4—15—59 ..................... Pro or L, W, D, s Two wells at this location. Surface Prm casing only, . 25. 441a ..... do ................ 3, 320 210 10 175 4—15—59 ..................... grc or L, Ic, D, S Well cased to total depth, R. rm 21.30. 18.333 Wayne Cowden ....... 3, 220 175. 3 6 124.3 4—15—59 To 0.5 Pro or L, W, S Well not used recently. Prm 22.423 ..... do ................ 3, 180 219. 4 6 104. 6 4—15—59 Tc . 6 Pro L, W, S Wig not used recently. Water salty, 21.31.18.322 Cowden and Smith- .. 3,310 ........ 6 P158. 8 4—15—59 Tc 1. 0 “Er L, W, S h 22. 28. 15. 334a L. J. Culley ........... 3,095 85. 6 6 75.4 4—17—59 Tc .9 Qal L, W, D, S Five wells at this location, two dry, tfiiree used. Water contains gypsum, 15. 334b _____ do ________________ 3,095 86.2 6 76.0 4—17—59 To 1.0 Qal L, Ic, D, S Surface casing only, R. Water con- tains gypsum, R 15. 3340 _____ do ................ 3, 095 88. l 6 86. 5 4—17—59 Tc .6 Qal L, W, S Well not in use. 22. 29. 11. 144 Mark Smith and Sons. 3, 090 170. 9 10 149. 2 4—14—59 Tc .1 Pro L, W, S Do. 33. 241 Mrs. Dublin. 3, 020 69. 9 6 55.0 4—14—59 Tpc .6 Prm (P) L, W, S Two wells at this location. 22. 29. 33. 2413 ._. .dO _____ _ _ . .. . 3, 020 56. 5 6 53. 0 4—14—59 Tc . 1 Prm (7) L,W,S Well not in use. 22. 30. 5. 431 Internation 1 Min- 3, 120 ......... 14 66. 4 4—14—59 Tc 2. 3 Prc N,N,N Unused supply well for potash re- erals and Chemical finery. 14 57. 1 4—14-59 Tc 1. 0 Prc N,N,N Do. 24 104. 8 4—14—59 Two 0 N,N,N Do. 6 60.7 2—19—59 Tc 1 3 Qg(?) L,W,D,S “Ranch Headquarters well." 5 73.0 2—19—59 Tc 3 3 Pro N,N,N Salty water, R. 6 45. 2 2—19—59 Tc 1. 0 Pro L,W,S CA. 6 P158. 2 2—19—59 Tpc 2.8 Er L,W,S Two wells at this location. 3, . 12 150. 9 219-59 c 2.7 “Er N,N,N 2, . 16 16. 2 2— 15-59 To .2 Qal T,Ic,I Well cased to total depth, R. 2, . 16 14. 8 2— 6—59 Tal 1. 1 Qal 'I‘,Ic,I Do. 14. 243 ..... do ................. 2, 980 145. 7 16 14. 7 2— 6-59 12111) 1. 8 Qal T,IC,I Do. 23.30. 2.444 C. H. and W. 0. 3,250 317.6 6 260.8 4— 3—59 To 1.0 Pro N,N,N Well filled. Replace by well 444a at James. same location. L, C . 2. 444a ..... do ................. 3, 250 318. 4 7 260. 5 4-20—59 Tc 1. 0 Prc L,W,S "Little Windmill well." Well cased to 314 feet. Drilled in 1959. 6. 424 James and Briones.... 2, 980 30. 0 6 6. 5 8-19—58 Tc . 0 Qal L,W,S “Nash Well.” 19.123 ..... do ................. 3,045 89.0 7 70.4 4— 7—59 To 1.3 Prc L,W,S “S£u%1AWell.” Surface casing only. 21.122 03H. and w. 0. 3,165 203.6 5 179 2 4— 6—59 To 1.3 Pro L,W,S “deiim wen." Cased to 160 feet. L, ames. . 23. 31. 6. 320 ..... do ................. 3, 300 212. 9 12 144. 7 2— 4-59 Tc 1. 0 ‘Er L,W,S,D Surface casing only, R. Poor quality. water, . 6. 444 ..... do ................. 3, 310 166. 4 6 105. 6 2- 4-59 To 1.8 'Er L,W,S,D "Ranch Headquarters well.” Poor quality water, R. 7. 222 ..... do ................. 3, 300 122. 4 10 1 1 "Hr L,IC,S Well not used recently. 7. 222a _ --do_. . 3, 300 94. 5 5 _. 2 "fir N,N,N Well dry. 7.240 .. .60.. 3,315 138.2 4 .7 2.8 'Er L,W,S “00an well.” L, CA. 17. 310 ..... do_- . 3, 305 354. 0 10 109. 4 3—27—59 . 9 'EF L,W,S “Unger well.” Well uncased. L, CA. 23. 31. 20. 340 C.JH., W , . 3, 480 361. 3 6 P256. 9 2—4—59 Tpc l. 2 "Fr L, W, S “E§rview well.” Well uncased, R ames. . 29.113 C.J H. and W. 0. 3,335 223.9 4 138. 4 3—26—59 To .7 Fr L, W, S “VIVJalker well.” Surface casing only. ames. , CA. 23. 32. 4. 222 ..... do ............... 3, 630 550 L, W, S “(Iléifton well." Surface casing only, 21.222 0. H., W. 0., and F. 3, 700 550 8 L., Ic, S “Swag well." Two wells at this 10- James. cation. Surface casing only, R. 21. 222a ..... do ............... 3, 700 550 6 L, W, S Well cased to total depth, R. 24. 30. 8. 113 Bill Eaton ____________ 3,280 191.9 6 176.0 3—23-59 Tc .0 Qg L, W, S, D “Ranch Headquarters well." Well cased to 190 feet. L, CA 12. 430 W. M. Snyder ......... 3, 510 500 6 367. 1 6—14—61 Tc . 6 '5r L, Ic, S “Poker well.” 18.231 Bill Eaton ____________ 3,200 451.6 6 227.8 3—19-59 Tc .4 1;“: or L, W, S “Two Iflilginill.” Well cased to 229 r feet. , . 23. 312 W. M. Snyder ......... 3, 425 428.1 6 423.1 3—26—59 Tc . 7 'Er L, W, S “New well." Well cased to total depth, R. Well reported to be 474. feet deep. L. 36. 333 ..... do ............... 3, 450 476. 8 6 445. 3 3—19—59 Tc 1. 2 QTu L, W, 8 “Wind: well." Well cased to 412 feet 2 GEOHYDROLOGY OF PROJECT GNOME SITE, NEW MEXICO A17 TABLE 2.—Records of wells in the Project Gnome area—Continued Alti Water level Measuring point tude Diam- De th Distance Type of Location No. Owner or name above Depth eter be ow Date of above Geologic pump, Remarks sea (feet) (inches) land- measure Descrip- land- source power, level surface ment tion surface and use (feet) datum datum (feet) (feet) 24. 31. 4.430 W M Snyder ........ 3, 626. 5 5 423. 6 3—13r59 ch 0.9 Pro L, W, S “Ingle well.” Well not cased. L CA1 17.111 do 85.0 7 68. 4 3—25—59 To 1.2 Qg L, W, S, D “Ranch Headquarters well. " We.l cased to total depth R. L, CA. 33.124 698.0 5 474.2 3—12-59 To 1.0 Pro L, W, S “Keyhole well ” Well cased to total depth, R. L, CA. 24. 32. 3. 322 550 10 .......................................... Fr L, W, S “New well. " Two wells at this lo cation. Surface casing only, R. 3. 322a 500 8 .......... 4—13-59 ...................... 'fir N, N, N We‘llll dry and caved in. Surface casing 10.344 60 6 33. 6 4-13-59 Tc 1. 0 Qal L, W, S, D “Ranch Headquarters well." Surface casing only, 24.32.33.422 366.4 12 313.4 2—18—59 Tc .6 L, W, S “Burro well.” 25.29. 2.111 140.0 8 100. 6 10—23—58 Ls .0 Pro ('2) N, N, N Potash test hole. Drilled to 857 feet. 16.444 200(?) 6 170.1 8-19-58 Tc 1.2 Pro L, W, S “Iaicktetttgveélg Well cased to total ep , . . 32. 211 110. 6 8 98. 7 3—24-59 Tc .9 Pro N, N, N Surfage casing only. Potash test hole. 25. 30. 7. 111 W. M. Snyder _________ 3, 170 385. 6 7 263. 3 3— 7—59 Tc .0 QTu L, W, S "Carper well." Well cased to 250 feet. 011 test hole converted to water well. L, C 7. 330 Ralph Lowe __________ 3,180 295.0 .......... 6—14—51 ...................... QTu N, N, N Drilled to supply water for oil tests. 8, 224 W. M. Snyder. _ _ _ 3, 220 343. 5 7 309. 7 8-19-58 Tc . 0 QTu N, N, N Three wells at this location. 8. 224a ..... do ........ 3,220 ............................................................ QTu N, N, N Hole crooked, R. .224b ..... do ........ 3, 220 ________ 7 P332. 55 6-14—61 Tc 1. 0 QTu L, W, S “Tomcat well. " 12. 113 _________________ __ 3,375 460.3 5 391.3 3—25—59 Tc .7 QTu N, N, N Drilled to supply water for oil test. L. 21. 333 J. G. Ross ............ 3,200 298.1 6 P2661 2— 5—59 To 1.0 QTu L, W, S, D Well cased to total depth, R. CA. 25.31.21.400 Mrs. E. R. Johnson 3,340 400 7 P318.0 2—17—59 Tc .4 QTu L, W, S, D Do. and others. 25.33. 20. 443 ........................ 3, 395 2g?)- 6 .......................................... F L, W, S 26. 29. 22. 340 J. G. Ross ............ 2,875 200(? 6 68. 7 8—19-58 Tc 2. 0 Pro L, W, S Well not used recently. 26.30. 5.334 El Paso Natural Gas 3,090 770 11 169.9 2—18—59 pr 1.9 QTu T, Ic, In, D Water well No.1 Pecos Turbine sta- Co. tion. Cased to total depth. 5.343 ..... do ............... 3, 100 775 11, 9 182. 6 8-18—58 Top 3. 3 QTu '1‘, Ic, In, D Water well N o 2 Pecos Turbine station. Cased to total depth. CA 8. 111 Mrs.dE. R. Johnson 3,085 400 7 163.8 2—18—59 Tom .3 QTu L, W, S “West well.” an others. 26. 31. 8. 310 Ross Estate ___________ 3, 230 309. 6 6 P2811 2—18—59 Tc 1. 4 QTu L, W, S, D “Ranch Headquarters well.” Two wells at this location. 8. 310a ..... do ............... 3, 230 324. 5 6 275. 8 8-18-58 Tc 1. 5 QTu N, N, N Well has never been placed in service cal characteristics and radioactivity background level of the water in the study area before the nuclear explo- sion. Results of chemical and radiochemical analyses are given in tables 3 and 4. Because of the lapse of time between the nuclear event and the original investigation of the nine privately owned wells within a 5-mile radius of the test site, a ' second investigation of the wells (and of an additional well drilled in May 1961) to establish their depth and water level and to determine the condition of pumps was made November 23 to December 9, 1961. For both investigations the pumps were removed by a contractor of the U.S. Atomic Energy Commission, and measure- ments were made by personnel of the U.S. Geological Survey. The owner, depth of well, depth to water, and condi- tion of the pump for each well within the 5-mile radius are listed in table 5. Depth and water-level data ob- tained from both investigations are included. OBSERVATIONS AT TIME OF NUCLEAR EXPLOSION At the time of the nuclear explosion, ground-water levels close to the site were under observation at U.S. Geological Survey test holes 1, 2, 4, and 5. Observations were also made in two wells near the Malaga Bend of the Pecos River south of Carlsbad, N. Mex., 9 miles West of the project site, and in others wells to the north in the Roswell artesian basin. The four observation wells at and near the project site had float-type continuous recording gages. The re cording gages consist essentially of a height—element mechanism to register the level of the water surface and a clock movement which feeds a chart at a constant rate while a marking stylus moves laterally across the chart and produces a graphic record of water level against time. VVater-level changes are transmitted to the height- clement mechanism and stylus by a wire line attached to a counterweighted float which rests on the water surface. The gages were equipped with a time—element mechanism which moved the chart at a rate of 0.1 inch per hour. The height element registered a graph change of 2 inches for each foot of water-level change in holes 1 and 4, and 10 inches for each foot of water-level change 1n holes 2 and 5. The recording gages were housed 1n metal shelters over the well casing. The shelters were securely bolted to a concrete platform, and the gages were fastened to the shelters. For several days before the explosion, the HYDROLOGY OF NUCLEAR TEST SITES A18 AK ms coaHA $N Am» 8A .A 8 V A .V o A c3 3N 8A a .u \ wnN A .V an 8 8. «A A .V AN onuwAuN g6 .omnoN Q. as com A #8 Ash §A 8 V A .u «A No «2. SA ci NvA A.V 3 “AN Ac. AA" N. 3 $INA|N §.AN.AN 3 AA. cs oNo.N on» va omN .A Ao.V ed AA o5 5m a: cd 3N AV 3 AMA A¢.V mm. A.V aN Sun IN ammAN on cum 3%. S. . AKA. A3. 8V A.V mA Nu . wmA . ANA N.N 3A mm. N 3 8V Ni m. Nd 313$ AAA .5 .8 uN AN m c 80.: 3“. N 8mm 8A.: A: V A.V cA 8n n 8N A ANN Nan Bnfi «.m mnN Nah an vA m. S $¢N|n AANHNM 3 ms SFN can. SPA §.A Ac V A.V AA :3. Sb mnA ad «AA AV 8 5N A... 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GEOI-IY‘DROLOGY 0F PROJECT GNOME SITE, NEW MEXICO A19 TABLE 4.—Radiochemical analyses of well water in the PrOJect Gnome area Algae activity as Beta Net Date equivalent activity as Radium extractable ' of SrW-YW Uranium as Ram alpha, Strontium-90 Location No. collection 058/1) (Dc/l) U equivalent (pc/l) (pc/l) Date (poll) Date (poll) determined determmed 22. 30. 32. 111 2—19—59 21 6— 8—59 72 7— 7—59 18 0. 3 <4. 0 <5 23. 30. 2. 444 3—26—59 71 6— 8—59 200 7— 6—59 38 l3 l7 <5 19. 123 2— 6-59 <21 6— 5—59 72 7— 6—59 9. 5 .6 <2 <5 21. 122 4— 1—59 <35 6— 9—59 120 7— 7—59 16 .5 10 <5 23. 31. 7. 240 4- 3—59 11 6— 9—59 12 6—23—59 6. 3 . 2 4. 3 __________ 17. 310 2~ 4—59 62 6— 8—59 100 7— 7—59 8. 6 . 1 3 <4 26. 340 2— 4—59 100 5—26—59 210 7— 7—59 16 . 8 <3 <5 29. 113 10—23—58 23 3—10—59 25 4—22—59 9. 0 <. 1 2. 5 <5 29. 113 4— 1—59 <25 6- 9—59 27 6—23—59 9. 9 .4 <2 <5 24. 30. 8. 113 10—24—58 3. 0 3— 9—59 3. 1 4—22—59 2. 8 . 1 <. 8 <5 8. 113 3—19—59 6. 0 6— 8—59 18 7— 6-59 1. 7 . 2 . 5 <5 18. 231 3—25—59 <36 5—26—59 320 7— 6—59 2. 8 . 9 4. 2 <5 36. 333 3—31—59 3. 9 6— 9—59 30 7— 6—59 5. 2 .2 3. 8 <5 24. 31. 4. 430 3—19—59 67 6— 8—59 250 7— 6—59 9. 2 4. 7 <2 <5 17. 111 3—31—59 <7. 4 6— 9—59 36 7—16—59 4. 1 1. 8 l. 2 <5 33. 124 3—31—59 110 6— 9—59 81 6—16—59 32 9. 8 57 <5 25. 29. 16. 444 2— 5—59 16 6— 5—59 98 7— 7—59 6. 6 . 1 <2 <4 32. 211 3—24—59 <52 5—26—59 670 7— 6—59 3. l . 7 2. 0 <5 25. 30. 7. 111 4—14—59 4. 5 8— 4—59 10 6—23—59 1. 0 2. 8 8. 4 <5 21. 333 2— 5—59 43 6— 5—59 74 7— 7—59 6. 2 . 2 1. 9 <4 25. 31. 21. 400 2—12—59 <12 6— 8—59 48 7— 6—59 9. 6 . 2 4 <6 26. 30. 5. 343 2—18—59 28 6— 8—59 68 7— 7—59 1. 1 . 4 <. 2 <4 gages were visited periodically and adjusted for time and gage height. The final check was made 11 hours be- fore the detonation. On December 11, about 24 hours after the explosion, records were collected at the four observation wells. The following effects were noted. The concrete platform, 6 inches thick and 6 feet square, around the well casing at test hole 1 contained cracks, as much as 1/8 inch wide, radiating outward from opposite sides of the well casing. Although no cracks TABLE 5.—Water wells within a 6-mile radius of Project Gnome site, Eddy County Depth Depth Date Condition Location No. Owner of to measured of pump well water 23. 30. 2. 444a C. H. and 318. 4 260. 5 4—20—59 Good. W. 0. James. 316. 4 259.0 11—25—61 Do. 19. 123 James and 89.0 70. 4 4- 7—59 Fair. Briones. 88. 2 71. 3 11—23-61 Fair to bad. 21.122 C. H. and 203. 6 179. 2 4—- 6-59 Fair. W. 0. James 204.5 177. 1 11—26-61 Fair to bad. 23. 31. 17. 310 _____ do ___________ 354. 0 109. 4 3—27-59 Good. 350. 3 108. 2 11-27—61 Do. 29. 113 _____ do ___________ 223. 9 138. 4 3-26—59 Do. 222. 8 140. 2 11—29-61 Do. 24. 30. 8. 113 Bill Eaton ________ 191. 9 176.0 3—23—59 Do. 193. 4 176. 3 12— 8-61 Do. 12.430 W. M. Snyder _____ 495. 6 364. 6 12— 7-61 Do. 18.231 Bill Eaton ......... 451. 6 227. 8 3—19-59 Fair 449.8 231. 9 12— 9-61 Do. 23. 312 W. M. Snyder _____ 428.1 423.1 3—26—59 Good. 456. 4 426. 2 12— 7-61 D0. 24. 31. 17. 111 ..... do ........... 85. 0 68. 4 3—25—59 Do. 80. l 66. 2 12— 7—61 Fair to good l were observed in the ground adjacent to the platform, beginning about 25 feet south of the well a. circular crack 30 to 40 feet in diameter and ranging in width from a hairline to one-fourth of an inch was observed. This crack appears to have formed over the perimeter of the filled-in mud pit used when the well was drilled. The metal shelter and gage housing at test hole 1 were undamaged. However, the recording gage had been badly shaken, and the record ended shortly after the explosion. The stylus recorded a partial trace of water-level rise, from which the maximum fluctuation was estimated. There was no apparent damage to the concrete plat- form, metal shelter, or gage housing at test holes 2 and 5. Both recording gages had been shaken and had not operated properly after the explosion. Incomplete traces of water-level changes immediately following the ex- plosion were apparent on both charts. The only disturbance at test hole 4 was minor mis- alinement of the recording gage. The recorder contin- ued to operate and traced water-level fluctuations that immediately followed the explosion. Figure 10 is a hydrograph of test hole 4 shewing the fluctuations of water level at the time of the nuclear explosion. The water level responded instantaneously to pressures exerted in the aquifer. The water level rose 2.2 feet, and the peak of the rise was followed immedi- ately by a sharp decline of water level. The decline grad- A20 ually slowed, and the water level in the well returned to normal after about 11 hours. A summary of water-level changes in the four obser- vation wells, at and immediately after the explosion, as indicated by recording gages, follows: Test hole 1 (2,000 ft from ground zero), estimated rise of 3.97 feet; test hole 2 (about 2 miles from ground zero), rise of at least 0.50 foot; test hole 4 (about 3,200 ft from ground zero), rise of 2.2 feet; and test hole 5 (about 4,000 ft from ground zero), rise of at least 0.40 foot. Water levels in all observation wells were measured with a steel tape on December 12, 1961, 2 days follow- ing the nuclear explosion. The water levels in the wells were essentially the same as those measured before the explosion. Only insignificant changes in altitude of the \measuring points at these wells resulted from the ex- plosion. Wells 1, 4, and 5 were resurveyed in January 1962. Test hole 1 was 0.09 foot lower than before the explosion, and test hole 4 was 0.04 foot lower. No change was found at test hole 5. INVESTIGATIONS AFTER THE NUCLEAR EXPLOSION WATER-LEVEL FLUCTUATIONS Water levels were observed in Geological Survey test hole 1 from September 1960 to June 1963. During most of this period a continuous water-stage recording gage was operated in the test hole. The range of fluctuations of water level from the “initial observations in Septem- ber 1960 to the detonation of December 10, 1961, was about 1 foot. Figure 11 is a hydrograph showing the fluctuations of water level observed in test hole 1 during the period of record. The depth-to-water measurements shown on the hydrograph are daily 'high measurements recorded on the first day of each month. No measurement is shown for February 1961, as grouting in the access shaft caused abnormal lowering and fluctuations of water level in the aquifer (fig. 9). During the period of record the general pattern was one of slowly rising water levels. The maximum varia- tion in depth to water was about 21/; feet. The general pattern of waterslevel fluctuation in test hole 2, also under observation from October 1960 to June 1963, was similar to that observed in test hole 1. However, the fluctuations of water level in test hole 2 were not as large as those in test hole 1. EFFECTS OF ATMOSPHERIC PRESSURE AND EARTH TIDES Water levels in wells tapping artesian aquifers re- spond to changes in atmospheric pressure. An increase HYDROLOGY OF NUCLEAR TEST SITES in atmospheric pressure causes the water level to de- cline, and a decrease in atmosPheric pressure causes the water level to rise. Water—level fluctuations pro— duced by earth tides, which are caused by the forces exerted on the earth’s crust by the sun and the moon, have also been observed in artesian wells. The effects of both atmospheric pressure and earth tides on water levels in the Oulebra Dolomite Member at the project site are recognized on the charts of the water- level recording gage in test hole 1. Figure 12 is a reproduction of this recorder chart for January 9—14, 1963. The atmospheric pressure, expressed in feet of water, was recorded on barographs at test holes 4 and 8. The effects of atmospheric pressure and earth tides on water levels, shown in figure 12, are insignificant in relation to the effects on water levels that would be caused by leakage of water from the aquifer into the Gnome cavity. The traces of the water level and of the barometric pressure are in general similar in their major fluctua- tions; however, in detail, they are dissimilar. The water levels in the aquifer show a fairly regular semidiurnal fluctuation of a few tenths of a foot, with high-water FIGURE 10.—Fluctuation of water level in US. Geological Survey test hole 4 at time of Project Gnome explosion, Project Gnome site. GEOHYDROLOGY OF PROJECT GNOME SITE, NEW MEXICO 439 441 DEPTH TO WATER. IN FEET BELOW LAND SURFACE 443 1961 FIGURE 11.—Water level in US. Geological Survey test hole 1, 1960—63, Project Gnome site. 1960 levels near midnight and noon, and low-water levels near 0600 and 1800. These semidiurnal fluctuations are similar to fluctuations caused by earth tides, observed I by Robinson (1939, p. 656) in a well about 9 miles north- west of the Project Gnome site. During periods of strong atmospheric pressure changes, such as January 11—13, 1963, the effects of earth tides are largely masked, and theprincipal cause of water-level fluctuations is barometric. The separate effects of atmospheric pressure and of , 2 FEET OF WATER ..- 10 11 1962 ‘1963 l earth tides upon water levels in the aquifer at the Project Gnome site. have not been thoroughly investi- gated. These cyclic phenomena, however, are recognized and are given consideration in the interpretation of changes of water level in the aquifer. ‘ CONCLUSIONS Initial geologic and hydrologic investigations, in- cluding an examination of all wells within: 15 miles of the Project Gnome site, showed that water: is obtained 13 ‘ 14 12’ JANUARY, 1963 ‘ Flam 12.—Fluctuations of water level in US. Geological Survey test hole 1 and inverted fluctuations of atmospheric pressure, January 9—14, 1963, Project Gnome site. 1 A22 from wells tapping rocks of various ages. Most of the ground water in the area is saline; however, because it is the principal source of stock water, and is widely utilized as such, it is a valuable resource. A brine aquifer (residual layer of the Rustler Formation) west of the site and in Nash Draw, from which no water is utilized for beneficial purposes, causes a deterioriation in the chemical quality of the Pecos River waters downstream from the site. The aquifers lie above the top of the mas- sive salt of the Salado Formation (the formation in which the Project Gnome nuclear device was deto- nated). No water is known to exist within the salt, and no fresh water is known to be in the underlying rocks. The Culebra Dolomite Member of the Rustler Forma- tion is the only aquifer at the Project Gnome site. This aquifer lies nearly 200 feet above the top of the massive salt and about 500 feet beneath the land surface. \Vater in the aquifer moves past the project site, at an estimated rate of one-half foot per day, in a generally southwest- ward to westward direction from recharge areas to the northeast, and ultimately discharges into the Pecos River. Observations of water level in the aquifer at the Project Gnome site, made for several months before the nuclear explosion, showed that natural fluctuations of water level were small. However, pressure changes in the aquifer, due to the release of water through drill holes and removal by bailing, were readily detected by a recording gage installed in an observation test hole at the site. VVater-level observations made at the time of and within a few days following the detonation showed no evidence of water leaking into or out of the aquifer at the Project Gnome site, suggesting that the Culebra Dolomite Member probably was not significantly rup- tured by the explosion. Moreover, there was no apparent abnormal change of water level in the aquifer beneath the member and above the top of the salt of the Salado Formation in test holes 2 and 5. Water-level records were examined for other wells in the Carlsbad area and to the north in the Roswell ar- tesian basin. In many of these wells the water level normally fluctuates a few tenths of a foot in response to natural earthquakes. In the two wells near Malaga Bend, 9 miles west of the Project Gnome site, fluctua- tions of 0.06 foot persisted for a few seconds after the nuclear explosion, but no fluctuation of water level was detected in any other well under observation in the R03well basin. Observations of water level at the Project Gnome site were continued after the nuclear explosion, and the net- work of points where the potentiometric surface can be observed was expanded by the use of three drill holes HYDROLOGY OF NUCLEAR TEST SITES as observation wells. Since the time of the nuclear ex- plosion no anomalous water-level fluctuation has been observed in the continuous record of water levels meas— ured in test hole 1. Water levels in the Culebra Dolomite Member at the Project Gnome site are affected by changes in atmos- pheric pressure and by effects attributed to earth tides, although these effects have not been intensively studied for this report. Data obtained from a pumping test on test hole 1 and from concurrent measurements in test holes 6 and 8 were used to estimate the potential yield of wells at the Project Gnome site. It was estimated that 100 gpm could be pumped from a well continuously for a period of 1 year without lowering the water level below the top of the aquifer. Larger quantities of water could be pumped for shorter periods of time, and smaller quan- tities could be obtained for much longer periods before the water level would be lowered to the top of the aquifer. SELECTED REFERENCES Adams, J. E.. 1944, Upper Permian Ochoa series of Delaware basin, West Texas and southeastern New Mexico: Am. Assoc. Petroleum Geologists Bull., v. 28, no. 11, p. 1596—1625, 4figs. Adams, J. E., and Frenzel, H. N., 1950, Capitan barrier reef, Texas and New Mexico: Jour. Geology, v. 58, no. 4, p. 289—312. Adams, J. E., and others, 1939, Standard Permian section of North America: Am. Assoc. Petroleum Geologists Bull., v. 23, no. 11, p. 1673—1681. Baltz, E. H., 1959, Diagram showing relations of Permian rocks in part of Eddy County, New Mexico: U.S. Geol. Survey TERI—1035, open-file report. Bjorklund. L. J., and Motts, W. S., 1959, Geology and water resources of the Carlsbad area, Eddy County, New Mexico: 13.8. Geol. Survey open-file report, 322 p., 15 p1s., 56 figs. Blanchard, W. G., J r., and Davis, M. J ., 1929, Permian stratig- raphy and structure of parts of southeastern New Mexico and southwestern Texas: Am. Assoc. Petroleum Geologists Bull., v. 13, no. 8, p. 957—995. 2 p1s., 10 figs. Cooper, J. B., 1960, Geologic section from Carlsbad Caverns National Park through the Project Gnome site, Eddy and Lea Counties, New Mexico: U.S. Geol. Survey TEI—767, open-file report. 1961, Test holes drilled in support of ground-water in- vestigations, Project Gnome, Eddy County, New Mexico— basic data report: U.S. Geol. Survey TEI—786, open-file report. 116 p., 12 figs. 1962, Ground-water investigations of the Project Gnome area, Eddy and Lea Counties, New Mexico: U.S‘. Geol. 'Survey TEI—802, open-file report, 67 p., 17 figs. Cooper. J. B., and others, 1962, Hydrologic and geologic studies for Project Gnome, preliminary report: US. Geol. Survey Project Gnome Report PNE—130P, 54 p. [Available from Clearinghouse for Federal Scientific and Technical In- formation, U.S. Dept. Commerce, Springfield, Va. 22151] GEOHYDROLOGY 0F PROJECT GNOME SITE, NEW MEXICO Crandall, K. H., 1929, Permian stratigraphy of southeastern New Mexico and adjacent parts of western Texas: Am. Assoc. Petroleum Geologists Bull., v. 13, no. 8, p. 927—944, 1 pl., 6 figs. Dane, C. H., and Backman, G. 0., 1958, Preliminary geologic map of the southeastern part of New Mexico: U.S. Geol. Survey Misc. Geol. Inv. Map I—256. Darton, N. H., 1928, “Red Beds” and associated formations in New Mexico. with an outline of the geology of the State: U.S. Geol. Survey Bull. 794, 356 p., 62 pls [1929] DeFord, R. K., and Lloyd, E. R., 1940, Editorial introduction, in West Texas—New Mexico symposium, pt. 1: Am. Assoc. Petroleum Geologists Bull., v. 24, no. 1, p. 1—14, 3 figs. Dickey, R. I.. 1940, Geologic section from Fisher County through Andrews County, Texas, to Eddy County, New Mexico, in DeFord, R. K., and Lloyd, E. R., [eds], West Texas—New Mexico symposium, pt. 1: Am. Assoc. Petroleum Geologists Bull., v. 24, no. 1, p. 37—51, 1 fig. Fenneman, N. M., 1931, Physiography of western United States: New York. McGraw-Hill Book Co., 534 p. Ferris, J. G., Knowles, D. B., Brown, R. H., and Stallman, R. W., 1962, Theory of aquifer tests: U.S. Geol. Survey Water- Supply Paper 1536—E, 174 p., 29 figs. Flawn, P. T., 1954, Texas basement rocks—a progress report: Am. Assoc. Petroleum Geologists Bull., v. 38, no. 5, p. 900~912,2figs. Gard, L. M., Jr., 1968, Geologic studies, Project Gnome, Eddy County, New Mexico: U.S. Geol. Survey Prof. Paper 589, 33 p., 15 figs. Gard, L. M., Cooper, J. B., and others, 1962, Hydrologic and geologic studies for Project Gnome: U.S. Geol. Survey Project Gnome Report PNE—130F, 196 p. [Available from Clearinghouse for Federal Scientific and Technical In- formation, U.S. Dept. Commerce, Springfield, Va. 22151] Hale, W. E., Hughes, L. S., and Cox, E. R., 1951, Possible im- provement of quality of water of the Pecos River by diver- sion of brine at Malaga Bend, Eddy County, New Mexico: Pecos River Commission, New Mexico and Texas, in coop- eration with U.S. Geol. Survey, Water Resources Div., Carlsbad, N. Mex., 43 p., 8 pls., 5 figs. Hale, W. E., and Clebsch, Alfred, Jr., 1958, Preliminary ap- praisal of ground-water conditions in southeastern Eddy County and southwestern Lea County, New Mexico: U.S. Geol. Survey TEM—1045, open-file report, 23 p., 3 figs. Hayes, P. T., 1958, Salt in the Ochoa series, New Mexico and Texas: U.S. Geol. Survey TEI—709, open-file report, 28 p., 4 figs. Hem, J. D., 1970, Study and interpretation of the chemical characteristics of natural water [2d ed.] : U.S. Geol. Survey Water—Supply Paper 1473, 363 p., 2 pls., 51 figs. Hendrickson, G. E., and Jones, R. S., 1952, Geology and ground— water resources of Eddy County, New Mexico: New Mexico Bur. Mines and Mineral Resources, Ground-water Rept. 3, 169 p., 6pls., 11 figs. Hughes, P. W., 1954, New Mexico's deepest oil test, in New Mexico Geologic Soc. Guidebook 5th Field Conf., south- eastern New Mexico: p. 124—130. Jones, C. L., 1954, The occurrence and distribution of potassium minerals in southeastern New Mexico, in New Mexico Geol. Soc. Guidebook 5th Field Conf., southeastern New Mexico: p. 107—112. A23 1959. Thickness, character, and structure of Upper Permian evaporites in part of Eddy County, New Mexico: U.S. Geol. Survey TERI—1033, open-file report, 19 p. 2 figs. King, P. B., 1942, Permian of west Texas and southeastern New Mexico, pt. 2 of DeFord and Lloyd [eds], West Texas-New Mexico symposium: Am. Assoc. Petroleum Geologists Bull., v.26, no. 4, p. 535—763, 2 pls., 2 figs. 1948, Geology of the southern Guadalupe Mountains, Texas: U.S. Geol. Survey Prof. Paper 215, 183 p., 23 pls., 24 figs. Kroenlein, G. A., 1939, Salt, potash, and anhydrite in Castile Formation of southeast New Mexico: Am. Assoc. Petroleum Geologists Bull., v. 23, no. 11, p. 1682—1693, 3 figs. Kulp, J. L., 1961, Geologic time scale: Science, v. 133, no. 3459, p. 1111. Lang, S. M., 1961, Methods for determining the proper spacing of wells in artesian aquifers: U.S. Geol. Survey Water- Supply Paper 1545—B, 16 p., 2 figs. Lang, W. B., 1935, Upper Permian formations of Delaware basin of Texas and New Mexico: Am. Assoc. Petroleum Ge- ologists Bull., v. 19, no. 2, p. 262—270, 7 figs. 1937, The Permian formation of the Pecos Valley of New Mexico and Texas: Am. Assoc. Petroleum Geologists Bull., v. 21, no. 7, p. 833—898, 29 figs. 1939, Salado Formation of the Permian basin: Am. As- soc. Petroleum Geologists Bull., v. 23, no. 10, p. 1569-1572. Lloyd. E. R., 1929, Capitan Limestone and associated forma- tions of New Mexico and Texas: Am. Assoc. Petroleum Ge- ologists Bull., v. 13, no. 6, p. 645—658, 1 fig., 2 tables. Maley, V. C., and Hufiington, R. M., 1953, Cenozoic fill and evaporite solution in the Delaware Basin, Texas and New Mexico: Geol. Soc. America Bull., v. 64, no. 5, p. 539—545, 3 pls., 2 figs. Meinzer, O. E., 1923a, The occurrence of ground water in the United States with a discussion of principles: U.S. Geol. Survey Water-Supply Paper 489, 321 p., 31 pls., 110 figs. 1923b, Outline of ground-water hydrology, with defini- tions: U.S. Geol. Survey Water-Supply Paper 494, 71 p., 35 figs. Moore, G. W., 1958, Description of core from A.E.C. drill hole no. 1, Project Gnome, Eddy County, New Mexico: U.S. Geol. Survey TEM—927. open-file report, 27 p. Needham, C. E., and Bates, R. L., 1943, Permian type sections in central New Mexico: Geol. Soc. America Bull., v. 54, no. '11, p. 1653—1667, 2 figs. Newell, N. D., and others, 1953, The Permian reef complex of the Guadalupe Mountains region, Texas and New Mexico— a study in paleoecology: San Francisco, W. H. Freeman and 00., 236 p. New Mexico Department of Public Health, 1955, Fluoridation in New Mexico—its present status: New Mexico State Den- tal J our., v. 5. no. 4, February 1955. New Mexico Geological Sdciety, 1954, Guidebook of southeast- ern New Mexico, 5th Field Cont, Southeastern New Mex- ico, 1954: 209 p. Nicholson, Alexander, Jr., and Clebsch, Alfred, Jr., 1961, Ge- ology and ground-water conditions in southern Lea County, New Mexico: New Mexico Bur. Mines and Mineral Re- sources Ground-Water Rept. 6, 123 p., 2 pls., 30 figs. Robinson, T. W., 1939, Earth tides shown by fluctuations of water levels in wells in New Mexico and Iowa: Am. Ge- ophys. Union Trans, pt. 4, p.‘ 656—666. A24 Robinson, T. W., and Lang. W. B.. 1938, Geology and ground- water conditions of the Pecos River valley in the vicinity of Laguna Grande de la Sal, New Mexico. with special ref- erence to the salt content of the river water: New Mexico State Engineer 12th-13th Bienn. Repts., 1934—38, p. 77— 100, 5 pls., 3 figs. Skinner. J. W., 1946, Correlation of Permian of west Texas and southeast New Mexico: Am. Assoc. Petroleum Geologists Bull., v. 30. no. 11, p. 1857—1874. Stipp, '1‘. F., and Hagler, L. B., 1956. Preliminary structure con- tour map of southeastern New Mexico showing oil and gas development: U.S. Geol. Survey Oil and Gas Inv. Map OM— 177 [1957]. Theis. C. V., 1935, The relation between the lowering of the piezometric surface and the rate and duration of discharge of a well using ground-water storage: Am. Geophys. Union Trans. pt. 2, p. 519—524. 3 figs. Theis, C. V., and Sayre, A. N., 1942, Geology and ground water, in [U.S.] Natl. Resources Planning Board, 1942, Pecos River Joint Investigation—Reports of the participating agencies: Washington, U.S. Govt. Printing Office, p. 27—38, 2figs. U. 5. GOVERNMENT PRINTING OFFICE: 1971 O - 428-472 HYDROLOGY OF NUCLEAR TEST SITES Theis. C. V.. and others. 1942. Ground-water hydrology of areas in the Pecos Valley, New Mexico, in [U.S.] Natl. Re- sources Planning Board. 1942, Pecos River Joint Investi- gation—Reports of the participating agencies: Washington, US. Govt. Printing Oflice. p. 38—75, 11 figs. [U.S.] National committee on radiation protection, 1959, Max- imum permissible body burdens and maximum permissible concentrations of radionuclides in air and in water for oc- cupational exposure: Natl. Bur. Standards Handbook 69: Washington, U.S. Govt. Printing Office, 95 p. [U.S.] National Resources Planning Board; 1942, Pecos River Joint Investigation—Reports of the participating agencies: Washington, U.S. Govt. Printing Ofiice, 407 p. [U.S.] Public Health Service, 1962, Drinking water standards: US Public Health Service Pub. 956, 61 p. Vine, J. D., 1959. Geologic map of the Nash Draw Quadrangle, Eddy County, New Mexico: U.S. Geol. Survey (FEM—830, open-file report. 1960, Recent domal structures in southeastern New Mex- ico: Am. Assoc. Petroleum Geologists Bull., v. 44, no. 12, p. 1903-1911, 7 figs. UNITED STATES DEPARTMENT OF THE INTERIOR PREPARED IN COOPERATION WITH THE PROFESSIONAL pApER 712—A GEOLOGICAL SURVEY U.S. ATOMIC ENERGY COMMISSION PLATE 1 92;}; a? F1296, RBQEZ. REES. , 9.32 E. R335; zetefioe‘ mama 32” 3:09 1. a i 3 e,“ 7’: {.3 t _ j; 1' (".3- ,. l I (l ’3 ' me ones e mi Comma ‘13:“ *‘ «A ,. 4g. ‘ " "fie-eel”. WWW?“ I 5 {<1 \ ,. x’ +—i—+—I—~4— ,1 E , ” EXPLANATION a E F — 3500 / m 42 may; 10/ “ WATER WELLS 3&0 // / O l ./ Well finished in rocks of Quaternary or Ter- 00 v / tiary age ’55" / , ,/ (W _ _ Well finished in rocks of Triassic age 571 \ . ‘.. ”ii! x.“ v W»? ~7,d:.—I_ t ;;,—— i .F. ~ _ .t: . , ' "i‘i'~ . . g ‘ ”,4, 3050 ; mg“ {3 Entersetmnel l erals one 607 j / i 60‘ ”we" ‘ E .15 , 1 3042 © 149.2 Chgmicg) Com __ 33074 36—7 3; " i ~ wit? W ~ F - Via 170.9 mine and {afloat / «9833le ‘ \l . . ,4 Y” ”x 3050 104.8 ' $53“? 2 a , Z“ V FF _, . 180.3 \ / IL 0 31w” 1 3019 3353 7‘5 L .55 ._ 5 ‘” . ’fi \( 3 {My 1509 f F 862 -1 , : F F ”a “739.97% 2;,“ » I i i‘ z ‘X e3 3-0 w W \ \ i \ \N~ E JV ‘1 )3 i 1 x‘" .“————— 0“ Well finished in rocks of Permian age \ , 00 2 3220 0 3399—1 / 298.1 /' Well data f 2;; g Number at left is altitude of potentiometric / l 1. M ‘ surface, infect above mean sea level. Upper 32223 right number is depth to water, in feet be- low land surface. Lower right number is I depth of well, in feet below land surface. I F, Blue number indicates number of wells at I I V" location. Measured depths are in feet and 1 / / c; _, tenths; reported depths are in feet. P, in- ll L” . ‘ dicates well being pumped at time of meas- / , ,. / I FFFFFF,» urement i l \ \ \ \\ ‘\ / / ’V ‘/ —— 3400 ———— “‘ * Water table or potentiometric contour for formations of Quaternary 0r Tertiary age .. Dashed where inferred or uncertain. Contour E I F's/N interval 50 feet. Datum is meansea level \ O W \ 144.7 3156 212.9 \. .3205 l 2 .3206 ‘ A .3220 \ \ 94.7 \ ——- —.—_ “I“ . z ”A ._. o 01 as 0290 O U1 U! o \ \ .. 0‘) .39. J>J> :\ l l l. Potentiometric contour for formations of Triassic age Dashed where inferred or uncertain. Con- tour interval 50 feet except where data are meager. Datum is mean sea level —l—l——l— 3050 ++ -l- -l— Water table or potentiometric contour for formations of Permian age Dashed where inferred or uncertain. Contour interval 50 feet. Datum is mean sea level H N N 4; \ \ \ \ urcguwtv\ ‘ COUNTY TE??? 3 insane Qmse‘e \Circle of 5-mile radius from Gnome site "‘ 5’ 2:5? 1; Approximate position of boundary between rocks of Quaternary and Tertiary ages, and older rocks {Saki take} Gnome site i l Approximate position of boundary between rocks of Triassic age and rocks of Permian age Majority of water-level measurements made August 1958 to April 1959 T24 53. / ”277:??ny 3‘1 F 0‘ (‘3 i (i / gffiy/ ,1 y~"’ y 3W.” I’ 313.4 3197 366.4 .E is... w z _ ‘ +/ \> mg «NAM an, “‘5 “”3”? )1 ‘v a 5 J) ’ j a: was , / t e x‘ mm: 3 . “a.“ m. z .. ,F F wa :1 mm lgfiiiv,“‘s‘5nFF ‘ ._; W M a": I yum-4 3, W \v 1% 3433.5ng 391.3 m away- 460.3 170.1 2.85359) 2000, a l icoyarr ”l". 25 S. .. .., .. .. 2934ofl I ’ x 298.1 F/ WWW. Wm "If”. We-.. ., 2887 (9&1 I I 110.6 1 . ~- ‘ \ "1, ..r 3. is? m m 4/“ R \ 61 ‘\\_-. xi" -. a. ~ . _ ‘ A W El Peso Nature my; ”will WélfWg—Zfigw 03 Co. turbinWF W 3‘ F , .44? 2950 __ _/ /-" :53 .ul (5‘ L169.9 \ 82.6 . Wm. M W mQ—quogl'gh—Z—ZSW Mfirmfi r-w—g knew: PM»! Wye/33% ----9 £9 avg-{N figs tw-«HI y-WW \ I \ 292110 1633 perm 75 e040 \ 29M \ j 900 400 \ ‘02 275.8 0 ‘ \“ \ E :3 ~‘ I . t . i a, F 2 QFWVFFF. a!“ \ \ 2' . " V mm; ,1. 5 .. 70m“? 5 _ 2w ‘11! £8 /a T. ’26 S. T. 26 S. E . WWW» W—«ww F 3( a- .-E-..,.JM.._.WW,1l,_.WW.. __ __ \ T\ .. n l . _ F H. F; F \ \_ l . “lbw”; 5 32.000“ "" um um «I... run- av- na- .. a... no” annual. now-w I uni-H“ 32°00 .,0 5’ REESE. .22: R2953. Race. . E X A 8 R315. “L“; R325. H.335; Base from New Mexico State Highway Compiled by James B. Cooper (1961) quadrangle maps. Lea County 1941, Contours west of the Pecos River from Hale Eddy County 1950 SCALE 1:125 000 and Clebsch (1958). Contours in Lea County from 2 o 2 4 5 8 10 MILES Nicholson and Clebsch (1961) TRUE NORTH r I. a: o z 9 L Lu 2 O x I APPROXlMATE MEAN 2 O 2 4 6 8 10 KlLOMETERS I———-—I DECLINATEON,1971 MAP SHOWING LOCATION OF WELLS AND WATER-LEVEL CONTOURS FOR GEOLOGIC F ORMATIONS IN THE PROJECT GNOME AREA, EDDY AND LEA COUNTIES, NEW MEXICO 428-472 0 ~ 71 (In pocket) ,r‘ 9““ . 1 sc!’; “1». “BRA“. , :75 9 ll'b VHA See Geohyd elegy of the Eastern Part of Pahute Mesa, Nevada Test Site, Nye County, Nevada GEOLOGICAL SURVEY PROFESSIONAL PAPER 712—B Prepared on behalf of the U.S. Atomic Energy Commission Geohydrology of the Eastern Part of Pahute Mesa, Nevada Test Site, Nye County, Nevada By RICHARD K. BLANKENNAGEL and J. E. WEIR, JR. HYDROLOGY OF NUCLEAR TEST SITES GEOLOGICAL SURVEY PROFESSIONAL PAPER 712—B Prepared on behalf of the US. Atomic Energy Commission Description of hydraulic testing in deep drill holes and of ground-water characteristics in the Pahute Mesa area UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1973 UNITED STATES DEPARTMENT OF THE INTERIOR ROGERS C. B. MORTON, Secretary GEOLOGICAL SURVEY V. E. McKelvey, Director Library of Congress catalog-ran! No. 72—6003“ For sale by the Superintendent of Documents, US. Government Printing Office Washington, DC. 20402 — Price $1.75 domestic postpaid or $1.50 GPO Bookstore Stock Number 2401-00303 CONTENTS Abstract ..................................................................................... Introduction. Purpose and scope .. Previous hydrologic investigations ............................... Acknowledgments... Geologic setting ...... Stratigraphic units and their hydrologic signifi- cance ....................... Structure .............. Aquifers and aquitards ........................................................... Rhyolitic lava flows. Characteristics of surface exposures .................... Characteristics based on cores, geophysical logs, and drilling records .................................... Characteristics based on hydraulic tests ............. Welded tufl’s ........................................................... Characteristics of surface exposures .................... Characteristics based on cores, geophysical logs and drilling records .................................... Characteristics based on hydraulic tests ............. Ash-fall tuifs, nonwelded or slightly welded ash- flow tuffs, and tuffaceous sediments .................. Characteristics of surface exposures .................... Characteristics based on cores, geophysical logs, and drilling records .................................... Page B1 1 WOOOON ooooczpco 15 16 16 Aquifers and aquitards—Continued Ash-fall tufl's, nonwelded or slightly welded ash-flow tuffs, and tuffaceous sediments—Continued Characteristics based on hydraulic tests .............. Regional movement of ground water .................................... Estimates of underflow beneath Pahute Mesa ............ Estimated annual recharge from precipitation and underflow ..... Estimated annual discharge ........................................... Local movement of ground water.. Head changes in drill holes ................ Relation between subsurface temperatures and flow patterns Water-level contour map ...................... Ground-water velocity ....... Chemical quality of water ...................................................... Relation of water chemistry and rock type ................. Water chemistry and regional movement of ground water" Engineering hydrology ............................................................ Rock media best suited for chamber construction ...... Water inflow to chambers ............................................... Water supply .................................................................... Areas favorable for drilling water-supply wells Summary and conclusions ............... . References ........................ ILLUSTRATIONS [Plates are in pocket] PLATE 1 . Water-level contour map showing faults and caldera boundaries, eastern Pahute Mesa. 2. Generalized geologic sections across Silent Canyon caldera, eastern Pahute Mesa. 3. Map showing direction of ground-water movement from eastern Pahute Mesa toward discharge areas in Oasis Val- ley and Amargosa Desert. FIGURE 1. Index map of the report area and locations of exploratory and large-diameter emplacement holes ...................... 2. Bouguer gravity map of Silent Canyon caldera 3. Map showing percentage of rock types with resistivities greater than 225 ohms— m2/m on 16-inch normal curve of electric log (rhyolite and densely welded tuff) in upper 2, 000 feet of saturated zone ................ 4-7. Graphs showing: 4. Relative specific capacity of the major rock types within the saturated zone .......................................... 5. Drawdown during 28-hour pumping test of hole UE—19h, August 4—5, 1965 .............................................. 6. Drawdown during 29-hour pumping test of hole U—20a—2, February 10—12, 1965. 7. Drawdown during a pumping test of hole UE—20e—1, June 4-5, 1964 ............................................................ 8. Fence diagram showing resistivity of rocks in upper 2,000 feet 0f saturated zone .................................................. 9. Map showing ranges of transmissivity 10. Map showing head changes with depth in exploratory test holes ..... 11. Water-chemistry map. 12. Graph showing inflow through small- diameter holes drilled into prospective chamber zones U-20f and U—20c III Page B17 17 19 Page B2 4 7 8 9 13 14 18 20 22 27 32 IV TABLE 1. . Resistivity calculations from 16-inch normal curve on electric logs made in upper 2,000 feet of zone of satu- menace woo-:1 CONTENTS TABLES Summary of lithology and thickness of the major Cenozoic rock units, Pahute Mesa, Nevada Test Site ........... ration in exploratory holes, Pahute Mesa ........................................................................... . Results from pumping tests of exploratory holes, Pahute Mesa . . Summary of porosity and fracture frequency in core samples of welded tuffs ...... Summary of porosity and fracture frequency in core samples of ash- fall and nonwelded ash-flow tufl’s ............ . Estimated average annual ground-water potential recharge from precipitation at Pahute Mesa and Rainier Mesa ..... .. .............................. . Head changes with depth in exploratory holes, Pahute Mesa ........ . Bottom~hole temperatures and thermal gradients in exploratory holes, Pahute Mesa .............................................. . Estimated ground-water velocity in volcanic rocks beneath Pahute Mesa 10. . Relative abundance of principal ions in selected ground-water samples from Pahute Mesa .................................. 12. Selected chemical analyses of water from exploratory holes and chambers, Pahute Mesa ..................................... Water-inflow, construction, and hydraulic-testing data for mined emplacement chambers within the saturated zone, Pahute Mesa .......... .................. Page B5 HYDROLOGY OF NUCLEAR TEST SITES GEOHYDROLOGY OF THE EASTERN PART OF PAHUTE MESA, NEVADA TEST SITE, NYE COUNTY, NEVADA By RICHARD K. BLANKENNAGEL and J. E. WEIR, JR. ABSTRACT A deep structural depression, the Silent Canyon caldera, underlies the eastern part of Pahute Mesa, Nye County, Nev. The caldera is elliptical in plan and measures about 11 by 14 miles; the greater axis trends in a north-northeast direction. Exploratory drilling revealed a Tertiary volcanic section of ash—flow and ash-fall tufl’s and rhyolitic lava flows which attained thicknesses in excess of 13,686 feet. Hydraulic tests made in deep drill holes indicated that these volcanic rocks are capable of transmitting water and that measurable permeability occurs at depths greater than 3,500 feet below the top of the saturated zone. Most movement of ground water beneath the mesa occurs through interconnecting fault and joint systems. Fractures are more common in rhyolitic lava flows and in densely welded ash-flow tuffs, and these fractures have a greater tendency to remain open than do those in ash-fall and nonwelded ash-flow tufi‘s. The yield of water to wells from intervals of ash-fall and nonwelded ash-flow tufi’s, particu- larly those that are zeolitized or argillized, is low. Hence, these rocks are considered the best media for mining of underground chambers in the saturated zone. In the Silent Canyon caldera, depth to water ranges from about 1,952 feet (alt 4,164 ft) in the western part to 2,350 feet (alt 4,685 ft) in the eastern part. In the extreme northwestern part of the Nevada Test Site, outside the caldera, the depth to water is about 850 feet (alt 4,700 ft). Stable and declining head potentials occur with depth in all but one of the holes drilled in the eastern part of the report area; variable heads in the upper 1,500 feet of the saturated zone and then increasing heads to total drilled depth occur in holes drilled in the western part. Pumping tests indicate that transmissivities range from 1,400 to 140,000 gallons per day per foot. The greatest transmissivities occur in holes drilled along the east margin of the caldera, where the principal rock type in the saturated zone is rhyolite. Water derived from drill holes at Pahute Mesa is sodium potassium type. These chemical constituents comprised over 90 percent of the total cations in more than half the samples that were analyzed. Ground water beneath Pahute Mesa moves southwestward and southward toward the Amargosa Desert through Oasis Valley, Crater Flat, and western Jackass Flats. The flow, across a 15-mile underflow strip which extends from the hy- draulic barrier on the west to the ground-water divide on the east, is estimated to be 8,000 acre-feet per year. Owing to the difficulty in obtaining accurate porosity data, estimates of ground-water velocity vary as much as two orders of magnitude — 5 to 250 feet per year. Based on the assumption that most ground-water movement occurs along intercon- nected fractures and that some movement occurs through interstices, a reasonable estimate of velocity is less than 15 feet per year. INTRODUCTION Geologic and hydrologic studies in the Pahute Mesa area were intensified in 1962 by the US. Geological Survey on behalf of the US. Atomic Energy Commission. The objective of these studies was to assist the Commission in selecting an area suitable for underground testing of larger nuclear devices at depths greater than were feasible within the then-existing limits of the Nevada Test Site. With the completion of hole PM—l in 1963, the de- cision was made to add eastern Pahute Mesa to the test site, and the land was obtained from the US. Bureau of Land Management and the US. Air Force. The Pahute Mesa area is about 130 miles north- west of Las Vegas, Nev., in Nye County, in the northwestern part of the Atomic Energy Commis- sion’s Nevada Test Site. (See fig. 1.) It is an elevated plateau of about 200 square miles with relatively gentle relief. Altitude ranges from 5,500 to more than 7,000 feet. The geological studies revealed a sequence of flat- lying volcanic rocks, and data from a gravity survey indicated a deep structural basin beneath the east- ern part of the mesa. An exploratory hole, PM—l (fig. 1) was drilled in May 1963 in the center of the structural basin to a depth of 7,500 feet. A thick sequence of zeolitized bedded tuffs having low trans- missivities, considered excellent test media, was penetrated in the hole. A deep water table, more than 2,000 feet below land surface, contributed to the favorability of the area. From 1963 through 1968, 19 exploratory holes were drilled in and near the eastern part of Pahute Mesa. This extensive B1 B2 F 530 000 E 550 000 L b 10 000 E 5 90 000 HYDROLOGY OF NUCLEAR TEST SITES E 610 000 E 63C} 000 . A ”6°30! 119w l N etoooti - i ‘ 3 NEVADA s 1 . livno : , NEVADA TEST SITE \\ 1., ,. 1 / \ ulh Vegas ,3\ 3752230" / \ t N 950 000 , / x \ - / /\ UE 19h ”5'13?“ \\ x __,A_ ............... NWT _ _ (,3 “(CW-2 .UE—ZOD \\ .3" 3 EXPLANATION V‘ V I" K x: 3’ UE-19h “ \\ 1") :7??? . ma; ‘3: :3 l\ Ngel' 3; 9 UE_19g5 l U E-19b—1 N 930000 ax _ . {I u 0' _ if " \stE 20; /'=‘:§§:§> f U_20g\\ k UE age, 0U_20g \ L __, fi-e / my 5? ,/ ‘ Emplacement hole “X /, N ‘l (Pr—1 E and number \ _ \\ 3/er ' \ l \ U—20f UE ZOQ‘Xf \ I “\UE-20f \ 73/ l \ Q \ l | ‘ l’ .._,r N CLoom _ Q T g l r 37°15 “37% UE-20d «3/8? ‘—20a—2 | 1 ‘3» U-20c ‘ -20a l l m —: UE-20c l I ‘s a \\ l .527 l v» I l l l l l | l I {I l N 890 000 \ a l2 \ l ‘ <|<fi \ 3 {fl [:1 \\ 22's: ‘3 4' _._.__.- ___._L._.._- ________ .__. ' .— ‘-\ AREA. 18 "'Tw—BT l u ‘ \ N 870 cm W; J ‘ . UE— 8r 20,000-foot grid based on Nevada coordinate system, central zone 0 5 MILES |__i__i_1___.L—l NEVADA TEST SITE \ o 5 10 15 MILES FIGURE 1.——-Index map of the report area and locations of exploratory and large-diameter emplacement holes. drilling program yielded data for evaluation of the subsurface geologic and hydrologic environment which is summarized in this report. PURPOSE AND SCOPE The purpose of the hydrologic investigation was to determine the water-yielding potential of volcanic rock strata in each exploratory or nuclear emplace- ment drill hole and, in particular, of those strata that were most favorable for construction of cham- bers. Many of the proposed tests require mined chambers in the saturated zone. To insure safety of miners, these chambers must be constructed in intervals of rock having low permeability. The long- range purpose of the investigation included deter- mination of (1) the distribution and hydraulic characteristics of the principal rock types; (2) local and regional directions of ground-water movement, including possible areas of recharge and discharge; (3) ground-water velocities; and (4) areas favor- able for development of water supplies. Hydraulic data, obtained from each exploratory or emplacement hole, and geological, geophysical, and geochemical data have been synthesized to present herein the current status of geohydrologic interpretations. The complexity of the geologic and hydrologic data offered considerable latitude for interpretation of underflow beneath Pahute Mesa, ground-water velocity ranges, and discharge in nearby areas. PART OF PAHUTE MESA, NEVADA TEST SITE, NEVADA PREVIOUS HYDROLOGIC INVESTIGATIONS Hydraulic tests at Pahute Mesa began in 1963 with those made in the PM—l exploratory drill hole. At that time, a wealth of information on testing techniques and on hydrology of volcanic rock strata was available from previous investigations in other areas at the Nevada Test Site. These investigations were divided into three phases (Winograd and others, 1971). To some extent, the investigations conducted in each phase were designed to fulfill the immediate needs of the Atomic Energy Commission in an expeditious manner. Briefly, the first phase (1957—59) included (1) the collection of hydrologic data from all existing wells and springs at or near the Nevada Test Site and (2) detailed studies of the hydrology of the tuff underlying Rainier Mesa through observations made in tunnels and shafts driven into the east face of Rainier Mesa. The sec- ond phase (1960—61) consisted mostly of the study of the hydrology of Yucca Flat, and the third phase (1962—64) involved collection of data from test holes drilled at Yucca Flat, Frenchman Flat, Jackass Flats, and the area adjacent to the test site south and southeast of Mercury. In addition to the reports prepared on behalf of the Atomic Energy Commission, several reports prepared cooperatively by the Geological Survey and the State of Nevada, Department of Conserva- tion and Natural Resources, Carson City, were of value during preparation of this report. ACKNOWLEDGMENTS The writers express appreciation to the US Atomic Energy Commission for the support given to the project. Particular acknowledgment is given to those members of the Office of Effects Evaluation and the Office of Engineering and Logistics, Nevada Operations Office, Las Vegas, Nev., who were di- rectly involved in overall coordination of the project. Interpretations of the geological structure and stratigraphy used in the report were developed by P. P. Orkild and other geologists of the US. Geolog- ical Survey. Finally, field assistance was rendered by many individuals employed by contractors to the Atomic Energy Commission, the scientific laboratories who conduct the nuclear tests, and other firms. GEOLOGIC SETTING Gravity data indicated the existence of a deep structural depression under the eastern part of Pahute Mesa. (See fig. 2.) Subsequent exploratory drilling confirmed this interpretation and revealed a Tertiary volcanic section of ash-flow and ash-fall B3 tufi‘s and lava flows at least 13,686 feet thick. This structural depression has been identified as the product of cauldron subsidence and named the Silent Canyon caldera (Noble and others, 1968; Orkild and others, 1968). The caldera, which occupies about 90 percent of the report area, is elliptical in plan, measuring approximately 11 by 14 miles; the great— er axis trends north-northeast, roughly parallel to the strike of basin-and-range faulting. The Silent Canyon caldera is masked by younger volcanic rocks except along the east boundary, where the wall of the caldera is exposed in some localities. Steepening isogals on the Bouguer gravity map and geological correlations based on subsurface data have approximately located the boundary or ring faults of the caldera. The southern part of the Silent Canyon caldera is intersected by the Timber Mountain caldera (Byers and others, 1968); thus, the boundary in that area is more complex. No pre-Tertiary rocks were penetrated in explora- tory holes drilled within the caldera or in the imme- diate vicinity outside the caldera. Paleozoic rocks are exposed about 10 miles north and 8 miles east- southeast of the caldera. A small stock of plutonic rock of Mesozoic age is exposed 5 miles east of the caldera (Orkild and others, 1968). STRATIGRAPHIC UNITS AND THEIR HYDROLOGIC SIGNIFICANCE The rocks which were emplaced during and after the collapse of the Silent Canyon caldera form key hydrogeologic units. Beyond the eastern, northern, and western limits of the caldera, precaldera rocks locally are aquifers. Within the caldera, the pre- caldera rocks are buried at such great depths they have little hydrologic significance. For hydrologic and engineering purposes, the rocks are grouped into precaldera, intracaldera, and postcaldera rocks. A summary of the rock units is shown in table 1. In the eastern part of the Silent Canyon caldera, fractured rhyolites and welded tuffs assigned to the lava and tufi‘ of Deadhorse Flat are the only aquifers of consequence. In general, the greatest values of relative specific capacity were measured in intervals of fractured rhyolitic lavas in the upper 2,000 feet of the saturated zone. In holes drilled near the east margin of the Silent Canyon caldera, rhyolitic lava flows comprise almost 100 percent of the rock in the upper 2,000 feet of the saturated zone. (See fig. 3.) Rhyolites, vitrophyres, and welded tufi‘s of the tuffs and rhyolites of Area 20 that are fractured and faulted constitute the significant aquifers in the western and central parts of the Silent Canyon B4 E 530 000 E 570000 E 590000 N 9/0 000 I 3/°22'30” w N 950 030 ‘ 7mg "TEE HYDROLOGY OF NUCLEAR TEST SITES II 610 000 E, 630 000 1 16°15 ' " " EXPLANATION —190 Bouguer gravity contour result- ing from variable density ‘WSITE \\ reduction of the Pahute Mesa gravity data Contour interval 2 milligals. Datum plane is 5,500feet (base N 93C} 000 \ / \\N\\‘_’2'OOS\K/ _\\ w of mesa) C23 Gravity low area Silent Canyon caldera boundary _I__T—I'_ N 9‘10000, Timber Mountain caldera complex . U E— 19gs Exploratory test hole and number 0 U — 19g Emplacement hole and number 3}"‘15’ N 890 000 N 870 000 20,000»foot grid based on Nevada coordinate system, central zone 0 5 MILES I—L__I_I_.L__I NEVADA TEsT SITE 0 5 10 15 MILES L—l—J__l FIGURE 2. — Bouguer gravity map of Silent Canyon caldera. caldera. With the exception of a concentration of rhyolitic lava flows in the southwestern part of the caldera, ash-fall and ash-flow tufl’s are predominant in this intracaldera unit. These tuffs are mostly zeolitized and are relatively impermeable; fractures and faults in these zeolitized rocks are generally resealed. STRUCTURE The major structure in the report area is the deep depression that has been designated the Silent Can- yon caldera. The features of this caldera include (1) a ring-fracture zone with vertical displacements ranging from 7,000 feet on the west to about 5,000 feet on the northeast, (2) a shallow broad subarcu- ate basin in the eastern and northeastern parts, (3) a north-northeast—trending deep basin in the western part, and (4) a horstlike feature in the center of the caldera, between the intracaldera deeps. Structure in the southern part of the caldera is complicated by intersection with the ringéfracture zone of the Tim- ber Mountain caldera. The principal structures are shown in figure 2 and on plates 1 and 2. Basin-and-range faulting was active throughout the evolution of the Silent Canyon caldera and prob- ably during deposition of the precaldera rocks. Most of the basin-and-range faults are expressed at the surface; however, in the western part, some faults are masked by the Thirsty Canyon Tufl’, the youngest postcaldera formation (Orkild and others, 1968). Most of the faults strike north-northeast and north; PART OF PAHUTE MESA, NEVADA TEST SITE, NEVADA B5 TABLE 1. —Summary of lithology and thickness of the ma jor Cenozoic rock units at Pahute Mesa, Nevada Test Site [After Orkild, Sargent, and Snyder (1969). Local units not formally named have been largely excluded from this table] System and series Time- rock unit Formation Member Description Thickness (feet) Distribution Hydrologic significance Tertiary Pliocene Miocene Postcaldera rocks Thirsty Canyon Tufl' Trail Ridge Spearhead Rhyolite of Ribbon Cliff Ash-flow tufl‘, reddish- to purplish- brown to pale-pink, densely to partially welded ; mainly devitrified but locally with glassy base; dis- tinctive white to buff pumice-rich ash-fall tuff at base. Ash-flow tuff, dark gray to dark red- dish brown, partially to densely welded, devitrified; thin glassy zone at base; thin buff pumice-rich ash—fall tuff locally at base. Trachytic lava flows, medium gray to brown, dense to vesicular. 0—200 0—300 0—265+ All Area 20 holes except PM-l and U—ZOg. In Area 19 only in U—19as. Upper part in all Area 20 holes except PM-l and U—ZOg; lower part only in UE—20e and UE-20f holes; undifferentiated in PM—Z. In Area 19 undifferentiated in U—19as and upper part in UE—19b—1, not recognized in other holes. Recognized only in UE—ZOj hole, Area 20. Above saturated zone. Do. Timber Mountain uff Ammonia Tanks Rainier Mesa Ash-flow tuif, gray, buff, pink, and black, partially to densely welded, glassy and devitrified. Ash-flow tufi', pale-red to dark-brown, nonwelded to densely welded; de- vitrified but glassy at base and top. In ‘Area 20 where lower part ap- proaches and is below top of zone of saturation, basal tufl' may be zeolitized, bedded. 0—325 0—1,400 All Area 20 holes except PM-l, PM-2, and U—20g. All Area 19 holes except UE—l9d. UE—19h, and UE—19i. All Area 20 holes except PM—Z. All Area 19 holes. Above saturated zone. Above saturated zone in Area 19 holes and in most Area 20 holes. In UE—20f basal part is zeolitized tuff aquitard; in UE-20i welded tufi' is aquifer. Lavas of Scrugham Peak quadrangle Paintbrush Tuff Rhyolite No. 5 Rhyolite No. 4 Tiva Canyon Tufl‘ of Blacktop Buttes Rhyolite No. 3 Rhyolite No. 1 Topopah Spring Quartz-rich rhyolite lavas, light-gray to brownish-gray, devitrified; gray to green vitrophyric lava flows. Quartz-bearing rhyolite lavas, light- to dark-gray: devitrified to vitro- phyric lava flow. Ash-flow tuft, gray to reddish-brown, nonwelded to densely welded; mainly devitrified but glassy at base and top. Zeolitized tuflt' at base in subsurface. Ash-flow tufi‘, pale-gray, nonwelded to partially welded, and bedded ash-fall tufE. Biotite rhyolite lavas, light- to dark- gray ; devitrified to vitrophyric lava flow. Pyroxene-bearing rhyolite lavas, light- to dark-gray; devitrified and vitrophyric lava. Ash-flow tuff, reddish-brown, densely welded, and bedded ash-fall tuff. 0—565 0—1,160 0-350 0—570 0—200 0—980 0—450 Recognized only in U—20a—2 hole, Area 20. Recognized only in UE-20c and UE—20d holes, Area 20. All Area 20 holes except PM-2, UE—20e, U—20g, and UE-20j. All Area 19 holes except UE—19e, UE-19gs, and UE—l9h. Recognized only in UE—19e, UE—19gs, and UE—19h, Area 19. Recognized only in U—20a—2, Area 20. Recognized in UE—19fs and UE—19i holes, Area 19. Recognized in UE—20c, UE—20d and UE-ZOf holes, Area 20. Above saturated zone. Do. Above saturated zone in Area 19. Welded tuft‘ aqui- fer in some Area 20 holes. Above saturated zone. Rhyolite aquifer. Above saturated zone. Welded-tuff aquifer. Intracaldera rocks Lavas of Scrugham Peak quadrangle Tuffs and rhyolites of Area 20 Upper rhyolite lavas of Area 20 Lower rhyolite lavas of Area 20 Bedded and ash- flow tufi's Lithic-rich ash-flow tufi' Rhyolite lavas, gray to light-green- ish-gray, massive to conspicuously flow-layered, devitrified to vitro- phyric ; flows locally are underlain by thin blocky flow breccias. Rhyolite lavas, gray to light-reddish- brown, massive to flow layered, devitrified to vitrophyric. Ash-flow and ash-fall tufl'. light-gray, yellow to orange-pink, lithic-rieh, zeolitized; intercalated with rhyolite flows of Area 20. Ash-flow tufi', gray to light-brown, zeolitized, nonwelded to partially welded. 0—1,850 0—1.700 0—2,100+ 0—2,100+ Greatest thickness in western part of caldera in vicinity of UE-20d, UE—20f, and UE—20h. A thin (382 ft) section may exist in UE-20j west of the caldera bound- ary: none recognized in PM—Z. Formation thins to east and not recognized in UE—19b—1, UE—19c, and UE—19d, Area 19. May in part be age equivalent of volcanic rocks of Deadhorse Flat. Rhyolite and welded tufl‘ aquifers and zeolitized tuff aquitards. B6 HYDROLOGY OF NUCLEAR TEST SITES TABLE 1. — Summary of lithology and thickness of the major Cenozoic rock units at Pahute Mesa, N evada Test Site — Continued System Time- . . and rock Formation Member Description T121323“ Distribution IZIYngloglc series unit Sign) canoe Rhyolite lavas Rhyolite lavas, light-brown to yellow- 0-4,300+ of Deadhorse ish-gray, devitrified, flow-laminated Flat to coarsely flow-layered; crystal- poor and crystal-rich comenditic lavas. Beddegltuif of Ash-fall and regrkfed tuft, sagd- 0—350 Rh 1 d 1d d Dea orse stone compos 0 v0 canic etri- . . yo ite an we 9 Flat tus, tufiaceous conglomerate, Greziisgftggilégfssfizfgzgf)": tufl' aquifers and Lava and tui’f of interlensing with lavas and welded :ized in an Area 20 holegs zeolitized tuff Deadhorse Flat tufi‘s of Deadhorse Flat; rocks are y ‘ aquitards. yellow, greenish-gray, and reddish m and are commonly zeolitized. x g Ash-flow tufi' of Ash-flow and ash-fall tuff, reddish- 0-1.600 a Deadhorse brown, brown, and green, non- ; Flat welded to densely welded, 1, devitrified, comenditic. c g E .5 cu ii 11 8 5 w "‘ = Grouse Canyon Ash-flow tufi. greenish-gray to pale- 0-1,700 a E ... . brownish-gray and red, densely _ Belted Range Tufi‘ welded, devitrified, comenditic. Irregularly deposited within Welded tufi’ aquifer and beyond limits of caldera. gutsgde caldera Tub Spring Ash-flow tuft, bluish-gray and brick- 0—1,450 ""1 5. red, nonwelded to densely welded, ~ devitrified, comenditic. Other rocks include: Rhyolite of Split Ridge. Rhyolite of Quartet Dome, bedded reworked and minor ash- flow tuft, ash-fall tufl’, etc. Pre- Rhyolite lavas, intrusives, and tufis not pertinent to Pahute Mesa hydrologic studies because of great caldera depth in subsurface and lack of control. Exceptions are in UE—20j, UE—20p, and I’M—2, which rocks are west of caldera limits. displacements range from a few feet to about 800 feet; most faults are downthrown to the west. Major faults near the east margin of the caldera provided the fissures or vents for the extensive rhyolitic lava flows that flooded the eastern part of the caldera. Secondary fault systems, stemming from the pri- mary basin-and-range fault system, trend in various directions and have only minor displacements. AQUIFERS AND AQUITARDS The rock units at Pahute Mesa that afford positive stratigraphic control on the surface and in the sub- surface are the widespread ash-flow sheets of the Belted Range Tuif, which probably are contempo- raneous with initial caldera collapse, and the major ash-flows of the Timber Mountain Tuff and Paint— brush Tufl", which are postcaldera in age. Heteroge- neous intracaldera rocks occur between these widespread ash-flow sheets within the saturated zone under Pahute Mesa. These intracaldera rocks include ash-fall and ash—flow tuffs and rhyolitic lava flows which emanated from fissures within the sub- siding caldera and from nearby volcanic centers. The vertical and horizontal distribution of these intracaldera ash-fall and ash-flow tufi's and lava flows over short distances is variable. These abrupt changes in rock types make subsurface correlations between test holes extremely difiicult. Interstitial permeability, although small, allows some ground-water movement; most of the move- ment under Pahute Mesa occurs in fractures. In a widespread sheet of welded tuff there may be inter- connected fractures over an area large enough to justify classification of a formation or member as an aquifer. Such welded-tuff units as the Rainier Mesa Member of the Timber Mountain Tufi' and the Topopah Spring Member of the Paintbrush Tufi' are aquifers where saturated. Fractures are common in some flows and absent in others. Factors controlling location of fractures appear to have been mode of tuff emplacement, cooling history, and proximity to faults. The depth at which intervals of high perme- ability may be penetrated in relatively closely spaced drill holes cannot be predicted. The combined thick- ness of intervals with measurable fracture perme- ability generally ranges from 3 to 10 percent of the total rock section penetrated in the saturated zone. The complex structure within the caldera, and especially the high incidence of fractures, often results in high measured water yields from rocks and members that normally are classed as aquitards. (A specific capacity of 0.1 gpm (gallon(s) per minute) per foot of drawdown per 1,000 feet of saturated rock is the arbitrary distinction between an aquifer and aquitard.) The Tub Spring and PART OF PAHUTE MESA, NEVADA TEST SITE, NEVADA E 530 000 ’11 (300 E 610 000 B7 E 630 000 N two 000 ' .“ 37°22’30” , / ' firiniAmi'iEsfiiTE ‘\ 116°15’ l \. " EXPLANATION N 950 00G ‘ \k 60 \ Line of equal percentage of rock types with resistivities greater than 225 ohms—mZ/m on 16— inch normal curve of electric log / U— 19b Interval 20 percent U E— 19b — l T . U E—20p N 930 000 N 910 000 Exploratory test hole and number 0 U-19g Emplacement hole and number 37°15 N 89C) 000 N 87C) 000 20,000-foot grid based on Nevada coordinate system, central zone 0 5 MILES l—I_l_|_r__l \ YUCCA FLAT ‘l FRENCHMAN FLAT K J AC KASS / FLATS /"Wx \ _ I , NEVADA TEST SITE \J O 5 10 15 MILES l—l_;_l FIGURE 3. —Percentages of rock types with resistivities greater than 225 ohms- m2/m on 16- inch normal curve of elec- tric log (rhyolite and densely welded tuff) in upper 2, 000 feet of saturated zone. Grouse Canyon Members of the Belted Range Tuff are aquitards throughout most of the Nevada Test Site (Winograd and others, 1971), and they are nearly impervious in at least three drill holes at Pahute Mesa. In the UE—19h and UE—20j drill holes, however, the Tub Spring is permeable; and in the UE—20j drill hole the Grouse Canyon is permeable. The UE—20j hole was drilled near the ring-fracture zone of the caldera, and the UE—19h hole was drilled in a faulted area. The results of hydraulic tests of 150- to ZOO-foot intervals in the various drill holes are summarized by yield and rock type in figure 4. Of the 297 tests that are plotted, 102 of the drill-stem tests were made in rhyolitic lavas, including rhyolite breccia and vitrophyre ; 41 were made in welded tufi' and 54 in bedded and zeolitized tufi'. The intervals isolated by straddle packers during some of these tests un- avoidably included two of these rock types, in which case the principal type was considered as represen- tative of the entire interval tested unless radioac- tive-tracer or other geophysical logging surveys indicated otherwise. In general, the lowest permeabilities were found where the principal rock was zeolitized tufl'. Rhyo- litic lavas, including rhyolite breccia and vitrophyre, B8 yielded water most freely in all drill holes; however, some of the rhyolitic lavas are relatively imperme- able. The welded tuff generally was more permeable than the zeolitized bedded tuff. 0.00-0.02 0.03-0.04 — Zeolitized beddei tuff 0.05-0.09 fl 0.10-0.5 I 0.00—0.02 0.03-0.04 Welded tuff , "'1 0.00-0.02 , _1 0.03-0.04 , I Rhyolite. rhyolite breccia. and vitrophyre 0.05—0.09 O.10-0.5 RELATIVE SPECIFIC CAPACITY, IN GALLONS PER MINUTE PER FOOT OF DRAWDOWN O 5 10 15 20 25 30 35 40 45 50 N U M BER OF TESTS FIGURE 4,—Relative specific capacity (150- to 200-ft inter- vals) of the major rock types within the saturated zone. RHYOLITIC LAVA FLOWS Rhyolitic lava flows have a wide range in water- yield potential. Whereas some of these flows are nearly impervious, some others have highly frac- tured or brecciated zones that may yield as much as 50 gpm per foot of drawdown. Eruption of lava flows into the subsiding caldera was fairly continuous in the eastern part and, later, sporadic in the western part. Depending on viscosity and topography, the flows spread for distances of several miles or formed more limited thick flows or steep-sided domes. The thickness of individual flows penetrated in wells ranged from a few feet to more than 1,000 feet. Both vapor-phase cavities alined along flowband- ing and interstitial spaces in rubble breccia con- HYDROLOGY OF NUCLEAR TEST SITES tribute locally to ground-water movement in lava flows. Most ground-water flow, however, occurs along faults and interconnected vertical and hori- zontal fracture systems that developed during cool- ing and shrinkage of the lava flow. Fracture systems which are highly interconnected typically occur along the outer margins of lava flows. Highly im- permeable intervals often occur in the central crys- talline parts of very thick flows of highly viscous lava. A series of overlapping flows, with little inter- ruption between flows, would result in slower cool- ing of a large mass of lava and, hence, in thick intervals having few fractures and low permeability. CHARACTERISTICS OF SURFACE EXPOSURES Surface exposures of rhyolitic lava flows at and near Pahute Mesa occur in postcaldera rocks above the saturated zone. The vertical structural zonation in widely separated flows is quite similar, and prob- ably lavas in the subsurface also have similar zona- tion. An idealized section of a rhyolitic lava flow mapped by F. M. Byers, J r., and others of the US. Geological Survey (written commun., 1964) con- tains an upper and lower envelope of rubble breccia, upper and lower glass zones, and a crystalline zone in the middle. Contacts between the zones are gra- dational, and each of the five zones is approximately equal in thickness. Porosities calculated from sur- face samples of the rubble breccia range from 28.9 to 70.2 percent; porosities of the crystalline and glassy zones range from 1.2 to 12.3 percent. The porosities of the brecciated zones of flows in the subsurface are likely to be lower owing to com- paction and to filling of pores with mineral matter. A rhyolitic lava flow with foliated interior and enveloped by breccia was mapped near Fortymile Canyon, south of Pahute Mesa, by Christiansen and Lipman (1966). Erosion in the area of the flow exposed the vent for the lava flow and the pre— eruption topography, and thus provided evidence “that brecciation of the lava flow occurred mainly during periods when the lava was spreading, where- as the body eroded its floor and walls much as does a glacier during periods when the flow was confined” (Christiansen and Lipman, 1966, p. 671). The authors noted that tuff adjacent to parts of the lava flow was compacted and indurated. This zone of induration or fusion affected the tuff locally for a thickness of 245 feet from the contact of the flow. Cooling of lava and indurated tufl’ at a contact zone results in shrinkage cracks and, ' hence, an interconnected fracture system. Radioactive-tracer surveys during pumping of some drill holes indicated major water-yielding zones at or near contacts be- tween lava flows and tufl". PART OF PAHUTE MESA, NEVADA TEST SITE, NEVADA CHARACTERISTICS BASED ON COREs, GEOPHYSICAL LOGS, AND DRILLING RECORDS Cores in exploratory drill holes generally were taken where a change in rock type was indicated by a change in the rate of penetration (drilling time) or was detected from examination of rock cuttings recovered from the hole. None of the drill holes was cored continuously, and few intervals of similar rocks, such as lava flows, were cored continuously. Less than 5 percent of the total section penetrated was cored. General descriptions based on cores and rock cuttings of rhyolitic lava flows assigned to the lava and tuff of Deadhorse Flat and the tuffs and rhyolites of Area 20, are satisfactory for gross stratigraphic correlations. However, the lack of con- tinuous cores through lava flows does not permit a statistical analysis of fracture systems or vertical structural zonation within isolated flows or series of flows. In addition, the percentage of core recov- ered from highly fractured intervals generally is low. The number of measurable fractures per foot of core, based on descriptions of 127 cored intervals from 12 drill holes, ranged from 0 to 4.3. The frac- tures, most of which resulted from shrinkage during cooling of the flows, show little or no vertical dis- placement and may be classed as joints. Vertical and high-angle (greater than 60°) joints are more com- mon than low-angle and horizontal joints. The joints are tight or slightly open in cores and often part when the core is removed from the core barrel. Slight vertical displacements noted along some fractures are related to flow structure or reflect nearby faulting. Much faulting has occurred at Pahute Mesa, and the number of faults mapped on the surface ranges from 0 to 10 per square mile. These faults and associated fractures generally re- main open in the competent lava flows and con- tribute to the overall permeability, especially the vertical permeability. Faults in the lava flows are difficult to detect from rock cuttings recovered from the drill hole. Probably, some of the most permeable intervals measured in drill holes are associated with faults rather than with the fractures related to flow structure. Most of the cores of rhyolitic lava flows recovered from drill holes contain some fractures; however, not all the intervals that are fractured are perme- able. In intervals of low permeability, fractures apparently are not interconnected or are poorly connected. In the UE—19b—1 exploratory hole, rhyo- litic lava flows are the principal rocks in the upper 2,000 feet of the saturated zone; the upper 1,000 feet B9 has highly permeable intervals, but the lower 1,000 feet is relatively impermeable. Data from a radio- active-tracer survey made in the UE—19b—1 hole, which was being pumped at a constant rate of 88 gpm, indicated that only 4 percent of the section penetrated in the saturated zone yielded water to the borehole. Most of the fractures in the rhyolitic lava flows are coated with manganese oxide, however, at depth, some are filled with quartz. The depths below the top of the saturated zone at which fractures remain open and interconnected are variable. Highly perme- able rhyolitic lavas occur at depths greater than 3,000 feet below the saturated zone in the UE—20f exploratory hole. In general, the intervals having the highest permeability occur in the uppermost 2,000 feet of the saturated zone. Cores recovered from various intervals within a rhyolitic lava flow may include vitrophyre and rub- ble breccia. The vitrophyres generally are highly fractured and permeable; permeability of the brec- cias is variable. Flowbanding is common in many rhyolite cores; layers range from less than 5 mm (millimeters) to more than 100 mm; dips range from horizontal to vertical. Some interstitial permeability may result from vapor-phase cavities that are alined with the flowbanding. D. L. Hoover and others (written commun., 1964) found such vapor-phase cavities from 3 by 100 mm to 10 by 50 mm in the lava flow penetrated in the U—20a—2 exploratory hole at a depth of 2,893 to 2,901 feet—within one zone of high permeability (table 7). Interstitial permeability in the lava flows, however, is not considered signifi- cant in ground-water movement under Pahute Mesa. The obtention of meaningful effective porosity data from samples of fractured rhyolitic lava flows is extremely difficult. Calculations of total porosity, using grain densities and bulk densities of cores, have been made by various geologists of the US Geological Survey. Calculated porosities of rhyolitic lavas of the tuffs and rhyolites of Area 20 range from 1.7 to 44.4 percent; the average porosity, based on 48 samples, is 15.5 percent. Calculated porosities of rhyolitic lavas of the lava and tuff of Deadhorse Flat are lower. Porosities in these rocks range from 1.2 to 26.2 percent, and the average porosity, based on 79 samples, is 11.1 percent. R. D. Carroll determined total porosities of rocks from geophysical logs made in drill holes. When grain density of rock samples taken from the hole were relatively uniform, Carroll determined poros- ities from density logs. Other logs used for deter- mination of porosity were neutron, electric, and B10 sonic logs. Total porosities of rhyolitic lava flows of the lava and tuff of Deadhorse Flat from density logs made in the UE—19d exploratory hole ranged from 3 to 15 percent —— that of most of the rhyolitic lava section was about 12 percent. Total porosities of an equivalent section of lava flows derived from neutron logs made in the UE—19b—1 exploratory hole ranged from 2 to 16 percent. Porosities of rhyolitic lava flows of the tuffs and rhyolites of Area 20, from laboratory calculations and from geophysical log determinations, generally were greater than those of the lava and tufl’ of Deadhorse Flat. Poros- ities of rhyolitic lava flows of the tuffs and rhyolites of Area 20 determined from geophysical logs made in the UE—20f exploratory hole ranged from 3 to 31 percent. No relation between porosity and type and degree of alteration of rhyolitic lava flows ——zeoli- tization, argillization, or devitrification—was es- tablished. Generally no significant changes in porosity were noted in parts of flows that were altered. The normal and lateral resistivity curves on electric logs made in exploratory holes at Pahute Mesa are excellent indicators of rock types. Borehole fluid resistivities are such that the apparent resistiv- ity of the 16-inch normal curve can be taken as the true resistivity of the formation at apparent resistivities less than 1,000 ohms-mQ/m. Resistivities of relatively impermeable zeolitized tuff units are usually less than 100 ohms-mZ/ m on the short normal curve of the log; apparent resistivities of densely welded tuff, vitrophyre, and rhyolitic lavas exceed 225 ohms-m2/m. Percentages of resistivity ranges in the saturated zone in drill holes, based on interpreta- HYDROLOGY OF NUCLEAR TEST SITES tion of electric logs, are shown in table 2. Alteration of rhyolitic lava flows may increase ion concentra- tion and result in low apparent resistivities (R. D. Carroll, written commun., 1965). Hence, contacts between rock types are most precise when they are based on interpretation and correlation of electric, sonic, density, and caliper logs. The caliper log often indicates rock type and fractured intervals and thus yields information re- lated to porosity and permeability. The caliper curve is usually smooth, and the diameter of the borehole is in gauge with the drill-bit size through sections of competent, nonfractured welded tufl‘ and rhyolites; borehole rugosity and angular caved zones are indi- cated on the log in highly fractured intervals. Prominent ledges and abrupt “washed-out” zones often occur at the contacts between rhyolitic lava flows and the less competent zeolitized tuffs. Fractured intervals can be detected on velocity logs; fractures may attenuate the sonic signal and can be recorded on the sonic log by “cycle skipping” (R. D. Carroll, written commun., 1965). Borehole rugosity and angular caved zones, indicative of fracturing, also may cause cycle skipping on the sonic log. The three-dimensional velocity log, used in conjunction with other logs, also is useful for identifying fractured intervals. Drilling time (the time required to drill a unit of depth of a geologic formation) is an excellent indi- cator of rock type and especially of the contacts between rock types. In exploratory holes, drilling times for 10-foot units of rhyolitic lava flows and densely welded tuff range from 10 to more than 260 minutes and average 70 minutes; drilling times for TABLE 2. — Resistivity calculations from 16-inch normal curve on electric logs made in upper 2, 000 feet of zone of saturation in exploratory holes, Pahute Mesa Depth below Percentage of interval Depth to Total depth of land surface having resistivity in Ex lorator water below hole below - - - phole y land surface land surface Egrlgglegmfltfiifg the(:)rligrl‘csaltnei2i/::)nge (ft) 0—100 100-225 >225 UE—19b—1 ............................................. 2,117 4,500 2,200-4,200 100 2,345 8,489 2,340—4,340 <1 >1 98 2,177 7,689 2,177—4,177 <1 99 2,240 6,005 2,200—4,200 76 16 8 2,305 6,950 22,300—4,300 24 11 65 2,045 7,500 2,350—4,100 25 33 42 2,112 3,705 32,100——3,700 18 10 72 2,258 8,000 2,260—4,260 62 25 13 2,112 7,858 2,110—4,110 92 8 865 8,781 1,050—3,050 93 4 3 2,066 4,500 2,100—4,100 7 93 2,075 5,348 2,075—4,075 73 5 22 1,822 6,395 1,820—3,820 64 14 22 ‘1,954 13,686 1,960—3',960 72 7 21 2,017 4,080 2,400—4,015 66 30 4 2,116 7,207 2,105—4,105 44 8 48 UE—20j .................................................. 1.270 5,690 1270-3270 78 15 7 1Depths of interval used for calculations may be limited by actual depths logged. 2Zone between 2,300 and 2,500 based on geology because no logs were available. 3Zone between 3,454 and 3,700 based on geology because no legs were available. ‘Water level after hole had been drilled to 4,543 ft below land surface. PART OF PAHUTE MESA, NEVADA TEST SITE, NEVADA 10-foot units of ash-fall tuff and nonwelded to partially welded ash-flow tufi' rarely exceed 80 min- utes and average 20 minutes. A positive indication of highly fractured, perme- able zones in rhyolitic lava flows during drilling is the complete or partial loss of drilling fluid and rock cuttings. Circulation is lost when fractures are sufl‘iciently open to accept both drilling fluid and rock cuttings without becoming plugged. Loss of circulation, particularly if accompanied by a sharp increase in deviation of the drill hole, may be indica- tive of a major fault. Major fractures in rhyolite also may deflect the drill bit, but generally the bit will dig into the updip side rather than drift along the plane of the fracture. Ledges in the drill hole often occur at the contact between ash-flow or ash-fall tuifs and rhyolitic lavas. These ledges create ideal circumstances for development of bridges—the result of caving of the walls of the borehole and accumulation of mate- rial—and thus prevent passage of drill pipe and testing tools. CHARACTERISTICS BASED ON HYDRAULIC TESTS Hydraulic testing in each exploratory drill hole involves geophysical logging, injection of known volumes of water into or withdrawing known vol- umes of water from intervals isolated by straddle packers, and test pumping. Equipment and testing techniques used by the US. Geological Survey to obtain hydrologic data in deep holes drilled at Pahute Mesa have been described by Blankennagel (1967 and 1968). The length of intervals isolated by straddle pack- ers is determined by hole diameter, hole condition, and lithology and generally ranges from 150 to 200 feet. Data on head changes and relative specific capacities (gallons per minute per foot of draw- down) of isolated intervals are obtained by injecting or withdrawing known volumes of water and mea- suring the rate of change of water levels with time. Relative specific capacity is the yield, in gallons per minute, accepted or released by a given length of bore (usually 200 ft) in response to a l-foot-of- water pressure differential. The rate at which the straddle-packed interval accepts water during a slug injection test, or yields water after swabbing, is calculated for the period 3—4 minutes after injection of water or cessation of swabbing. The choice of 3—4 minutes is an arbitrary one, but similar or identical yield rates are derived at later times. Moreover, similar or identical rates are derived if the rate is calculated at points of equal head rather than equal time. B11 The term “relative specific capacity” is employed rather than the more familiar term “specific capac- ity” because of the following consideration. A com- parison of the specific capacity values derived from drill-stem tests of permeable intervals with those from pumping tests of the same intervals has shown that the injection or swabbing-test data for perme- able zones may be low by a factor of as much as 50. For low yielding zones, on the other hand — that is, those with relative specific capacity less than 0.05 gpm per foot —— injection and swabbing—test data yield information which is comparable to that which could be obtained if these holes were pumped. Therefore, because the values for the permeable in- tervals are not absolute values, in comparison with those obtained for the relatively impermeable zones, the term “relative specific capacity” is utilized in this report. Most of the water pumped from exploratory drill holes is obtained from zones of fracture perme- ability that constitute only 3 to 10 percent of the total section penetrated in the saturated zone. Where high relative specific capacities are measured in the 150- to ZOO-foot intervals of rhyolitic lavas isolated by straddle packers, one or more thin zones of inter- connecting fractures that range from less than 10 feet to several tens of feet in thickness generally account for most of the permeability in the interval. Delineation of these highly permeable intervals in drill holes is done by means of radioactive-tracer surveys While pumping or injecting water at a con- stant rate, and by examination of electric, fluid resistivity, caliper, and temperature logs. Submersible pumps were used during testing of exploratory holes when drilling records indicated that pumping rates of 50 or more gallons per minute could be sustained. The principal justification for pumping exploratory drill holes was to delineate permeable zones and to obtain data on the water yielded by each zone. These data were obtained from detailed radioactive-tracer surveys and temperature logs made in the hole while pumping at a constant rate. Most of the holes in which these surveys were made were pumped for periods of 12 or 24 hours to stabilize production prior to the geophysical logging; during these periods water-level drawdown was measured. These pumping-test data often proved inadequate because of the heterogeneous aquifer systems, time limitations, and erratic measurements caused by foaming of detergents used during drill- ing. Nevertheless, the data were useful for determin- ing approximate transmissivities, specific capacities, and the potential water yield of wells. No observa- tion wells were available; hence, all calculations were. based on single-well tests. B12 Pumping tests were made in 14 exploratory holes at Pahute Mesa. In 12 of these holes, the major water production was obtained from fractured zones in rhyolitic lava flows; production was obtained from fractured welded tuff in the UE—19e and UE—20j drill holes. Transmissivities from pumping tests generally ranged from 1,400 to 140,000 gpd per ft (gallons per day per foot) ; specific capacities ranged from 0.7 to 67 gpm per foot of drawdown (table 3). The highest transmissivities were mea- sured in those holes where rhyolitic lava flows were the major rock types penetrated in the upper 2,000 feet of the zone of saturation. In drill holes UE—19h and U—20a—2, where rhyo- HYDROLOGY OF NUCLEAR TEST SITES litic lava flows are the predominant rock, trans- missivities are relatively high, and drawdown graphs during the time of measurements (figs. 5 and 6) indicate that the water-bearing fractures probably are well connected. In drill holes where rhyolitic lavas are the major water-producing rocks, but where ash-fall and ash-flow tuffs are the prin- cipal rocks penetrated in the upper 2,000 feet of the saturated zone, transmissivities are generally lower and permeability barriers may be interpreted from drawdown plots at lower pumping rates and in less time. In the UE—20e—1 drill hole, rhyolitic lava flows comprise less than 25 percent of the rock type penetrated in the saturated zone; an abrupt TABLE 3. —-— Results from pumping tests of exploratory holes, Pahute Mesa [<, less than: >, greater than] hDIepthlof D h f Dept}; 30 Pumping-test data 0 e e water eow ' ‘ _ ' Expfiniatory lalld ow fringing” land Yield Dr aw- 08823212; 1“???ng Duration Remarks 0 9 surface (ft) surface (gpm) down (gpm (gpd test (ft) (it) (ft) per ft) per ft) (hours) UE—18r1 ................ 5,004 1,629 1,372 240 19 13 23,000 47 Test interrupted by pump and gen- erator failures. TW—81 .................... 5,490 , 1,068 400 <8 >50 185,000 35 (perforated) UE—19b—1 ............. 4,500 2,190 2,117 97 <2 >50 56,000 12 4,520 2,421 2,345 59 10 6 12,000 ( ?) 33 7,689 2,560 2,177 100 10 10 20,000 ( ?) 22 6,005 2,475 2,240 56 53 1 8,400 24 4,779 2,565 2,305 130 32 4 11,000 24 4,508 2,650 2,045 185 37 5 30,000 24 3,705 2,322 2,112 185 <3 67 140,000 28 8,000 2,896 2,258 140 200 .7 1,400 17 4,500 860 2,066 186 23 8 18,000 29 4,493 2,446 2,075 60 <2 >40 >44,000 ( ?) 25 6,395 1,500 1,822 94 13 7 8,300 28 Impervious bound- ary nearby. UE—20f .................. 13,686 4,456 1,954 100 210 .5 >1,000 ( ?) 47 Most permeable zone cased off. UE—20h .................. 7,207 2,506 2,116 85 5 17 ............ 38 423 51 8 11,000 30 UE—20j .................. 5,690 1,740 1,270 56 >3 16 59,000 23 Impervious bound- ary nearby. 1South of Pahute Mesa. steepening of the drawdown graph (fig. 7) after 40 minutes of pumping is indicative of a probable reduction in fracture permeability away from the borehole. The steepening of the drawdown graph (fig. 5) for hole UE—19h drawdown test did not occur until after 650 minutes at nearly twice the pumping rate. Lowest transmissivities and specific capacities were recorded in holes UE—19e and UE—19i, where rhyolitic lava flows made up only 5 percent of the rocks in the saturated zone, and in hole UE—20f, where the upper 2,500 feet of the saturated zone was cased and cemented. In hole UE—20f, the inter- val from 4,456 to 13,686 feet was pumped; this test indicated that open, water-bearing fractures occur at depths greater than 2,500 feet below the top of the saturated zone, or about 4,500 feet below land surface. WELDED TUFFS Data on hydraulic properties of welded tuffs at Pahute Mesa are not as numerous as those collected for other rocks. The combined percentage of ash-fall and nonwelded ash-flow tufi's and rhyolitic lava flows penetrated in the upper 2,000 feet of the saturated zone in 17 exploratory holes is more than seven times greater than the percentage of welded tuffs. Densely welded tuffs have physical character- istics like those of rhyolitic lava flows. Like the rhyolitic lava flows, they have a wide range in water-yield potential; interconnecting fractures afford the principal avenues for ground-water move— ment. PART OF PAHUTE MESA, NEVADA TEST SITE, NEVADA B13 .. | I IIIIIII l IIIIIIII I IIIIIIII {3 Static water level: 2,112 feet below land surface u‘ 0 o O O E 2 _ O O O 0 ~‘ _ o _ . ‘ ~0 E Transmissivity: 140,000 gallons per day per foot W 0 Average pumping rate: 185 gallons per minute g 4 — Specific capacity: 67 gallons per minute per — < foot of drawdown ‘8 5 l I I | I I 1 1 l | I 4 l I I I I I L I I I l I 1 I 1 10 100 1000 2000 TIME. IN MINUTES SINCE PUMPING BEGAN FIGURE 5.—Drawdown during 28-hour pumping test of hole UE—19h, August 4—5, 1965. The principal members of the Belted Range Tuff -—the Grouse Canyon and Tub Spring—are rela— tively impermeable except in those areas that have been disturbed by faulting. Welded tuffs of the intracaldera rocks do not have the areal extent and uniformity of the Widespread ash-flow sheets of the Belted Range and the postcaldera ash flows. With some exceptions, most of the welded tufi‘s assigned to the intracaldera rocks are relatively impermeable. Welded tuffs of the Paintbrush Tuff, specifically the Tiva Canyon and Topopah Spring Members, are relatively permeable but occur Within the saturated zone only in the southwestern part of the caldera. CHARACTERISTICS OF SURFACE EXPOSURES The Widespread postcaldera ash—flow sheets that formed Pahute Mesa are well exposed in deep canyons and ravines and along the face of the mesa; excellent exposures also are found in surrounding areas within and outside the Nevada Test Site. Hence, most of these ash-flows have been mapped in considerable detail, and descriptions are given in publications such as those by Carr (1964), Noble and others (1964), Orkild (1965), and Lipman and others (1966). One of the most comprehensive dis- cussions on ash flows (deposits of ash resulting from flowage of a highly heated mixture of volcanic ash and unsorted material containing gas) is that by Ross and Smith (1961). They noted that columnar joints are common in welded tuffs, and joints are more closely spaced in the zones of most intense welding; similar jointing does not usually occur in the nonwelded parts of flows. Horizontal platy joint- ing commonly occurs in or near the zone of maximum compaction. Because of the uniformity of the Widespread post- caldera ash-flow sheets and the common occurrence of columnar jointing in the welded zones, these rocks generally are good aquifers. Welded tuffs, however, have lower compressive strength than 10 r I I I | | | I I | I | I Iii | | I I I | | I I I I I 12 — — _ o z 14 — — I- 0 III _ o 0 Static water level: 2,066 feet below land surface _ u. E 16 — — z‘ _ _ 5 g 18 — ._ < _ _ S 20 _ Transmissivity: 18,000 gallons per day per foot fl Average pumping rate: 186 gallons per minute — Specific capacity: 8 gallons per minute per ‘i foot of drawdown 22 — °_ .— 0 _ I I | | 1 I I l I I I I | I l | I I I I | I I I I I I 24 l 10 100 1000 TIME, IN MINUTES SINCE PUMPING BEGAN FIGURE 6.— Drawdown during 29-hour pumping test of hole U—20a—2, February 10—12, 1965. B14 rhyolitic lavas, and fractures commonly are resealed or healed where they occur at depths greater than 1,000 feet below the top of the saturated zone. For these reasons the welded tuffs of the Belted Range Tuff and most of the welded tuffs in the lava and tuff of Deadhorse Flat and the tuffs and rhyolites of Area 20 are poor aquifers. Ash-flow tufl’s range from nonwelded and poorly consolidated to densely welded. Interstitial porosity of the nonwelded tufl' may exceed 50 percent (Ross and Smith, 1961), and interstitial permeability of these rocks may exceed 2 gpd per square foot. Inter- stitial porosity and permeability decrease with an increase in the intensity of welding. Pore space generally is not entirely eliminated in the thoroughly HYDROLOGY OF NUCLEAR TEST SITES from 4.2 to 38.4 percent (table 4); the more in- tensely welded samples ranged from 4.2 to 13 per- cent. Samples usually are described as partly welded, moderately welded, or densely welded. Because of the direct relationship between porosity and the degree of welding, Ratté and Steven (1967) sug- gested that a numerical value of porosity would more effectively express the degree of welding than the undefined terms listed above. They would class- ify rocks having less than 10 percent porosity as densely welded and those having more than 10 percent as partly welded. Most welded tuffs are devitrified, a postdeposi- tional alteration that changes both the matrix and the pumice fragments into crystalline material. 0 I I I I I III| I II IIIII] I T IIIIIII 2 Static water level: 1,824.8 feet below land surface Transmissivity: 8,300 gallons per day per foot (from analysis of water-level recovery) Average pumping rate: 94 gallons per minute Specific capacity: 7 gallons per minute per foot of drawdown 10'— lilll 1 ll 14 l 1 l I E! ¢—Low pumping rate——> m 000 0 LL \<:b\o\°° 0000 o O 0 0° E 6 — ‘\\\7~ _ i Impervious \\\\ ° 3 \ O boundary \\°o 3 < n: D lllllI L I III 100 2000 TIME, IN MINUTES SINCE PUMPING BEGAN FIGURE 7,—Drawdown during a pumping test of hole UE-20e—1, June 4—5, 1964. welded zones, but porosity may be only a few per- cent, and interstitial permeability may be less than 2><10-5 gpd per square foot (Winograd and others, 1971). Interstitial porosity and permeability of the nonwelded and partially welded tuffs may be large enough to be significant in regional ground-water movement; however, interstitial porosity and perme- ability of the densely welded tufi's are too low to contribute significantly to ground-water movement. Ground water moves through densely welded tuffs principally along primary and secondary joints and other fractures associated with faults. CHARACTERISTICS BASED ON CORES, GEOPHYSICAL LOGS, AND DRILLING RECORDS Total porosity from core samples of welded tuffs in the saturated zone under Pahute Mesa ranged However, porosity ranges of vitric and devitrified welded tuff are similar. Lithophysal cavities and gas pockets occur in some welded tuffs and, locally, may contribute to interstitial porosity and permeability. All core samples recovered from the welded zones of the postcaldera ash-flow sheets contained frac- tures; the number of fractures ranged from 0.5 to 2.5 per foot of core and averaged 1.4 per foot. Many core samples of welded tuffs from the intracaldera rocks contained no open fractures, but fractures that were sealed with quartz and other minerals were common. A measure of the degree of welding of tuf’f can be determined from resistivity curves of electric logs. Most resistivities of moderately welded tuffs are between 100 and 225 ohms-m2/m; but densely welded PART OF PAHUTE MESA, NEVADA TEST SITE, NEVADA B15 TABLE 4. — Summary of porosity and fracture frequency in core samples of welded tufis [Compiled from unpublished data prepared by US. Geological Survey] Exploratory Number Range in Average Range in Average . hole and, in total total number of number of Formation parentheses, cafes porosity porosity fractures fractures number of cores (percent) (percent) per ft per ft Tiva Canyon UE—20d (1) , 4 72-172 14 0.5—2.2 1.4 Member of UE—20f (1), Paintbrush Tufl‘. UE—20c (2) Topopah Spring UE—20d (1) , 9 12.8—36.2 22 .7—2.5 1.4 Member of UE—20f (1) , Paintbrush Tufl'. UE—20c (7) Tuffs and rhyolites UE—19i (1) , 4 13.7—31.1 22.9 0— .1 <.1 of Area 20. UE—20e—1 (1) , UE—20h (2) Lava and tufl” of UE—19b—1 (4) , 14 12.9-33.5 21.4 0—>4 >.7 Deadhorse Flat. UE—19e (5), UE—19gs (4), UE—19i (1) Grouse Canyon UE—19e (1) , 14 4.2—24.0 12.1 0—3.2 .9 Member of UE—19gs (10), Belted Range Tufi’. UE—19i (1) , UE—20e—1 (1) , UE—20j (1) Tub Spring UE—l9fs (1) , 5 8.1—38.4 19.6 0—7.5 1.8 Member of UE—19h (2), Belted Range Tufi". UE—20j (1) , PM—2 (1) tufl's and tuff Vitrophyres have physical properties like those of rhyolitic lavas, and, their resistivities are in the same range as those of rhyolitic lavas. In general, geophysical logs indicate that the param- eters for identification of rhyolitic lavas in the subsurface also apply to the densely welded zones of tufi's, and very often the rocks cannot be dis- tinguished from one another. However, the fact that the densely welded zones usually grade vertically into less-welded zones and nonwelded zones facili- tates their identification on the basis of geophysical logs. Drilling rates in most welded tuffs range between the rates for ash-fall and nonwelded tufl’s and the rates for competent rhyolitic lava flows. Drilling characteristics and the parameters for detection of highly fractured, permeable zones in densely welded tuf’fs and rhyolitic lava flows are similar. CHARACTERISTICS BASED ON HYDRAULIC TESTS Welded tufi‘ was the principal rock in at least 41 intervals that were isolated by straddle packers in drill holes and tested by adding or withdrawing water. Relative specific capacities of 26 of the inter- vals tested (63 percent of the total) ranged from less than 0.01 to 0.02 gpm per foot of drawdown. Relative specific capacities of 0.1 to 0.5 gpm per foot of drawdown were measured in seven of the inter- vals tested, and of this total, five were for rocks of the Grouse Canyon Member in the UE—20j drill hole, and one was for the rocks of the Tub Spring Member in the UE—19h drill hole. The UE—20j hole was drilled near the ring-fracture zone of the cal- dera, and there was much evidence of faulting in core samples recovered. Data are illustrated in fig- ure 4. The only consistently permeable welded tui’fs are those assigned to the Tiva Canyon and Topopah Spring Members. These units occur within the satu- rated zone in the southwestern part of the caldera, and, where tested, relative specific capacities ranged from 0.04 to 0.15 gpm per foot of drawdown. The welded tuf’fs in these widespread ash-flOw sheets are aquifers in other parts of the Nevada Test Site, such as Jackass Flats in the southwestern part, where the Topopah Spring Member is the principal aquifer penetrated in production wells (R. A. Young, 1972). Welded tuff was the principal aquifer in only one of the holes test pumped (UE—20j) where produc- tion was from highly fractured welded tuffs and zeolitized bedded tufl‘s. ASH-FALL TUFFS, NONWELDED 0R SLIGHTiY WELDED ASH-FLOW TUFFS, AND TUFFACEOUS SEDIMENTS Although their origin and mode of emplacement differs, ash-fall and nonwelded or slightly welded ash-flow tuffs have similar physical properties and hydraulic characteristics. Interstitial porosity and permeability of these rocks, and of the reworked tufi‘s, are greater than those of the rhyolitic lavas and densely welded tufi'. However, fractures resealed more readily in these relatively incompetent rocks; B16 hence, fracture porosity and permeability generally are much lower than those of the rhyolitic lavas and densely welded tuffs. Yield of water to wells from ash-fall and nonwelded ash-flow tuffs is low, and for this reason these rocks are considered the best media for mining of chambers within the saturated zone. In the western part of the caldera, tufl’s are the predominant rocks in the saturated zone. The thick- nesses of zeolitized bedded tuffs which are inter- layered with rhyolitic lava flows in the tufi‘s and rhyolites of Area 20 range from 10 feet to more than 850 feet; an exceptional thickness of 1,750 feet was penetrated in the PM—l drill hole. Zeolitized nonwelded ash-flow tuffs attain thicknesses greater than 600 feet, and in the PM-l drill hole lithic-rich nonwelded ash-flows have a thickness of more than 2,000 feet. In the eastern part of the caldera, rhyolitic lava flows are the predominant rocks in the saturated zone; thicknesses of ash-fall and ash-flow tuffs be- tween these lava flows are not as great as in the western part of the caldera. The thickness of zeoli- tized bedded tuff ranges from 25 feet to 170 feet; zeolitized nonwelded ash-flow tuffs range in thick- ness from 20 feet to 280 feet. Intervals that have undergone argillization gen- erally are thinner; the average thickness of these intervals is 10 feet. However, a few intervals with thicknesses in excess of 200 feet are known. CHARACTERISTICS OF SURFACE EXPOSURES Most ash-fall and nonwelded ash-flow tuifs that are exposed at the surface in the report area do not occur in the saturated zone. East of the caldera, however, ash-fall and ash-flow tuffs assigned to the Belted Range Tuif crop out. These rocks, where penetrated within the caldera, are at great depths and are highly impermeable. Surface exposures of incompetent ash-fall and ash-flow tuffs generally are poor; the rocks are best observed in roadcuts and in gullies along the flanks of mesas. The topographic expression of the out- crops is that of gentle rounded hills, but cliffs may form where there is considerable case hardening. Wind erosion causes a peck-marked or honeycombed effect in some areas. Ash-fall tuffs are varicolored, fine grained with lapilli of various diameters, generally well sorted, thin to thick bedded, vitric and devitrified, zeolitic, and friable to well cemented. The nonwelded ash- flows are varicolored, massive, poorly sorted, vitric and devitrified, and locally zeolitized. Reworked tuffs are fine to coarse grained, well sorted to poorly sorted, thin bedded to massive, and often cross— HYDROLOGY OF NUCLEAR TEST SITES bedded. Fractures and minor fault displacements are common, but many are resealed. CHARACTERISTICS BASED ON COREs, GEOPHYSICAL LOGS, AND DRILLING RECORDS In the saturated zone, the formations containing the greatest thicknesses of ash-fall and nonwelded ash-flow tuffs and of reworked tuffs and volcanic sediments are the tuffs and rhyolites of Area 20 and the lava and tuff of Deadhorse Flat. Brief descrip- tions of these units, based on cores and cuttings, are given in table 1. Most ash-fall and nonwelded ash-flow tuffs in the saturated zone are zeolitized (glassy fragments are altered to zeolites), and some are argillized; re- worked tuffs and volcanic conglomerates, sandstones, and siltstones often are silicified. Vitric tuffs, in which glass shards are unaltered, are more common in postcaldera units that occur above the saturated zone. Total porosity of cores of ash-fall and nonwelded ash-flow tuffs ranges from 12 to slightly more than 50 percent. The average total porosity from 84 core samples of these rocks of the tuffs and rhyolites of Area 20 is 26 percent; porosity of similar rocks from the lava and tuff of Deadhorse Flat and from the Paintbrush Tuff averages more than 35 percent (table 5). There is a nonuniform decrease in poros- ity to a depth of more than 6,000 feet; however, porosity decreases rapidly below a depth of 7,000 feet. In the UE—20f exploratory hole, ash-fall and nonwelded to slightly welded ash-flow tuffs to a depth of 7,000 feet have total porosities ranging from 25 to 32 percent; from 7,000 feet to 11,000 feet total porosities range from 11 to 16 percent (R. D. Carroll, written commun., 1965). Samples of ash-fall tuff assigned to informal local units in the U—12e and U—12b tunnel systems under Rainier Mesa, east of Pahute Mesa, were analyzed for interstitial porosity and interstitial permeability (Thordarson, 1965). The interstitial porosity of the zeolitic tuff ranged from 25 to 38 percent; inter- stitial permeability ranged from 0.0004 to 0.02 gpd per square foot. The interstitial porosity and perme- ability of vitric tuff were high compared with those of zeolitic tufl". Fracture frequencies in cores of ash-fall and non- welded ash-flow tuffs ranged from 0.0 to 2.0 frac- tures per foot of core. The average from 82 cores, less than 0.2 fracture per foot, is considerably lower than the average fracture frequencies for rhyolitic lava flows and densely welded tuff, which develop fracture systems upon cooling and shrinking. Fur- ther, the fractures in the incompetent ash-fall and nonwelded ash-flow tuffs commonly are resealed. PART OF PAHUTE MESA, NEVADA TEST SITE, NEVADA B17 TABLE 5. — Summary of porosity and fracture frequency in core samples of ash-fall and nonwelded ash-flow tufis [Compiled from unpublished data. prepared by US. Geological Survey] Exploratory hole and, in parentheses, number of cores UE—19c (1), 13 UE—19gs (1), UE—19h (1), UE—19i (3), UE—20c (2), UE-20d (4), UE—20f (1) UE—19e (4), 84 UE—19fs (5), UE—19gs (3), UE—19h (2), PM—l (9), U—20a (5), UE—ZOc (9), UE—20d (8), UE—20e—1 (18), UE—20f (10), UE—20h (10), UE—20j (1) UE—19c (1) , 3 UE—19e (2) UE—19c (1), 12 UE—19d (1), PM—2 (9), ‘ UE—20j (1) Number of cores Formation Paintbrush Tuif Tufi‘s and rhyolites of Area 20. Lava and tufl“ of Deadhorse Flat. Belted Range Tufl‘ Average number of fractures per ft 0.07 Range in number of fractures per ft 0.0—0.3 Average total porosity (percent) 35.5 Range in total porosity (percent) 28.0—44.9 12.3—43.9 26.2 .0-1.4 .18 29.6-50.3 37.7 .0— .1 (.10 19.4—38.6 28.3 .0- .3 .06 Dips in cores of bedded tuffs are usually erratic and range from 0° to 90°. These dips may reflect irregular depositional surfaces or slump features. The resistivities of relatively impermeable zeol- itized tuffs are generally less than 100 ohm-meters on the short normal curve on electric logs made in drill holes at Pahute Mesa. Resistivities of vitric ash-fall tuft, nonwelded to slightly welded ash-flow tufl’, and tufi’aceous sediments are similar to those of the zeolitized units. The lowest resistivities occur in intervals that have been altered by argillization; these intervals, from hydraulic testing data, are the least permeable. Bedded tuff and nonwelded tuff generally are indi- cated on the curves of caliper logs by large “washed- out” or caved intervals; hole size is generally greater than drill-bit size. Prominent ledges and abrupt “washed-out” zones commonly occur at the contact of these rocks with the more competent rhyolitic lava flows and densely welded tufl‘s. CHARACTERISTICS BASED ON HYDRAULIC TESTS Zeolitized bedded tuff was the principal rock in 54 intervals isolated by straddle packers in drill holes and tested by adding or withdrawing water; these intervals ranged in thickness from 150 to 200 feet. Relative specific capacities in 41 of the intervals tested (76 percent of the total) ranged from less than 0.001 to 0.02 gpm per foot of drawdown (fig. 4). Intervals with relative specific capacities that ranged from 0.05 to 0.5 gpm per foot of drawdown were measured only in UE—20h and UE—20j drill holes. In general, the highly argillized intervals were the least permeable. REGIONAL MOVEMENT 0F GROUND WATER Ground water beneath Pahute Mesa moves south- westward and southward toward the Amargosa Desert through Oasis Valley, Crater Flat, and west- ern Jackass Flats. (See pl. 3.) This area is consid- ered part of a single ground-water basin informally designated by Winograd, Thordarson, and Young (1971) as the Oasis Valley—Fortymile Canyon basin. The Oasis Valley—Fortymile Canyon ground-water basin comprises an area of intense late Miocene and early Pliocene volcanism. The Silent Canyon and Timber Mountain calderas are the principal volcano- tectonic Structural basins in the area; the younger Timber Mountain caldera, including its moat area, occupies most of the southern two-thirds of the ground-water basin. The Black Mountain caldera (Christiansen and Noble, 1965) lies west of the Silent Canyon caldera, and several other calderas have been mapped in the surrounding area. Volcanic cones project above the alluvial fill in Crater Flat. Paleozoic carbonate rocks may underlie volcanic rocks beyond the ring fractures of the calderas, but they probably do not underlie the volcanic rocks within the calderas; and if carbonate rocks underlie the moat areas, they would be found at great depths below land surface. For this reason, it does not ap- pear that the carbonate aquifer in western Yucca B18 Flat and northern Jackass Flats has lateral continu- ity with carbonate rocks that may underlie areas outside the calderas. The hydrologic limits of the basin are not fully defined and probably do not correspond everywhere with the surface-drainage boundaries. Highly im- permeable elastic rocks underlie the topographic divide that trends southward along the Belted Range (pl. 3). The eastern limits of the basin probably coincide with this divide. The western limits of the basin at Pahute Mesa may coincide with the west margin of Gold Flat in the area of Quartz Mountain. The ground-water gradient to the southwest at Pahute Mesa is interrupted by a barrier (pl. 1) HYDROLOGY OF NUCLEAR TEST SITES which trends parallel to the north-northeast strike of basin-and-range faulting. The exact strike and full linear extension of this barrier are not firmly established. Seismic studies after some nuclear ex- plosions have disclosed aftershock epicenters along the barrier lineament. Some of the epicenters are several miles south of the intersection of the Silent Canyon and Timber Mountain calderas, beyond the south end of the barrier as shown on plate 1; hence, the barrier may project farther into the moat area of the Timber Mountain caldera. Part of the barrier coincides with the western ring-fracture system of the Silent Canyon caldera. A ground-water drain parallels the barrier on the east and is possibly con- E 530 000 E 550000 F. 570 000 E 590 000 E 610000 E 630 000 M 9-10 000 . “5°30 11915' ’ a 1 H! EXPLANATION . ‘ . 1 l . . . . . I,,_ , _,~ ,. . , v , _\ Rock types With reSIStIVIties less ‘ 1/ ’ NEVADA TEST SITE \_ than 225 ohms—ml/m on 16— 37“22’30” 3-7 / \\ \ » inch normal curve of electric / / \ log . , N /’ \ \ N 950 ooo , .s .wm ,, \f , / \ \‘ . U E— 2.0 p, .. N 930 000 " E-19gs f Rock types with resistivities greater than 225 ohms—m2/m on 16—inch normal curve of electric log l 5 l l U-19b UE-19b—1 .UE-20c 1u Exploratory test hole and number 0 U-20C Emplacement hole and number N 919 00 ._ /;* . a. m . 3’ 15 $5} U—20a—2l \ N 8900001- \ €44 _______ ‘7" AREA 18 ’ \\ l N 870 000 .UE—l8r JACKASS 20,000-foot grid based on Nevada FLATS/ coordinate system, central zone 0 5 MILES L__.L._.I_L.~_l_l '\__\ ‘\. . NEVADA TEST SITE ’r O 5 10 15 MILES L—I—.~J__l FIGURE 8. —— Resistivity of rocks in upper 2,000 feet of saturated zone. PART OF PAHUTE MESA, NEVADA TEST SITE, NEVADA trolled by the caldera ring fractures. The hydraulic gradient west of the barrier is southeastward, and leakage occurs across the barrier into the drain. ESTIMATES OF UNDERFLOW BENEATH PAHUTE MESA Ground water moves in volcanic rocks beneath Pahute Mesa through interconnecting faults and joints. As many as 10 faults per square mile have been mapped on the surface in postcaldera ashflows that mask the structural basin. The density of faults in intracaldera rocks probably is many times greater. The highly complicated structure and distribution of different rocks in the subsurface result in great variations in transmissivities over short distances. The fence diagram (fig. 8), compiled from interpre- tation of resistivity data obtained from 16—inch nor- mal curves of electric logs made in drill holes, shows the distribution of rocks in the upper 2,000 feet of the saturated zone. Rocks with resistivities less than 225 ohm-meters include zeolitized and bedded ash- fall tuff, tuffaceous sediments, and nonwelded and partly welded ash-flow tuff. Rocks with resistivities greater than 225 ohm-meters include rhyolitic lava flows, vitrophyre, and densely welded tufl’. Greatest transmissivities were measured in holes drilled along the east margin of the Silent Canyon caldera, where the principal rock in the saturated zone is rhyolitic lava (table 3 and fig. 9). Transmissivities, based on data from single wells that were pumped on Pahute Mesa for periods of 12 to 47 hours, range from 1,400 to 140,000 gpd per ft. Because of these extreme values, a geometric mean, rather than an arithmetic mean, is considered to yield a more reasonable value for average trans— missivity. A geometric mean of 10,000 gpd per ft is used for calculation of underfiow. Data from hydraulic tests in drill holes, especially from radioactive-tracer surveys, indicate the com- plexity of flow patterns beneath the mesa. These radioactive-tracer surveys show that most water is yielded from fractured zones that constitute only 3 to 10 percent of the total section penetrated in the saturated zone. Further, the depths at which these fractured zones occur are highly variable and are not always correlative between holes that are drilled even as close as 100 feet apart. Despite the tortuous flow patterns and changes in head potentials with depth, discussed under “Head Changes in Drill Holes,” ground-water flow beneath Pahute Mesa is essentially lateral with potential for downward leakage in the eastern part of the caldera and upward leakage in the western part. The hy— draulic gradient ranges from 25 to 100 feet per mile B19 (pl. 1); a gradient of 50 feet per mile is used to compute underfiow from Pahute Mesa. The equation for computing underflow per mile of flow-section width is Q:0.00112TI, where Q is the quantity of underflow, in acre-feet per year; 0.00112 is the factor for conversion of gallons per day to acre-feet per year; T is the transmissivity, in gallons per day per foot; and I is the ground-water gradient, in feet per mile. By using the values for transmissivity (table 3) and gradient (preceding paragraph) given above, the estimated underflow beneath Pahute Mesa is 560 acre-feet per mile per year. The length of underflow strip, about 15 miles, is the approximate distance between the hydraulic barrier on the west and the ground-water divide on the east—near test well TW—8. The total estimated underflow passing southwestward beneath Pahute Mesa is about 8,000 acre-feet per year. ESTIMATED ANNUAL RECHARGE FROM PRECIPITATION AND UNDERFLOW Precipitation in southern Nevada depends to a large extent upon altitude and location (R. F. Quir- ing, U.S. Weather Bur., written commun., 1965). Pahute Mesa lies principally within a transition zone approximately 50 miles Wide, with a deficit-pre- cipitation zone to the west and an excess-precipita- tion zone to the east. It has a precipitation-altitude relation as shown in table 6. An estimate of potential recharge to the ground- water reservoir beneath eastern Pahute Mesa and adjacent areas was made by using the method de- vised by Eakin and others (1951). The metho'J entails measurements of areas in which the average annual precipitation ranges between specific limits; a percentage of the average annual precipitation on each area represents the recharge to the ground- water reservoir from that area. Percentages of an- nual precipitation used for computing potential recharge are listed in table 6. The area planimetered for estimation of potential recharge to the ground- water reservoir lies within the coordinates of lat 37°10’ N. and 37°22’ N. and long 116°12’ W. and 116°33’ W. (See pl. 3.) The recharge to the ground-water reservoir be- neath Pahute Mesa from precipitation may be greater or less than the estimated annual potential recharge because of the complicated structure and rock types. Welded tuffs that occur on the surface over most of the mesa are highly fractured; how- ever, percolation of water to the deep water table may be restricted by intervals of poorly permeable bedded or nonwelded tuffs that range in thickness from 200 to more than 600 feet. In a few holes that were drilled with air as the circulating medium, B20 E" 530 000 E. 570 000 N 970000‘ ' T , . e . ; l l 3/“22’30” ;~» N 950 000' TPMTE ‘6 N 930 000 I) () m J] 000 E. 610 000 1 v HYDROLOGY OF NUCLEAR TEST SITES E 630 000 1 16“ 1 5’ i l EX P LA N ATI O N ‘ Transmissivity, in gallons per day per foot Less than 10,000 10,000 to 40,000 Greater than 40,000 Boundary of transmissivity range a U E—20e- l 8300 Exploratory test hole and number Number is transmissim‘ty, in gallons per day per foot 0 U —19as Emplacement hole and number N 910 00 . 37’15’ — N 890 (1)00 . if" } .731; . . l I . 70 (MT) W—v fl N 8 ’” . 23,000 20.000-foot and based on Nevada UE—18r coordinate system, central zone 5 M|LES NEVADA TEST SITE \J 0 5 10 15 MILES L__A_*|__A FIGURE 9. — Ranges of transmissivity. perched or semiperched water was detected at depths of 500 or more feet. Conversely, in the eastern part of the mesa, where rhyolite is the principal rock type in the subsurface, vertical permeability through fractures may be great, and precipitation may con- tribute considerably to recharge. Estimates of average annual potential recharge to Kawich Valley and Gold Flat are 3,500 acre-feet and 1,600 acre-feet, respectively (Eakin and others, 1963). As much as 1,000 acre-feet may enter Kawich Valley as underflow from the north, and 2,000 acre- feet may enter Gold Flat from the north. Most of the ground water from Kawich Valley probably moves into the eastern part of Pahute Mesa; pos- sibly one-third of the underflow from Gold Flat may enter the ground-water system beneath part of Pahute Mesa (pl. 3). On the basis of conservative estimates, about 5,500 acre-feet of underflow from these areas enters the ground-water system beneath the mesa. Including the average annual potential recharge from precipitation, the total volume of re- charge is about 8,000 acre-feet. ESTIMATED ANNUAL DISCHARGE From Pahute Mesa, ground water moves south- westward and southward toward the Amargosa Desert through Oasis Valley, Crater Flat, and west- ern Jackass Flats (pl. 3). Malmberg and Eakin (1962) estimated the total PART OF PAHUTE MESA, NEVADA TEST SITE, NEVADA B21 TABLE 6. —Estimated average annual ground-water potential recharge from precipitation at Pahute Mesa and Rainier Mesa Precipitation Altitude of Area of Precipitation Estimated Estimated zone zone zone (acre-ft recharge recharge (in.) (ft) (acres) per yr) (percent) (acre-ft per yr) 10—13 ....................................... 7,000—8,000 25,000 25,000 7 1,750 8—10 ................ 6,000—7,000 95,000 71,000 2 1,400 Total.L ............................................................ 120,000 96,000 3,150 average annual ground-water discharge from Oasis Valley by evapotranspiration and subsurface outflow to be about 2,400 acre-feet. Recent studies in Oasis Valley made by W. A. Beetem and R. A. Young, US. Geological Survey, indicate that the discharge from Oasis Valley might be two or more times greater than the estimates by Malmberg and Eakin (W. A. Beetem, oral commun., 1968). Minor eastward underflow may occur across the ground-water divide into Yucca Flat. The annual inflow into Yucca Flat probably does not exceed 200 acre-feet (Winograd and others, 1971). There- fore, of the estimated 8,000 acre-feet of underflow beneath the south margin of Pahute Mesa, possibly as much as 5,000 acre-feet moves toward the Amar— gosa Desert through Crater Flat and western Jackass Flats. As much as 2,000 acre-feet of potential recharge to the ground-water system results from precipita- tion at Timber Mountain, Buckboard Mesa, western Jackass Flats, and Crater Flat. Hence, the total dis- charge to the Amargosa Desert is about 10,000 acre-feet. LOCAL MOVEMENT OF GROUND WATER The depth to water under the eastern part of Pahute Mesa and within the limits of the Silent Canyon caldera ranges from about 1,950 feet (alt 4,160 ft) in the western part to 2,350 feet (alt 4,685 ft) in the eastern part. In the extreme northwestern part of the Nevada Test Site (pl. 1), outside the caldera, the depth to water is about 850 feet (alt 4,700 ft). Geologic structure, spatial distribution of tuff and rhyolitic lava flows, and location of recharge areas are major factors contributing to the complex flow patterns interpreted from drill-hole data on the mesa. Geologic structure and the distribution of major rock types in the subsurface are shown on plates 1 and 2 and in figure 8. Head changes with drilled depth are stable or decline in all but one of the holes drilled in the eastern part of the report area; head changes with drilled depth are variable or increasing in holes drilled in the western part of the report area. Head changes in drill holes are summarized in figure 10 and listed in table 7. HEAD CHANGES IN DRILL HOLES Rhyolitic lavas are relatively competent rocks; hence, compressibility of these rocks is moderate, and faults and joints may remain open even at great depths. Horizontal permeability occurs along flow layers and fractures; vertical permeability occurs only along fractures. In most of the drill holes in the eastern part of the caldera, where rhyolitic lava flows comprise 90 to 100 percent of the rock section in the saturated zone, heads are relatively stable through depths ranging to 2,500 feet below the top of the saturated zone; heads decrease, or probably decrease, below these relatively stable intervals to total drilled depth. Ground water flows essentially laterally; however, the flow paths probably are tor- tuous in that most of the water flows through zones of fracture permeability that are randomly distrib— uted and comprise only 3 to 10 percent of the drilled section. Water leaks downward along fractures to depths greater than 6,000 feet below the top of the saturated zone. Other factors that influence the flow pattern in this area are: ( 1) Underflow from Ka- wich Valley which enters from the northeast, and (2) recharge from precipitation which enters the saturated zone from higher altitudes on the east. Rhyolitic lava flows of the lava and tuff of Dead- horse Flat, that flooded the eastern part of the cal- dera, pinch out abruptly to the west where they interfinger with ash-fall and ash-flow tuffs. Where the percentage of rhyolitic lava flows decreases, head changes with depth in drill holes are less pronounced, and a transition zone between decreasing heads with depth and increasing head with depth is approached. A reduction in vertical permeability occurs where lava flows interfinger with tuffs. Hence, vertical per- meability is a major factor controlling the pattern of head changes with depth beneath Pahute Mesa. Isolated structural highs, consisting mostly of tufl“, were present along the east margin of the caldera during eruption of the lava flows. Areas with a low percentage of rhyolitic lava in the saturated zone were penetrated by drill holes UE—19e and UE—19i (table 2 and fig. 3). The areal extent of the high penetrated in the UE——19e hole probably was minor; heads in this hole decrease with depth, as do the heads in surrounding drill holes where the section B22 consisted mostly of rhyolitic lavas. Tui’l’s in the area of UE—19i were of greater areal extent; UE—19i was the only drill hole in the eastern part of the caldera in which heads increased with depth. Ash-fall and ash-flow tufi‘s comprise the greatest percentage of the rock section in the upper 2,000 feet of the saturated zone in drill holes in the west- ern and central parts of the caldera. Rhyolitic lavas account for less than 1 to 22 percent of the total section; exceptions are drill holes U—20a—2 and UE—20h, in which a regional nose of rhyolitic lavas, thinning northward, was penetrated. The rhyolitic lavas in the western and central parts of the caldera are lenticular bodies of variable thickness. These lava flows are separated by thick E 53C) 000 E 550000 k 1.3.690 E 590 000 HYDROLOGY OF NUCLEAR TEST SITES sections of ash-fall and ash-flow tuffs that have low permeabilities. The tufl‘s are relatively incompetent and, hence, are more sensitive than the rhyolitic lavas to compression by weight of rock overburden. Fractures are more likely to be rescaled, volume and porosity are reduced, and pressures are increased. Vertical permeability in some areas may be low enough to create confined aquifers. In holes drilled in the western and central parts of the caldera, heads usually are variable from the top of the saturated zone through intervals of rhyo- litic lava flows that have high permeabilities and then increase with depth to the total drilled depth. Ground-water flow is essentially lateral with upward leakage. :5 610 000 EXPLANATION N 970000; ~» 7 We“: 7 , : J, l f : : i 2 : 37’“22’30” L77 : ,/ I \és l l f UE-19h‘l l I l (“g \0 CD -_- C -. m u'o o m l ... N L330 000 \ L qk ‘: U_20f ue-zofi‘yw ~ UE—20f ‘ 1! O m I. ” ’EVXISA :FfisfiiTE iU-19g ,f 7 j) 9 IUE—19gs :f’ Relatively stable or variable heads to depths as much as \. 2,500 feet below top of satu- \ rated zone, and decreasing or ‘\ probable decreasing heads \ with greater depth ”—1.2? 7 ‘ [40. i® 9 32 Relatively stable or variable heads to depths as much as 2,500 feet below top of satu- rated zone, and increasing or probable increasing heads with greater depth Numbers near arrows indicate measured head changes. Quer'bed where doubtful U—19b UE-19b—1 1,, 1,1,9,“ , _. Approximate boundary between areas with decreasing and increasing heads w ,_. ’ “A i 1/\\ N 9: 0:0, “9% .’: \ :75 5" 2.5, D l N 890 000 *4 .UE—20j ou-20a Exploratory test Emplacement hole hole and number and number N 870 000' 20,000-foot grid based on Nevada coordinate system, central zone 0 5 MILES NEVADA TEST SITE \/ 0 5 1o 15 MILES |—~_‘—J—_J HR] FIGURE 10,— Head changes with depth in exploratory test holes. PART OF PAHUTE MESA, NEVADA TEST SITE, NEVADA B23 TABLE 7. ——-Head changes with depth in exploratory holes, Pahute Mesa, [All depths in feet below land surface. Intervals isolated with packers in PM—l and PM-2 were too impermeable to yield data on head changes] . . . 1 Exploratory 3322'; i1? 1:35:3113151’11 ”7332;313:273” P""°1‘1‘1"1'§51§%§dt”“ ”will. $335? (5’ hole open hole packers interval (5) interval (5) permeability UE—19b—1 ................ 2,117 2,176—4,060 2,117 Rhyolite .............................. 2,610—2,640, 2,720—2,760 UE-19c .................... 2,345 2,319—2,874 2,3190) Rliyolite and rhyolite receia. 2,884—3,284 2,348 .......... do ............................... } 31050—3375 4033-4231 2,360 Rhyolite .............................. UE—19d .................... 2,177 2,500—3,483 2,177 Rhyolite .............................. 33123323 332 d 3’ y 9 _ 4,626—4,784 2,180 330° 335° 4,810—4,968 2,200 5,823-6,583 2,210 UE—19e .................... 2,240 2,619—2,779 2,232 Welded tufl’ and _ moulded we. } 21633731649319, 4,802—6,004 2,253 .......... do ............................... ’ ' UE—19fs ................... 2,305 2,750—3,218 2,302 Rhyolite .............................. _ 3,520—3,838 2,304 .......... do .................... 23389539359330 4,146—4,480 2,309 Rhyolite and tuff... ..... ’ ’ UE—19gs .................. 2,045 2802-2970 2,043 Rhyolite ...... . _ 4,636—4,834 2,050 Welded tuff ................ . 2344283847$§0 6,920—7,118 2,049 Tufi'aceous sandstone ....... ’ ’ UE—19h .................... 2,112 2,200-2,396 2,109 Rhyolite and welded tufi 2 ,37e8—2, 388 2408-3705 2,112 do st.)’ UE—19i ..................... 2,258 2910-3068 2,220 3,460—3,618 2,258 3,4303331437‘7’235 4,100—4,258 2,218 ’ ’ U—20a—2 ................... 2,066 2067-2608 2,064 2,492—2,682 2,066 2,895—3,085 2,065 2,400—2,682, 3,090—3,280 2,064 2,895—3,085, 3,648-3,838 2,042 3,648—3,838 4,048—4,238 2,051 4,355.4,500 2,033 UE—20d .................... 2,075 2,578—2,776 2,078 4,118-4,316 2,013 2578-2330 UE-20e—1 ................ 1,822 2,774—2,972 1,828 Rhyolite .............................. 3,480—3,678 1,835 Rhyolite and vitrophyre 4,020-4,218 1,828 Zeolitized bedded tufi‘ 3,550—3,660 and welded tuff. 4,540—6,395 1,820 .......... do ............................... UE-20f .................... 11,954 géggjggg 143$ Rhyoliite and vitrophyre 3338—3536 11954 3’145g638l344366 4350-5249 1,857 ’ ’ 8,972-9,170 1,846 UE—20h .................... 2,116 2575—2743 2,111 2741-3210 2,116 3,350—3,518 2,111 3,042—3,170, 3,705—3,8‘73 2,114 ? 4,040—4,060 3,892—4,060 2,116 4,070—4,238 2,117 .......... do ............................... , UE—20j ..................... 1,270 1,858—2,056 1,245 Welded tufl‘ ........................ ~ 32:7 d y - 1 12 9 2,670—2,868 1,261 2’026‘9’ggg5g'57 2957-3155 1,270 ’ ’ 3359—3832 1,273 do 4,023—5,690 1,264 Rhyolite and tufi‘ ............... UE—18r” ................... 1,372 1,440—5,004 1,372 Welded tuff ........................ 1,660—1,675 .......... do 2,350—2,360 Rhyolite .............................. 3,550—3,560 TW—s2 ....................... 1,068 1,320-5,490 1,068 Rhyolite and welded turf 1,290—2,010 ‘Depth to water when hole was drilled to 4,543 ft. After water was 1,772 ft. hole was cased to 4,490 ft and drilled to 13,686 ft, depth to 2South of Pahute Mesa. B24 RELATION BETWEEN SUBSURFACE TEMPERATURES AND FLOW PATTERNS Geothermal gradients, listed in table 8, were computed by dividing the difference between the bottom-hole temperature and the mean annual air temperature by the depth at which the bottom-hole temperature was obtained. Because of caving, slough- ing, or other poor hole conditions, the bottom-hole temperatures were not always obtained at the total drilled depth. Temperature logs were made several hours or days after completion of the drilling and, hence, represent transient rather than equilibrium temperatures. However, the data are considered suf- ficiently accurate to establish the relation between temperature and flow pattern. Geothermal gradients (Levorsen, 1954) are expressed in degrees Celsius per 100 feet. Temperatures in the subsurface do not appear to contribute to head differences with depth in holes drilled within the boundaries of the Silent Canyon caldera at Pahute Mesa. There is a relation, how— ever, between temperatures and head changes in drill holes. Thermal gradients are lowest in those holes where relatively stable heads occur; gradients are highest where the differences in heads are great- est, irrespective of the direction of head change. If the maximum head changes are equivalent, thermal gradients in holes having heads that increase with depth may be quite similar to those in holes having heads that decrease with depth. (See table 8.) Exploratory drill holes PM—Z and UE—20j are west of the Silent Canyon caldera. The high geo- thermal gradient of 131°C per 100 feet in PM—2 probably results from remnant heat in a massive granitic intrusive body that was penetrated below a depth of 8,400 feet. WATER-LEVEL CONTOUR MAP The water-level data used to construct the water- level contour map (pl. 1) represent the composite level of several water-bearing zones in each hole. The lenticularity and random vertical distribution of rhyolitic lava flows and ash-fall and ash-flow tuffs in the saturated zone in the caldera preclude any distinction and correlation between water levels of different aquifers in the various wells. Inaccu- racies in the map exist because of head differences with increasing depth and the differing depths of drill holes; nevertheless, the map reflects the hy- draulic gradient and direction of ground-water flow under the eastern part of Pahute Mesa. Composite water levels from wells drilled in the eastern part of the Silent Canyon caldera are considered more reliable than those from wells HYDROLOGY OF NUCLEAR TEST SITES drilled in the western part. Depths of exploratory holes in the eastern part range from 3,700 to 8,489 feet. Composite water levels either remained con- stant or decreased only slightly during drilling of the deeper holes. Depths of exploratory holes in the western and central parts of the caldera range from 4,500 to 13,686 feet. Composite water levels fluctu- ated during drilling and generally rose with increas- ing depth. A head difference in excess of 142 feet was measured in the deepest hole drilled in the area; for uniformity, a composite water level obtained after that well was drilled to a depth of 4,500 feet was used for construction of the water-level contour map. Local variations in ground-water flow lines prob- ably are very common because of the heterogeneous rock section and the prevalence of faults which cause differences in conditions of local confinement. The hydraulic gradient is to the southwest and to the southeast toward a north-south trending drain; it is quite flat in the eastern part of the caldera but steepens in the western part. A definite relation exists between the slope of the hydraulic gradient and the subsurface geology. In the eastern part of the caldera, where the gradient is gentle, principal rocks in the saturated zone are rhyolitic lava flows. The flat area in the central part of the caldera, between exploratory holes U—20a—2, UE—20h, and PM-l, reflects the nose of a rhyolitic lava flow in the subsurface that has an axis striking roughly between holes U—20a—2 and UE—20h. The flat area also occurs in the area of a major north- striking horst that bisects the caldera. The steepen- ing of the hydraulic gradient in the western part of the caldera corresponds to an increase in the per- centage of poorly permeable zeolitized tuff in the saturated zone. The caldera boundary fault on the west brings into juxtaposition the intracaldera rocks with pre- caldera rocks having lower permeabilities, thus creating a limited barrier to the flow of ground water. GROUND-WATER VELOCITY Ground-water velocity, V, beneath Pahute Mesa was estimated using the equation VzQ/Ap, where Q is the underflow, in cubic feet per day; A is the cross-sectional area of flow, in square feet; and p is fracture and interstitial porosity. The value for Q, based on data presented earlier in this report, is 8,000 acre-feet per year, or about 1x106 cubic feet per day. The area of underflow (A) used in the computation is assumed to be 15 miles long and 3,500 feet thick, or 277x10“ square feet. The magni- tude of effective fracture porosity in the heteroge- PART OF PAHUTE MESA, NEVADA TEST SITE, NEVADA neous volcanic rocks is difl‘icult to estimate; hence, velocities were computed using effective porosities of 0.5, 10, and 20 percent. The estimated velocities based on these values are shown in table 9. Thicknesses of volcanic rocks within the saturated zone are known to be greater than 11,000 feet in the western part of the caldera and greater than 6,000 feet in the eastern part. Data from hydraulic tests indicate that most ground water occurs within the upper 2,500 feet of the saturated zone, but many drill holes penetrated permeable zones at depths be- tween 3,600 and 5,000 feet below the water table. However, many holes were not drilled to depths greater than 2,500 feet below the top of the satu- rated zone, so there is no control on depths to per- meable intervals. Hence, a mean value of 3,500 feet B25 is considered realistic for computation of area of underflow (A). Values for effective fracture and interstitial po- rosity are difficult to estimate accurately, because rocks with greatly different physical properties in- terfinger within the area carrying underflow. In much of the eastern part of the caldera, where rhyo- litic lava flows comprise the greatest percentage of rocks in the saturated zone, the low ranges of poros- ity appear most valid. In this area, ground water may move almost exclusively along faults and joints; further, interconnected fractures probably total less than 10 percent of the vertical thickness of the satu- rated zone. Conversely, greater porosity may prevail in the rocks of much of the western part of the cal- dera where ash-fall and ash-flow tuffs and volcanic TABLE 8.—Bottom-hole temperatures and thermal gradients in exploratory holes, Pahute Mesa Formation temperature Total Maximum head difierence Exploratory depth Depth of D a Gradient in exploratory hole hole (ft) $?:?;3 logélztgd (maximum) (103: 331. (ft) UE-19b-1 ................... 4,500 4,060 6—15—64 34.4 0.53 Stable. UE—19c ................ 8,489 4,510 5— 7—64 46.6 .75 41; decreasing with depth. UE—19d ............ 7,689 6,560 6—25—64 61.1 .74 32; decreasing with depth. UE—19e ............ 6,005 4,880 8—23-64 46.6 .69 21; decreasing with depth. UE—19fs ..... 6,950 4,400 8—20—65 41.1 .64 7; decreasing with depth. UE—19gs 7,506 7,500 5- 4—65 61.6 .65 7; decreasing with depth. UE—19h 3,705 3,456 7—31—65 31.1 .53 Stable. UE—191 8,000 7,968 9— 3—65 73.8 .77 40; increasing with depth. PM—l 7,858 7,808 5— 1—64 65.5 .68 Unknown. PM—2. 8,782 5,438 8—10-64 83.8 1.31 Increasing with depth. U—20a— 4,500 4,500 2—17—64 41.1 .63 33; increasing with depth. UE—20d 4,493 4,490 8—14—64 46.1 .74 65; increasing with depth. UE—20e—1 6,395 6,200 5—27—64 57.2 .72 15; increasing with depth. UE—20f ................ 13,686 12,270 6—25—64 121.0 .88 142; increasing with depth. UE—20h ................ 7,207 7,195 8—16—64 50.0 .52 Relatively stable. UE—ZOJ ....................... 5,690 4,550 10—10—64 46.1 .73 9; increasing with depth.1 1Heads decrease 28 ft in upper 2,000 ft of saturated zone and then increase 9 it to total depth. sediments comprise the greatest percentage of the rock in the saturated zone. Total porosities of these rocks may be more than three times greater than those of rhyolitic lavas; and, in the western part, interstitial permeability may be more important in ground-water movement. Fractures probably combine with pores to form an interconnecting system; thus, although most ground water moves through the underflow strip along fractures, two systems of permeability are in- volved. Hydraulic test data indicate that some ground-water movement, however small, is related to interstitial permeability. Low-permeability blocks between fractures probably result in velocities that vary as much as 2 or 3 orders of magnitude over short distances. Admittedly, any value of porosity used for computation of ground-water velocity be- neath the mesa cannot be valid throughout the cross- sectional area of the underflow strip. A median value of 10 percent porosity probably is the most acceptable 'value if credence is given to the assump- tion that most ground-water movement occurs along interconnected fractures and that some water moves through interstices. CHEMICAL QUALITY OF WATER Chemical analyses of water samples collected in tunnels and from springs and wells at and near the Nevada Test Site were reported by Clebsch and Barker (1960), Moore (1961), Eakin and others (1963), Schof’f and Moore (1964), and in various other open-file publications of the Geological Survey. The discussion of water chemistry in this report is concerned principally with the analyses of samples collected from holes drilled at Pahute Mesa, the rela- tion of water chemistry to the principal rocks in the volcanic sequence, and general comments on water movement inferred from water chemistry. The principal chemical constituents in water sam- ples from holes drilled at Pahute Mesa are listed in table 10, and selected analyses are plotted as modi- fied pie diagrams in figure 11. Most samples repre- 326 TABLE 9 — Estimated ground-water velocity in volcanic rocks beneath Pahute Mesa1 Assumed effective fracture and inter- stitial porosity (percent) 0.5 0.7 250 20 Velocity Feet Feet Years per day per year2 per mile Remarks Assumes movement only through fractures. 10 .04 15 350 Principal movement through fractures; some interstitial permeability. 20 .02 7 750 Assumes significant movement through interstitial permeability. lAssumes underflow of 8,000 acre-feet per year (1X106 ft3 per 2day) and ungfiggggegtrip 15 miles Wide and 3,500 feet thick or 277x10“ ft2 sent a composite of waters from various contributing zones in a single drill hole. They were collected dur- ing the final stages of pumping tests made shortly after termination of drilling operations. Pumping rates ranged from 56 to 215 gpm, and the duration of the pumping tests ranged from 12 to 48 hours. In some holes, lower contributing zones were un- tapped because of low pumping‘rates, and some sam- ples probably contained drilling additives that could not be removed from the reservoirs during the shorter pumping periods. Despite the conditions at the time these samples were collected, the chemical data appear to represent the aquifer water. Analyses of water samples collected as above described were compared with those analyses of water samples col- lected from drill holes UE—19e, U—20a—2, and UE—20h after these holes had been completed as supply wells and had been pumped for periods of 6 to 12 months. No major changes were found in the two sets of analyses of water samples. In some drill holes, where differences in head po- tential with depth and data from radioactive-tracer surveys indicated natural crossflow, samples were collected at selected depths to determine possible variations in water quality of the major contribut- ing zones. This survey was made approximately 2 years after the holes had been drilled to total depth and was only partially successful because caving of the walls in some boreholes prevented access of the wire-line sampling tool and because many drill holes were plugged with cement or mud or were in use as water-supply wells. Samples thus obtained are in- cluded in table 10. Samples were collected at critical depths in drill holes UE—19d and UE—20e—1. The flow pattern in the UE—19d drill hole was essentially lateral from the top of the saturated zone to a depth of 3,400 feet below land surface; below this depth the head poten- tial decreased, and there was downward crossflow. A radioactive-tracer survey indicated that perme- able intervals below 3,400 feet were not tapped dur- ing a pumping test made in the hole. Samples were collected with the wire-line tool at depths of 3,380 HYDROLOGY OF NUCLEAR TEST SITES and 4,200 feet. The chemical character of the sam- ples was almost identical. The flow pattern in UE—20d was more complex. A highly permeable in- terval of densely welded tuff or rhyolite at a depth of approximately 3,000 feet below land surface (which had a lower head potential than those of the intervals above and below) accepted fluid from the shallower and deeper contributing zones. Samples collected at a depth of 2,920 feet in the area of down- ward flow and at 3,200 feet in the area of upward flow closely resemble each other chemically (table 10). Testing of specific water—bearing intervals was re- stricted to one sample each in the UE-20e—1 and UE—19c drill holes because of impenetrable bridges in the holes. The analysis of the water sample from UE—20e—1 compared favorably with the analysis of a composite sample collected during a pumping test. There was considerable divergence, however, in the concentration of chemical constituents in the spot sample and the composite sample from the UE—19c drill hole. Water samples of leakage or inflow from walls at the top of a chamber mined in the saturated zone in the U—19as emplacement hole were high in dis- solved solids. The samples were collected under unfavorable conditions and were probably contami- nated by cement and drilling additives. Chemical analyses of these samples are shown in table 10. Because the concentration of chemical constituents is abnormally high compared with that for other samples, these data were not plotted in figure 11. RELATION OF WATER CHEMISTRY AND ROCK TYPE All holes drilled at the mesa were spudded and drilled to total depth in volcanic rocks. Water from the volcanic rocks at the Nevada Test Site has been classified as sodium potassium type. In this classifi- cation, sodium predominates (potassium generally is minor to negligible), and sodium and potassium to- gether are 60 percent or more of the total cations. The water samples reported by Schofl' and Moore (1964) were derived principally from contributing zones in volcanic tuff, and some water contained small amounts of calcium, magnesium, sulfate, and chloride. Water derived from drill holes at Pahute Mesa is a similar sodium potassium type. These chemical constituents comprised more than 90 per- cent of the total cations in more than half the water samples that were analyzed. The chemical composition of the ground water beneath Pahute Mesa can be related to the pattern of head changes with depth (fig. 10). In the eastern part of the caldera, rhyolitic lava flows and densely welded tufl’s constitute 40 to 100 percent of the V01— PART OF PAHUTE MESA, NEVADA TEST SITE, NEVADA E 530 000 E 550 000 E 570 000 E 590000 1,1930 N g70000,“ ..,‘ . .. ”V, l i l i 1 a ' l l ‘ l l l a s 1 . n" c i l / ' NEVA 37°22'30"»~ i / / i l "'BA"'i'i:sT sire ' \‘ B27 E 630 000 E 610 000 ' 116°]?! EXP LAN ATl ON Ca+ Mg&HCO, +C03 u S0.+Cl 'Na+K \ E 100 o 100 w Plotting scheme and scale, in N 950 000 9"- percentage of milliequivalents per liter Diagrams, except where indi- cated by “are of composite water samples collected dur- ing pumping tests 'Diag'ram representing spot sample collected from drill hole with wire- line tool. Silent Canyon caldera boundary Timber Mountain caldera complex ‘, c U E-20c — _l Exploratory test hole and number N 910000r .. ,. ., . 3/°15' o U_20c l i l i Emplacement hole and number l N 890 000 r—' V , N 870 000 :..,, 20,000-foot grid based on Nevada coordinate system. central zone 0 5 MlLES L.~_~ \ NEVADA TEST SITE 0 5 10 15 MILES FIGURE 11.—Water-chemistry map. canic rocks in the upper 2,000 feet of saturation, and head potentials decrease with depth. Water from drill holes where these conditions prevail is domi- nated by sodium and bicarbonate (table 11). Cal- cium and sulfate are abundant but accessory constituents of several of the waters. In the western part of the caldera, zeolitized bedded ash-fall and ash-flow tuffs constitute the principal volcanic rock types in the saturated zone, and head potentials increase with depth. Water from holes drilled in these areas contains even greater relative amounts ofsodium than much of the ground water in the eastern part of the caldera. (See table 11.) The diminished amounts of calcium may be due to increase in zeolitized tuf’fs. Among the anions, bicarbonate is the most abundant, but sulfate and chloride typically contribute greater relative amounts than in most caldera waters to the east. Bicarbonate is dominant in the southwestern part of the caldera, where rhyolitic lavas are the principal rock types in the saturated zone. The concentrations of chemical constituents in ground water in the western part of the caldera are relatively uniform and somewhat greater than that in the eastern part; dissolved solids in water from the western part range from 206 to 336 mg/l (milli- grams per liter) and average 280 mg/l. In the east- ern part of the caldera, dissolved solids range from B28 HYDROLOGY OF NUCLEAR TEST SITES TABLE 10. — Selected chemical analyses of water [Chemical analyses in milligrams o h a Date Major 5 "' “J E Hole 0f 62:51.13: producing E a 5 EA '2 A 5 sample (f?) zone(s) me p 5: 1; a g E»; 3 M 33'». 53 collection (ft) go 863' §< 11;“ 5:02 .50 as 5a .52 wv Ev “v ow “V .—V “v fly '5». e m < J: E 5 2 :72 «1°: UE—lfir2 ..... 1—29-68 1,629—5,004 32.2 45 (0.1 0.08 (0.01 26 1.0 0.18 81 U—l9as ........ 6— 7—65 3,195 ...... 6 .04 .03 (.02 2.0 .1 (.02 200 6— 7—65 3,195 ...... 66 .09 .14 (.02 1.2 (.1 (.02 189 6— 7—65 3,195 84 .08 03 (.02 1.6 (.1 (.02 263 UE—19b—1 ................ 6—21—64 2.190—4,500 31.6 47 .03 35 .05 20 1.7 ...... 43 10-13—64 2,190—4,500 2,740 30.0 41 .04 .41 1.2 24 2.4 ...... 42 UE—19c .................... 5— 7—64 2,421—4,520 3,040—3,075 38.3 41 (.01 .04 .01 1.0 .1 .05 29 3— 9—66 3,050 3,040—3,075 31.1 30 (.01 .04 .90 13 .1 09 141 UE—19d .................... 3—24—64 724—4,500 3,300.3,480 32.2 28 .03 .59 .03 29 2.9 29 173 6—27—64 2.560—7,689 3,300-3,480 45.0 58 .02 06 (.01 44 5.0 ...... 150 6—27—64 2,560—7,689 3,300—3,480 45.0 49 .05 .06 (.01 29 1.4 ...... 148 3— 9-66 3,380 3,300—3,480 34.4 55 (.01 (.01 .58 58 2.8 19 153 3— 9—66 4,200 3,300-3,480 34.4 55 (.01 .02 56 57 2.8 19 153 UE—l9e .................... 4—22—65 2,475—6,005 2,650—2,690: ...... 50 .52 09 02 5 (.1 ...... 50 4,970—4,990 8— 1—66 2,475—6,005 2,650—2,690; 35.0 56 (.01 02 .02 3.7 (.1 .02 43 4,970—4,990 UE—19fs ...... 8—18—65 2,565—4,779 ............ 37.7 56 .02 (.01 03 11 1.6 02 29 UE-19gs ..... 3—27—65 2,650—4,508 2914(2) :03,970 : 41.6 46 .04 .07 01 12 (.1 ...... 68 , 7 8- 2—66 2,650—7,500 2,940: 3,970; 41.6 50 01 (.01 01 2.8 (.1 .02 84 4.270 . 9— 2—65 2,896—8,000 3,460—3,618 47.2 39 .08 (.01 .05 5.0 (.1 04 75 10-14-64 2,066—4,500 26.6 41 .05 .13 (.01 5.9 (.1 ...... 58 3—10-66 2,066—4,500 ...... 48 (.01 .09 .01 6.1 .1 .03 55 3— 8—66 2,920 2,570—2,720: ...... 37 (.01 .05 .01 1.4 (.1 .03 81 4 250—4,460 3— 8—66 3,200 2,570—2,720: 41.6 46 .04 .07 01 1.4 (.1 .03 83 4.250—4,460 7-27—66 2,446—4,500 2,570—2,720; 40.0 47 (.01 (.01 (.01 4.3 .1 07 88 4,250—4.460 7-28—66 2.446-4,500 2,570—2,720 ; 40.0 52 (.01 (.01 .02 21 .1 13 68 4,250—4,460 8-12—66 2,446—4,500 2,570—2,720; ...... 45 .09 (.01 39 8.5 .1 (.01 107 4,250—4,460 UE—ZOe—l ................ 6— 5—64 1,825—6,395 3,550—3,660 47.2 44 .82 .13 (.01 4 (.1 112 3— 8—66 2,600 3.550—3,660 32.8 36 .01 .02 (.01 .2 (.1 83 UE—20f.... 8—11-64 4,456-13,686 4,570—4,680 48.8 47 07 .56 .14 4.8 (.1 113 U—20f ....... 3-28-66 4,026 4,030 41.1 36 04 (.01 01 .8 .1 69 5—27—66 4,025 4,025 41.6 39 .05 (.01 .02 .4 (.1 69 UE—20h ....... 8—26—65 2,506—7,207 4,000.4,070 32.2 49 .02 (.01 .03 .6 (.1 64 UE-20j ........ 10—21-64 1,740—5,690 2,050—2,250; 38.9 44 .01 4.8 (.01 46 1.2 ...... 138 2,960—3.830 T‘V—S2 ...................... 10—15—64 1.068—1,860 1,300—1,780 26.1 45 (.01 (.01 (.01 8.8 .9 ...... 30 g2“?!ugéogaitugimfiegg'analysis or collection. 117 to 248 mg/l and average 200 mg/l. Water from UE—19d contains 550 mg/l dissolved solids, very high for the area, and is not included in the ranges and averages just given. The generally smaller amounts of dissolved solids in water in the eastern part of the caldera may indicate a relatively short contact time with the rocks, or nearness to areas of recharge. Indeed, the mineralization in water from some of the drill holes is similar to that of the perched water collected during tunnel mining be- neath Rainier Mesa (Clebsch and Barker, 1960). Recharge from precipitation is greatest in the east, where altitudes are greatest; however, most water probably enters the report area as underflow from valleys to the northeast and north. Water from drill hole UE—20j, west of the caldera boundary, is rich in sodium and has nearly equal amounts of calcium plus magnesium, bicarbonate, sulfate, and chloride (table 11). The higher-than— normal sulfate content may be due to extensive hydrothermal alteration in older volcanic rocks pen- etrated in the drill hole. The drill hole is approxi- mately 3.5 miles southeast of, and dowu gradient from, the PM—2 drill hole, in which a granitic intru- sive body was penetrated at a depth of 8,380 feet. Altered dacite in the intrusive contact zone consists of microcrystalline aggregates of chlorite, sericite, quartz, calcite, and pyrite. Dacitic lava flows and flow breccias penetrated at depths from 2,870 to 8,270 feet in PM—2 are pyritized and calcitized. Water collected from test well TW—8, south of the eastern part of the caldera, and water from the UE—18r drill hole, south of the western part of the caldera in the moat area of the Timber Mountain caldera, are of the sodium bicarbonate type but con- tain a significant quantity of calcium plus magne- PART OF PAHUTE MESA, NEVADA TEST SITE, NEVADA from exploratory holes and chambers, Pahute Mesa per liter; <, less than] B29 Hardness as 03003 m E E O) Q) ll) 3 3 5'55 5 E 5% 3 E .3 ”:3 3 :1 W '5 o 0 A G A A A '6 54° - o o 31 EA .5; E3 88 2,3; .434 :3- 28 fig 2?. 3% 3?. E 35:58 3M 5.: W1 .20 am 802 So '51:. 232 we. 32:: ~34: 0 13555:: 954., :‘v %V s»; '2‘. EV Ev iv 31‘, gv 'Evofl is g 39.49” :1: n. A w o m m o 1:. z a. c: o z w n. 3.1 0.10 0.11 0 252 24 7 8 2 9 0.6 <0.01 313 69 0 449 8.0 5.5 .61 .06 39 174 26 19 80 1.0 <.01 526 0 889 9.3 5.8 62 .07 8 05 27 11 80 2.5 <.01 491 3 o 828 8.4 7.1 85 .04 82 261 51 12 70 12 <.01 723 4 0 1,150 9.3 2.9 ...... .03 0 130 24 13 3.2 .1 <.01 242 57 0 316 6.7 3.0 ______ .03 0 150 21 6 8 3.2 .4 <.01 229 70 0 314 7.4 <1 .04 05 0 66 4.6 2 6 1.2 1.8 <.01 117 3 0 124 7.6 .2 28 ...... 0 400 <1 7 7 4.3 .2 <.01 390 33 0 644 7.9 3.6 30 .04 0 424 63 39 4.4 30.0 <.01 560 85 0 889 7.5 3.7 ............ 0 380 80 33 6.9 .1 <.01 572 131 0 851 7.9 4.0 ____________ 0 320 66 30 7.5 <.1 <.01 525 79 0 777 7.6 3.7 .38 0 481 60 19 5.0 .5 <.01 581 156 0 911 7.9 4.3 .38 0 489 57 20 4.9 .6 <.01 578 154 0 906 7.9 1.2 .15 0 86 20 11 1.2 3.4 .01 190 1 0 232 7.4 .8 .06 01 0 80 16 3 7 5.3 1.7 <.01 169 9 0 204 8.2 3.0 .02 ...... 0 86 9.0 6.3 3.6 2.2 <.01 186 34 0 202 8.1 .6 ............ 0 146 36 9.0 2.0 1.3 <.01 248 30 0 345 8.2 1 5 .04 01 0 123 43 22 3.0 .6 <.01 270 7 0 388 8.2 5 .05 01 0 98 70 7.0 5.5 .2 <.01 230 13 0 228 7.7 1 9 ...... 01 0 108 28 11 2.6 1.2 <.01 206 15 o 285 6.9 .2 .06 0 106 27 11 2.7 1.4 <.01 201 16 0 286 7.7 .2 .08 0 122 40 23 3.1 1.3 <.01 252 4 0 384 8.2 1 .08 ...... 0 120 42 24 3.1 1.7 <.01 282 4 0 351 8.3 1 7 .00 01 0 137 44 23 2.8 <.1 <.01 286 11 0 405 7.8 5 .00 01 5 143 53 8.8 2.4 .3 <.01 281 53 0 394 8.4 2.6 .08 .01 4 192 40 24 3.0 <.1 <.01 327 22 0 487 8.5 3.8 ............ 0 130 43 57 4.6 1.0 <.01 336 1 0 515 7.7 2.0 .07 ...... 2 119 42 20 4.5 .5 <.01 245 1 0 352 8.5 2.0 ............ 0 164 48 40 5.0 .1 .02 368 12 0 519 7.2 .6 02 .01 10 103 30 6 6 3.9 <.1 .07 240 3 0 298 8.9 .8 00 .01 14 98 23 7 0 3.7 7 .12 194 1 0 297 9.1 1.8 08 <1 0 107 30 15 2.7 1 3 <.01 231 2 0 301 8.1 6.4 ............ 0 150 135 115 2.2 9 <.01 583 120 0 904 7.0 3.3 ............ 0 78 16 7 8 .7 3.6 01 149 26 0 197 6.8 TABLE 11.—Relative abundance of principal ions in selected ground-water samples from Pahute Mesa [On the basis of milliequivalents per liter] Percentage of total cations Percentage of total anions Exploratory Calcium Sodium BIN"- Chloride We NUS. plus b31659 Sulfate 553354.. magnesxum potassium carbonate plus nitrate 58 75 14 11 95 81 7 12 72 68 18 14 99 63 19 18 66 70 10 20 83 68 21 11 93 45 41 14 90 62 21 17 sample) 2 98 54 23 23 UE—ZOe—l.... 1 99 44 18 38 UE—20f (lower zone) 95 53 20 27 UE—20h 99 59 21 20 UE-20j. 72 28 33 39 UE—18r 72 82 10 8 TW—81.. 73 66 17 1‘7 ‘South of Pahute Mesa. sium, sulfate, and chloride ions. Mineralization is greater in UE—18r than in TW—8, but the chemical constituents of both waters are similar. WATER CHEMISTRY AND REGIONAL MOVEMENT OF GROUND WATER Chemical data for water samples collected from wells and springs at Oasis Valley, western Jackass Flats, and north-central Amargosa Desert corrobo— rate the direction of ground-water movement. The water from springs and wells in Oasis Valley is of the sodium bicarbonate type, similar to the water beneath Pahute Mesa. Amounts of sulfate and chlo- ride are similar to those in waters beneath the west- ern part of the caldera and are similarly subordinate to bicarbonate. Dissolved-solids content of water from Oasis Valley is greater than that of water be- neath Pahute Mesa. ENGINEERING HYDROLOGY Underground testing of nuclear devices at Pahute Mesa involves emplacement of devices far below the top of the saturated zone, commonly in mined cham- ‘ bers at the emplacement depth. During excavation B30 of the chambers and preparations for detonation of the nuclear device, only a small influx of water can be tolerated. A variety of hydraulic tests has been made to determine the water-yielding potential of the strata penetrated by each exploratory hole and, in particular, of those strata with the lowest water yield or those most favorable for development of chambers. Five chambers, of different dimensions, have been mined successfully at depths as much as 2,600 feet below the top of the saturated zone (table 12). At each site an emplacement hole, usually 72 inches in diameter, was drilled near the exploratory hole. Drilling of the emplacement hole was interrupted several hundred feet above the interval proposed for mining. Casing with an outer diameter of 10% inches was tack-cemented, and a 97/3-inch hole was drilled and cored through the critical interval to per- mit geophysical logging and hydraulic testing. The exact intervals for mining were based on these geo— logic and hydraulic-testing data. Emplacement holes were drilled to desired depths, and 48-inch casings were cemented in place. Pumpage from exploratory holes has been the principal water supply for construction and drilling at Pahute Mesa. The greatest water yields generally were obtained from holes drilled in areas where fractured rhyolitic lava flows and densely welded tuffs predominate in the saturated zone. A perma- nent water supply could be developed from wells in areas away from probable damage from nuclear events and in areas where these rocks predominate. ROCK MEDIA BEST SUITED FOR CHAMBER CONSTRUCTION Ash-fall and nonwelded ash-flow tuff which have been zeolitized or argillized are hydrologically the most desirable rocks for chamber construction. Al- though interstitial porosity and permeability of these rocks are greater than those of the rhyolitic lavas and densely welded tuffs, fractures reseal more readily in these relatively incompetent rocks. Hence, fracture permeability generally is much lower in ash-fall and nonwelded tuffs than that of the rhyo- litic lavas and densely welded tuffs. The yield of water to wells from ash-fall and nonwelded ash-flow tuffs is low. Intervals of 200 feet of ash-fall and nonwelded ash-flow tuff in exploratory holes having relative specific capacities less than 0.03 gpm per foot of drawdown are considered most favorable for cham- ber construction. Those intervals having relative specific capacities ranging from 0.03 and 0.04 gpm per foot of drawdoWn are considered marginal, and those having greater than 0.04 should be mined only HYDROLOGY OF NUCLEAR TEST SITES where the head is small. Rhyolite and densely welded tufl' are not considered favorable rock types for min- ing chambers. Although these indurated rocks may be impermeable immediately adjacent to the bore- hole, open water-bearing fractures a short distance from the borehole might be encountered during min- mg. WATER INFLOW T0 CHAMBERS Early estimates of inflow to chambers were made to establish hydrologic safety limits for miners. One method was developed by W. E. Hale (written commun., 1964). For this method, permeability was derived, approximately, from values of relative spe- cific capacity. Estimates of inflow were graphed for two sizes of chambers, 50x10><10 feet and 20x10><10 feet, and for heads of water from 0 to 3,000 feet. Inflow, plotted from these graphs, proved highly conservative, based on actual measurements during mining operations. Presently, hydraulic-test data are used to compute the transmissivity and storage coefficient of the tested interval using a method of analyzing slug tests by type curves developed by Cooper, Brede- hoeft, and Papadopulos (1967). Utilizing these data, Dudley (1970) developed a technique to compute the gradually declining discharge at constant drawdown to a large-diameter opening in the interior of an aquitard, which is bounded above and below by re- charge boundaries. By treating the chamber as a large-diameter well pumped at constant drawdown, the nonsteady discharge can be computed by the method of Hantush (1961) for analyzing flowing wells under leaky artesian conditions. For this tech- nique, the assumption is made of the instantaneous appearance of a full-sized chamber. Obviously, this assumption is not met. Hence, the calculated inflow during early stages will be higher than that actually found. Since the gradual construction of the chamber retards early inflow, the dissipation of hydraulic head is also retarded, and the inflow calculated for later times will be less than the actual inflow. For safety reasons, pumping systems should be designed to handle the maximum potential inflow. Hydrologic and construction data for the cham- bers mined at Pahute Mesa are shown in table 12. Prior to each of the mining operations, a series of radiating small-diameter holes was drilled through ports in the 48-inch casing into the prospective chamber zones. These holes usually were 1.5 inches in diameter and about 25 feet long and were sloped about 35° downward from horizontal. Measurements of water inflow from these holes into the casing showed a significant decrease with time. A 30- to 40— percent decline of inflow occurred during the first PART OF PAHUTE MESA, NEVADA TEST SITE, NEVADA B31 TABLE 12.— Water-inflow, construction, and hydraulic-testing data for mined emplacement chambers within the saturated zone, Pahute Mesa Chamber size (ft) Hydraulic-testing data for D h to D th t chamber zone EXDLOol'lztory chameblg‘ center water 0 Relative specific Thickness of Eggfifi (ft)1 (ft)1 Diameter Height capacity (gpm per straddle packed foot of drawdown) interval (ft) U—19as ..................... 3,100 2,192 14 35 0.003 198 10 U—19e ....................... 4,765 2,240 24 46 .0005 68 5’110 U—19g ....................... 3,010 2,058 26 42 .03 118 224 U—'20c ........................ 4,600 2,097 24 31 .002 58 3214 U—20f ....................... 4,040 1,930 16 35 .01 198 70 1All depths are referred to land surface. 2Virtually stable values, in gallons per hour, after 1-3 months of continuous inflow. aInflow to these chambers was anomalously high because of some vertical leakage through annular cement around 48-in. casing. few days (fig. 12). The decline after 1 week was slight. These small-diameter holes were used for pressure grouting in attempts to seal any water- bearing fractures before mining. The benefits of the pressure-grouting operations were questionable. Nevertheless, water inflow during mining operations presented no serious problems. Inflow rates to the chambers, 1 to 3 months after completion of mining operations and when inflow was virtually stable, were obtained from pumpage data (table 12). The greatest volume of inflow, 224 gph (gallons per hour) was measured in U—19g. Hydraulic-testing data indicated that the relative specific capacity of the interval mined in this hole was 0.03 gpm per foot of drawdown. Intervals mined in the other emplacement holes had relative specific capacities no greater than 0.01. Zones 150 to 200 feet thick which were tested in vertical exploratory holes had relative specific capac- ities ranging from less than 0.001 to 0.78 gpm per foot of drawdown. An interval having a relative spe- cific capacity less than 0.03 gpm per foot of drawdown is considered safe from a hydrologic standpoint for mining at depths as great as 3,000 feet below static water level. An interval having a relative specific capacity of 0.03 to 0.04 gpm per foot of drawdown is considered of marginal safety for chamber con- struction under most hydraulic heads anticipated for Pahute Mesa chambers. And an interval having a relative specific capacity greater than 0.04 gpm per foot of drawdown should be mined only where the head is small. WATER SUPPLY Exploratory drill holes at Pahute Mesa are used on a temporary basis to supply water for drilling and construction operations. These drill holes even- tually are plugged with cement or instrumented for nuclear events scheduled in nearby emplacement holes. The few drill holes not directly related to nuclear testing are virtually water-tight and non- productive or are located where they will probably be destroyed by future nuclear events. To assure a permanent supply of water from wells (those not likely to be disrupted by a nuclear detonation), the wells must be drilled at distances at least three times the total depths of the emplacement holes away from all emplacement holes. Within the areas that are available, selection of well sites should be based on rock types in the saturated zone, proximity to faults, and accessibility. As an alternative to drilling supply wells on Pahute Mesa, construction water could be piped or hauled from test well TW—8, an existing supply well off the mesa, or from UE—18r, drilled in the moat area of the Timber Mountain caldera complex. Whether piping or hauling water would be more feasible than drilling new supply wells on the mesa is principally a matter of comparative costs—an economic question is beyond the scope of this report. AREAS FAVORABLE FOR DRILLING WATER-SUPPLY WELLS Pumping tests were made in 14 exploratory holes at Pahute Mesa. The greatest water yields generally were obtained from holes drilled in areas where the more competent rocks, fractured rhyolitic lava flows and densely welded tufl’s, predominate in the upper 2,000 feet of the saturated zone. Resistivity curves on electric logs made in holes drilled at the mesa are useful for defining the competency of rocks; appar- ent resistivities of densely welded tufi‘, vitrophyres, and rhyolitic lavas generally exceed 225 ohm-meters. Figure 3, a contour map based on the percentage of rocks with resistivities greater than 225 ohm-meters in the upper 2,000 feet of the saturated zone, can be used to locate favorable areas for wells. A well drilled 2,000 feet into the saturated zone within the contour intervals in which high-resistivity rocks comprise 40 to 100 percent of the rock section should penetrate an adequate thickness of favorable rocks and produce a satisfactory supply of water. Holes drilled in areas of major faults—principally near boundary faults of the Silent Canyon caldera (pl. 1)—may have high water yields even though less favorable rocks occur in the saturated zone. In B32 HYDROLOGY OF NUCLEAR TEST SITES 200 I 1 fl I | | I I \\ g 0 150 — ._ I n: Lu 0. (I) Z o \O\\ 3' 100 —— \\\ _ < \\\ U \\\ E \\\\ .. \\\\ g o \\\ a: U-20c. 7 holes producing water, \0\ E 50 — May 29-31.1967 ~\ — A “‘\\“ U—20f, 22 holes producing water, May 25-28, 1966 o 1 llllllll 1 llllllll l IIIJJLLl L I 0.1 1.0 10 100 400 TIME. IN HOURS SINCE INFLOW BEGAN FIGURE 12. — Inflow through small-diameter holes drilled into prospective chamber zones U—20f and U—20c. exploratory hole UE—20j, west of the caldera bound— ary fault, bedded and welded tuifs of the Belted Range Tuff are permeable. Cores from that hole show that the permeability is associated with faults. The Belted Range Tuff is relatively impermeable where it is not highly faulted. SUMMARY AND CONCLUSIONS The Silent Canyon caldera, beneath the eastern part of Pahute Mesa, is elliptical in plan, measuring approximately 11 by 14 miles. The greater axis trends in a north-northeast direction, roughly paral- lel to the strike of basin-and-range faults. It is asym- metrical in cross section; the amount of collapse in the ring-fracture zone ranges from about 7,000 feet on the west side to about 5,000 feet on the northeast side. A north-trending structural high separates two distinct intrabasin deeps within the caldera. Pre—Tertiary rocks were not penetrated in explor- atory holes drilled within the caldera or in the im- mediate Vicinity outside the caldera. The rocks that afford positive stratigraphic control on the surface and in the subsurface at Pahute Mesa are the wide- spread ash-flow tuffs, which are probably con- temporaneous with initial caldera collapse, and the major ash—flows, which are postcaldera in age. Het- erogeneous intracaldera rocks occur between these widespread ash-flow sheets in the saturated zone under Pahute Mesa. These intracaldera rocks in- clude ash-fall and ash-flow tuffs, and rhyolitic lava flows which emanated from fissures within the sub- siding caldera and from nearby volcanic centers. The variable vertical and horizontal distribution of these rocks over short distances makes subsurface correlations between test holes extremely difficult. Although interstitial permeability of the volcanic rocks is small, it does allow for some ground-water movement; most ground water moves along the hy— draulic gradient in fractures. Rhyolitic lavas are relatively competent rocks; hence, compressibility of these rocks is moderate, and faults and joints may remain open even at great depths. Most of the cores of rhyolitic lava flows recovered from drill holes con- tain some fractures; however, not all the intervals that are fractured are permeable. In the intervals of low permeability, the fractures apparently are not interconnected or are poorly connected. Densely welded ash-flow tuffs have physical characteristics similar to those of rhyolitic lava flows, and, like the lava flows, they have a wide range in water-yield po- tential, with interconnecting fractures affording the principal avenues for ground-water movement. In- terstitial porosity and permeability of ash-fall and nonwelded ash-flow tuffs are greater than those of the rhyolitic lavas and densely welded tuffs. How- ever, fractures are healed more readily in these rela- tively incompetent rocks, and secondary fracture porosity and permeability are much lower than those of the rhyolitic lavas and densely welded tuffs. The yield of water to wells from intervals of ash-fall and nonwelded ash-flow tuffs is low, and for this reason, from the hydrologic standpoint, these rocks are con- sidered ideal media for mining of chambers within the saturated zone. The depth to water under the eastern part of PART OF PAHUTE MESA, NEVADA TEST SITE, NEVADA Pahute Mesa, within the limits of the Silent Canyon caldera, ranges from about 1,952 feet (alt 4,164 ft) in the western part to 2,350 feet (alt 4,685 ft) in the eastern part. In the extreme northwestern part of the Nevada Test Site, outside the caldera, the depth to water is about 850 feet (alt 4,700 ft). The regional hydraulic gradient is to the south- west and south. It ranges from a low of about 25 feet per mile in the northeastern part of the caldera to 100 feet per mile in the western and southwestern parts; the average gradient is 50 feet per mile. The steeper gradients in the western and southwestern parts of the caldera correspond to an increase in the percentage of poorly permeable ash-fall tufi' in the saturated zone. The total underflow beneath Pahute Mesa is about 8,000 acre-feet per year; ground-water velocity ranges from 0.7 to 0.02 foot per day, or 20 to 750 years per mile. From Pahute Mesa, ground water moves southwestward and southward toward the Amargosa Desert through Oasis Valley, Crater Flat, and western Jackass Flats. The chemical composition of the ground water beneath Pahute Mesa can be related to the pattern of head changes with depth. In the eastern part of the caldera, rhyolitic lava flows and. densely welded tufi‘s constitute 40 to 100 percent of the volcanic rock types in the upper 2,000 feet of the saturated zone, and head potentials decrease with depth. Water from drill holes where these conditions prevail is dominated by sodium and bicarbonate but contains appreciable amounts of calcium and magnesium. In the western part of the caldera, zeolitized bedded ash-fall and nonwelded ash-flow tuffs constitute the principal volcanic rock types in the saturated zone, and head potentials increase with depth. In water from holes drilled in these areas, sodium and potas- sium together are 90 to 99 percent of the total ca- tions. The diminished amounts of calcium may be due to ion exchange when water from the rhyolite areas moves into areas that consist predominantly of zeolitized tuffs. Among the anions, bicarbonate is the most abundant, but sulfate and chloride contrib- ute greater relative amounts in most water from the western part of the caldera. The systematic and detailed hydraulic testing in deep drill holes at Pahute Mesa probably has never been done on a similar scale elsewhere in the world. The techniques and tools applied in the testing should have broad applications wherever deep-well testing is done. A typical hydraulic-testing schedule usually begins with geophysical logging. After com- pletion of the geophysical logging, a pumping test is made to measure the gross yield of all the rocks ex- B33 posed in the Well bore. During the pumping, radio- active-tracer and temperature surveys are made to locate the zones where water enters the hole. After the pump has been pulled out of the hole, intervals in the drill hole are selected for a series of injection or swabbing tests. Injection or swabbing tests are made by adding knOWn volumes of water to, or with- drawing known volumes of water from, intervals isolated with straddle packers and then observing the rate of decline or rise in water level resulting from this injection or Withdrawal of water. From the rate of change in water levels with time, the yield of the various intervals at maximum draw- down can be computed. The yield of the intervals tested is stated as “relative specific capacity,” ex- presSed in gallons per minute per foot of drawdown and defined as the volume of inflow during a 1-min- ute interval divided by the difference between static water level and the mean water level during the 1-minute interval. Data obtained during these tests can also be used to determine the transmissivity of the aquifer immediately adjacent to the interval of borehole being tested. In addition, static water levels (formation pressures) of selected intervals are ob- tained. Successful water injection and swabbing tests have been made in intervals isolated with straddle packers at depths to 9,000 feet. Temperatures at these depths have exceeded 794°C. Underground testing of nuclear devices at Pahute Mesa involves emplacement of devices far below the top of the saturated zone, commonly in mined cham- bers at the emplacement depth. During excavation of the chambers and preparations for detonation of the nuclear device, only a small influx of water can be tolerated. The intervals to be mined are selected in the least permeable rocks, on the basis of data collected during hydraulic testing of exploratory holes. Ash-fall and nonwelded ash-flow tufi' which have been zeolitized or argillized are hydrologically the best rock types in which to construct the cham- bers. Intervals of 200 feet in exploratory holes hav- ing relative specific capacities less than 0.03 gpm per foot of drawdown are conSidered most favorable. Those intervals having relative specific capacities ranging between 0.03 to 0.04 gpm per foot of draw- down are considered marginal, and those greater than 0.04 should be mined only where the head is small. Rhyolite flows and densely welded tufi‘ are not considered favorable rock types for mining of cham- bers. Although these indurated rocks may be im- permeable immediately adjacent to the borehole, open water-bearing fractures a short distance from the borehole may be encountered during mining. B34 Five chambers, with diameters ranging from 14 to 26 feet and heights ranging from 31 to 46 feet, have been mined successfully at depths as much as 2,600 feet below the top of the saturated zone. At each site an emplacement hole, usually 72 inches in diameter, was drilled near the exploratory hole. Drilling of the emplacement hole was interrupted several hundred feet above the interval proposed for mining. Casing with an outer diameter of 10% inches was tack-cemented, and a 97/8-inch hole was drilled and cored through the critical interval to per- mit geophysical logging and hydraulic testing. The exact intervals for mining were based on these hy- draulic testing and geological data. Early estimates of inflow to chambers were made to establish hydrologic safety limits for miners. Permeability was derived, approximately, from val- ues of relative specific capacity, and estimates of inflow were graphed for chambers of different dimensions. Actual measurements during mining operations showed these estimates to be highly con- servative. Presently, hydraulic-test data are used to compute values of transmissivity and storage co- efficient for the tested interval. These data are ap- plied by a type-curve method developed by Hantush (1961) for the constant-drawdown, leaky artesian inflow to a well to determine inflow to a chamber. Measurements of water inflow to chambers show a decline of 30 to 45 percent of inflow during the initial 3 to 4 days. The decrease is very gradual after 1 week. Inflow rates to the five chambers mined at Pahute Mesa ranged from 10 to 214 gph 1 to 3 months after completion of chamber excavations and when inflow was virtually stable. On the basis of data from hydraulic testing, areas favorable for drilling water—supply wells were deter- mined. The greatest water yields generally were ob- tained from holes drilled in areas where fractured rhyolitic lava flows and densely welded tuffs pre- dominate in the upper 2,000 feet of the saturated zone. A satisfactory supply of water can be obtained where these rocks comprise about 40 percent or more of the rock section. Where less favorable rocks occur in the saturated zone, satisfactory water-sup- ply wells can be developed in areas of major faults, especially near the caldera boundary. South of Pahute Mesa the moat area of the Timber Mountain caldera complex is an excellent potential source of water supply. REFERENCES Blankennagel, R. K., 1967, Hydraulic testing techniques of deep drill holes at Pahute Mesa, Nevada Test Site: U.S. Geol. Survey open-file rept., Interagency Report Special Studies I—1, 50 p. HYDROLOGY OF NUCLEAR TEST SITES /. 1968, Geophysical logging and hydraulic testing, Pahute Mesa, Nevada Test Site: Ground Water, v. 6, no. 4, p. 24—31. Byers, F. M., Jr., Orkild, P. P., Carr, W. J., and Quinlivan, W. D., 1968, Timber Mountain Tuff, southern Nevada, and its relation to cauldron subsidence: Geol. Soc. America Mem. 110, p. 87—98. Carr, W. J., 1964, Structure of part of the Timber Mountain dome and caldera, Nye County, Nevada, in Geological Survey Research 1964: U.S. Geol. Survey Prof. Paper 501—B, p. Bl6—B19. Christiansen, R. L., and Lipman, P. W., 1966, Emplacement and thermal history of a rhyolite lava flow near Forty- mile Canyon, southern Nevada: Geol. Soc. America Bull., v. 77, p. 671—684. Christiansen, R. L., and Noble, D. C., 1965, Black Mountain volcanism in southern Nevada [abs.]: Geol. Soc. America Spec. Paper 82, p. 246. Clebsch, Alfred, Jr., and Barker, F. B., 1960, Analyses of ground water from Rainier Mesa, Nevada Test Site, Nye County, Nevada: U.S. Geol. Survey open-file rept., TEL—763, 22 p. Cooper, H. H., Jr., Bredehoeft, J. D., and Papadopulos, I. S., 1967, Response of a finite diameter well to an instanta- neous charge of water: Water Resources Research, v. 3, no. 1, p. 263—269. Dudley, W. W., Jr., 1970, Nonsteady inflow to a chamber within a thick aquitard, in Geological Survey Research 1970: U.S. Geol. Survey Prof. Paper 700—C, p. 0206— C211. Eakin, T. E., and others, 1951, Contributions to the hydrology of eastern Nevada: Nevada State Engineer, Water Re- sources Bull. 12, 171 p. Eakin, T. E., Schofl’, S. L., and Cohen, Philip, 1963, Regional hydrology of a part of southern Nevada—a recon- naissance: U.S. Geol. Survey open-file rept., TEI—833, 40 p. Hantush, M. S., 1961, Drawdown around a partially pene- trating well: Am. Soc. Civil Engineers Proc., Jour. Hydraulics Div., v. 87, (HY4), p. 83—98. Levorsen, A. I., 1954, Geology of petroleum: W. H. Freeman and 00., p. 398—406. Lipman, P. W., Christiansen, R. L., and O’Connor, J. T., 1966, A compositionally zoned ash-flow sheet in southern Nevada: U.S. Geol. Survey Prof. Paper 524—F, 47 p. Malmberg, G. T., and Eakin, T. E., 1962, Ground-water appraisal of Sarcobatus Flat and Oasis Valley, Nye and Esmeralda Counties, Nevada: Nevada State Engineer, Ground Water Resources, Reconn. Ser. Rept. 10, 39 p. Moore, J. E., 1961, Records of wells, test holes, and springs in the Nevada Test Site and surrounding area: U.S. Geol. Survey open-file rept., TEI—781, 22 p. Noble, D. C., Anderson, R. E., Ekren, E. B., and O’Connor, J. T., 1964, Thirsty Canyon Tufi of Nye and Esmeralda Counties, Nevada, in Short papers on geology and hy- drology: U.S. Geol. Survey Prof. Paper 475—D, p. D24—D27. Noble, D. C., Sargent, K. A., Mehnert, H. H., Ekren, E. B., and Byers, F. M., Jr., 1968, Silent Canyon volcanic center, Nye County, Nevada: Geol. Soc. America Mem. 110, p. 65—75. Orkild, P. P., 1965, Paintbrush Tufl‘ and Timber Mountain Tufl’ of Nye County, Nevada, in Changes in stratigraphic nomenclature by the U.S. Geological Survey, 1964: U.S. Geol. Survey Bull. 1224—A, p. A44—A51. PART OF PAHUTE MESA, NEVADA TEST SITE, NEVADA Orkild, P. P., Byers, F. M., Hoover, D. L., and Sargent, K. A., 1968, Subsurface geology of Silent Canyon caldera, Nevada Test Site, Nevada: Geol. Soc. America Mem. 110, p. 77—86. Orkild, P. P., Sargent, K. A., and Snyder, R. P., 1969, Geo- logic map of Pahute Mesa, Nevada Test Site and vicin- ity, Nye County, Nevada: U.S. Geol. Survey Misc. Geol. Inv. Map I—567, 5 p. Ratté, J. C., and Steven, T. A., 1967, Ash flows and related volcanic rocks associated with the Creede caldera, San Juan Mountains, Colorado: U.S. Geol. Survey Prof. Paper 524-H, 58 p. Ross, C. S., and Smith, R. L., 1961, Ash-flow tufl's—Their origin, geologic relations, and identification: U.S. Geol. Survey Prof. Paper 366, 81 p. B35 Schoff, S. L., and Moore, J. E., 1964, Chemistry and move- ment of ground water, Nevada Test Site: U.S. Geol. Survey open-file rept., TEI—838, 75 p. Thordarson, William, 1965, Perched ground water in zeol- itized-bedded tufl", Rainier Mesa and vicinity, Nevada Test Site: U.S. Geol. Survey open-file rept., TEI—862, 90 p. Winograd, I. J., Thordarson, William, and Young, R. A., 1971, Hydrology of the Nevada Test Site and vicinity, southeastern Nevada: U.S. Geol. Survey open-file rept., 429 p. Young, R. A., 1972, Water supply for the Nuclear Rocket Development Station at the U.S. Atomic Energy Com- mission’s Nevada Test Site: U.S. Geol. Survey Water- Supply Paper 1938, 19 p. * U.S. GOVERNMENT PRINTING OFFICE: 1973—515—659/22 PROFESSIONAL PAPER 7 I 2—B UNITED STATES DEPARTMENT OF THE INTERIOR PREPARED ON BEHALF OF THE PLATE 1 GEOLOGICAL SURVEY 116530, US. ATOMIC ENERGY COMMISSION N 960 000, f £759?qu \V F 570 00o ,, E 580 000 2230" E 590 000 ‘ ‘ \‘ . ' " ' ' 7% WV“, , . " a" ( KT. ' E V I u §\\ ‘1’ ' ’ ,NW\\N ‘ I.\ ' .Lf, .fifiQ? ,5: IKNVI IN, (I Ip}\. /‘ r I , U f‘,‘ \ I, II aw \ I W V IN I 160'K7Tfi0” C“ 1’4“ \‘\‘ ,, \ \C\£\(\%J\r 5/9230” E, 570 ()OO E 540 000 “II goooUE-ggp‘ 4653‘, I / ‘I ‘ E 620 000 116°15‘ , N 96x1 000 I\/ N 950 000 N 940 000 N 94:) COO ‘ [a “I . 4, , ”I ,w . )‘fi/ \ N 5'3,“ I’ III, , , / ¥ 1 « i L 1: I’ l ‘er ’ i V «fa, g0" I 2 ,4 "\w, UEI—fi?d g 2240 r ” [(4679 ‘_ I‘ ‘\ I MS I x Wick Mon n ea In ‘ N 930 000 N )( I; IleIZICI 6.,UE-f20 C ,,»44157‘ , "7;???“ 2126 N 920 000 4 Q ‘13 ,4 O O CO N 900 OOCI ‘ N 890 000 41,71 , , , , , aféfsé’aiin ber‘ , , w ,. M f ‘ _ . . , ,d .\ ; - , f Nufitb (Sage Mermcleflthr‘tg) water, In feel belOw land 4):, p\<‘“ “ “I; m 2 :“silrfaeeyfiy’m 3?: beld‘gfi/ line is altitude of water level, ~ \ E I: , ‘ Infamy; ve mean {sea leggel? , * It . 15;: ,, ‘ 4300— —E . A E , , . -,,Vfi/7I. Tuner .. , 4 LEE: , _ . N 88 000 . , x 7 Water-level contouf‘ L N \ who flak/41mg? of'water level. Dashed ”where ap'pm‘x' f, I N 880 000 " my located. Contour interval, 100 feet. Datum , nrsea'lepel I “ ~"7 ‘ , " I y " 'Apfil‘o’ximate Bosatlon’ of inferred hydrauhc barrio \ Bafliiwefimfipénfi withmajorfaultIzorireamappe’djo, , isui‘face fiflaconfcirme’d in-péliffiby Subsurface e0- ('1 - aidm‘cqucawelatmg , _ , C ' ' t» ' ‘—r 1' .-'»"’—" 5‘ \ ‘ Dashed Ville/re approxfl‘rimtely located; dolly/Ii'wheré 6777‘. I‘ llandrba'rwtm downthrow ”ide offa Bf“ , ‘ I I ‘ ,J: n” , . ’ CO _ , ,7» ' ,> , , , I ” I v ,I’" , I . ’ ' _ /ii i; 1‘ ‘ \ I , 5‘le ‘ IVA 509g L9 P‘ ,I‘ \‘ ,, \ :_,,,_r,:,1 95 " f A , f ‘I/ x_,> , 91752774515 “New" A A / 7 _ :’ Ling; of'g’eoI’Ggioseotion shoWnypnwpla‘te’ 2/ . 3 " “ " 'I’ \_ 1‘ ‘ \I ‘ N EN 000 ' ' , , ~ , ~7 7* , 21L“ , , ‘ , 1 N 870 000 ‘ , ‘7 6 1:0,, , ‘ /‘ Z\ " U I I A I , ( ‘/« <;_/{ [71, \ ‘ 5 730,,” 50" LE , E E, L ’7 ‘- » F: >~ ,‘ , I ‘~ I A -- 3 . ‘ ,I ,I . , , ‘ 116°37'30” E 540000 30 E 550 000 E 560000 E 570000 E 580 000 22,30” E 610000 E 60 000 3700730,, L \A L L ‘ . l 1 6° 1 5’ Base from U.S.. Geologlcal Survey SCALE 1:48.000 ., , N 30, 116°15’ H d | b R K 10,000-foot grid based on Nevada 1 1/2 0 1 2 3702é]1360ll37 3O S‘IIEENEOEA‘NQEON Y ID 0eg Y - - Blankennagel and coordinate system, central zone I_I I_I Hf 1_I ,_. ,______; i +—____fii3 MILES % Q g J- 15- WeIr, Jr. 1970. Structural 80$ (17$:wa 8,53,": geology modlfled from Orkild, Sargent, 1.5 O 1 2 3 KILOMETERS QVSZU ”9000‘ 01,6200 and Snyder (1969) I:I_I:L_I:I_I:I_:I_I;——j , i L I: «m» (30' <9 I“ WWII,“ CONTOUR INTERVAL 20 AND 40 FEET 15’ 0e 9L g, m "“ "3 DATUM Is MEAN SEA LEVEL 9%” 53¢ 3,6 MAP LOCAT'ON (Sgt: §F§7 $207 ‘7 O 0 37°07'30” INDEX SHOWING OUADRANGLE LOCATIONS WATER-LEVEL CONTOUR MAP SHOWING FAULTS AND CALDERA BOUNDARIES, EASTERN PAHUTE MESA, NEVADA TEST SITE, NYE COUNTY, NEVADA * u.s. GOVERNMENT PRINTING OFFICE: 1973—515—659/22 PREPARED ON BEHALF OF THE U.S.ATOMIC ENERGY COMMISSION UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY Area of upward crossflow from precaldera and intracaldera rocks into aquifers in I Area of downward crossflow from rhyolite aquifers into basal intracaldera and I I I‘ upper part of intracaldera rock units and postcaldera rocks I precaldera rock units 7‘ Z 5 z 2 Z Z _ ~ — — 0 WEST z 2 z z z D 9 g 5 D ; EAST - 9 0 ~ — o z '- Lu m 2 u D '— ; m 0 T U m m m z I '— UJ Iu U) m I A M 3 in) “1‘ E B m m UE 19 m A m In In ' e _ _ I "’ "’ U—19as‘ , UE 19b 1 7000 7000’ |\) SECTION I I N I 2 E o z m m E 6000’ C 6000’ 5000’ w! Ash-fall and ash-flow of Dead Horse Flat 5000’ 4000’ 4000' Area 20 tuffs and rhyolites of PROFESSIONAL PAPER 712—B PLATE 2 EXPLANATION Postcaldera rocks Described in table 1 Tuffs of the Belted Range Tuff tuff and Rhyolites and densely welded ash-flow tuff of Dead Horse Flat and Area 20 3000' 3000' 2000' 2000' 1000, 1000' SEA LEVEL SEA LEVEL 1000' 1000' 2000' 2000’ 3000, 3000' 4000, _ 4000' SOOO’J 5000’ 6000’ — 6000’ Intracaldera rocks Precaldera rocks Described in table 1 Contact between intracaldera rocks Dashed where approximately located Contact between intracaldera and precaldera and postcaldera rocks Dashed where approximately located PM-l Exploratory test holes and emplacement hole Crossbar is at total depth ———‘fi— — — — Fault Arrows indicate relative direction of displacement Trace of geologic sec- tions shown on plate 1. Dashed where approximately located 7000’— \\ 7000' .7 \ Water level \ Queried where data are lacking 8000’ J_ 8000’ 1 Area of upward crossflow from precaldera and intracaldera rocks into aquifers in . Area of downward crossflow from rhyolite aquifers into basal intracaldera and I " upper part of intracaldera rock units and postcaldera rocks TIT precaldera rock units ' z 5 Z Z _ SOUTHWEST Z z 5 g g 2 Z — 9 QB NORTHEAST ‘ 9 ,t < o ; - 0 g 5 Lu In D I. 0 | z 0 o p u] Lu m U) B/ B 2 o m < [u m z o m m “1 UJ In an m “J Lu UE—19d I 7000’ In (I) m m 7000 U E-19gs 6000’ U E-18r (Projected) 5000’ 4000’ 3000’ 2000' 1000' _ SEA LEVEL— #5::{3 ‘32::le — «its 1000' — _ 2000I— — 3000' VERTICAL EXAGGERATION X 2 SCALE 1:48 000 Adapted from P. P. OrkIId and others (1968) 1/2 3 MILES I 3 KILOMETERS | I I I I I DATUM IS MEAN SEA LEVEL CANYON CALDERA, EASTERN PAHUTE MESA, NEVADA TEST SITE, NYE COUNTY, NEVADA * U.s. GOVERNMENT PRINTING OFFICE: 1973-515—659/22 GENERALIZED GEOLOGIC SECTIONS ACROSS SILENT 6000’ 5000’ 4000’ 3000’ 2000’ 1000’ SEA LEVEL 1000’ 2000’ 3000’ UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY 116° 45’ PREPARED ON BEHALF OF THE US. ATOMIC ENERGY COMMISSION PROFESSIONAL PAPER 712~B PLATE 5 ’ , , 30' 15' 116°OO’ 3/ 50 E \\\\ ’/ : I _— 37’3OI, E \o‘ MOUNT 3 E E I 5 \ I HELEN S E ,2 I 7/ j \x‘ I \C I ’z E 0 : : 3T 9.“ i I; ‘2 I ?\/ \\C I E I V0 :: :2 7/2 I O : — , G) t ’ E I a E I : 5 l (3‘ SC \1 : | v \\\\'I/’ : P4 \“HHH 7/” E A? HUT \\‘\\\ /" > \‘ E \x V E I,“ E S /\ [I’m [k E I M 6 \\‘\ \Z \3 3‘ \\ :_ I S 4 S / m5 E ‘ \§ I \ 1 \\ C: \2\ —>’I"I .\\\ lll‘ EH“? 2: l Co'.'°o R QT», ’1 /\ l)- : 0 : 3‘ 1 3' 0 . > \ /“l4L : \x 1 e >_ I '_ .0 I g v I—Ig EXPLANATION RUN . \ I: : Q- 2 o ' [j \ 2 9 8 I o \/\l|Il'H|ll”“H// .5 .. I‘ E 5 OI /T \\\\‘\ [W . \ 11/, E E Lake A . I d‘ f 5 BLACK . . E] E g: I o r/ pprox1mate Ioun ary o E A N Al N o ' : o Sllent Canyon caldera 2 QUARTZ MOU. T A MOUNTAIN . / .1 z I E / E TOLICHA ' .. g - I _l \ I I I J I J I Q 1 C I ”HI FIE/AK ' . ° / ‘I PA H UTE M ESA I I l Inferred hydraulic barrier L o \ I I ' 0 ‘I ‘7 15 15 — —: 9 . . o / \‘ I I I I I Z . . o o . o x / g]. I ”0,, \ h _, I "H” Approximate boundary of \\I r . . "/1 \ l/ “U... ,, I 5 _: Timber Mountain caldera complex 5 w” ' . \ 3 ’nn‘ H]: SP 2 :_ , Nu u,“ I \I ”nunnnn"; 3 5 5 '3 ; .L_L _I_.L _I_I. _L_L _1_L _L_.L , d c 1, : i : 1 , . ’3, S ‘00,“ \c 0, 3"2/ 5 3 I 5 8 3 Periphery of resurgent dome of 9: “In”? E 3‘ S ' ; , I E I“ 2, Timber Mountain caldera : ’III‘”: i 5 ~ 5 ”z 5 \\“\‘\ \ 5/ E ;I\\‘/_ I ’. I f ¢§ ooooaqooooooooooooooo f 5‘ \ ’ol/ Hmi‘ ; 3 1; I 3 71,”; Approximate boundary of 3 C: -L.L ‘LL \ 1m / I : I ZONIHHUU/‘é" Black Mountain caldera //,H\ y \\ :\ i E I // it 7 “IQ E I 2! Ill ‘ F/ SUM/I, \<< 0,: I :¥ ,/ =I V 2 I 5' QC I’M/l 1, I 5 I Oasis Valley dlscharge area :1 \\\\ I any 2’ I S 3 I 0, \Q a, 1 3 l i 3, I E 3‘ I S I Topographic and ground—water 5‘ E TIMBER \ T E I s I divide 5 3 I0 I \ll , \ a \ ”a0 a 3 MOUNTAIN 5“ I; 3 S: // 2 \u/ T: E S I I \“I' ”'0 & 7/,;\“\ E 1 1° 2 \C I ’ 2 I 3 < 3 0 fi— ; \il, : A 4) : G l | / \I, k ’x x‘ C, \\\II:“\\\ 2 ’III\ . 5:: ll'll\' 0 I I \\\\: //’/III\\\”/’ N7} ’6 $ \ummh’ . . a ,(‘< \l I (0e 0,} I I _ 37000, Dlrection of ground-water 37°00 _. WW —I—.— 11’ Six ‘5‘ (1,], Q”, I I movement BULL FROG \\_\ at”: at: ' Ill!!! 2 Sq : I I \\\“"'"""IIIIIIIII‘\ Ill/0 I 1’ 5S:: l I \\\ 3 1 \C ”U HILLS \‘ 6/ 2 \\\x:\ll,"/\< i \\\\ . “\‘u‘lll‘ll’l/ ’11,, I Z 5 F :\ 0° I l S \‘\I‘I\I III|I\\ a,” ’ I’M, . Z _: t; 0“. I. 9 \l, S \\\ [H]! ‘ '- 2 o 7I \\\‘\ Q 1 3 9° \\\ ”Hm” I“ IIIIII|I\\ :l“ \\ um, 0. ; \I “00 - 3 3 \\\ m “ \\ 3 ”in“ l ‘ I \ I ,- A \ \ _ _ 5 :7 3 5 0‘ P INNAC ES RID G E _: Q, : YIJCC/A PASS 0‘ _: ’7 \\\\"< Z’IV‘O 5\ N [’0 ' o‘LS", 0, :- 06 3 E \ E " 51-” a \v, ”I 5 : i E a)" 5 5 a : . \II I _.‘.' 1 I: Z 3, I : I : // C \C \\II/I E [I \\ 5 Beat? 3 5 2 [0’0 : 5 : 9%0 0“”? a“ I I \ \ \ _ // I - — ‘_ _\ \ \\\\\ 7,. ”-6/ y E : 1 ‘2 1/0,, 2. Z .— ,/- \\\I\\\\\\ 1 I m, : I :_ // i 2 : : 3 S “III/I 3‘ ”a, E 1 : 0 ’w — : : ’: x H 5 E E 0, 1 E : ’ 1‘ C ‘. _-_ ’IIIIIHHH \ T I \\ l’ I,“ : \‘I/ o I; : \‘ Z T . //I\ R — \ — - ’I , : . \‘ 1,3 : '7 I: C 3’ . l 2 I '3 ’2 / ’mIIIIk 5 _ / I”, l’é I >' I )0 5 g I; I ”/H|\\\\ . '— ‘: 5 O I: I '5’ | 2 CRATER 6¢ : 9A I o LINC_0LN _c0UNT_Y_ E A N“ . 0 CLARK COUNTY 4/4 3, ”4°”; 7, 7 c, : Lu | ’0 1 ¢ >' c : m z E E : v" E I 3 _: :— I; JACKASS Q" I E 5 FLAT 5 :— FLATS I “ml/”u I 4 3/ 5 3 \. F \\\I/: I II [C 4’ “NC 00 \\\\/E : E : I I \\‘\ 5 «7 3 5 3 I IRAN GER . ’9 § 2 3 :3 35 I m \ I 3| :’IIIII\HH 6‘ i g 5 5 g]; I \\ ' 3: MOUNTAINS 36°45 -—— : : ‘ 1 06‘ 5 S I 0‘0 $0 I: I 31!] \\\\‘III|||IIII 1—- 36°45’ \ S ‘ ~ I \ / 4 3’10 S : I \\\\IIIIIMI///l \\\\\\\\ E . ////‘\\\\ III —/ 0’ \\\‘ E :\ IIIIIIII \\\\\'I////l \ \\ I I . ” 5 a a SKULL o“ m... I‘ . \ ¢ 0 0,, \IIIII, \\ l ' \ 3 3 I ”IIIIIIH‘ ”a \ 3 5 . I ’z \ \ : I : ’2’ 06‘ Z: i I I I 01/. I0’, C“\/l/6\p 6‘ ll”? ‘ I \\\\ >01 3 "< \4 6‘4 | ' ImIII‘I s : 4° \04 )‘ ' N E VA D A \/‘\\\'/,J\\\\ _ T E S T __ hV_ ‘ \\ 7HHIIIIII|III|IIIH T ‘ _.—.__ .A.___‘ _ A. .. _,.. \ ’: 0/94; 7 IT? ‘\ '— ‘CSPOTTE ‘ \\ : l \ i /‘q\\ \lIIIH‘H‘I‘I” SPECTER :, \ Me'cury :2 q \\\\\\ E \ .Lalhrop WeIIS \\\‘\\\ ’ \‘ II \\\|\\\\ \"I l/ \\\‘I\I|\\\\\\ : : ’0 ‘ I 3 ""H 2 \\ 2.1.; RANGE \ - 3 RANGE I C 104 I \ I f; C’ I 00 E I 116 5 30 15' 116°OO’ SCALE 1:250 OOIO 5 Q 5 10 15 2O 25 MILES I I I I I I I 5 O 5 10 15 20 I I I I I 25 KILOMETERS I gl MAP SHOWING DIRECTION OF GROUND-WATER MOVEMENT FROM EASTERN PAHUTE MESA TOWARD DISCHARGE AREAS IN OASIS VALLEY AND AMARGOSA DESERT, NYE COUNTY, NEVADA * U-S- GOVERNIMENT PRINTING OFFICE: 1973—~515—659/22 a 7 DAY" “MY 6 9.5, Hydrogeologic and Hydrochemical Framework, South-Central Great Basin, Nevada-California, with Special Reference to the Nevada Test Site 1 40‘!“ GEOLOGICAL SURVEY PROFESS‘iHO‘iVAL PAPER 712-C Prepareg’ on behalf of the US. Aégmic Energy Commission .- Ei‘“““-—-——_._~__ EUOCUMENTS DERARHMENT? i ; OCTISIQE i ? LIBRARY 1; H NIVERSITY OLQAI Irma/1:113 I SEP‘19 137E U.S.S.D HYDROGEOLOGIC AND HYDROCHEMICAL FRAMEWORK, SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA, WITH SPECIAL REFERENCE TO THE NEVADA TEST SITE Big Spring, Ash Meadows, Nev. Spring emerges from lake beds. Discharge about 1,000 gallons per minute; water temperature 82°F (28°C). Paleozoic carbonate and elastic rocks form hills east of spring. ‘ -,;_ m. ,‘ , Crystal Pool, Ash Meadows, Nev. Spring emerges from lake beds. Orifice (center of photo) dips steeply northward (to left) beneath travertine(?) lip. Discharge, 2,800 gallons per minute, is largest of pool springs in the area; water temperature 88°F (31°C). Hydrogeologic and Hydrochemical Framework, South-Central Great Basin, Nevada-California, with Special Reference to the Nevada Test Site By ISAAC J. WINOGRAD and WILLIAM THORDARSON HYDROLOGY OF NUCLEAR TEST SITES GEOLOGICAL SURVEY PROFESSIONAL PAPER 712-C Prepared on behalf of the US. Atomic Energy Commission UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON21975 UNITED STATES DEPARTMENT OF THE INTERIOR STANLEY K. HATHAWAY, Secretary GEOLOGICAL SURVEY V. E. McKelvey, Director Library of Congress Cataloging in Publication Data Winograd, Isaac Judah, 1931— Hydrogeologic and hydrochemical framework, South Central Great Basin, Nevada-California. (Hydrology of nuclear test sites) (Geological Survey Professional Paper 712-C) Bibliography: p. Includes index. Supt. of Docs. no.2 ll9.16:712—C 1. Water, Underground—Great Basin. 2. Water chemistry. 3. Groundwater flow—Great Basin. 1. Thordarson, William, joint author. 11. United States Atomic Energy Commission. 111. Title. IV. Series. V. Series: United States Geological Survey Professional Paper 712—C. GBlO25.N6W55 551.4’9’0979334 75—619105 For sale by the Superintendent of Documents, US. Government Printing Office Washington, DC. 20402 Stock Number 024-001-02651—1 CONTENTS Abstract _______________________________ Introduction Purpose and scope ______________________ History of the investigation and previous reports __ __ Acknowledgments Well-numbering system ___________________ Geographic setting _________________________ Physiography _________________________ Economic development ___________________ Climate Geologic setting __________________________ Precambrian and Paleozoic stratigraphy _________ Mesozoic stratigraphy _ _ _. _ _ _ _ _ _ _ _ _ _ _‘ ______ Cenozoic stratigraphy ____________________ Structural geology ______________________ Principal aquifers and aquitards Aquifers ____________________________ Lower carbonate aquifer ________________ Character in outcrop ________________ Character based on cores, drilling records, and geophysical logs _________________ Drill-stem and pumping tests __________ Upper carbonate aquifer ________________ Welded-tuff aquifer ___________________ Character in outcrop ________________ Character based on cores and drilling records _ Pumping and bailing tests ____________ Bedded-tuff aquifer ___________________ Lava-flow aquifer ____________________ Valley-fill aquifer ____________________ Aquitards ___________________________ Lower clastic aquitard _________________ Character based on outcrop, cores and drilling records Hydraulic tests ___________________ Regional evidence of low transmissibility _ __2 Upper elastic aquitard _________________ Tuff aquitard Character based on observations in underground workings Character based on core and electric-log analyses Hydraulic tests ___________________ Lava-flow and other minor aquitards _________ Distribution and saturated thickness of aquifers and aquitards Yucca Flat Other valleys _________________________ Movement of ground water Perched ground Water ____________________ Nevada Test Site ____________________ Spring Mountains ____________________ Intrabasin movement Yucca Flat ________________________ Frenchman Flat _____________________ Other valleys _______________________ Interbasin movement Evidence for interbasin movement __________ Page Cl wmwmmehmwmp—‘H 17 20 30 31 31 34 34 34 36 37 38 39 39 42 42 43 43 44 45 46 46 47 47 49 49 50 50 52 53 53 57 62 62 62 Movement of ground water — Continued Interbasin movement — Continued Influence of clastic aquitards and major shear zones on interbasin movement ______________ Northeastern Yucca Flat—western Emigrant Valley _______________________ Northwestern and west-central Yucca Flat _a_ Southern Indian Springs and Three Lakes Valleys Southwestern Mercury Valley _: _________ Ash Meadows ____________________ General significance of the hydraulic barriers _ Construction and interpretation of the potentiometric map of the lower carbonate aquifer ______ Notes on construction _______________ Interpretation of major features of potentiometric map ________________________ Lower carbonate aquifer transmissibility derived from map _____________________ Depth of ground-water circulation __________ Ash Meadows ground-water basin ________________ Discharge area ________________________ Ground-water discharge ________________ Character and geologic control of spring discharge Upward leakage from lower carbonate aquifer beneath valley northeast of spring line ____ Estimates of evapotranspiration _________ Possible upward leakage south of Lathrop Wells Magnitude of possible underflow across the discharge area ___________________________ Areal extent of the ground-water basin __________ Inferred from isohyetal map _ _ _ _ _ _ _ _ ; _____ Inferred from potentiometric map __________ Inferred from major geologic features _ _ _ _ _ ; _ _ ._ Extent of basin Relation to Las Vegas ground-water basin ________ Relation to Pahrump Valley ground-water basin Sources of recharge to the lower carbonate aquifer ___ Precipitation _______________________ Underflow from the northeast _____________ Downward leakage from Cenozoic rocks _______ Quantity derived from northwest side of basin __ _ Oasis Valley—Fortymile Canyon ground-water basin _____ Probable hydraulic connection between central Amargosa Desert and Furnace Creek Wash—Nevares Springs area in Death Valley ___________________________ Ground-water chemistry, hydrochemical facies, and regional ' movement of ground water ________________ Previous interpretation of ground-water chemistry _ __ Hydrochemical facies Variations of dissolved-solids content with depth in the lower carbonate aquifer __________________ Sources of sodium and sulfate ions in water of the lower car- bonate aquifer _____________________ Sodium __' ________________________ Sulfate __________________________ Hydrochemical evidence for regional ground-water flow— V Page 063 63. 66 67 68 70 70 71 71 71 73 74 75 75 78 78 84 84 84 85 85 85 86 86 88 89 90 92 92 92 93 93 94 95 97 97 98 102 103 104 105 108 VI CONTENTS Page Page Ground-water chemistry, hydrochemical facies, and regional Ground-water chemistry, hydrochemical facies, and regional movement of ground water — Continued movement of grount water — Continued Hydrochemical evidence for regional ground-water flow Hydrochemical evidence for regional ground-water flow — Continued — Continued Hydraulic,connection between Pahrump Valley and Summary Of hydrochemical evidence on regional Ash Meadows _____________________ C108 movement of ground water _____________ C112 . . . , Ground-water velocity ______________________ 113 Direction 0f ground-water movement WIthm the lower Velocity of movement from the tuff aquitard into the lower carbonate aquifer beneath Nevada Test Slte _ _ _ 109 carbonate aquifer in Yucca Flat _____________ . 113 Estimates of downward crossflow from the tuff Velocity within the lower carbonate aquifer beneath central aquitard into the lower carbonate aquifer _____ 109 Yucca Flat _________________________ 114 m __________ R. Ground-water movement in Amargosa Desert ._ _ .. _ 111 Evidence bearing on possible flow through integrated solu- Upward crossflow in the east-central Amargosa tion channels or highly permeable fracture zones of Desert _______________________ 111 regional extent _______________________ 115 Sources of water in the central Amargosa Desert 111 33:33:}nfithi ff: further-study- : : : : : : : : : : : : : : : ii: Possible source of spring discharge at Furnace Creek References _____________________________ 119 Wash-Nevares Springs area, Death Valley ____ 112 Index ________________________________ 125 ILLUSTRATIONS FRONTISPIECE Photographs of Big Spring and Crystal Pool, Ash Meadows, Nev. Page PLATE 1. Hydrogeologic map of Nevada Test Site and vicinity, southern Nevada ________________________ In pocket 2. Hydrogeologic maps and fence diagram of Yucca Flat, Nevada Test Site, southern Nevada. ___________ In pocket 3. Map showing ground—water chemistry and hydrochemical facies, Nevada Test Site and vicinity __________ In pocket FIGURE 1. Index map _______________________________________________________________ C5 2. Graphs showing normal and means of monthly precipitation __________________________________ 7 3. Map showing mean annual precipitation ______________________________________________ 8 4—8. Photographs showing: 4. Intensely fractured Silurian dolomite in the Spotted Range ______________________________ 14 5. Secondary openings along bedding and joint planes in Cambrian(?) carbonate rocks, Titus Canyon, Death Valley National Monument, Calif _______________________________________________ 16 6. Structure in cores of Pogonip Group, well 88—66, Yucca Flat _____________________________ 18 7. Sealed, vuggy, and open fractures in cores of carbonate rock from the Pogonip Group, well 88-66, Yucca Flat 18 8. Sealed and vuggy fractures and slickenside with red clay in limestone core from the upper part of the Carrara For- mation, well 79—69a, Yucca Flat ____________________________________________ 19 9. Diagrammatic semilog graphs of water-level drawdown and recovery during single-well constant-rate pumping test in the lower carbonate aquifer f ______________________________________________ 21 10—12, Semilog graphs of drawdown and residual drawdown of water level during pumping test: 10. Test well 66-75, September 11—13, 1962 _________________________________________ 24 11. Well 67—68, September 11—14, 1962 ____________________________________________ 25 12. Well 67—73, February 24—26, 1963 _____________________________________________ 26 13. Semilog graph of drawdown of water level during pumping test in test well 75—73 (Frenchman), May 9, 1962 _____ 27 14. Semilog graphs of drawdown of water level for three different rates of pumping in well 79—69 ______________ 27 15—17. Semilog graphs of drawdown and residual drawdown of water level during pumping test: 15. Test well 84—68d, November 21—23, 1966 _________________________________________ 28 16. Test well 87—62, August 10—11, 1962 ___________________________________________ 29 17. Well 88—66, March 16—20, 1962 ______________________________________________ 30 18. Diagrammatic section of ash-flow tuff, showing relation of joint density, interstitial porosity, coefficient of interstitial permeability, and coefficient of fracture transmissibility to degree of welding _____________________ 32 19—21. Photographs showing: 19. Columnar jointing in ash-flow tuff, Tiva Canyon Member of Paintbrush Tuff, at Busted Butte, western Jackass Flats ______________________________________________________ 33 20. Relation of joint density to degree of welding in ash-flow tuff, 4 miles east of Shoshone, Calif. along Charles Brown Highway _________________________________________________________ 33 21. Zone of lithophysal cavities in ash-flow tuff, Topopah Spring Member western Jackass Flats ___; ______ 33 22. Semilog graph of drawdown of water level during pumping test in well 74—61, December 18, 1958 ___________ 35 23. Semilog graph of drawdown of water level during pumping test in well 74-57, February 18—22, 1964 __________ 35 24. Semilog graphs of drawdown and residual drawdown of water level during pumping test in well 81—67, October 2—4, 1959 36 FIGURE TABLE 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 99999999? CONTENTS VII Page Semilog graphs of drawdown and residual drawdown of water level during pumping test in well 74—70b, September 9—11, 1959 ________________________________________________________________ C38 Semilog graph of recovery of water level during 133-day shutdown in well 74—70b, December 23, 1960, to May 4, 1961 39 Semilog graphs of drawdown and residual drawdown of water level during pumping test in well 74—70a, August 27-29, 1959 40 Semilog graphs of drawdown and residual drawdown of water level during pumping test in well 83—68, September 20—21, 1960 ________________________________________________________________ 41 Semilog graph of residual drawdown of water level during pumping test in well 91—74, November 23—25, 1959 _____ 42 Diagrammatic section showing perched and semiperched ground water in the tuff aquitard of Rainier Mesa ______ 51 Map showing hydrogeology of Frenchman Flat __________________________________________ 58 Map and sections showing hydrogeology of Emigrant Valley __________________________________ 64 Map and sections showing hydrogeology of southern Indian Springs Valley _________________________ 68 Map showing hydrogeology of southeastern Amargosa Desert __________________________________ 76 Map and graph showing major springs at Ash Meadows ____________________________________ 79 Photograph showing solution notches marking water levels as much as 4 feet above 1966 water level of pool at Devils Hole 83 Photograph showing possible former stand of water about 20 feet above 1966 water level on south wall of Devils Hole 83 Trilinear diagram showing chemical types of the ground water at Nevada Test Site and vicinity ____________ 99 Graph showing regional variations in Na+K and Ca+Mg within the lower carbonate aquifer _______________ 105 Graph showing regional variations in Na+K, HCO; +C03, and 804 +Cl within the lower carbonate aquifer _____ 108 TABLES Page Stratigraphic and hydrogeologic units at Nevada Test Site and vicinity ___________________________ C10 Intercrystalline porosity and permeability of cores from lower carbonate aquifer, Nevada Test Site and vicinity ____ 17 Pumping-test data for aquifers in Nevada Test Site and vicinity _______________________________ 22 Interstitial porosity and permeability of cores from the lower clastic aquitard, well 89—68, Yucca Flat __________ 41 Interstitial porosity and permeability of cores from the tuff aquitard, Nevada Test Site __________________ 45 Hydraulic gradients in Cenozoic hydrogeologic units, Yucca Flat _______________________________ 54 Spring discharge at Ash Meadows in 1953 and 1962 _______________________________________ 80 Chemical constituents of ground water in the Nevada Test Site and vicinity ________________________ 100 Classification of hydrochemical facies at the Nevada Test Site and vicinity _________________________ 102 Chemical analyses of water from test wells 89—68 and 67—68, Yucca Flat and Mercury Valley, Nye County ______ 102 Chemical analysis of water from test well 68-69, Mercury Valley, Nye County _______________________ 107 Chemical analyses of water from three depth intervals in test well 73—66, Rock Valley, Nye County ___________ 107 Summary of deuterium content of water from major springs, southern Great Basin, Nevada-California _________ 110 Estimated ground-water velocity in tuff aquitard, Yucca Flat, Nye County _________________________ 114 Estimated ground-water velocity in lower carbonate aquifer beneath central Yucca Flat and Specter Range, Nye county 115 HYDROLOGY OF NUCLEAR TEST SITES HYDROGEOLOGIC AND HYDROCHEMICAL FRAMEWORK, SOUTH- CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; WITH SPECIAL REFERENCE TO THE NEVADA TEST SITE By ISAAC J. WINOGRAD and WILLIAM 'Il-IORDARSON ABSTRACT Intensely fractured Precambrian and Paleozoic carbonate and clastic rocks and block-faulted Cenozoic volcanic and sedimentary strata in the Nevada Test Site are divided into 10 hydrogeologic units. Three of these — the lower clastic aquitard, the lower carbonate aquifer, and the tuff aquitard — control the regional movement of ground water. The coefficients of fracture transmissibility of these rocks are, respectively, less than 1,000, 1,000 to 900,000, and less than 200 gallons per day per foot; interstitial permeability is negligible. Solution caverns are locally present in the carbonate aquifer, but regional movement of water is controlled by variations in fracture transmissibility and by structural juxtaposition of the aquifer and the lower clastic aquitard. Water circulates freely to depths of at least 1,500 feet beneath the top of the aquifer and up to 4,200 feet below land surface. Synthesis of hydrogeologic, hydrochemical, and isotopic data suggests that an area of at least 4,500 square miles (including 10 inter- montane valleys) is hydraulically integrated into one ground-water basin, the Ash Meadows basin, by interbasin movement of ground water through the widespread carbonate aquifer. Discharge from this basin — a minimum of about 17,000 acre-feet annually — occurs along a fault-controlled spring line at Ash Meadows in east-central Amargosa Desert. Intrabasin movement of water between Cenozoic aquifers and the lower carbonate aquifer is controlled by the tuff aquitard, the basal Cenozoic hydrogeologic unit. Such movement significantly influences the chemistry of water in the carbonate aquifer. Ground-water velocity through the tuff aquitard in Yucca Flat is less than 1 foot per year. Velocity through the lower carbonate aquifer ranges from an estimated 0.02 to 200 feet per day, depending upon geographic position within the flow system. Within the Nevada Test Site, ground water moves southward and southwestward toward Ash Meadows. INTRODUCTION In 1957, the US. Atomic Energy Commission detonated the first of a series of underground nuclear ex- plosions at Nevada Test Site. Underground testing was started to prevent atmospheric fallout, a by-product of earlier series of surface and aerial detonations at the Test Site. Since 1957, the US. Atomic Energy Commission ' has detonated many nuclear devices in a variety of un- derground geologic and hydrologic environments. Although such testing minimizes contamination by at- mospheric fallout, data for evaluating the possible con- tamination of ground-water reservoirs in the vicinity of these detonations were lacking in 1957. Yet, the sole source of water at Nevada Test Site and vicinity was from wells and springs. To fulfill its obligation toward public safety, to pre- vent, if possible, even local contamination of a valuable natural resource, and to defend itself against possible damage claims, the US. Atomic Energy Commission in 1957 asked {the US. Geological Survey to study the oc- currence and the movement of ground water beneath the Nevada Test Site. Specifically sought was an evaluation of the potential for contamination of ground water in and near the Test Site. PURPOSE AND SCOPE The purposes of this investigation were to (1) define the hydraulic character and subsurface distribution of the major aquifers and aquitards, (2) identify and describe the principal areas of recharge to and discharge from the major aquifers, and (3) determine the rate and the direction of ground-water movement within the ma- jor aquifers and aquitards. Of these objectives, the third was of prime importance for an evaluation of the rate of movement of various radionuclides from the vicinity of an underground nuclear detonation. The accuracy of the velocity estimates, however, rested heavily upon the other two study objectives. The Nevada Test Site occupies a small part of two ground-water basins — the Ash Meadows and the Oasis Valley—Fortymile Canyon basins. Consequently, the ob- jectives are discussed for a region several times the size of the test site. C1 C2 The scope of the report is broad in view of the com- plexities of the geology, the vastness of the study area, and the absence of previous detailed hydrogeologic studies of similar terrane. Yet, the types and quantity of data obtained during this investigation are seldom available in hydrogeologic studies. In addition to stan- dard hydrologic data, a wealth of geologic, geophysical, geochemical, and isotopic data were used to supplement interpretations of the hydrologic data. To a first ap- proximation, therefore, the objectives of the study are believed to have been accomplished. The development of ground-water supplies was an im- portant byproduct of the investigation; more than half the test holes are used as water wells. This report does not discuss the exploration for, and development of, new water supplies, although many of the data and inter- pretations will aid others in such tasks. HISTORY OF THE INVESTIGATION AND PREVIOUS REPORTS Hydrologic data for this report were collected and in- terpretations were made over an 8-year period, 1957—64. The work was done in three phases: (1) the period 1957—59, (2) the period 1960—61, corresponding in part with the moratorium on both surface and underground nuclear testing, a ban in effect from November 1958 through September 1961; and (3) the period 1962—64, coinciding with the renewed nuclear testing at Nevada Test Site. The initial phase of the study was devoted to two ma- jor tasks. First, hydrologic data were collected from all existing wells and springs at and in the vicinity of Nevada Test Site. Second, the hydrology of tuff underly- ing Rainier Mesa was studied in detail in more than 5 miles of tunnels, drifts, and shafts driven into the east face of that mesa. This phase of the work, done under the direction of Mr. Alfred Clebsch, Jr., resulted in several reports. Clebsch and Winograd (1959) evaluated the regional hydrology of the test site area, and J. E. Moore (1961 and 1962) and Clebsch and Barker (1960) tabulated data on most existing wells and springs of the area. Hood (1961) analyzed pumping tests of four wells, and Clebsch (1961) analyzed the significance of tritium- age measurements of ground water from supply wells and springs. Thordarson (1965) described the hydrology of Rainier Mesa, and Clebsch (1959 and 1960) evaluated potential water-supply contamination from the un- derground nuclear testing beneath Rainier Mesa. Schoff and Winograd (1961 and 1962) described the hydraulic data obtained from six core holes drilled into carbonate rocks in northern Yucca Flat. The second phase of the program (1960—61) was prin- cipally a study of the hydrology of Yucca Flat. It was prompted by the US. Atomic Energy Commission’s plan to utilize Yucca Flat as an underground testing area if nuclear testing were resumed. To acquire an un— HYDROLOGY OF NUCLEAR TEST SITES derstanding of the hydrology and subsurface geology of the valley, six test holes ranging in depth from 1,700 to 2,300 feet were drilled in Yucca Flat. Before the drilling, gravity and some seismic surveys were made to aid in selection of the drill sites. The geology of the ridges sur- rounding the valley was mapped concurrently with the drilling. Mapping provided stratigraphic and structural background for interpretation of the stratigraphic se- quence penetrated by the drill holes. Test drilling, begun in April 1960 and completed in September 1961, was under the general supervision of Mr. Stuart L. Schoff and under the field direction of Mr. I. J. Winograd. The lithologic, hydrologic, and physical- property data obtained from five of these holes were summarized by Price and Thordarson (1961), Thordar: son, Garber, and Walker (1962), Garber and Thordarson (1962), J. E. Moore and Garber (1962), and J. E. Moore, Doyle, Walker, and Young (1963). A brief synthesis of the test-hole data, emphasizing how the data related to the regional flow of ground water, was presented by Winograd (1962). In September 1961, soon after completion of the test drilling in Yucca Flat, the moratorium on the testing of nuclear weapons ended. The US. Atomic Energy Com- mission immediately requested new test areas that would permit testing at greater depths. To meet this re- quest, the U.S. Geological Survey expanded its hydrologic, geologic, and geophysical studies to encom- pass unexplored areas of the Nevada Test Site. Con- tamination of ground water through underground testing in the Cenozoic strata in Yucca Flat was considered only a slight possibility. Independent studies by geochemists of the US. Geological Survey and the Lawrence Radia- tion Laboratory showed that ion-exchange capacity of the Cenozoic strata at depths of proposed underground testing would probably prevent most radionuclides from moving more than a few hundred to a few thousand feet from the point of detonation. Moreover, much of the Cenozoic tuff to be used as a host for most of the events was an aquitard of extremely low transmissibility. However, the quest for deeper sites and the suggestion that the widespread Paleozoic carbonate rocks are highly transmissive dictated that further studies be made of those deeper aquifers. To provide a more complete understanding of the regional flow of ground water within the Paleozoic car- bonate rocks, the US. Geological Survey began a second drilling program early in 1962 and completed it by mid- 1963. Ten test holes, ranging in depth from 900 to 5,500 feet were drilled. Eight of the holes tested the Paleozoic strata. Only 2 of the 10 holes were drilled in Yucca Flat; the others were drilled in Indian Springs Valley, Frenchman Flat, and Jackass Flats. To obtain stratigraphic information or a water supply, Los Alamos Scientific Laboratory, Lawrence Radiation Laboratory, SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE and Reynolds Electrical and Engineering Co. drilled several dozen additional test holes in Yucca Flat. In the 10 holes drilled specifically for hydrologic information, a wide variety of hydrologic, geologic, and physical- property data were obtained from each aquifer penetrated. But in some of the stratigraphic test holes, drilling methods and time considerations precluded even a determination of the static water level in the principal aquifer. The third phase of the work, devoted to collection of data from the various test holes drilled from 1962 to 1964, began under the general supervision of Mr. Stuart L. Schoff. Mr. William E. Hale succeeded Schoff as general supervisor. Mr. Isaac J. Winograd and Mr. Richard A. Young directed the field efforts at Mercury, Nev. Several reports describe the third phase of the study. Schoff and J. E. Moore (1964) discussed the chemistry of ground water at the Nevada Test Site and demonstrated how water-quality data might be utilized as an indepen- dent tool for determining the direction of ground-water movement. Winograd’s (1963) review of the hydrology of the area between Las Vegas and the Amargosa Desert emphasized the development of a new water supply in southern Indian Springs Valley. Walker and Eakin (1963) made a reconnaissance of the Ash Meadows—Amargosa Desert discharge area, and Eakin, Schoff, and Cohen (1963) made a reconnaissance of the valleys surrounding the test site. Winograd and Eakin (1965) and Eakin and Winograd (1965) summarized the regional significance of the subsurface data from the holes penetrating the Paleozoic carbonate rocks. Winograd and Thordarson (1968) described structural control of ground-water movement within the carbonate rocks. The availability of construction data, lithologic and geophysical logs, water analyses, cores and cuttings yield and production records, and water levels for all the wells and test holes discussed in this report has been summarized by Thordarson, Young, and Winograd (1967). Some of the subject matter of this present report has been described briefly in several of the reports previously listed. The present report, however, presents the first detailed analysis and synthesis of the hydrologic data collected during the second and third phases of the Survey’s work at the test site. In addition, it integrates, for the first time, appropriate facets of the geologic and geophysical studies made concurrently with and in par- tial support of the hydrologic study program. The geologic data, in particular, were very useful in the evaluation of regional flow patterns. ACKNOWLEDGMENTS This report depends on the work of many people and organizations. First, we thank the US. Atomic Energy C3 Commission, Nevada Operations Office, for its support of the US. Geological Survey’s studies at the Nevada Test Site. Particularly acknowledged are Messrs. O. H. Roehlk, R. L. Kinnaman, and R. T. Russell of the Operational Safety Division. Of the 15 men who helped collect the subsurface data that form the backbone of this report, particular acknowledgment goes to Messrs. C. E. Price, R. F. Nor- vitch, M. S. Garber, and R. A. Young. Mr. C. E. Price prepared a series of geologic, hydrologic, and drilling checklists that were utilized throughout both drilling programs and were a great help in standardization of data collection. Mr. R. F. Norvitch supervised the pump- ing tests on half the test holes. In addition, he analyzed the step—drawdown tests in four of those wells. Mr. M. S. Garber, assisted by Mr. A. C. Doyle, modified and calibrated existing instruments for accurate measure- ment of water levels at depths to 2,800 feet below land surface. Mr. R. A. Young supervised the drilling operations and, in Winograd’s absence, served as acting field party chief. Throughout the field effort and to a lesser degree dur- ing the report-preparation phase, the authors benefited greatly through technical discussions with many in- dividuals in the US. Geological Survey. The specific contributions of these colleagues are acknowledged in the body of the text. Here we only briefly list the general area of help offered by them. Mr. W. E. Hale raised provocative questions throughout the period 1962 to 1965. His intense interest in the work led to a significant improvement in many facets of the field efforts. Mr. W. A. Beetem suggested utilization of chemical data as a tool in deciphering the regional flow system, and he and his associates collected and analyzed most of the water samples from which such an analysis was eventually made. Messrs. S. W. West, S. L. Schoff, Alfred Clebsch, Jr., G. F. Worts, Jr., and O. J. Loeltz offered continued technical advice and personal encouragement to the authors. The authors benefited greatly from numerous discussions of the regional geology and geophysics with many colleagues; they especially thank Messrs. R. L. Christiansen, Harley Barnes, and F. G. Poole. Thanks also go to Messrs. F. A. McKeown, P. P. Orkild, F. N. Houser, D. L. Healey, and E. B. Ekren for their interest and helpful discussions. The editorial assistance of Virginia Glanzman and Billy Robinson is gratefully acknowledged. Numerous individuals working for the prime contrac- tor, the testing laboratories, and other firms were a con- stant source of help, including Messrs. R. W. Newman, Willard Martin, Leonard Palmer, Emmett Herbst, Robert R. Gunny, Ross McDonald, and Merv Boggs. WELL-NUMBERING SYSTEM Wells and test holes referred to in this report are iden- C4, Lified by the Nevada coordinate system, central zone, or by township, range, and section. All the holes within or in the immediate vicinity of Nevada Test Site are iden- tified by the 10,000-foot grid of the Nevada coordinate system, central zone, the system used by the US. Atomic Energy Commission and its contractors. The first two digits of the north coordinate and the first two digits of the east coordinate of this grid are used to iden- tify the well. Thus, a well at coordinates N. 671,051 feet and E. 739,075 feet is identified by the numbers 67—73. Where more than one well is in the same 10,000-foot grid, one hole will be designated by four numbers, and all others by consecutive letters after the fourth number — for example, 67—73, 67—73a, and 67—73b. The alphabetical designation does not necessarily indicate the sequence in which the holes were drilled. Wells in the Amargosa Desert, in Pahrump Valley, and elsewhere along the periphery of the study area are identified by township, range, and section. In the part of the study area in Nevada, the townships with a few ex- ceptions are south of the Mount Diablo base line; the ranges are all east of the Mount Diablo meridian. Therefore, these geographic designations are not given in the well designation. For example, a well in the NW 14 sec. 27, T. 16 S., R. 51 E., is identified simply by 16/51—27b. The letters a, b, c, or d, which follow the sec- tion number, refer respectively to the northeast, northwest, southwest, and southeast quarter sections. Double letters that follow a section number identify a well site in a 40-acre tract. Thus the well number for location SWIANEIA sec. 34, T. 19 S., R. 53 E., is 19/53—34ac. A number after the letter was used by Walker and Eakin (1963) in the Amargosa Desert to designate the number of wells in a quarter section. Wells in California are readily identified by a capital N that follows the township designation. In California, the townships are north and the ranges east of the San Ber- nardino base line and meridian, respectively. GEOGRAPHIC SETTING The study area generally lies within the area bounded by lat 36°20’ and 37°30’ N. and long 115°10’ and 116°45’ W. (fig. 1). It encompasses about 7,100 square miles of Clark, Lincoln, and Nye Counties, Nev., and Inyo Coun- ty, Calif. This area is within the south-central part of the Great Basin section of the Basin and Range physiographic province defined by Fenneman (1931). Some botanists consider the region a part of the Mohave Desert (Jaeger, 1957). The Nevada Test Site, an area of about 1,400 square miles (all in Nye County) in the cen- tral part of this region, is the area of detailed study. PHYSIOGRAPHY In the region are two of the largest valleys in southeastern and south-central Nevada and two of the HYDROLOGY OF NUCLEAR TEST SITES highest mountain ranges. The Las Vegas Valley, border- ing the study area on the southeast (fig. 1), is about 40 miles long and as much as 20 miles wide; the valley trends south-southeast, and its floor ranges in altitude from 2,000 to 3,000 feet. The Amargosa Desert, a valley that forms the southwestern part of the study area, is ap- proximately 50 miles long and as much as 20 miles wide. This valley also trends southeast, and-its floor generally ranges in altitude from 2,000 to 3,000 feet. The east- central part of Death Valley, one of the largest intermon- tane valleys of the Great Basin, lies in the southwest cor- ner of the study area (fig. 1). Smaller intermontane valleys within the study area include, from east to west, Pahranagat Valley, Desert Valley (also called Tikaboo Valley), Three Lakes Valley, Indian Springs Valley, Emigrant Valley, Frenchman Flat, Yucca Flat, Pahrump Valley, and Jackass Flats. The floors of these north-nort west trending basins range in altitude from 3,000 to 4,500 feet. The two predominant mountain ranges are the Spring Mountains, bordering the study area on the south, and the Sheep Range, forming the eastern border (fig. 1). The Spring Mountains trend northwest, are about 45 miles long, and are up to 18 miles wide. These moun- tains, which merge with the flanking bajadas at altitudes ranging from 5,000 to 6,000 feet, reach an altitude of nearly 12,000 feet. The Sheep Range trends north, is about 45 miles long, and is as much as 8 miles wide. The maximum altitude of the Sheep Range is nearly 10,000 feet. The northern third of the study area includes, from east to west, the Pahranagat, Timpahute, Groom, and Belted Ranges, and Pahute Mesa. These four ranges trend northward and range in altitude from 6,000 to 9,000 feet. Pahute Mesa ranges in altitude from 5,000 to 7,000 feet. These uplands, although small in comparison with the imposing Spring Mountains and Sheep Range, are nevertheless prominent features in comparison with the numerous ridges and mesas lying within the central part of the region. The centrally located ridges and mesas are generally less than 6,000 feet high. The area is a superb example of Great Basin topography. The contrast in slope between the valley floors and the flanking ridges is generally striking even where the relief between them is small. Most of the basins contain playas, and some contain' badlands developed on exhumed pluvial lakebeds. Pediments, which are characteristic of some intermontane basins, are usually absent; where present, the-pediments are dis- rupted by normal faults. ' Las Vegas and Pahranagat Valleys are tributary to the Colorado River. Jackass Flats and the Amargosa Desert are connected to Death Valley via the Amargosa River ,(fig. 1). Drainage in most of the remaining valleys within the study area is to playas. C5~ SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE 115°oo' 1 16°00' 1 16°30’ EXPLANATION Nevéda Test Site boundary .314";- fl'll J '1‘ m2¢ Yucca Flat (altitude 3924 ft) 1958—64 (mean) I l FIGURE 2. — Normal and means of monthly precipitation. C7 08 HYDROLOGY OF NUCLEAR TEST SITES 117°30' 117°00’ 30' 116°00' 30' 115° 00’ 114°3o' I TONOPAH ' ' l l I 1 (any) (£3.70 TONOPAH (AIRPORTD $13.03 ADAVEN 38°00' — ' _ 30’ — _ 37000! _ I . . .. . . . .. ' ' ‘ .— '::.::::'-. Lii-fiwmnopie- . """ I‘ZIIQQWEU-SIII 3" ‘ 1,78 ‘ ' _ DEATH VALLEY$ 4 , “P : ‘Fn A """"""" '- AiVEfiASI . . . . . . . . 1.351135 E‘GAs_V(A'| nPiJ'RT): .: ‘_‘ 36°00’ — I. . _ $5.39 BOULDER CITY l l l | l Modified from Weedfall (1963). Compiled 10 20 30 4'0 5'0 MILES by graphical addition of seasonal isohyetal fl i—Ill i I [I 1 1| maps by R. Fl Quiring (1965) I 0 10 20 30 40 50 KILOMETERS EXPLANATION RANGE OF ANNUAL PRECIPITATION, [N INCHES \§ -— 4 — @1303 ADAVEN __ 5—.— & Weather station 2 Lines Of equal mean annual Mean annual precipitation listed Less than 4 8-12 16— 0 2 —28 ' ' ' ' ' precipitation, m inches next to station; adjusted to \ Interval variable 30-year period, 1 931—60 12—16 20— 24 Greater than 28 Nevada Test Site boundary A FIGURE 3. — Mean annual precipitation. that several wet periods, or pluvials, occurred during the the evidence for pluvials in the study area were past 70,000 years. The last major pluvial probably presented by Mehringer (1965) and by Wells and closed about 9,000 years ago. Recent reviews of some of Jorgensen (1964). SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE GEOLOGIC SETTING The Nevada Test Site region is geologically complex. It lies within the miogeosynclinal belt of the Cordilleran geosyncline, in which 37,000 feet of marine sediments ac- cumulated during the Precambrian and Paleozoic Eras. Except for a few small intrusive masses, no rocks of Mesozoic age are found within the study area. The region is also within a Tertiary volcanic province in which ex- trusive rocks, locally more than 13,000 feet thick, were erupted largely from caldera centers. Quaternary detrital sequences, largely alluvium, fill most of the low- lying areas in the region. Two major periods of deformation affected the region. The first orogeny occurred in late Mesozoic and perhaps early Tertiary time and resulted in folding and thrust faulting of the Precambrian and Paleozoic rocks. During middle to late Cenozoic time the region underwent nor- mal block faulting, which produced the Basin and Range topography. Displacements along major strike-slip faults, measured in miles, occurred during both periods of deformation. The description of stratigraphy and structure which follows pertains chiefly to the Nevada Test Site but is applicable in general terms to most of the area of figure 1. Where differences in the general geology of a specific part of figure 1 and that at the Nevada Test Site exist, they are noted at appropriate places in the text. The out- line of stratigraphy and structure presented below is taken from the following sources: Albers (1967); Harley Barnes (U.S. Geol. Survey, written commun., 1965); Barnes and Poole (1968); Burchfiel (1964, 1965); Ekren (1968); Ekren, Rogers, Anderson, and Orkild (1968); Fleck (1970); Hinrichs (1968); Longwell (1960); Longwell, Pampeyan, Bowyer, and Roberts (1965); No- ble (1968); Orkild (1965); Poole, Carr, and Elston (1965); Ross and Longwell (1964); Secor (1962); Stewart (1967); and Vincelette (1964). PRECAMBRIAN AND PALEOZOIC STRATIGRAPHY During Precambrian and Paleozoic time, 37,000 feet of marine sediments were deposited in the study area. The region was then part of an elongated subsiding trough, the Cordilleran geosyncline, which covered most of westernmost North America. The eastern part of this trough, dominated by carbonate and mature clastic sediments, is called the miogeosyncline. The miogeosynclinal sediments throughout the Nevada Test Site and the surrounding region have been divided into 16 formations. Names, thicknesses, and gross lithologic character of these formations are summarized in table 1. For detailed stratigraphic descriptions the reader is referred to Burchfiel ( 1964). Because of the generally uniform miogeosynclinal sedimentation, 15 of the 16 formations of table 1 (ex- C9 cluding the Devonian and Mississippian rocks) are probably representative of the lithology and the relative thickness of Precambrian and Paleozoic strata in the region extending several tens of miles beyond Nevada Test Site. In addition to the uniform lithologic character of the formations throughout the study area, the vertical dis- tribution of clastic and carbonate lithologies within the 37,000-foot sequence is significant. The Precambrian to Middle Cambrian strata, 10,000 feet thick, are predominantly quartzite and siltstone; the Middle Cam- brian through Upper Devonian strata, 15,000 feet thick, are chiefly limestone and dolomite, the Devonian and Mississippian rocks of the Yucca Flat area, about 8,000 feet thick, are chiefly argillite and quartzite; and the Pennsylvanian and Permian rocks about 4,000 feet thick, are chiefly limestone. Thus, the Precambrian and Paleozoic sedimentation was marked by two major se- quences of elastic and carbonate sedimentation. Minor clastic rocks — the Dunderberg Shale Member of the Nopah Formation, the Ninemile Formation, and the Eureka Quartzite — occur within the lower carbonate se- quence. A lateral variation in lithology and thickness of Devo- nian and Mississippian rocks contrasts with the lithologic uniformity of other parts of the stratigraphic section. In western Yucca Flat, Jackass Flats, and areas to the west and northwest, the Devonian and Mississip- pian strata are composed chiefly of elastic rocks (quart- zite, siltstone, argillite, and conglomerate), as much as 8,000 feet in thickness, called the Eleana Formation (table 1). However, in the Spotted Range and the Indian Springs Valley, rocks of equivalent age are predominantly carbonate, and they aggregate, about 1,000 feet in thickness. Preliminary work by Poole, Houser, and Orkild (1961) indicated that the southeastward transition from clastic to carbonate lithology was probably gradational, but that postdepositional thrust or strike-slip faulting may have obscured the transition. For this report the clastic Eleana Formation will be considered representative of the Devonian and Mississippian rocks in Yucca Flat, Jackass Flats, and northwestern Frenchman Flat. The predominantly car- bonate Monte Cristo Limestone and part of the Bird Spring Formation of the Spring Mountains are ten- tatively considered representative of time-equivalent rocks in the Spotted Range and Indian Springs Valley. No major unconformities occur within the miogeosynclinal column. Several disconformities are present but are not marked by deep subaerial erosion of the underlying rocks. MESOZOIC STRATIGRAPHY Rocks of Mesozoic age in the study area consist of C10 HYDROLOGY OF NUCLEAR TEST SITES TABLE 1. — Stratigraphic and hydrogeologic units at Nevada Test Site and vicinity Maximum System Series Stratigraphic unit Major lithology thickness Hydrojgrstologic Waterbeanng charactenstips and extent 0' (feet) saturation Quaternary and Holocene, Valley fill Alluvial fan, fluvial, 2,000 Valley-fill Coefficient of transmissibility ranges from 1,000 to Tertiary Pleistocene, fanglomerate, lakebed, aquifer 35,000 pd per ft; average coefficient of interstitial and and mudflow deposits permealiility ranges from 5 to 70 pd per sq ft; Pliocene saturated only beneath structurally eepest parts of Yucca Flat and Frenchman Flat. Basalt Olew‘ Mesa Ba‘sgslitcufilggv s, dense and 250 Water movement controlled by grimary (cooling) and ' secondary fractures and possi ly by rubble between Rhyolite of Shoshone Rhyolite flows. 2.000 Lava-flow flows; intercrystalline porosity and permeability Mountain aquifer negligible; ezganated 8%fficiient of transmissijbilitly ‘ ranges from to 10, gp per ft; saturate ony B“fi&:€g§“ll 3353“ “0W5“ 250 beneath east2,000 Formation may locally be aquifers In northern Yuc- tuffaceous beds of welded ash flow, ash-fall 08 Flat. Calico Hills tuff, tuff breccia, tuf- faceous sandstone; hydrothermally altered at Calico Hills; matrix of tuff and sandstone com- monly clayey or zeolitic. TuffofCrater Flat Ash-flow tuff, nonwelded 300 to partly welded, in. terbedded with ash-fall tuff; matrix commonly clayey or zeolitic. Miocene an Rocks of Pavits Spring Tuffaceous sandstone and 1,400 Oligocene siltstone, claystone; fresh-water limestone and conglomerate; minor gypsum; matrix commonly clayey, zeolitic, or calcareous. Oligocene Horse Spring Formation Fresh-water limestone, 1,000 conglomerate, tuff. SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE C11 TABLE 1. — Stratigraphic and hydrogeologic units at Nevada Test Site and vicinity — Continued Maximum , . . . System Series Stratigraphic unit Major lithology thickness Hvdwgeobglc Water-bearing characteristics and extent 0' ' ' (feet) ”n" saturation1 Cretaceous to Granitic stocks Granodiorite and quartz (A minor Complexly fractured but nearly impermeable. Permian monzonite in Stocks, aquitard) dikes, and sills. Permian and Tippipah Limestone Limestone. 3,600 Upper Complexly fractured aquifer; coefficient of Pennsylvanian carbonate transmissibility estimated in range from 1,000 to aquifer 100,000 gpd per ft; intercrystalline porosity and permeability negligible; saturated only beneath western onetthird of Yucca Flat. Mississippian Eleana Formation Argillite. quartzite, mm 7,900 Upper Complexly fractured but nearly impermeable; co- and Devonian glomerate, conglomer- elastic efficient of transmissibility estimated less than 500 ite, limestone. aquitard gpd per ft; interstitial ermeability negligible but owmg to poor hydrau ic connection of fractures probably controls ground-water movement; saturated only beneath western Yucca Flat and Jackass Flats. Upper Devils GateLimestone Limestone, dolomite, >1,380 . minor quartzite. Devonian —— ? — Middle Nevada Formation Dolomite. >1.525 Devonian and Undifferentiated Dolomite. 1,415 Silurian Upper Ely Springs Dolomite Dolomite. 305 Eureka Quartzite Quartzite, minor lime- 3140 Middle Stone~ Q A . . . ~ . = ntelopeValley Limestone and Silty 1,530 Ordovman ? _‘ s. Limestone limestone. A . - . .. Complexly fractured aquifer which supplies major Lower ‘2' Ninemile Formation Clilritsdflyeedgelild limestone, 3-35 springs throughout eastern Nevada; coefficient of hen ‘ Lower transmissibility ranges from 1,000 to 1,000,000. pd no. Goodwin Limestone Limestone, >900 carbonate per ft; intercrystalline porosny and permeabi ity .f negligible, solution caverns are present locally but N F , ‘ aq‘“ er regional ground-water movement is controlled by opah ormation Dolomite, limestone. 1.070 fracture transmissibility; saturated beneath much Smoky Member . . of study area. U er Halfpint Member Limestone, dolomite, pp silty limestone, 715 Dunderberg Shale Shale, minor limestone. 2‘25 Member Bonanza KingFormation Limestone, dolomite, Banded Mountain minor siltstone. 2,440 C b , Member am rian PapooseLakeMember Limestone, dolomite, Middle minor siltstone. 2.160 Carrara Formation Siltstone, limestone, in- 1.050 terbedded. Upper 1,050 feet predominantly limestone; lower 950 feet 950 predominantly siltstone. Lower Zabriskie Quartzite Quartzite. 220 Complexly fractured but nearly impermeable; supplies no major springs; coefficient 'of Wood Canyon Formation Quartzite, siltstone, shale, 2.285 Lower transmissibility less than 1.000 and per ft; in- minor dolomite. elastic terstitial porosity and permeability is negligible, _ . _ . . aquitard‘ but proba ly controls regional ground-water move- Stirling Quartzne QuartZite, Siltstone. 3.400 ment owing to poor hydraulic connection of frac- Precambrian tures; saturated beneath most of study area. JohnnieFormation Quartzite, sandstone, 3,200 siltstone, minor lime- stone and dolomite. “Coefficient of transmissibility has the units gallons per day per foot (gpd , _ ‘ _ J'l‘he three Miocene sequences occur in separate parts ofthe per ft) Width of aquifer; coeffiment of region. Age correlations between them are uncertain. They are “The Noonday('?i Dolomite, which underlies the Johnnie Formation, is considered part of the lower clastic aquitard. permeability has the units gallons per day per square foot (gpd placed vertically in table to save space. per sq ft) of aquifer. several small granitic stocks. No Mesozoic sedimentary rocks occur within the study area. Several thousand feet of Triassic and Jurassic rocks crop out in the southeastern one-third of the Spring Mountains and in the ridges east and northeast of Las Vegas; however, these strata are not known to underlie the Nevada Test Site or its immediate surrounding area. CENOZOIC STRATIGRAPHY Cenozoic volcanic and sedimentary rocks are widely distributed in the region. Tertiary volcanic and associated sedimentary rocks aggregate as much as 6,000 feet in thickness in Yucca Flat, 8,500 feet in western Frenchman Flat and eastern Jackass Flats, more than 5,000 feet in western Jackass Flats, and more than 13,500 feet beneath Pahute Mesa. The volcanic rocks are of both pyroclastic and lava-flow origin and include several rock types. The most common rock types, in order of decreas- ing abundance, are ash-flow tuff, ash-fall tuff, rhyolite lavas, rhyodacite lavas, and basalt. The tuffs are com- monly of rhyolitic and quartz-latitic composition. The Tertiary sedimentary rocks associated with the volcanic Cl2 strata include conglomerate, tuffaceous sandstone and siltstone, calcareous lacustrine tuff, claystone, and fresh-water limestone. The Tertiary rocks are largely of Miocene and Pliocene age, but some are Oligocene. The Quaternary strata generally aggregate less than 2,000 feet in thickness and consist of valley-fill deposits and minor basalt flows. The Cenozoic strata at Nevada Test Site have been divided into 12 formations and numerous members. These strata are listed in table 1, which also provides in- formation on their thickness, lithologic character, and areal extent. The formations and members are represen- tative of the Cenozoic rocks beneath Yucca Flat, Frenchman Flat, and Jackass Flats; the table is not representative of the volcanic rocks in the Pahute Mesa and Timber Mountain areas of the Nevada Test Site. Yucca Mountain, Pah Canyon, and Stockage Wash Members of the Paintbrush Tuff have been omitted from table 1 because of their limited areal extent and probable absence within the zone of saturation. The table is based on the work of Harley Barnes (written commun., 1965), Orkild (1965), and Poole, Carr, and Elston (1965). The terminology for the pyroclastic rocks described in this report is that of Ross and Smith (1961) and Poole, Elston, and Carr (1965). Several general characteristics of the Cenozoic pyroclastic rocks, lava flows, and associated sediments are summarized as follows: 1. Areal extent, thickness, and physical properties of each of the Cenozoic volcanic formations vary widely. This irregularity is characteristic of volcanic rocks and is a function of their modes of emplacement, prevailing wind directions, and the topographic relief at the time of their extrusion. Ac- cordingly, the descriptions of lithology and thickness of the Cenozoic formations in table 1 are considered representative only of Yucca Flat, Frenchman Flat, and Jackass Flats. 2. Tertiary rocks generally overlie Precambrian and Paleozoic rocks with angular unconformity. A con- glomerate or breccia commonly lies at the base of the Tertiary section on a weathered surface of older rocks. Locally, joints in the older rocks are filled with detritus derived from the overlying basal Ter- tiary rocks. Evidence of the development of karst terrane on the carbonate rocks beneath the uncon- formity is absent. 3. The oldest Tertiary rocks were deposited upon a paleotopographic surface of moderate relief developed upon Precambrian and Paleozoic strata. Harley Barnes (written commun., 1965) reports that this erosion surface had a maximum relief of about 2,000 feet. By partly filling the topographic lows, the oldest Tertiary rocks reduced the relief of HYDROLOGY OF NUCLEAR TEST SITES the area. By late Miocene time, the relief was con- siderably reduced, as evidenced by the widespread distribution of ash flows of the Paintbrush Tuff. 4. The Miocene and Oligocene rocks up through the basal Wahmonie Formation are of both pyroclastic and sedimentary origin and consist principally of nonwelded ash-flow tuff, ash-fall tuff, tuff breccia, tuffaceous sandstone and siltstone, claystone, and freshwater limestone; lava and welded ash-flow tuff are of minor importance in the area considered. The Pliocene and Miocene rocks above the Wahmonie Formation, in contrast, consist chiefly of welded ash-flow tuff. Nonwelded ash-flow tuff, ash-fall tuff, and tuffaceous sandstone are relatively minor in these younger rocks. 5. The bulk of the Miocene and Oligocene sedimentary rocks appears to be restricted to Frenchman Flat, eastern Jackass Flats, Rock Valley, and Mercury Valley. These strata make up the Rocks of Pavits Spring and the Horse Spring Formation and also are present in the Salyer Formation. Miocene and Oligocene sedimentary rocks are of minor oc- currence in Yucca Flat and western Jackass Flats, although the entire section of Tertiary strata in the latter valley has yet to be explored by drilling. 6. The Miocene and Oligocene rhyolitic tuffaceous rocks up through the Wahmonie Formation are generally massively altered to zeolite (clinoptilolite, mordenite, and analcime) or to clay minerals; a vertical zonation of the zeolite minerals in these rocks was described by Hoover (1968). The Miocene and Pliocene rhyolitic tuffs above the Wahmonie Formation, by contrast, either are glassy or have devitrified to cristobalite and feldspar, but they are less commonly altered to zeolite or clay. STRUCTURAL GEOLOGY The structural geology of the region is complex, and details on the general tectonic setting of the study area are available in only a few published reports cited above. About half of these papers are devoted primarily to a single structural feature of the region, the Las Vegas Valley shear zone. The outline of structural geology presented below provides the information needed for subsequent discussions of the disposition of the aquifers and aquitards and the hydraulic barriers within the prin- cipal aquifers. Harris (1959) demonstrated that a large positive area (Sevier Arch) probably existed in much of southeastern Nevada and western Utah from late Jurassic to early Late Cretaceous; thus, Jurassic and Cretaceous strata were probably never deposited within most of the study area. SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE The Precambrian and Paleozoic miogeosynclinal rocks were first significantly deformed during late Mesozoic and perhaps early Tertiary time. The deformation was marked by uplift and erosion and subsequent folding, thrusting, and strike-slip faulting that made the region mountainous. Beginning with the Miocene volcanism and continuing through the Quaternary, large-scale normal block faulting has disrupted the Tertiary volcanic and sedimentary strata as well as the previously deformed Precambrian and Paleozoic rocks. The normal faulting caused the Basin and Range structure reflected by the topography in the region today. In late Tertiary and Quaternary time, the resulting valleys have been largely filled by detritus aggregating several hundred to a few thousand feet. Currently active normal faulting is in- dicated by fault scarps cutting alluvial fans and by the absence of extensive unfaulted pediments. Some evidence indicates that strike-slip faulting occurred dur- ing Tertiary time, some time after deposition of early Miocene tuff (Ekren and others, 1968). This faulting may possibly reflect periodic rejuvenation of strike-slip faults formed during the late Mesozoic orogeny. Widespread erosion of the miogeosynclinal rocks oc- curred during and after the late Mesozoic orogeny but before block faulting. Before the first deformation of the region, the Precambrian and Lower Cambrian clastic rocks were buried at depths of at least 15,000 feet in the eastern half of the study area and about 27,000 feet in the western half. Today, these strata are exposed in several areas. They form the bulk of the northwest one- third of the Spring Mountains, a significant part of the Groom and Desert Ranges, and the bulk of the Funeral Mountains. Their distribution, a function of geologic structure and depth of erosion, exercises significant con- trol over the regional movement of ground water. Plate 1 shows the areal extent of dominantly clastic pre-Tertiary strata and the relation of these strata to some major thrust faults and folds. In contrast to the miogeosynclinal rocks, the postdepositional distribution of Tertiary rocks has been controlled principally by fairly simple block faulting and erosion. The northwestern part of the area is a faulted and eroded volcanic plateau of which Pahute and Rainier Mesas (fig. 1) are remnants. In the remainder of the area, ridges of pre-Tertiary rocks interrupt the con- tinuity of the once extensive ash-flow sheets. Thurst faults are perhaps the most spectacular of the tectonic features of the region. Thrust faulting displaced the pre-Tertiary rocks laterally a few thousand feet to several miles. Locally, imbricate thrusting repeatedly stacked the miogeosynclinal strata upon one another. Some major thrust faults, though folded, crossfaulted, and eroded can be followed in outcrop or reconstructed for miles (pl. 1). C13 Some workers (Burchfiel, 1965; Secor, 1962) believe that the major thrust faults, which commonly have dips of 35°-50°, flatten with depth and follow less competent strata, specifically the shales of the Carrara Formation; that is, the thrusting is of the décollement type, where the sedimentary rocks slide over the crystalline base- ment. Vincelette (1964) and Fleck (1970) rejected the décollement hypothesis; they presented evidence that the relatively steep. dip of the major thrust faults remains unchanged with depth. Strike-slip faults and shear zones cut and offset the thrust faults in several places within the region. The best documented of these is the Las Vegas Valley shear zone (Longwell, 1960). This zone (structural feature 13 on pl. 1) is expressed topographically by a valley that extends from Las Vegas nearly to Mercury, a distance of about 55 miles. The amount and the direction of movement along this shear zone has been estimated from structural and stratigraphic evidence to be 15 to 40 miles. Other strike- slip zones, most of which are of smaller displacement than the Las Vegas Valley shear zone, have been mapped in Death Valley, the Spring Mountains, and the Amargosa Desert and at the Nevada Test Site. Some of these faults may be structurally related to the Las Vegas Valley shear zone (E. B. Ekren, written commun., May 1966). Normal faults, numbering in the thousands within the study area, are the most common tectonic features of the region. Generally the displacement along these faults is less than 500 feet, but it is thousands of feet on some. The normal faults are responsible for the characteristic Basin and Range topography of the region. Several large anticlines and synclines occur within the area (Longwell and others, 1965; Tschanz and Pampeyan, 1961). Approximate axes bf some of these folds are shown on plate 1. These broad folds were formed before the beginning of extensive sedimentation and volcanism in the Miocene; they parallel other features of the late Mesozoic deformation and probably formed during that episode. Thrust, strike-slip, and normal faults and the folds that may influence the regional movement of ground water are shown on plate 1. Most of the structures shown were taken directly or by inference from the geologic maps of Clark and Lincoln Counties (Longwell and others, 1965; Tschanz and Pampeyan, 1961), from un- published data on the Amargosa Desert by R. L. Christiansen, R. H. Moench, and M. W. Reynolds (U.S. Geol. Survey), and from unpublished data on the Yucca Flat area by Harley Barnes (U.S. Geol. Survey). PRINCIPAL AQUIFERS AND AQUITARDS Ground-water hydrology of the region: can be most ad- vantageously discussed by grouping; the numerous 014 geologic formations and members into units of hydrologic significance. Accordingly, the 29 formations listed are grouped into 10 hydrogeologic units (table 1) in order of decreasing age as follows: Lower clastic aquitard; lower carbonate aquifer; upper clastic aquitard; upper carbonate aquifer; tuff aquitard; lava- flow aquitard; bedded-tuff aquifer; welded-tuff aquifer; lava-flow aquifer; and valley-fill aquifer. AQUIFERS Of the six aquifers listed in table 1, the lower car- bonate and the valley-fill aquifers have the widest areal distribution and are the principal aquifers within the region. In the Las Vegas Valley, only the valley-fill deposits are presently tapped for water supply because of the great depth to the underlying carbonate rocks. In parts of the Nevada Test Site where the valley fill is un- saturated or absent, the lower carbonate aquifer provides the sole source of ground water. Within the Amargosa Desert, on the other hand, some irrigation wells may tap both the valley-fill and the lower car— bonate aquifers. The remaining four aquifers have a limited occurrence within the zone of saturation, although locally, as in western Jackass Flats, the welded-‘tuff aquifer is the sole source of water. Geologic character and hydraulic properties of the aquifers vary widely. The lower and the upper carbonate aquifers and the welded-tuff aquifer store and transmit ground water chiefly through secondary openings developed along fractures. The bedded-tuff and the valley-fill aquifers, on the other hand, store and transmit water chiefly through primary or interstitial openings. LOWER CARBONATE AQUIFER The lower carbonate aquifer comprises the carbonate rocks of Middle Cambrian through Devonian age — that is, all the formations from the upper half of the Carrara Formation through the Devils Gate Limestone (table 1) .1 These carbonate strata aggregate about 15,000 feet in thickness, but as a result of the deep erosion of the deformed miogeosynclinal rocks this thickness is rarely present in any one location. The saturated thickness of the carbonate strata, which ranges from a few hundred feet to several thousand feet, is due to the combined in- fluence of geologic structure, erosion, and depth to water table. In general, however, because of the great aggregate thickness and stratigraphic position of the rocks com- posing the lower carbonate aquifer, several thousand feet of the aquifer occurs within the zone of saturation throughout most of the study area; the aquifer is com- pletely unsaturated or eroded only in the vicinity of the outcrops or buried structural highs of pre-Middle Cam- brian elastic rocks (pl. 1). ‘In southern Indian Springs valley, carbonate rocks of Mississippian, Pennsylvanian, and Permian age (Monte Cristo(?) Limestone and Bird Spring Formation) are also included in the lower carbonate aquifer. HYDROLOGY OF NUCLEAR TEST SITES CHARACTER IN OUTCROP Outcrop studies provided qualitative information on the nature of the intercrystalline (or matrix) openings, the character of fractures and bedding-plane partings, the occurrence of caverns, and the stratigraphic control of secondary openings in the carbonate strata. The intercrystalline porosity of the carbonate rocks is extremely low. Hand specimens examined ranged from fine grained to coarsely crystalline, and the calcite or dolomite crystals composing the rock were tightly in- tergrown. Vugs as much as 0.4 inch in diameter were observed in some hand specimens, but no interconnected vuggy porosity was noted. The carbonate rocks are highly fractured and locally are brecciated. Most outcrops exhibit three or more sets of joints, one or more high-angle faults, and one or more brecciated zones. The joints, and most of the faults, are high-angle fractures (fig. 4). Brecciation commonly oc- curs along faults of only a few feet displacement and does not necessarily reflect movement of large magnitude. Strike and frequency of the faults and joints vary con- siderably from area to area. Even within an area of a few square miles the strike of the high-angle faults may differ from fault block to fault block. Harley Barnes and FIGURE 4. — Intensely fractured Silurian dolomite in the Spotted Range. Bedding-plane opening above pencil. SOUTH-CENTRAL GREAT BASIN, NEVADA—CALIFORNIA; NEVADA TEST SITE associates (written commun., 1965) tabulated the fre- quency distribution of 933 high-angle faults mapped in eight 71/2-minute quadrangles covering the Yucca Flat area. The dominant strikes range from N. 20°—30° W. to N. 20°—30°E. Burchfiel (1965) tabulated the strikes of 384 faults in six different parts of the 15-minute Specter Range topographic quadrangle. The dominant strikes ranged from about N. 75° E. to N. 45° W. Differences in character and frequency of joints in Yucca Flat were documented by Messrs. David Cum- mings, R. P. Snyder, and D. L. Hoover (written com- mun., Nov. 1963). They measured the length and orien- tation of 1,845 joints in the Tippipah Limestone of Per- mian and Pennsylvanian age (the upper carbonate aquifer of this report) in west-central Yucca Flat in a plot 300 by 400 feet (Nevada coordinates, central zone, N. 855,200 ft; E. 651,000 ft). The slope of the rock surface is about 5° eastward. The rock is a thin-bedded to very thick bedded (3—18 in.) finely crystalline limestone. Although faults are numerous in the area, only one nor- mal fault, having a length of 15 feet and a displacement of a few feet, was observed. In addition to outcrop measurements, Messrs. David Cummings, R. P. Snyder, and D. L. Hoover (U.S. Geol. Survey) studied 280 feet of core from six holes in or within 150 feet of the study plot; the holes were as much as 200 feet in depth. Their major findings pertinent to this study follow: 1. Most fractures were filled with secondary calcite, calcareous clay, or calcareous clay containing iron oxide. 2. Two major joint sets and three minor sets were mapped. The major joints ranged from 0.8 to 9 feet in length and from 0.1 to 0.4 inch in width. The minor joints ranged from 0.15 to 6 feet in length and were less than 0.1 inch in width. 3. The number of joints in core ranged from 5 to 43 per foot; the median value was 18 per foot. Dip of the fractures ranged from nearly horizontal to vertical but was generally steeper than 45°. 4. Selected stratigraphic intervals were traced from one drill hole to another, but joint frequencies and dips in cores from a given stratigraphic interval could not be correlated between holes. 5. The maximum dimension of 72 percent of individual limestone blocks bounded by fractures was less than 0.3 foot; 97 percent had a maximum length less than 0.7 foot; and 99 percent had a dimension less than 1 foot. Joint patterns in Banded Mountain in northeastern Yucca Flat and in the Ranger Mountains in southern Frenchman Flat were studied by P. J. Barosh (written commun., Aug. 1965). He distinguished both local and throughgoing joints in the Paleozoic rocks. The local joints differ in number and direction from bed to bed, and some are confined to a particular bed or group of C15 beds. The local joints generally have consistent trends in an area a few tens of feet in diameter. Most of the local joints are perpendicular to the bedding, but some are parallel to the bedding. Throughgoing joints sets described by Barosh cut many beds without change in direction or number. These joints have consistent trends within areas 50 to several hundred feet in diameter and commonly have a moderately uniform spacing of 0.5 foot to 2 feet. Most of these joints are nearly perpendicular to bedding, although some Ideally intersect bedding at oblique angles. The throughgoing joint sets are parallel to an associated fault set. Barosh also found that joint density bears a strong relationship to rock type. Fine-grained carbonate rocks have the greatest joint density of any studied. The joints generally cut this rock into blocks ranging from 1 inch to a few inches on a side. Medium-grained carbonate rocks are cut into blocks ranging from a few inches to 1 foot on a side. Coarse-grained carbonate rocks are cut into blocks commonly ranging from 6 inches to 2 feet on a side. In outcrops, subaerial chemical and mechanical weathering have increased the fracture porosity within a few feet to tens of feet of the surface. Near-surface solu- tion widens fractures and may remove the calcite that lines or fills them. Because of the influence of subaerial weathering, a qualitative estimate of the fracture porosity expected in the subsurface cannot readily be made from outcrop study. Significant differences in the degree of fracturing of the carbonate rocks above and below low-angle faults were noted. The fracturing and brecciation is most in- tense where the carbonate rocks compose segments (klippen) of the upper plate of low-angle thrust faults. Such plates crop out in several parts of the study area. R. H. Moench (written commun., Mar. 1965) described a few such plates in the unnamed hills bordering Pahrump Valley on the northwest, where faults separating the up- per from the lower plates are nearly horizontal or are parallel to the bedding of the lower plate. The rocks com- posing the upper plate are described by Moench as “thoroughly brecciated.” Secor (1962) noted numerous “landslide deposits” of carbonate-rock detritus along the southwest margin of the Spring Mountains adjacent to Pahrump Valléy. The landslide masses commonly rest on carbonate bedrock, cover earlier thrust and normal faults, and are themselves partly buried by younger alluvium. They are commonly monolithologic, are thoroughly recemented, and represent stratigraphic units of Cambrian through Permian age. Secor believed that these masses may have moved as much as 7 miles from their source. The textureof the landslide masses he described ranges from large blocks of undeformed rock, in which bedding can be traced for hundreds to C16 thousands of feet, to completely disordered angular brec- cias with average particle diameter of less than 1 inch. One slide described by Secor (1962) may be as much as 2,000 feet thick. A megabreccia of carbonate rock overly- ing Tertiary tuffs a few miles east of Shoshone, Calif. was examined by the senior author of this report. In con- trast to the thoroughly cemented landslide block described by Secor, the megabreccia east of Shoshone locally contains considerable porosity between blocks composing the deposit. In summary, outcrop evidence indicates that the klippen(?) and landslide plates of car- bonate rocks may be zones of above-average porosity and fracture transmissibility where they occur within the zone of saturation. The cited outcrop studies of joints and faults illustrate the heterogeneous nature of 'the fractures cutting the Paleozoic carbonate rocks. In many parts of Nevada Test Site, the carbonate rocks have been subjected to more intense structural deformation than the rocks described. Secondary openings are present locally along bedding planes in the carbonate rocks, but widespread develop- ment of openings along such planes is absent. Some of the bedding-plane openings may be due entirely to sub- aerial mechanical and chemical weathering, but some may be due, in part, to solution of the rock within the vadose zone or in the zone of saturation. For example, downslope slippage of strata along bedding planes has probably produced some of the openings visible along these planes. Elsewhere, solution (presumably in the subsurface) has dissolved small smooth tabular openings that are strung out along otherwise tightly closed bed- ding and joint planes (fig. 5). Veins of banded calcite, 1 to several inches thick, occur locally along bedding planes. At places, solution of this calcite has left tabular openings several feet long and of unknown depth along the bedding plane (fig. 4). In general, the openings seldom extend more than several feet along the plane. Neither stratigraphically controlled regional solution of the carbonate rocks nor significant solution below the major Tertiary—pre-Tertiary unconformity is evident. The absence of stratigraphically controlled solution is not unexpected because of the absence of disconformities marked by significant erosion in the Paleozoic rocks older than Late Devonian. No field evidence suggests that sinkholes or karst topography exist in, or were developed on, the carbonate rocks beneath the Tertiary—pre-Tertiary unconformity. Field and subsur- face evidence from a few holes indicates that the frac- tures within the uppermost part of the carbonate rocks are commonly filled by tuffaceous or lacustrine detritus derived from the Tertiary rock above the unconformity. All the carbonate-rock formations composing the lower carbonate aquifer contain small isolated caves. These caves seldom exceed 20 feet and are generally less than 10 feet in maximum dimension. In some outcrops HYDROLOGY OF NUCLEAR TEST SITES FIGURE 5. — Secondary openings along bedding and joint planes in Cambrian(?) carbonate rocks, Titus Canyon, Death Valley National Monument, Calif. Openings in middle ground, up to 10 feet long. the caves are abundant and may be found every few tens of feet within a particular stratum; in nearby outcrops of the same formation, they may be totally absent. The caves range from nearly rectangular to roughly spherical. Some have small openings that widen inwardly. Locally, the caves tend to develop along major fault zones, but fault zones seemingly do not characteristically control formation of these caves. Most of the small caves examined probably originated as weathered-out joint or fault blocks. The character- istic smallness of the caves and their lack of interconnec- tion with adjacent caves probably precludes a solution origin. The most convincing evidence that the small caves are not due to solution is their general absence on dip slopes, although they are locally well developed on cliff faces of a given formation. Mechanical and sub- aerial chemical weathering on cliff faces can readily dis- lodge blocks outlined by steeply dipping joints, whereas such dislodgement is much more difficult on dip slopes. If the caves were of solution origin they should be visible on dip slopes. Caves with openings smaller than their in- terior dimensions may be solely due to solution; these caves cannot be readily ascribed to weathering out of joint blocks. In contrast to the many unconnected caverns of minor dimension seen in outcrop at the Nevada Test Site, Devils Hole and Gypsum Cave represent two major solu- tion features developed within the carbonate aquifers. Devils Hole is a water-filled funnel-shaped cavern at Ash Meadows, in the SWWSE 1/; sec. 36, T. 17 S., R. 50 E., about 23 miles southwest of Mercury (figs. 1 and 34). The cavern is at the south end of a ridge composed of the Bonanza King Formation. At ground level, the SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE northeastward-trending opening is about 70 feet long and 30 to 40 feet wide. At water surface, about 50 feet below the general land surface, the pool of water is about 40 feet long and 10 feet wide (Worts, 1963). Since 1950, speleologists using scuba equipment have explored the cavern several times (Halliday, 1966, p. 273). A sketch map of the cavern (Halliday, 1966, p. 281) shows several rooms and passageways. The speleologists reported that at depth the cavern follows a fault having a dip of 70° and a width of about 20 feet (Worts, 1963). Intensive search in July 1965 for two missing scuba divers in- dicated that the solution passages probably extend more than 315 feet beneath the pool level (Las Vegas Sun, June 23, 1965), or more than 365 feet below the land sur- face. Devils Hole seems to be structurally controlled (Worts, 1963) by a nearly vertical fault, which strikes about N. 40° E. The major fault zone, exposed along the entrance to the cavern, is as much as a yard wide and consists of a breccia of carbonate rock completely cemented by calcium carbonate. Gypsum Cave, a world-famous archeological site, is in the SE14 sec. 11, T. 20 S., R. 63 E., about 13 miles east- northeast of Las Vegas. A detailed description of this cave and of its archeological significance was presented by Harrington (1933). Gypsum Cave was formed in car- bonate rocks of Permian age. The cavern received its name from deposits of selenite crystals, which are abun- dant in one of the rooms. The cave mouth is in a low limestone spur and is about 150 feet above the adjacent bajada. The entrance is about 70 feet wide and about 15 feet high. Overall, Gypsum Cave is about 300 feet long and as much as 120 feet wide in its widest part. In addition to Devils Hole and Gypsum Cave, several other large caves in the Spring Mountains and one near the crest of Worthington Mountain east of Penoyer Valley (Sand Spring Valley) have been reported. The size and extent of these caves are unknown. CHARACTER BASED ON CORES, DRILLING RECORDS, AND GEOPHYSICAL LOGS The subsurface character of the lower carbonate aquifer was studied through microscopic (binocular) ex- amination of cores and cuttings, laboratory measure- ment of the intercrystalline porosity and permeability of cores, and examination of drilling-time logs and selected geophysical logs. Study of cores and cuttings, as a guide to the type and amount of secondary porosity, was sub- ject to the limitations common to most subsurface geologic work, namely: (1) In most holes the footage cored was only 5 to 90 percent of the hole depth; (2) ma- jor openings along fractures or bedding planes are not sampled by coring and vuggy openings covering an area equal to or greater than the diameter of the hole are com- monly destroyed during coring; (3) open or vuggy frac- C17 tures successfully cored are not necessarily connected to other open fractures in the formation; and (4) cuttings generally are poor indicators of secondary porosity. Despite these limitations, examination of cores and cut- tings and the laboratory analyses of the cores yielded valuable qualitative information on the subsurface character of the aquifer. Intercrystalline, or matrix, porosity and permeability of cores from formations composing the lower carbonate aquifer are extremely low, as shown in table 2. The total- porosity data were obtained by comparing bulk density and grain density of the cores and, therefore, include the sum of the unconnected, or isolated, intercrystalline porosity and the effective, or interconnected, porosity. The effective porosity was determined by the mercury injection or water-saturation method. Permeability tests were run using Denver, Colo. tap water. Although the number of cores analyzed is small, they represent several hundred feet of cores (from eight test holes) examined both megascopically and microscopically. Several dozen additional measurements of total porosity of outcrop and core samples (not included in the tabulation) confirm that the total-porosity values tabulated are represen- tative. Four types of fractures were distinguished in the cores: (1) fractures filled with breccia or clayey gouge; (2) slickensides; (3) fractures sealed with calcite, dolomite, or other minerals; and (4) fractures partly filled with calcite or dolomite. The first three types of fractures con- tained little or no effective (interconnected) porosity, although they commonly contained isolated vugs. Frac- tures filled wtih breccia or clayey gouge are probably fault planes. Breccia generally consists of angular car- bonate rock fragments densely cemented by calcite or dolomite. Vuggy porosity is locally present in breccias, but in general the vugs are isolated. Breccias, though not common, filled the thickest of the sealed fractures observed; zones ranged in thickness from a few inches to many feet. Fractures filled with clayey or shaly gouge were common in intervals of interbedded carbonate rock and shale (fig. 6). The effective porosity of the gouge- filled fractures is probably moderate, but the permeability of the gouge is no greater than that ex- pected in shale or clay. The slickensides were tightly sealed or contained a fill- TABLE 2. — Intercrystalline porosity and permeability of cores from lower carbonate aquifer, Nevada Test Site and vicinity [Total porosity, in percent, determined from grain and bulk densities; effective porosity, in percent, determined by mercury-injection or water'saturation methods; permeability, in gallons per day per square foot, determined using Denver, Colo, tap water. Analyses by US. Geol. Survey, Denver, Colo] Porosity or Number of permeability samples Range Median Mean Total porosity ________ 16 0.4—]2.4 5.5 54 Effective porosity ______ 25 0.0—9.0 1.1 2.3 Permeability _________ 13 0.00002—0.1 .00008 .01 C18- FIGURE 6. — Structure in cores of Pogonip Group, well 88-66, Yucca Flat. Left core (depth 2,980 ft), folded limestone (light gray) and shale (medium gray). Right core (depth 2,983 ft), fault breccia of limestone fragments. In both cores the fragmented limestone is tightly cemented by clayey or shaly gouge. ing of gouge material. These fractures averaged three per 100 feet of core logged. Most of them dip at angles between 25° and 90°, are generally less than 0.04 inch thick, and extend entirely across the cores. Fractures sealed with calcite or dolomite veinlets or, to a lesser extent, with quartz, iron and manganese oxides, or clay are the most abundant type of fractures in the cores. They amounted to an estimated 95 percent of all the fractures found, and in some holes the carbonate veinlets constituted an estimated 5 percent of the volume of rock cored. These fractures averaged about 10 per foot of core and ranged from 1 to 30 per foot of core. ' Most of them dip at angles greater than 45° and are less than 0.1 inch thick. They rarely attain a thickness of 0.4 inch. The larger veinlets extend entirely across the cores, whereas the smaller veinlets form complex branching or angular patterns among the larger veinlets. The veinlets are commonly offset a fraction of an inch to several inches at their point of intersection with other veinlets. Some of the fractures have only a thin crust of iron oxide, manganese oxide, or clay. These fractures appear to be joints — that is, fractures with little or no visible dis- placement parallel to the fracture plane. The fractures sealed with calcite, dolomite, or other minerals contain isolated vugs, but effective porosity is negligible. The partly filled fractures range from those containing only a thin coating of calcite or dolomite on the fracture planes to fractures filled with vuggy calcite or dolomite veinlets (figs. 7 and 8). The vuggy veinlets probably originated by solution of the carbonate veinlets that fill HYDROLOGY OF NUCLEAR TEST SITES FIGURE 7. — Sealed, vuggy, and open fractures in cores of carbonate rock from the Pogonip Group, well 88—66, Yucca Flat. Cores are from depths of 2,705 to 2,787 feet below surface. most of the fractures in the cores. Some of the open frac- tures lined with carbonate minerals also probably originated by solution along fractures, which was followed by some mineral precipitation. Other open frac- tures were probably never sealed and are in the process of being filled. That the secondary porosity in the vuggy veinlets and along some of the unfilled fractures is due to solution is suggested by the following: (1) Better development of porosity along the intersection of frac— tures; (2) variable width of the individual fractures; (3) vugs that appear to be due to solution of fossils; (4) a decrease with depth in the abundance, degree of openness, and continuity of vugs; and (5) better develop- ment of porosity in limestone than in dolomite. Partly filled fractures are less abundant than those sealed by carbonate veinlets and amount to 1 to 5 per- cent of the fractures found. They averaged about five per 100 feet of core and ranged from zero to one fracture per 2 feet of core. Fractures containing vuggy veinlets are probably twice as abundant in holes penetrating limestone as in holes penetrating dolomite. In general, the vuggy veinlets dip at high angles, are 0.02 to 0.1 inch thick, and rarely attain a thickness of 0.4 inch. Most of them extend entirely across the cores. Most of the partly filled fractures in the cores (fig. 7) were parallel, but a few intersected. Total porosity of the partly filled fractures is es- timated to average 0.1 percent and to range from 0 to 1 percent of the volume of the core. Effective porosity of the fractures with vuggy veinlets is even lower because most of the vugs are probably isolated within the matrix of carbonate vein material and are therefore either very poorly, or not at all, hydraulically connected. SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE FIGURE 8. — Sealed and vuggy fractures and slickenside with red clay in limestone core from the upper part of the Carrara Formation, well 79—69a, Yucca Flat. Core is from 1,411 feet below surface and is 21/5 inches in diameter. In general, core examination suggests that secondary porosity is better developed along fractures with minor displacement than along fault planes. Stylolites also are common partings in cores, but they are generally completely sealed with clayey residues. They averaged three per foot of core and ranged from zero to 30 per foot. They are most conspicuous and probably most abundant in limestone. They dip at low to high angles to the core axis and are probably parallel to bedding planes. Effective porosity along the stylolites appeared to be negligible. In only one test hole, well 67—68, was significant solution widening of stylolites noted in limestone. R. D. Carroll of the US. Geological Survey (written commun., 1966) used geophysical logs to estimate the fracture porosity of the lower carbonate aquifer for test wells 88-66, 73-66, and 89—68. The interpretations perti- nent to test well 88-66 were presented by Moore, Doyle, Walker, and Young (1963). R. D. Carroll estimated the total (that is, inter- crystalline and fracture) porosity of selected fractured intervals utilizing density and neutron logs in conjunc- tion with caliper logs. He also used a group of geophysical logs (density, neutron, sonic, and electric) to estimate intercrystalline (matrix) porosity in nearly un- fractured intervals adjacent to the fractured zones. He then arrived at an estimate of fracture porosity by sub- tracting values of matrix porosity from those of total porosity. His values of total fracture porosity ranged from 1 to 12 percent, and matrix porosity ranged from 1 to 6 percent. His laboratory values for matrix porosity C19 ranged from 0.2 to 4.5 percent. For nine intervals ranging in length from 7 to 300 feet, he was able to calculate both total and matrix porosity. The average fracture porosity of those nine intervals ranged from 0 to 10 percent. These values excluded data for intervals that caved and for zones containing interbedded shale and carbonate strata. In general, the zones of highest fracture porosity calculated by Carroll for selected intervals in wells 88-66, 73—66, and 89—68 corresponded with the most permeable intervals in those holes. However, in other holes highly fractured zones, delineated by the sonic logs, yielded little or no water to the bore during drill- stem tests. The high fracture-porosity values estimated by Carroll do not necessarily contradict the considerably lower effective fracture porosity estimated from the cores. First, geophysical-log analysis cannot differentiate between the effective and noneffective porosity in the fractures filled with vuggy veinlets. Second, much of the total porosity detected by the density and neutron sondes may represent highly porous gouge fillings rather than void space. Third, and this is considered by Carroll (oral commun., Feb. 1, 1967) to be the major cause of the. large differences in values of porosity, geophysical log- ging devices are inherently unable to measure accurately porosity differences of a few percent in rocks of extremely low porosity. Because of inhomogeneities in the forma- tion and in the well bore, neutron, density, and sonic logging devices tend to register high porosity values for such rocks. Values thus obtained should be considered upper limits; in rocks with low porosities, the values may be in error by several hundred percent. Drilling records do not indicate the penetration of any major caverns in the lower carbonate aquifer. By December 1966, approximately 16,000 feet of the lower carbonate aquifer had been penetrated in 26 holes drilled in 10 widely separated parts of the study area. Six thou- sand feet (about 37 percent) of this drilling was monitored at six drill sites by instruments that con- tinuously recorded drilling rates. Two test holes, well 79-69a in southern Yucca Flat and well 65—62 in northeastern Amargosa Desert, each penetrated a single void about 2 feet long. The void in well 79—69a was reported by the driller, whereas that in well 65—62 was recorded by instruments. The interval in well 65—62 cor- responded with a zone of lost circulation. Whether these voids are distal ends of solution caverns or fractures opened by solution cannot be determined. The absence of major caverns is particularly signifi- cant because about 5,000 feet, or roughly 30 percent, of the total footage of carbonate rock drilled was penetrated at 13 drill sites beneath the Tertiary—pre- Tertiary unconformity. Study of the Tertiary—pre- Tertiary unconformity in outcrop also indicated the C20 absence of any prominent solution alteration of the Paleozoic carbonate rocks beneath the unconformity. Daily records of holes drilled with mud indicate that lost circulation commonly occurred during drilling of the carbonate rock aquifers. The rate of fluid loss generally changed abruptly with depth. The abruptness of the fluid loss, the absence of caverns, and the extremely low intercrystalline permeability of the rock suggests that the mud was lost into partly filled fractures. Conversely, in some holes drilled with air a sudden increase in water entry into the bore was noted occasionally. This marked entry of water probably indicates the penetration of the first major water-bearing fracture. DRILL—STEM AND PUMPING TESTS Hydraulic properties of the lower carbonate aquifer were measured by drill-stem and pumping tests of 10 wells. These wells were drilled to depths of 1,300 to 4,200 feet by the air-rotary or cable-tool method. In these wells, the lower carbonate aquifer was tapped at depths ranging from 735 to 1,500 feet below the surface to 3,700 to 4,206 feet below the surface. The construction record of each hole is presented by Thordarson, Young, and Winograd (1967). Drill-stem tests were made in 8 of the 10 wells to identi- fy intervals containing water-bearing fractures, to detect changes in head with depth, to pinpoint the source of water collected for chemical analysis, and to determine the capacity of the pump needed for the pumping test. The drill-stem testing techniques were primarily of two types — additive swabbing tests of open, (that is, un- cased) holes and swabbing of select zones isolated by packers. However, oil field drill-stem tools of dual shut- in type (described by Dolan and others, 1957, and by Ammann, 1960) were also used. The drill-stem tests showed that the fracture transmissibility of the carbonate aquifer, as reflected by specific-capacity data, varies greatly with depth in each of the holes. Many straddle-packed intervals yielded only a fraction of a gallon per minute in response to a drawdown of hundreds of feet. Other intervals in the same hole were swabbed at a rate of 50 gpm (gallons per minute) without lowering the water level more than a few feet. None of the eight holes drill-stem tested showed a uni- form pattern of increase or decrease in fracture transmissibility, and open fractures were present as much as 1,500 feet beneath the top of the aquifer and 4,200 feet below land surface. In some holes the transmissibility increased markedly with depth; in others the most permeable zones were near the top of the zone of saturation. In test well 88—66, for example, ad- ditive tests showed a negligible yield for the Pogonip Group in the interval of depth between 2,550 and 2,703 feet; yield increased about tenfold after penetration of HYDROLOGY OF NUCLEAR TEST SITES the interval between 2,703 and 2,896 feet. Another ten- fold increase in yield was measured after penetration of the interval between 3,095 and 3,295 feet (J. E. Moore and others, 1963). In test well 75—73, on the other hand, the only permeable zone of consequence in the Pogonip Group was in the interval between 1,217 and 1,448 feet; the top 105 feet and the bottom 405 feet of the saturated rock tapped apparently yielded little or no water to the bore. The drill-stem tests of the three holes (73—66 in Rock Valley, and 88—66 and 87—62 in Yucca Flat) that penetrated the lower carbonate aquifer beneath the Tertiary—pre-Tertiary unconformity indicate negligible to moderate permeability immediately below the uncon- formity. In these holes the zones of greatest relative specific capacity are about 75, 725, and 240 feet, respec- tively, below the unconformity. Three wells drilled into the aquifer in the Amargosa Desert, NWIA sec. 27, T. 16 S., R. 51 E., penetrated an aggregate thickness of about 560 feet of the lower car- bonate aquifer. Flowmeter surveys made during injec- tion of water into these wells indicated that the wells in- tersected only a few water-bearing fractures. Of equal in- terest, the survey of one hole indicated that a depth in- terval whose core contained open fractures several feet in length was nearly impermeable. Pumping tests of 48 to 86' hours duration at constant discharge rates were made in each of the 10 wells penetrating the lower carbonate aquifer at Nevada Test site (table 3). Step-drawdown tests, at three different discharge rates, were made in four of the wells. At only one well site (79-69a) was a separate observation well (79—69) available in which to measure drawdown, and unfortunately, drawdown was not detected in the obser- vation well because of extremely high transmissibility coupled with a moderate pumping rate. Submersible-type pumps with a check valve im- mediately above the pump assembly and a second check valve several hundred feet above the first valve were used in all the wells. Water levels were measured with an electrical sonde in a 1-inch ID access line. Changes as lit- tle as 0.02 foot in water level are detectable with this in- strument. The semilog graphs of water-level drawdown and recovery for the pumping tests are characterized by a steep first limb and a gently sloping second limb (figs. 9—17), which make interpretation of hydraulic properties of the aquifer difficult. The steep initial limb usually lasts 10 to 60 minutes, gradually decreases in slope, and changes into the gentle limb, which generally persists for several hundred to more than 1,000 minutes. With in- creasing time, the gentle limb either continues declining (or rising during recovery), without detectable change in slope, or bends sharply downward (or upward during recovery). SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE C21 I/t'= TIME SINCE PUMPING STARTED/TIME SINCE PUMPING STOPPED 1 10 1000 10,000 40 I 1111111 ‘I-—+1—1—I_=.1_1_+ IIIIII I 1111111 0 I (5:1ij _ 240180 _ 60 EXPLANATION \\ 20 p. T- coefficient of transmissibility, \ I." ~ in gallons per day per foot \ T- 2:4 O — E . s . Z If, a- well discharge, in gallons per minute \ '- w 80 ‘0 >.~ : 1‘ \ . w 1 _ 51.: so so \ _ > g Fm‘ "Mb Time since pumping stopped, in minutes \ 8 _ Lu E 100 A: = 90 60 g a Slope of curve, in feet per log cycle A: 3150A E o _ \ _ I E Drawdown \ 3 5 _____ z o 120 so g Recovery \ O z o 3 a O _ _ < 0 r: 3 o E 1‘0 — First limb usually of 10 to 60 minutes duration. \ 100 .1 0 Steepness probably reflects one or more of the A' ‘ 11‘2 b g _ following: (I) Crowding of flow line near bore Second 'm _ 9 due to partial penetration; (b) crowding of flow Aa= 32 9 lines near bore due to negligible flow area in Third limb I: 150 — water—bearing fractures; (c) zone of low trans— 120 missibility adjacent to bore Second limb usually of 300 to 2000 minutes duration. Third and fourth limb, where present, reflect one or Limb used to calcualte transmissibility using both of the following. (a) Change in transmis— 00"" modified nonequilibrium formula of Jacob (1950) sibility; (b) hydraulic barrier ”Mb no 1 11111111 I 11111111 IIIIIIII | I 1111140 1 10 100 1000 10,000 TIME (”SINCE PUMPING STARTED, IN MINUTES FIGURE 9. — Diagrammatic semilog graphs of water- level drawdown and recovery during single- well constant- rate pumping test in the lower carbonate aquifer. The slope of the steep initial limb on the drawdown curves is about 5 to 20 times greater than that of the gen- tle secondary limb; on the recovery curves, the steep in- itial limb is about 10 to 60 times steeper than the second limb. A second anomalous feature of the semilogarithmic curves is the rapid rate of recovery of water level after pumping. Theoretically, the recovery of water level at the conclusion of a pump test should take as much time as the drawdown. However, in each pumping test of the carbonate aquifers, the recovery was faster than the drawdown at equal times, particularly in the time inter- val 0 to 200 minutes. The more rapid rate of recovery results in an early limb that has a markedly steeper slope than the corresponding limb on the drawdown curve. Conversely, in every well the second, or gentle, limb of the recovery curve is gentler than the corresponding se- cond limb on the drawdown curve. Similar anomalous recovery is also noticeable on semilog plots for a pump test of a fractured carbonate aquifer near Eureka, Nev. (Stuart, 1955, figs. 5 and 7). Dual-limb semilog curves resembling those for the lower carbonate aquifer have been described in the ground-water and petroleum-reservoir engineering literature for the following flow geometries: ( 1) Leakage from an overlying or underlying semipervious confining layer; (2) a sloping water table; (3) an aquifer that thickens with distance from the well, (4) turbulence at the well face (Baker, 1955), (5) aquifer damage by drill- ing, the so- c-alled skin effect, (6) partial penetration; and (7) a zone of below-average transmissibility adjacent to the bore. Only the last two flow geometries, plus a related condition, appear important during pump tests of the lower carbonate aquifer. , The configuration of the semilog curves is probably due to one or more of the following: ( 1) Three- dimensional flow near the bore due to partial penetra- tion of the aquifer; (2) a zone of reduced transmissibility, C22 HYDROLOGY OF NUCLEAR TEST SITES TABLE 3. — Pumping-test data for aquifers in Nevada Test Site and vicinity Well Stratigraphic unit Depth interval (feet) Thick- ness (feet) De th Coefficient of transmissibility’ Estimated :3 Specific (gpd per ft) ‘ . - capacity1 ‘ $33331 ”gelling” 232:; (gpm per EStimated Calculated Calculated Remarks foot of front from from (percent) (13:2) drawdown) specific drawdown recovery capacity curve curve Lower carbonate aquifer 79—69a 79—69 67473 67~68 66775 87462 84 -68d Carrara Formation (upper half). ___do _______ Bonanza King Formation (Banded Mountain Member). Bonanza King(?) Formation. Nopah(?) Formation Nopah Formation (Smoky Member). Pogonip Group _ _ - do _______ Silurian(’?) dolomite. Devils Gate Limestone and Nevada Formation, undifferentiated. Devonian(‘.’) dolo- mite and calcar- eous quartzite. 1,54071,701 1,540—1,650 1,020—1,301 1,333v1,946 7851, 168 737-1,490 2,55o3,422 1,103—1,853 3,137—3,400 3,700—4,198 2,821—3,026 161 110 281 383 753 872 750 263 498 205 14-, Unconfined 1,540 530 900,000 ____ ____ Well 79‘69a is 100 ft northwest of well 79—69. During two pumping tests neither drawdown nor recovery could be measured owing to very high aquifer transmissibility and low pumping rates (60 and 212 gpm) Carrara Formation tapped by well is probably part of upper plate of low- angle thrust fault that crops out a few miles west of well. 2 _ _ _ _ _ _ do _____ 1,540 6.2 6,000 ('0 (“) Step-drawdown analysis in- dicates specific capacity of 11.7 gpm per foot of draw- down and water entry chiefly from interval 1,607—1,623 ft. Carrara For- mation tapped by well is probably part of upper plate of low-angle thrust fault that crops out a few miles west of well. 5 Confined 839 4.8 8,000 20,000 53,000 Step-drawdown analysis in- dicates specific capacity of 113 gpm per foot of draw- down. 20 613 } _ _ _ do _____ } Stepvdrawdown analysis in- 785 6.0 6,000 39,000 86,000 dicates specific capacity of 16.7 gpm per foot of draw- down. I!) ___do _____ 737 4.5 4,000 11,000 27,000 Step-drawdown analysis in- dicates specific capacity of 10.8 gpm per foot of draw- down. 10 Confined 2,055 .4 700 1,300 5,300 Water yielded principally from interval 3,176—3,4l2 t. 10 Unconfined 1,103 .7 600 3,800 _ _ _ _ _ _ _ _ <5 Confined 1,735 30i 60,000 _ _ - _ _ _ - _ Density changes in water _ column, due to anom- alously high water temperature, completely masked water-lave flue tuations due to pumping. Air—line measurements per mitted approximation of specific capacity. <20 ___do _____ 1,968 .8 1,000 ____ 3,500 ____. Unconfined <5 -__do _____ 1,626 .4 700 2,400 __-- ____ Bedded-tuff aquifer 81—67 90774 90775 Bedded tutti?) of Piapi Canyon Group. ,‘_do ,,,,,,, ___do _______ 113854.800 490- 670 89671 ,091 180 195 , , _ _ Confined 1,569 0.9 1,000 1,300 2,100 See Hood (1961) for pumping— test details; aquifer is probably bedded tuff or nonwelded ash.flow tuff. . , A - Unconfined(?) 490 .4 200 _ - _ - - _ - - Constant~rate pumping test not made; specific capacity based on measurements made after 90 min. pum - ing; aquifer is probab y bedded tuff or nonwelded ash-flow tuff. , A , , _ _ _. do _____ 896 .6 400 _ _ _ _ _ _ _ - Constant-rate pumping test not made; specific capacity reported after 30 min. of pumping; aquifer is probably bedded tuff or nonwelded ash-flow tuff. Welded-tuff aquifer 73758 81 —(i9 Topopah Spring Member of Paintbrush Tut'f. ,__dn ,,,,,,, ,A_d() _______ 741— 887 928— 1,475 15074.67?) 146 168 40 Confined”) 741 56 100,000 See re- See re- Drawdown of 6.9 ft measured marks. marks. with air line and test pressure gage in first 3 min. of pumping test at rate of 387 gpm; additional draw down not detectable in sub- sequent 57 min. of pumping test. 100 -__do _____ 928 22 40,000 68,000 ____ Step‘drawdown analysis suggests considerable head losses at face of bore; losses are probably due to poor gun perforation of casing. I00 __-do _____ 1,507 .1 200 See re- 50 Well tested by bailing; data marks. on recovery of water level given by Moore and Garber (1962). SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE C23 TABLE 3. -— Pumping-test data for aquifers in Nevada Test Site and vicinity — Continued Depth Coefficient of transmissibility2 Depth 'l‘hick- Es‘ima‘F’d - ‘°- 12:23:35] (gpd per m Well Stratigraphic unit interval ness pg‘pggrjitggrn “giggle 33:; (gpm per Estfimated Calfculated Calfculated Remarks (feet) (feet) foot of rom mm mm (percent) (12:21]) drawdown) specific drawdown recovery capacity Curve curve Lava-flow and welded-tuff aquifers 74—61 Basalt of Lose—1,150 11 1 100 Unconfined Combined test of lava-flow Kiwi Mesa 1 039 2 5 4 000 28 000 girlid welded-tuff aquifers}; ' ' ' ’ ‘ ‘ __ easurements ma e wit 'l‘opopah Spring 1,150715129 179 4:3 Confined test pressure gage and air Member of line. Paintbrush Tui'f. Valley-fill aquifer 747701) Valley fill 689—1200 511 .51] Unconfined 689 1.7 1,000 2,400 2,500 See Hood (1961) for pumping- test details. Value of 1,700 1,700 gpd per ft from recovery during 133-day shutdown; other values from 48-hr pumping test. 7477011 _ _ _ do _______ 6837 900 217 1:3 _ _ _ do _____ 683 4.0 3,000 7,700 11,000 See Hood (11961) for pumping- test detai s. 75~72 _ _ , do _______ 714— 870 156 , , _ _ _ _ _ do _____ 714 1.3 800 v A _ _ _ _ _ _ Specific capacity and static galtler level reported by n er, 811~f§8 _ A do _______ 1,60571870 264 80 100 -__ do _____ 606 1.9 1,000 13,200 12,200 See Price and Thordarson £1961!) for pumping-test etai s. 91774 _ _ _ do ,,,,,,, 107- 371 264 101) _ _ _ do _____ 107 12 10,000 _ - _ _ 33,500 Specific capacity after about 21 hr of pumping; driller‘s 1ng indicates mostly clay e ow 235 ft. Ell 314,, A , _ do ,,,,,,, 114— 542 428 100 _ z - do _____ 114 30 30,000 _ _ _ - _ _ _ _ Specific capacity and static water level reported by driller; driller’s log suggests chief aquifer in depth inter- val 114—200 ft. ‘ Specific capacity computed at 100 min. of pumping. several tens to hundreds of feet thick, surrounding the bore; or (3) crowding of flow lines, within several tens to a few hundred feet of the bore, due to small cross- sectional flow area in the water-bearing fractures tapped by the bore. Any of these three flow conditions would result in abnormally high time-dependent pressure losses near the bore, which would be reflected by the steep initial limb of the time-pressure curves. Near a partially penetrating well, the flow lines con- verge on the bore not only radially but also spherically ; this convergence, or crowding, of the flow lines results in head losses that greatly exceed those for radial flow. For example, Muskat (1937, fig. 77) indicated that only 40 percent of the head loss of radial flow occurs within 5 feet of the bore, but as much as 95 percent of the head loss of spherical flow (near zero penetration) occurs within 5 feet; intermediate head losses (not illustrated by Muskat) occur for penetrations between 0 and 100 per« cent. Muskat’s figure 77, when replotted on semilog paper, closely resembles the dual-limb time-pressure curves of the carbonate aquifer. The steep initial limb reflects the abnormal head losses near the bore, whereas the secondary limb reflects aquifer conditions farther from the bore, where flow is more nearly radial. A dimen- sionless semilog plot for partial penetration presented by Hantush (1964, fig. 11) also closely resembles the plots for the lower carbonate aquifer. Hantush’s illustration -‘ See text for discussion of methods of computation of coef- ficient of transmissibility, in gallons per day per foot (gpd per ft), and time-drawdown curves (figs. 10—17 and 22—29) for per- tinent construction data and length of pumping tests. ‘Time-drawdown curves (fig. 14) indicate a positive boun- dary of very high transmissibility at 35 min.; the “zone” of high transmissibility is probably that tapped by adjacent well 767698. indicates that after a certain time has elapsed, the slope of the gentle, or second limb of the curve approaches that for a fully penetrating well, except where well penetra- tion is nearly zero. Time-pressure curves by Nisle (1958) for a partially penetrating well tapping a confined aquifer also closely resemble those for the lower car- bonate aquifer. From Nisle’s mathematical study he concluded that the standard modified nonequilibrium approach of Jacob (1950) could be used to compute aquifer transmissibility directly from the second limb of the curve. Nisle’s study is of particular importance because the time—pressure curves he presented are for the pumped well, whereas Hantush’s curves present time-pressure behavior for observation wells only. Geologic sections through each well test pumped at Nevada Test Site suggest that the wells penetrate 5 to 20 percent of the aquifer (table 3). A zone of reduced transmissibility adjacent to the bore is a second possible cause for the dual-limb time- pressure curves. Loucks and Guerrero (1961) presented many semilogarithmic curves showing the effect of vary- ing the radius of a zone of reduced transmissibility and of changing the ratio of the transmissibility of the aquifer and of the zone adjacent to the bore. They assumed radial-flow geometry. Their curves closely resemble those for the lower carbonate aquifer. The steep initial limb of their curves reflects the low C24 HYDROLOGY OF NUCLEAR TEST SITES t/f'= TIME SINCE PUMPING STARTED/TIME SINCE PUMPING STOPPED 1 10 100 1000 10.000 10 I l | 1 I I I I I | I I I I I I | | I I I I I I I I I 0 ’I As'= 1.5 4 _ 280 2 0 I x x\ _ xx (264) (154) . 35 "xxX r= —1 5 . = 27,000 gpd per ft xx ‘ 15 . x 5 o x E _ X EXPLANATION Lu 0 x “- E o x T= coefficient of E u, 20 . transmissibility — 10 >3 “- I: E . X 0=154 gpm I; . _ — o 0 z ' 3 ‘ X g 25 ° . 280 _ 15 It 3 . Time since pumping stopped, (29 n. in minutes E 0 — o — z - x 3 E . o O 3 ’0. Drawdown 2 CI 30 . — 20 3 s. 0 Z 0. x x D E _ 5.. Recovery A g (264) 154) < g ”'5. T=—3(7 =11,0009pdp0rft E . . g 35 \‘u x 25 .1 0 § __ Formation tested: Smoky Member of Nopah Forma- Pump discharge 9 tion was adjusted _ 3 ‘Interval tested: 737—1,490 feet (unconfined). Packer I I I: 40 — tests indicate intervals 1,093—1,184, 1,234—1,284, k 30 and 1,388—1,488 feet yield little or no water Percent penetration: 10 percent (estimated) _ Type of completion: Machine slotted 8 5/8— and — 7 5/8—inch—OD casing and open hole 45 I I [ I l l l I | I I I I I I I I l I I I I I I I I I I I I I L 35 1 10 100 1000 10,000 TIME (t) SINCE PUMPING STARTED, IN MINUTES FIGURE 10. — Semilog graphs of drawdown and residual drawdown of water level during pumping test in test well 66—75,,September 11—13, 1962. ' transmissibility of the inner zone, whereas the gentle limb reflects that of the outer zone, the true formation transmissibility. The transition between the two limbs is gradual as it is in semilog curves for the lower carbonate aquifer. A zone of reduced transmissibility adjacent to— some of the wells tapping the lower carbonate aquifer appears possible owing to the probable heterogeneity of the water-bearing fractures. However, the presence of such a zone adjacent to eight of nine wells in which draw- down was measurable seems unlikely. A third possible cause for the abnormally high head losses shown by the first 60 minutes of the time-pressure data is the very small cross-sectional flow area in the water-bearing fractures near, and tapped by the bore. Within several tens to a few hundred feet of the bore, the flow lines may converge toward a single fracture or perhaps toward a few fractures that connect with the bore; the resulting head losses may therefore be con- siderably greater than those near a hole tapping an aquifer producing from interstices. These head losses are hereafter called head losses due to ramming, or ramming effect, to emphasize the physical crowding of the flow lines in the fractures near the wells. Head losses due to the postulated ramming effect are expectable in most wells tapping the lower carbonate aquifer. The relative importance of partial penetration, a zone of reduced transmissibility, or the ramming effect on the pressure losses near each well site cannot be determined with available data. At some sites, all three factors may be operative. For example, the head losses that result from a zone of reduced transmissibility and from the ramming effect should apply not only to radial flow toward a fully penetrating well but also to three— dimensional flow toward a partially penetrating well. Of the three flow geometries, partial penetration probably occurs at each well site. Ramming effect also probably occurs at most well sites owing to the small cross- sectional flow area in the water-bearing fractures. Par- SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE C25 t/t'= TIME SINCE PUMPING STARTED/TIME SINCE PUMPING STOPPED 1 10 100 1000 , 10,000 5° I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I 0 A: = 3.8 _ (264) (455) ‘ 2,st T= T: 6,000 gpd per ft 0 55 I “My .. 5 350 _ . 105 l X x x x — E I- 9 x E Lu u, so 10 >‘ IL . x 0'1 2 IL! 1 — — S 0 o g a g 55 . EXPLANATION _ 15 g D l . T= coefficient of transmissibility E 0 — _ E . 0:455 gpm 3 I z 3 70 ° _ 0 . (264) (455) 20 (a) Z . 7'= T = 39,000 god per ft 350 0 c3) _ \' . Time since pumping stopped,_ 3 D A: = 3.1 in minutes I: 3 . o 75 — _I I: N 25 ‘3 Formations tested: Nopah (?) Formation and Bonanza Drawdown g _ King(?) Formation 785—1,l68 feet (unconfined) ' x _ 9 and 1,333-1,946 feet (confined). Packer tests Recovery 3 indicate intervals l,047—1,260, 1,555—l,6l3 and . As= 6.6 I 80 * l,626—l,674 feet yield little or no water to bore x 30 Percent penetration: 20 percent (estimated) _ Type of completion: Gun-perforated 10 3/4- inch—OD 0y casing opposite upper aquifer; open-hole completion _ of lower aquifer A: _ 19 85 I Illlllll I IIIIIII I I IIIIII llIIII35 1 10 100 1000 10,000 TIME (1’) SINCE PUMPING STARTED, IN MINUTES FIGURE 11. — Semilog graphs of drawdown and residual drawdown of water level during pumping test in well 67-68, September 11—14,‘ 1962. tial penetration and ramming appear chiefly responsible for the steep initial limb of the time-pressure curves. Whatever the exact cause of the steep initial limb, the gentle second limb represents aquifer conditions at a greater distance from the bore than that represented by the steep limb. The authors cannot explain the abnormally rapid recovery rate, nor did they find an applicable explana- tion in the literature. The anomalous recovery rate does not appear to be due to the entry of lower density water into the well during the pumping period. Such a density decrease, which would result in a water column longer at the end than at the start of pumping, could be due to warming of the water column during pumping and (or) to entry of gas into the water column. Differences in am- bient water temperature between the top and bottom of the water column in wells at the test site have been measured and are as much as 35°F, though they are usually less than 20°F. Such temperature differences, when coupled with water-column lengths of 500—1,400 feet, are probably responsible for post-pumping static water levels which were higher than pre-pumping levels in wells 84—68d and 88—66 (Moore and others, 1963, table 6; D. B. Grove, written commun., Oct. 1, 1967). However, temperature changes alone cannot explain the rapid recovery of water level, which in some wells (84—68d and 88—66, figs. 15 and 17) was as much as 40 feet greater than drawdown at comparable times. Some mechanism other than temperature-induced density changes must be operating. Outgassing has not been noted in the sampled waters. Until the significance of the anomalous recovery rate is understood, the authors elect not to utilize transmissibility values obtained from the recovery curves. The constant slope of the second limb of the semilog curves (figs. 10—17) suggests that the fracture transmissibility of the lower carbonate aquifer may be homogeneous on a gross scale even if locally non- homogeneous. Within the time interval (as much as 2,000 min.) represented by the gentle limb, no promi- C26 ,HYDROLOGY OF NUCLEAR TEST SITES t/r'= TIME SINCE PUMPING STARTED/TIME SINCE PUMPING STOPPED 1 10 100 1000 10,000 30‘ I llIIIII | I IIIIII l(264)I(I)IIIIIl I IIIIIIIO A = “I, w.‘—-s * ‘ . - - _ T=—2— = 53,000 gpd per ft _ N- ._ 300 f ’1 x x X 14 4o . x 10 " I; — EXPLANATION _ E l- . T= coefficient of transmissibility z w x _ 3 5° 0: 400 gpm 20 > (I Z 0 Lu _. _ l _ > 0 300 O z . . . 0 .. o Time smce pumping stopped, “J 'L 60 ' - c 30 “3 2 in minutes 3 o ’( 0 a. . g 0 _ e Drawdown _ g 2 E o 0 X 3 2 0 70 Recovery x ‘0 E Z . o a — - 3 Q . o g 3 0.. , D E 8° . . (264) (400) 5° " 0 = ——= < . . . T 52 20,000 gpd pm ft x g _ Formation tested: Banded Mountain Member of AI = 5-2 _ 2’ Bonanza King Formation K . n I: 90 —Interval tested: 1,021—1,301 feet (confined at well); v . k ' ' a 60 interval l,020—l,217 feet yields little or no water M'""'.'°"°" (9f , . measuring dovnce Percent penetration: 5 percent (estimated) - Type of completion: Machine slotted 13 3/8 — and 8 5/8—inch—OD casing 100 I I l I I | l I l l I l l | I I l l I | l I I | | | I l | | | l 70 1 10 100 1000 10,000 TIME (t) SINCE PUMPING STARTED, IN MINUTES FIGURE 12. — Semilog graphs of drawdown and residual drawdown of water level during pumping test in well 67—73, February 24—26, 1963. nent changes in slope could be detected even on expanded-scale plots of the time pressure data shown in figures 10—17. The absence of such breaks in slope may suggest that the water-bearing fractures are reasonably well connected and that fractures of differing transmissibility, if present, are randomly distributed. Or, stated differently, after a large volume of aquifer is sampled by pumping, the areas of low fracture transmissibility that may have been intersected are ap- parently balanced by regions of high transmissibility; the resultant time-pressure curve resembles that of a grossly homogeneous aquifer. That this often cited qualitative explanation is experimentally sound was proven through model studies of fractured aquifers by Warren and Price (1961) and Parsons (1966). The work of Nisle (1958) indicates that transmissibility of partially penetrated and confined isotropic aquifers can be determined by applying the slope of the second limb of the time-pressure curves to the modified nonequilibrium formula of Jacob (1950). The coefficients of transmissibility calculated using the modified nonequilibrium formula are listed in table 3. To estimate the coefficients of transmissibility of four wells having no clearly defined drawdown curves, specific-capacity data were applied to a conversion chart relating these two parameters (Walton, 1962, fig. 4). The coefficients of transmissibility derived from the drawdown curves of six wells range from 1,300 to 39,000 gpd per ft. Their median value is about 7,000 gpd per ft, and their mean value is 13,000 gpd per ft. The coef- ficients of transmissibility derived from the secondary limb of the recovery curves range from 1.7 to 2.5 times those computed from the drawdown curve (table 3). The difference is due to the anomalous recovery rate cited earlier. These values are considered inaccurate. Coefficients of transmissibility estimated from the specific-capacity data of 10 wells range from 600 to 900,000 gpd per ft. Their median value is 5,000 gpd per ft, and their mean value is about 100,000 gpd per ft. The ,mean value is strongly influenced by the very large SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE C27 IIIT IITIITIII I Ilillll EXPLANATION T: coefficient of tnnsmissibility a: 30 gpm 10 20 O Drawdown Formation tested: Pogonip Group Interval tested: 1,103—1,853 feet (unconfined). Packer tests indicate interval 1,217—1,448 feet is chief producing zone Percent penetration: 10 percent (estimated) Type of completion: Machine slotted 7-inch—OD casing to 1,516 feet; open hole below DHAWDOWN DURING PUMPING TEST, IN FEET IIlI T lIIIIIlI = (264) (so) = 3,800 99¢ per A: = 2.1 ° C llllllll llllllll 2.1 100 1000 TIME (1‘) SINCE PUMPING STARTED, IN MINUTES FIGURE 13. — Semilog graph of drawdown of water level during pumping test in test .well 75—73 (Frenchman), May 9, 1962. 10. I lllllll llllllll IIIIIIII I [Illl '_ ° . EXPLANATION _ I I- o . 3 2° . . Drawdown u. 2 ° . ° Formation tested: Carrara(?) Formation, '_ _ . . ' . upper limestone part — g . . pumping rate: 200 ppm; specific Interval tested: 1,540—l,650 feet (un— ; . . ' . capacity 6.2 9pm W n confined); interval l,607—l,623 feet 2 3° . . . . chief producing zone _ 3 ' 0 ° ° ' " ' ' Percent penetration: Unknown IL 0 _ . . Type of completion: Machine slotted E . . 16 5/8—inch-OD casing s . - - 0 - D . . Pumping rate: 255 9pm; specific g e . capacity 5.5 ppm per ft 0 _ ° . . ' o o e 0.0. fl 0 e 3 . : so 0 O O — . Pumping rate: 300 9pm; specific —- ' . . capacity 5.1 pm per ft 1 lllllll |.7’999999 | IIIIIII I I llllll 60 1 10 100 1000 10,000 TIME (1) SINCE PUMPING STARTED, IN MINUTES FIGURE 14. — Semilog graphs of drawdown of water level for three different rates of pumping in well 79—69. specific capacity of a single well (79—69a). The magnitude of error in estimating coefficients of transmissibility from specific capacity is shown by the data of table 3. Nevertheless, in the absence of draw- down curves for four of the wells (table 3), such a value is useful to obtain minimum values of transmissibility. For the six wells with clearly defined drawdown curves, the coefficients of transmissibility estimated from the C28 HYDROLOGY OF NUCLEAR TEST SITES r/t’= TIME SINCE PUMPING STARTED/TIME SINCE PUMPING STOPPED 1 10 100 1000 10,000 0 T I IIIIII I I I’fI'V'IIIT ”‘x' I IIIIII I I I IIIII o X P 61 27 x _ EXPLANATION I . 12 " E ‘0 _ T: coefficient of transm' ‘"'ty ‘0 m X I— . a: 76 gpm g a _ ° — ~ u. " E 0 E 61 m . 80 - . . . so > 0 Tlme since pumping stopped, ‘ 8 E ' in minutes W n. _ o _ I n. ° Drawdown E 0 120 " 120 I: E o x x g I ' Recovery 2 3 _ o x _ o . , E 7 g . T= l————26‘;(2 6) = 2,400 gpd per ft X D o 160 -. . ' x 160 5 D ' e . I E k A: = 8.2 D g Formation tested: Devonian(?) dolomite and calcareous nuartzite N i Interval tested: Dolomite, 2,821—2,924 ft; quartzite, 2,924—3,026 ft; aquifer con- 8 200 — fined. Flowmeter survey while pumping indicates major inflow from calcareous 200 a quartzite E _ Percent penetration of lower carbonate aquifer: Less than 5 percent (estimated) Type of completion: Open 6-inch hole; overlying tuft” aquitard cased and _ cemented off 240 I lllilllI I lllllllI I lllllll l llllll 240 1 10 100 1000 10,000 TIME (1) SINCE PUMPING STARTED, IN MINUTES FIGURE 15. — Semilog graphs of drawdown and residual drawdown of water level during pumping test in test well 84—68d, November 21—23, 1966. specific capacity data are about V6 to 1/2 of the values derived from the second limb of the drawdown curve (table 3). That the wide range in transmissibility (table 3) need not be randomly distributed areally but rather is struc— turally controlled is suggested by the tests of wells 79—69 and 79—69a in southern Yucca Flat and of three wells drilled in northwestern Amargosa Desert (in NW M: sec. 27, T. 16 S., R. 51 E.). These five wells tap the lower car— bonate aquifer in the upper plate of low—angle thrust faults. Specific-capacity data for these wells suggests minimum coefficients of transmissibilities that range from 25,000 to 900,000 gpd per ft. The transmissibilities of these wells probably reflect the intense fracturing and disaggregation common to thin thrust plates in outcrop and shown by the cores from these wells. These two groups of wells also suggest that the lithology of the aquifer (dolomite versus limestone), at least locally, is subordinate to structure as a control on transmissibility. Wells 79-69 and 79—69a tap limestone strata of the Carrara Formation in the upper plate of a thrust fault. But at the Amargosa Desert well sites, the limestone of the Carrara Formation occurs only in the lower plate of the thrust; the Bonanza King Formation occurs in the upper plate. The major fracture transmissibility exists in the dolomite strata of the Bonanza King Formation. Of the wells listed in table 3 (except wells 79—69 and 79—69a), six tap dolomite, dolomite and limestone, or dolomite and quartzite, and only two tap limestone; hence, no relationship of transmissibility to lithology can be drawn from these well data. No attempt was made to determine the storage coef- ficient of the lower carbonate aquifer from the pump-test data. Such a determination is of questionable value even when obtained from a fully penetrating pumped well (Ferris, 1959). However, core examination suggests that the effective fracture porosity of the lower carbonate aquifer is probably a fraction of 1 percent; accordingly, the storage coefficient under unconfined conditions is not likely to exceed 0.01. Because of the extremely low effective porosity of the carbonate rocks, the specific storage under confined conditions is governed by the bulk modulus of compression of the rock and probably ranges between 10‘5 and 10‘6 per foot. On the other hand, where the aquifer is several thousand feet thick the storage coefficient may be as large as 10‘3. SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE r/r’s TIME SINCE PUMPING STARTED/TIME SINCE PUMPING STOPPED 1 10 100 1000 10,0” 20 I “"I If I l A F 7T3 l | I I I I I I I I I | I I I I I I I I l 0 l s . I (264) (so) _ mo I x.,. h 7.3 ‘ 3.500 '9‘ 9" " EXPLANATION _ 0 ‘ x \ ’- ‘5 ,, T= coefficient of w ‘0 ,, transmiuibility 20 If ’_ . O = 98 [pm 5 m _ _ . II. I f E z . w _~ 00 x 16 _ U > a ‘ Time aince pumping stopped, 8 E . in minutes '3“ a. Drawdown E U 80 __. z . x , '° 3 I 3 _ ’ Recovery _ g n 0 § ° o o 100 x N E g ' Formation tested: Devils Gate 3 ( _ a Limestone and Nevada Forma- _ _J g . tion < 0 Interval tested: 3,700—4,l98 feet 3 12° .. (confined) — 1W 5 0...... " Percent penetration: < 20 percent : _ .0“. ~. (estimated) "k” run“) 7 Type of completion: Open ._ .." " 7 SIB-inch hole 140 I lllllll 1 I IllllI I llIllll 1 |llllI|12° 1 10 mo 1000 10. TIME (I) SINCE PUMPING STARTED, IN MINUTES FIGURE 16. —— Semilog graphs of drawdown and residual drawdown of water level 'Iuring pumping test in test well 87-62, August 10—11, 1962. Two additional features of the semilog curves of the pump-test data warrant brief mention. The semilog curves of five wells (wells 67—68, 67—73, 75—73, 84—68d, and 88—66) exhibit a third or fourth limb in addition to the steep initial limb and the gentle secondary limb. (For some of these wells the additional limbs are discern- ible only on expanded-scale plots of figs. 10-17). These limbs, where present, have a steeper slope than the se- cond limb. This condition suggests a lateral change in aquifer character. The steeper limbs reflect a reduction in aquifer transmissibility, which may be due to one or more of the following: (1) A hydraulic discontinuity within the aquifer, which may reflect either termination of the aquifer along a major fault or the presence within the aquifer of a gouge-lined fault zone; (2) thinning of the aquifer near structural highs; and (3) a lateral decrease in fracture 'transmissibility unrelated to aquifer thickness. Whatever the cause, these limbs, in contrast to the second, suggest a marked decrease in aquifer transmissibility. Similar limbs are shown by Stuart (1955, figs. 5, 9, and 11) for tests of Paleozoic carbonate rocks near Eureka, Nev. Evidence that fault zones, rather than solution- widened joints, are locally the principal water-bearing fractures in the lower carbonate aquifer is suggested by the pump test of one well in the study area and by a pump test of the Fad shaft in the Eureka mining district, Eureka County, Nev. The air-rotary method was used to drill well 67-68 through the Nopah(‘?) Formation (saturated in the depth interval 785 to 1,168 ft below land surface). Though the bore penetrated no caverns and no mud was used to drill this part of the hole, water pumped from the hole during the first tens of minutes of the first of a series of pump tests was muddy. In addi- tion, the specific capacity of this well (not the transmissibility) increased during later pumping tests. Both the muddy water and the increase in specific capacity are probably due to the washing out of gouge from a fault zone rather than to the production of water from a joint. A similar discoloration of water and atten- dant increase in specific capacity was reported by Stuart (1955) during a pumping of the Fad shaft after penetra- tion of a fault zone in a drift 2,250 feet below land sur- face; other faults in the same mining district appear to be barriers, however (Nolan, 1962, p. 57—61). In summary, the drill-stem and pumping tests yielded the following significant information on the lower car- bonate aquifer: 030 HYDROLOGY OF NUCLEAR TEST SITES t/!'= TIME SINCE PUMPING STARTED/TIME SINCE PUMPING STOPPED 1 10 100 1000 10,000 4° I I I I I I I I I I I I I I T T I I I I I I I I o x x X Ar= 3 '— 240 l xx — . ‘80 so I “‘8‘ (264) (60) - p 32 x T= T = 5,300 gpd per ft x . ‘0 l 20 O " I- x In __ __ III 0 x u. I; E - w so ' " 40 g u. . I, z _ ‘ _ g ‘ I 0 E o x 3 g 100 so '1 . 0 3 z o. . x - (9 ~— 0 _ a: z 0 3 _ . O I ‘ EXPLANATION '. z 3 120 — -, so 2 z T: coefficient of '. (264) (60) 8 g transmissibility ' T = —12— = 1,300 gpd per ft 3 0 _ T < g 0 = 60 gpm \ A 12 x g ,= E 140 I h.“ 100 .I a < 32 3 —Time since pumping stopped, ‘4 a . . u: m minutes Formation tested: Pogonip Group II 160 .—— o —— Interval tested: 2,550-3,422 ft (confined) 120 Drawdown Percent penetration: 10 percent (estimated) Type of completion: Slotted 6 S/8—inch—OD _ " casing; open principally to interval 3,176— _ Recovery 3,412 feet 180 l | | l l | l l | I l l I l l | I I l l I I l I I I 140 1 10 100 1000 0,000 TIME (t) SINCE PUMPING STARTED, IN MINUTES FIGURE 17. — Semilog graphs of drawdown and residual drawdown of water level during pumping test in well 88-66, March 16-20, 1962. 1. Water-bearing fractures are sparse, but they are open to depths of at least 1,500 feet beneath the top of the aquifer and up to 4,200 feet below land surface. There is no apparent decrease in fracture yield to this depth. 2. Transmissibility of the carbonate aquifer beneath the Tertiary-pre-Tertiary unconformity is not above average. 3. The coefficient of transmissibility of the aquifer ranges from about 1,000 to 900,000 gpd per ft. 4. Eight of 12 pumping tests (including four made at Eureka, Nev.) indicate that the aquifer is cut by one or more negative hydraulic boundaries. UPPER CARBONATE AQUIFER The Tippipah Limestone of Pennsylvanian and Per- mian age is the upper carbonate aquifer of this report (table 1). It is 3,600 feet thick, but it has been eroded from most of the study area. The upper carbonate aquifer is saturated only beneath the western one-third of Yucca Flat at altitudes below 3,800 feet; elsewhere in the study area these carbonate rocks are absent beneath the valleys or occur in ridges well above the regional water table. In western Yucca Flat the upper carbonate aquifer is separated from the lower carbonate aquifer by as much as 8,000 feet of the upper clastic aquitard, the Eleana Formation. Carbonate rocks equivalent, in part, to the Tippipah Limestone occur within the zone of saturation at and near the village of Indian Springs in southern Indian Springs Valley. These rocks (Bird Spring Formation) are tentatively considered to be part of the lower carbonate aquifer, despite their stratigraphic position, because the strata equivalent to the Eleana Formation are very thin in this area and are principally of carbonate lithology. The term “upper carbonate aquifer” is restricted to Yuc- ca Flat. As of December 1966, no hydraulic tests had been made of this aquifer. However, outcrop examination and exploration by shallow core holes indicate that its water- SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE bearing character is probably similar to that described for the lower carbonate aquifer. The upper carbonate aquifer, though of limited poten- tial value as a source of water supply for part of western Yucca Flat, does not play a role in the regional move- ment of ground water beneath Nevada Test Site because its saturated extent beneath the regional water table is restricted to small areas. WELDED-TUFF AQUIFER The welded-tuff aquifer includes the Topopah Spring and Tiva Canyon Members of the Paintbrush Tuff and the Rainier Mesa and Ammonia Tanks Members of the Timber Mountain Tuff (table 1). The Paintbrush Tuff and the Timber Mountain Tuff compose the Piapi Can- yon Group defined by Orkild (1965). Only the Topopah Spring Member has been tapped by wells within the zone of saturation. As of December 1966, this tuff was the sole aquifer used for water supply in Jackass Flats. The welded-tuff aquifer occurs throughout much of the Nevada Test Site, but it is a potential source of water supply only in the structurally deepest part of the inter- montane basins, where it occurs within the zone of saturation. CHARACTER IN OUTCROP Physical characteristics of ash-flow tuffs that could affect the movement of ground water include jointing, relation of joint density to degree of welding, horizontal partings within the flows, nature of the basal and the up- per parts of the flows, and vertical variation of in- terstitial porosity and permeability within a flow. These characteristics are largely a result of the mode of origin of the ash-flow tuffs. The general discussion of the origin of these rocks and the terminology used were extracted from the works of Ross and Smith (1961) and Smith (1960). Ash-flow tuffs are the consolidated deposits of volcanic ash emplaced through flowage of a turbulent mixture of gas and pyroclastic materials. The deposits consist principally of glass shards and pumice fragments usually less than 0.15 inch in size, although some flows consist of ejecta of coarser size. The deposits are characteristically nonsorted and exhibit no bedding; this characteristic contrasts with the generally pronounced bedding of ash-fall tuff deposits. Some ash flows are only a few feet thick, but in general they are tens, and some are hundreds, of feet thick. After emplacement of an ash flow, compaction or welding of the ash may result in an average 50-percent reduction in porosity of the original flow. Welding is defined as the process that promotes the union or cohe- sion of the molten glassy shards and pumice. Welding within a single ash flow is variable. Smith (1960) distinguished three zones of variable thickness in C31 most ash flows: the zone of no welding, the zone of par- tial (or incipient) welding, and the zone of dense welding. The zone of dense welding is commonly un- derlain and overlain by zones of partial welding, which are sandwiched between zones of no welding. In some thin flows that were exceptionally hot, the entire unit of tuff may be densely welded; in others, welding may be absent or minor. The degree of welding directly affects the interstitial porosity of the ash-flow tuff. Measurements reported by Ross and Smith ( 1961) for ash-flow tuff in central New Mexico show an inverse relation between degree of welding and porosity. In the nonwelded base or top of a fresh ash flow, the interstitial porosity may be greater than 50 percent; in the densely welded part, it may be less than 5 percent (fig. 18). Columnar jointing characterizes the zones of dense and of partial welding; these joints form in response to tensional forces active during cooling of the flow (figs. 18—20). The columnar-joint spacings range from a few tenths of an inch to many feet; the more closely spaced joints are usually in the zone of most intense welding. The joints are usually vertical, but departures from the vertical are not rare. Cooling joints are uncommon in the nonwelded parts of the ash flow (fig. 20). Reduction in porosity that accompanies welding also results in a prominent foliation within the rock. Pumice fragments, randomly oriented in the nonwelded zone, are flattened and subparallel in the zone of partial welding. Transition from the zone of partial welding to the zone of dense welding is marked by further flattening of the pumice fragments and a marked reduction of their porosity. Foliation is probably responsible in part for Horizontal partings commonly seen within zones of dense welding in outcrop. These partings parallel the dip of the ash flow and can be mistaken for bedding planes. The partings may also mark the contact of adjacent flows in a multiple-flow simple cooling unit. The basal contact between the ash-flow tuffs and the underlying strata (bedded tuff) is generally gradational, but locally the flows incorporate rubble from the un- derlying terrain. Ross and Smith (1961) mentioned that rubble is common near the base, but it may occur well up in the flow and give an impression of being bedding (fig. 18). Gases within the densely and partially welded zones form cavities (lithophysae) that are roughly circular, may be partly lined with primary and secondary minerals, and are as much as 1 inch in diameter (fig. 21). Cavities formed within flattened pumice fragments are also common (fig. 19); these hollows (miarolitic cavities) may not be gas cavities but rather large pumice fragments that have been altered by vapor-phase crystallization and weathering. Locally, both types of C32 HYDROLOGY OF NUCLEAR TEST SITES EXPECTED VERTICAL RANGE INTERSTITIAL '"TEMT'T'AL FEET POROSJITY COIEF FIICJJEMT 0F 18" — (mam) PE RfMIEAII'll L ITY (and per sq m > H > 2 IE) _ ZONE OF NO WELDING LEAK" AOUITAW . _ (May be gay or subjected to vapor-phase (cm of H-mmm crystallization and devitrifination) <11. ’4 per fill) —————————— ".5 (112— ————————————-——————————-—— HO ”' , 12- - ZONE 0F PARTIAL WELDING (May be glassy or subjected to vapor-ohm m crystallization and devitrification) .4 § 1... _ meiosn-TWF WIFE-I g ”miscibility emailed by m g and second-Ivy M Ire-libel! E d W M 1.2- ‘0 “m2 _ _________________ E . _ '5 pd per it) 9 2 ZONE or DENSE WELDING ; :5 50 > 2 (Zeolitized) u ... EXPLANATION — .— . — D V 4 § 9 0 ° Transitional contact between 4 q A b I l” o 0 0 zones of welding V 4 A 4 D 0 Do Basal rubble zone Primary and secondary joints Lithophysal cavities < , Less than; >, greater than Greatly exaggerated FIGURE 18. — Diagrammatic section of ash-flow tuff, showing relation of joint density, interstitial porosity, coefficient of interstitial permeability, and coefficient of fracture transmissibility to degree of welding. Simple cooling unit illustrated; in multiple-flow compound cooling unit, two or more zones of dense welding may be present. cavities together may constitute as much as 10 percent of the volume of a welded tuff. An ash-flow tuff may consist of a single emplacement of ash, multiple emplacements that were deposited in rapid succession and were cooled as a unit, or multiple emplacements that were not deposited in rapid succes- sion. A flow consisting of a single emplacement or of mul- tiple emplacements that cooled together is called a sim- ple cooling unit (Smith, 1960). Units emplaced at time intervals whose duration precluded uniform cooling are called compound cooling units (Smith, 1960). This dis— tinction is important hydrologically, because fractures play an important role in the movement of ground water through ash-flow tuffs. In the simple cooling unit, for ex- ample, only a single densely welded fractured zone is commonly present; whereas in a compound cooling unit, two or three such zones may be present. The preceding paragraphs described, in general terms, the origin of ash-flow tuffs and their principal primary and secondary features as outlined in current literature. Similar features of the ash-flow tuffs at the Nevada Test Site are described below with reference to their water— bearing character. The joints in outcrop in the ash—flow tuffs of the Piapi Canyon Group are polygonal cooling joints and secon- dary joints caused by later forces such as compaction of the underlying porous bedded tuff or regional stresses. Both types of joints are largely restricted to the dense brittle welded tuff and die out or markedly decrease in frequency within the underlying and overlying partly welded zone (fig. 20). The spacing of the cooling joints ranges from a fraction of an inch in the glassy densely welded zones to a few feet in the zone of partial welding; the nonwelded zone contains few Visible joints. The SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE C33 FIGURE 20. — Relation of joint density to degree of welding in ash- flow tuff, 4 miles east of Shoshone, Calif. along Charles Brown Highway. Joints in black vitrophyre (upper one-third of photograph) are spaced 14 to 1/2 inch apart. Well-defined joints are absent in basal nonwelded part of ash flow (lower one-third of photograph). Light meter in foreground, 4 inches long. FIGURE 19. —— Columnar jointing in ash-flow tuff, Tiva Canyon Member of Paintbrush Tuff, at Busted Butte, western Jackass Flats. Foliation marked by eroded pumice. polygonal structure is generally obscured by secondary jointing, except in the youngest welded tuffs. Most of the primary and secondary joints dip 70° to vertically. The strike of the secondary joints is highly variable, but the dominant strike parallels that of the major normal faults. Most of the joints in outcrop were tightly closed. The interstitial porosity in the ash-flow tuffs of the Piapi Canyon Group is inversely related to the degree of welding (fig. 18). No interstitial porosity was visible (with hand lens) in the densely welded tuff, and very lit- tle was visible in the partially welded tuff. Lithophysae, much like the vesicles in basalt, are usually unconnected or poorly connected. Rubble zones at the base of ash-flow tuffs, in contrast to rubble zones beneath some basalt flows, are not highly permeable. The rubble in outcrop is completely sur- rounded by and incorporated into the ash-flow material. Hence, permeability of the rubble zone is controlled by that of the ash matrix. The horizontal partings are locally a few tenths of an inch wide and tens of feet long. Because the partings . . ~ . FIGURE 21. — Zone of lithophysal cavities in ash-flow tuff, Topopah parallel the foliatlon w1th1n the welded zone or the con- Spring Member, western Jackass Flats. C34 tact between flows, they may represent breakage along a plane of primary weakness after the removal of over- burden from a prestressed rock; therefore, the partings are not likely to be open at depths of the regional water table (700 to 2,000 ft). CHARACTER BASED ON CORES AND DRILLING RECORDS Laboratory determinations of interstitial porosity and permeability of 20 core samples of the Topopah Spring, Tiva Canyon, and Rainier Mesa Members are sum- marized diagramatically in figure 18. Interstitial porosity ranges from 3 to 48 percent. Interstitial permeability ranges from 0.000007 to 2.0 gpd per sq ft. These data indicate that the interstitial permeability of the densely welded zones is extremely low and probably prevents movement of ground water through these rocks. They also indicate that interstitial porosity and permeability of some nonwelded and par- tially welded tuff may be significant to ground-water movement. In general, both interstitial porosity and in- terstitial permeability vary inversely with degree of welding (fig. 18). Secondary fractures in the cores are joints, thin brec- ciated zones, vuggy veinlets, and slickensides. The frac- tures are commonly sealed by gouge, clay, or mineral ox- ides. They dip at high angles, are less than 0.04 inch in width, and extend completely across the cores. The total number of fractures commonly averages 1 per foot and ranges from 0 to 20 per foot. The volume of fracture fillings is generally less than 2 percent of the core; only a fraction of this percentage contains vuggy or other porosity. Fault planes and associated breccia zones might also be considered as potential zones of high transmissibility. However, the breccia zones in cores were usually tightly sealed. Records of holes drilled on Rainier Mesa and in Yucca Flat suggest open and interconnected fractures in the welded-tuff aquifer. When mud was used as the cir- culating medium in rotary drilling, circulation losses oc- curred repeatedly during drilling of the Rainier Mesa Member. The water-bearing fractures are probably restricted principally to the zone of partial to dense welding. Frac- tures are unlikely to remain open in the friable, non- welded parts of ash-flow tuffs. For these reasons, the ash- flow tuffs of the Paintbrush Tuff and the Timber Moun- tain Tuff are termed the “welded-tuff aquifer.” The name emphasizes the location of the chief water—bearing zones within the welded part of an ash-flow tuff. PUMPING AND BAILING TESTS The hydraulic properties of the Topopah Spring Member were tested in three wells by pumping and in one well by bailing. Wells 73—58, 74—57, and 74—61 are in HYDROLOGY OF NUCLEAR TEST SITES Jackass Flats, and well 81—69 is in southern Yucca Flat. The methods of testing were the same as those used in tests of the lower carbonate aquifer. These four tests in- dicate a wide range in specific capacity and transmissibility (table 3). The specific capacity ranges from 0.1 to 56 gpm per foot of drawdown, and the transmissibility ranges from 200 to 100,000 gpd per ft. The transmissibility of the three wells in Jackass Flats ranges from 28,000 to 100,000 gpd per ft. Semilog plots of two of these tests are given in figures 22 and 23. The semilog plot of well 74—61 (fig. 22) is marked by an initial steep limb that gradually flattens into a gentle limb within the first 20 to 30 minutes of the pumping ,test; the gentle limb persists for about 70 minutes. The dual-limb configuration is probably due to the same faco tors causing such time-pressure behavior in the lower carbonate aquifer. The time-drawdown curve of well 74—57(fig. 23) is uni- que among the curves for Nevada Test Site wells in that the semilog plot does not exhibit the initial steep limb on the time-pressure curve. Absence of the initial steep limb may be due to the full penetration of the aquifer; however, other interpretations are possible. Negative hydraulic barriers are evident on the semilog curve for well 74—57 (fig. 23) and in expanded—scale plot for well 74—61, and one was reported by R. A. Young (oral commun., Sept. 1966) for well 73—58. This condition suggests that the welded-tuff aquifer, as well as the lower carbonate aquifer, is locally compartmentalized. Positive evidence of compartmentalization of the Topopah Spring Member is presented on geologic maps of Jackass Flats (McKay and Williams, 1964; and Lip- man and McKay, 1965), which show that the Topopah Spring Member crops out and is entirely above the water table within 1 to 11/2 miles of the three wells tapping this aquifer. The significance of these boundaries to the long- term use of the Topopah Spring Member as an aquifer in Jackass Flats is the subject of a report by R. A. Young (1972). Hydraulic tests of welded tuffs older than the Topopah Spring Member (the Grouse Canyon and the Tub Spring Members of the Indian Trail Formation, and the Tuff of Crater Flat; table 1) suggest that these older tuffs are aquitards in Yucca, Frenchman, and Jackass Flats. BEDDED-TUFF AQUIFER The bedded-tuff aquifer comprises ash-fall tuff in- terbedded with Tiva Canyon and Topopah Spring Members of Paintbrush Tuff and the Grouse Canyon Member of Indian Trail Formation (table 1). Locally, as at Rainier Mesa, the ash-fall tuffs aggregate several hun- dred feet in thickness, although they are generally less than 100 feet thick. Because these ash-fall tuffs are highly variable in thickness and extent, they have not been assigned formal geologic names. DHAWDOWN DURING PUMPING, IN FEET DRAWDOWN DURING PUMPING, IN FEET SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE C35 20 I II III I ||I||l| l ll||l|| l lllllll EXPLANATION _ o _ T: coefficient of transmissibility 0:133 gpm. Well was pumped 2,155 minutes at 150 25 gpm, then 2,160 minutes at 180 gpm before this _ test; recovery period of only 367 minutes separa- o ted the tests, but change in water level during _' last 100 minutes of recovery was less than 0.2 _ foot .30 Drawdown .— 0 _ Formations and intervals tested: Basalt of Kiwi Mesa ._ (l,039—l,15'0 ft); unconfined. Topopah Spring ' Member (1,150—1,329 ft); confined 35 Estimated penetration: 45 percent of Topopah — 0 Spring Member _ Type of completion: 12 3/4—inch—OD casing, machine perforated in intervals 1,077—1,097 feet and l,244-l,300 feet 40 ° Measurements made with test pressure gage and air — ° line 0 _ . _ O 45 O 0 O 50 '. '0 , (264) (133) A: = 0.8 = — — = 28,000 gpd per ft _ M 1.25 _ X 55 l II III | lllllll | l|||ll| l lllllll 1 10 100 1000 10,000 TIME (r) SINCE PUMPING STARTED, IN MINUTES FIGURE 22. — Semilog graph of drawdown of water level during pumping test in well 74—61, December 18, 1958. 25 llllIlll ||l|||l| lllllll III Ill 0 — _ 264 697 _ As= 2.7 T= -————( )2; ) = 68,000 and per ft 30 N N EXPLANATION A: = 5-5 \ 35 T= coefficient of \ transmissibility 0 = 697 gpm A5 = 1 1.0 40 — . Drawdown Formation tested: Topopah Spring Member (928-1,475 ft); confined(?) — Percent penetration: Aquifer fully penetrated — Type of completion: Gun and jet perforated 13 3/8— and l l 3/4-inch-OD casing ‘5 | ||l|l|ll | llllllll l l|||l|| III III 1 10 100 1000 10,000 TIME (I) SINCE PUMPING STARTED, IN‘MINUTES FIGURE 23. — Semilog graph of drawdown of water level during pumping test in well 74-57, February 18—22, 1964. C36 HYDROLOGY OF NUCLEAR TEST SITES t/t'= TIME SINCE PUMPING STARTED/TIME SINCE PUMPING STOPPED 0 1 10 100 1000 10,000 I I I I | I I I I I | I I I | I | l I I | I I I I I I I I 0 Ar= 5‘1 (264) 40) _ 1,100 T: 5.: = 2,100 god ”of ft EXPLANATION i x T: coefficient of ,_ it,( ~ transmissibility W 10 81 x. — 10 “.fi , x = ,_ "xx 0 40 gpm E In u: — X " _ >: LL . Wow at surface ’1 I z x 3‘ uI -_ 20 o .. Time since pumping stopped, _ 20 S g in minutes U _ o " W n. u: 2 '— x o _ 0 3 x Drawdown Z n. ‘ x - 0 30 x x —— 30 3‘. Z a E Recovery 3 _ o x Z a — E o x 0 g D . 3 4o \' 8 r— (264) (40) _ ‘0 E g . .— —_8.—1 — 1,300 and par ft 0 < _ _ .I g A3= 8.1 x < Formation tested: Bedded tuff(?) ll 8 50 — of Piapi Canyon Group (1,685— I 50 E) . - Iu 1,800 ft), confined \ As: 2.5 cc _ EstImated penetration: Unknown . Type of completion: Slotted — 6-inch—ID casing 60 l IIIIIIII l IIIIIII I |||Ill I I |Illll6° 1 10 100 1000 10,000 TIME (I) SINCE PUMPING STARTED, IN MINUTES FIGURE 24. — Semilog graphs of drawdown and residual draWdown of water level during pumping test in well 81—67, October 2—4, 1959, The ash-fall tuffs are fine grained to granule sized, poorly to well sorted, and highly‘friable. Locally, they have been either reworked by running water or deposited in standing water. They are distinctly stratified and possibly should be classified as tuffaceous sandstones, siltstones, and mudstones. The highly friable nature of these rocks appears to preclude the existence of open joints or faults within them; open fractures were not seen in hundreds of feet of tunnels driven in these rocks beneath Rainier Mesa. Laboratory studies of cores and hand specimens by Thordarson (1965) indicated that interstitial porosity of 48 samples averaged 40 percent, and interstitial permeability of 11 samples ranged from 0.07 to 4.1 gpd per sq ft. The permeability of 3 samples not cited by Thordarson ranged from 0.7 to 18 gpd per sq ft. The cited permeability values are representative of only vitric ash- fall tuffs; that is, the glass shards composing the tuff are unaltered. Where the glass shards are altered to zeolite or clay minerals, the permeability is reduced by several orders of magnitude. In general, the bedded tuffs of the Piapi Canyon Group are unaltered and locally constitute aquifers. The bedded tuffs in and below the Wahmonie yon Member, are generally altered to zeolite or clay; these rocks compose part of the tuff aquitard described in this report. The bedded-tuff aquifer was probably tapped by three wells at the Nevada Test Site (81—67, 90—74, and 90—75). Results of pumping tests of these wells are tabulated in table 3, and the semilog plot of the test of well 81—67 is presented in figure 24. The pumping-test data indicate that the coefficients of transmissibility of the bedded- tuff aquifer range from 200 to 1,000 gpd per ft (table 3). LAVA-FLOW AQUIFER The Basalt of Skull Mountain, the Rhyolite of Shoshone Mountain, and the Basalt of Kiwi Mesa com- pose the lava-flow aquifer. The lava flows composing these three units, respectively, aggregate as much as 250, 2,000, and 250 feet in thickness (table 1). These strata are restricted to the Jackass Flats area. Their areal dis- tribution is shown on the geologic maps by McKay and Williams (1964) and Lipman and McKay (1965). The rocks possibly lie within the zone of saturation beneath parts of east-central Jackass Flats. Well 74—61 penetrated 111 feet of the Basalt of Kiwi Mesa and 179 Formation, except those associated with the Grouse Can- feet of the underlying Topopah Spring Member of Paint- SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE brush Tuff (table 3). The transmissibility of the basalt is some fraction of that for the two aquifers, which is about 28,000 gpd per ft. Outcrop examinations of the Basalt of Kiwi Mesa and analogy with the water-bearing characteristics of basalts in other areas suggest that the movement of ground water through the basalt flows tapped by well 74—61 may be controlled by permeability and porosity developed along bedding planes and cooling joints. A transmissibility of several thousand gallons per day per foot would not be unlikely for wells tapping basalt flows composing either the Basalt of Kiwi Mesa or the Basalt of Skull Mountain. Ground-water flow through the Rhyolite of Shoshone Mountain is probably controlled by secondary fractures; by analogy with the rhyolites beneath Pahute Mesa, the water-bearing fractures in rhyolites beneath Jackass Flats are probably sparse. VALLEY-FILL AQUIFER Alluvial-fan, fluvial, fanglomerate, lakebed, and mudflow deposits in depressions created by post- Pliocene block faulting are the valley fill. It constitutes the major aquifer used for water supply in Frenchman Flat, western Emigrant Valley, and Amargosa Desert. In Yucca Flat, western Jackass Flats, and Mercury Valley, the valley fill is either unsaturated or only locally saturated. The valley fill is at least 1,870 feet thick beneath central Yucca Flat (well 83—68) and at least 1,200 feet thick beneath central Frenchman Flat (well 74—70b); it is 1,040 feet thick at well 74—61 in central Jackass Flats and is unsaturated. Because of the great depth to water in these valleys (690 to 1,915 ft), the saturated thickness of the valley fill is but a fraction of the thickness cited. Faulting and erosion have exposed only the uppermost part of the valley-fill aquifer. Thus, outcrop examination of these strata as a means of studying their subsurface character was not useful. W. P. Williams, W. L. Emerick, R. E. Davis, and R. P. Snyder (written com- mun. June 1963) mapped and sampled the valley fill to a depth of 550 feet in a shaft (U2j) beneath the northwestern part of Yucca Flat. A few of their obser- vations follow: 1. A marked and irregular variation exists in the percen- tage of detrital lithologies with depth. 2. The deposit contains large amounts of cobbles and boulders. At several intervals these materials con- stitute 65 to 75 percent of the deposit. At all depths in the shaft, except the 40- to 80-foot depth, they are the most abundant size-class material. Though minor, clay and silt are consistently present con- stituents. 3. The valley fill is generally poorly stratified to non- stratified and poorly sorted. C37 4. Strata are mostly horizontal and are commonly dis- continuous from one wall of the shaft to the other. 5. Caliche is a common cementing material at all depths in the shaft. At no place is the valley fill so in- durated or cemented that it cannot be dug easily with a miner’s pick. Price and Thordarson (1961) and Thordarson, Garber, and Walker (1962) presented data on valley fill penetrated by test wells 83—68 and 84—67 in central Yuc- ca Flat; their descriptions agree with those of W. P. Williams, W. L. Emerick, R. E. Davis, and R. P. Snyder (written commun., June 1963), who also presented porosity data for the valley fill. They cellected 42 samples from depths ranging from 500 to 1,500 feet in 20 drill holes and in the U2j shaft. The total interstitial porosity of the samples ranged from 16 to 42 percent and averaged 31 percent. Lake beds, chiefly of illite, mixed-layer clay minerals, and montmorillonite, were penetrated in the depth in- terval 548 to 770 feet at the base of the valley fill in test well 81—69 in southern Yucca Flat (Moore and Garber, 1962). Data from other test holes in Yucca Flat do not in- dicate lake beds at the base of the valley fill elsewhere in the valley. Six wells at the Nevada Test Site penetrate the valley- fill aquifer. Table 3 summarizes the results of pumping tests of those wells. The time-drawdown and the recovery plots of the tests of four of the wells are presented in figures 25—29. All the pumping tests were of the pumping well only. Transmissibility of the valley-fill aquifer ranges from about 800 to 34,000 gpd per ft (table 3). The transmissibility and the saturated-thickness data suggest average interstitial permeabilities of 5 to 70 gpd per sq ft. A relatively short (48 hr) pumping test of the valley- fill aquifer gives reasonably correct coefficients of transmissibility. Figure 25 summarizes the results of a 48-hour pumping test of well 74—70b; coefficients of transmissibility of about 2,400 and 2,500 gpd per ft are indicated by drawdown and recovery data. Figure 26 summarizes the recovery of water level in this well dur- ing a subsequent shutdown period of 133 days. The 133- day recovery curve indicates a coefficient of transmissibility of 1,700 gpd per ft, or about 70 percent of that derived from the 48-hour test. Pumping-test data for the valley-fill aquifer were not routinely corrected for dewatering. The dewatering in wells 91—74, 83—68, 74—70b, and 74—70a amounted respec- tively to about 5, 10, 20, and 20 percent of the saturated thickness penetrated by each well. Using the procedure outlined by Jacob (1963), corrections of the drawdown in wells 83—68 and 74—70b for decrease in aquifer thickness due to dewatering were made and resulted in increases in transmissibility of only 10 to 15 percent; similar correc- C38 t,/t'= TIME SINCE PUMPING START HYDROLOGY OF‘ NUCLEAR TEST SITES ED/TIME SINCE PUMPING STOPPED 301 10 100 1000 10,000 I IIIIIII I IIIIIII I lllllll I IIIIII° A53 19.5 _ '3,on r_ (260) (‘31) ‘ _ _——13.. 2,“ '0 90! ft ‘0 10 N 5 _ “5 A: = 13.0 _ w . Looking air line; “- y. no muromonu Z t‘ so 20 ‘. m > ,———)(——\ I: z . \ x W -‘ _ I xx — 3 ‘29 53 x o : ° " E 2 60 . \ 30 o 2 - ° -. z 0 — I z " _ g I MARK!“ o - 3 70 —— . . , 40 Z 0 (264) (131) 3 z T = coefficient of 77' ——'—“.2 = 2.400 and pct 11 0 g mmmhibility ‘ I; O — _ ( g a: :31 3pm N:1t.2 g E so —— I 50 4 o " “ 53 g - Till-e inc. pumping stopped, _ 5 in minutes g 90 __ . —__ Formation tested: Viley H (689— x K‘- 60 andown 1,200 ft); unconfined Estimated penetration: 60 percent . “ x Type of completion: Slotted lo-inah- . _ Recovery ID caning x o 100 I I I l I I I I I I I I I I I I I I I I I I I I I I I I I I I 70 1 10 100 1000 10,000 TIME I!) SINCE PUMPING STARTED, IN MINUTES FIGURE 25. — Semilog graphs of drawdown and residual drawdown of water level during pumping test in well 74—70b, September 9—11, 1959. tions for the other two wells in the valley fill did not appear essential. Semilogarithmic plots of the drawdown and the recovery data obtained during pumping tests of the valley-fill aquifer superficially resemble those for carbonate-rock and welded—tuff aquifers. However, significant differences exist. In two of the three wells for which both recovery and drawdown data are available, 74—70b and 83—68, the coefficients of transmissibility from either type of data are nearly equal (table 3 and figs. 25 and 28); and recovery in these wells was not more rapid than the drawdown. In the third well, 74—70a, the coefficient of transmissibility derived from the recovery curve is only 50 percent greater than that indicated by the drawdown curve (fig. 27). Wells tapping the valley-fill aquifer penetrate an es- timated 15 to 100 percent of the aquifer. Incomplete penetration and the resulting crowding of flow lines near the bore may explain the initial steep limb of the time- pressure curves. Leaky-aquifer conditions may also in— fluence the pressure response of the valley-fill aquifer to pumping. AQUITARDS Four of the 10 geohydrologic units of this report are aquitards (table 1). The lower clastic aquitard and the tuff aquitard have the widest areal distribution within the zone of saturation and play a key role in the regional movement of ground water. The lower clastic aquitard is the hydraulic basement for ground-water movement within the miogeosynclinal carbonate rocks throughout the study area, whereas the tuff aquitard effectively separates the Cenozoic from the Paleozoic aquifers in Yucca Flat, Frenchman Flat, Jackass Flats, and other valleys. The upper clastic aquitard occurs within the zone of saturation chiefly in western Yucca Flat and northern Jackass Flats, whereas the lava-flow aquitard is restricted to a small area in the central part of the Nevada Test Site. Aquitards differ widely in their geologic origin and SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE 039 m IIIIIII llllllll llllllll I Illlll E w .0 .. u. E w. _ 0 < x E 002 D (264) (55) (n T- 'T - 1,700 gpd per ft 0 - _ E 4 m A:*8.G 3 O .i w _ __ a E 52 6“ EXPLANATION : T= coefficient of transmissibility '- — 0 = 55 gpm during 5 l/z-month period preceding E shutdown; avenge pumplge during December, a 69. ' / about '40 gpm. Pumping at wells 73-70 and _ a 74-70: remained reuombly study during re- _ / covery period _ X l Recovery lllllll lllllll llIllI llllllll 7 001 10 100 10“ “3,000 TIME") §|NCE PUMPING STOPPED, IN DAVS FIGURE 26. — Semilog graph of recovery of water level during 133-day shutdown in well 74—70b, December 23, 1960, to May 4, 1961. their physical properties; all have low fracture transmissibility, usually less than 500 gpd per ft for several thousand feet of saturated rock, and negligible interstitial permeability. The specific capacity of wells tapping these rocks is usually less than 0.1 gpm per foot of drawdown per 1,000 feet of saturated rock. In this report, the arbitrary dividing line between an aquifer and an aquitard is a specific capacity of about 0.1 gpm per foot of drawdown for 1,000 feet of saturated rock. Specific capacity of the aquitards rarely exceeds this value, and that of the carbonate and the welded-tuff aquifers may locally be as low as 0.2 gpm per foot of drawdown for several hundred feet of saturated rock. Thus, the value chosen is an approximate dividing point between the most permeable aquitard and the least permeable fractured aquifer. The very low transmissibility of the aquitards limits the type and the duration of hydraulic tests that can be run in, wells tapping these units and prevents routine analysis of the hydraulic data. The specific-capacity values cited for the aquitards should be considered only as a crude measure of the transmissibility of water- bearing fractures in these units. Specific-capacity values given in the following pages are probably maximum values of this parameter. LOWER CLASTIC AQUITARD The lower clastic aquitard comprises all siltstone, quartzite, shale, and sandstone of Precambrian through Early Cambrian age — that is, the clastic rocks of John- nie Formation, Stirling Quartzite, Wood Canyon Forma- tion, Zabriskie Quartzite, and the lower half of the Carrara Formation (table 1). These clastic rocks aggregate about 10,000 feet in thickness, although such a thickness is not present everywhere owing to the effects of faulting, folding, and erosion. The outcrop pattern of the lower elastic aquitard and its inferred distribution within the uppermost part of the zone of saturation are shown on plate 1. CHARACTER BASED ON OUTCROP, CORES AND DRILLING RECORDS In outcrop, the elastic rocks of the lower elastic aquitard resemble rocks of the lower carbonate aquifer in several ways. First, when viewed through a hand lens, the rocks seem to have negligible interstitial, or matrix permeability. Second, the clastic strata are highly frac- tured and are locally brecciated; locally, quartzite seems noticeably more fractured than the more porous siltstone, shale, and sandstone. Third, secondary C40 HYDROLOGY OF NUCLEAR TEST SITES t/t'= TIME SINCE PUMPING STARTED/TIME SINCE PUMPING STOPPED 1 10 100 1000 10,0 10 | | | I | I I I | I | I I I I | I | | | I | I I I I I I I I I I 08 _ ' (264) (160) I “=33 rs—T=11,0009pdporh — 148 EXPLANATION 1 5 5 T T=coefficient of t— 46 transmieibility m _ In .— " a = 160 gpm ; m _ a 20 — to >‘ z x 25 5 - _ . Time since pumping stopped_ a g in minutes 0 _ w E 25 . ‘5 I 3 ’ Drawdown g a. _ __ I E Recovery 0 3 Z 0 3° ' x Formation tested: Valley fill - 20 3 o o O z . . I (683-900 ft); unconfined D g _ 15 xx Estimated penetration: 15 2 o ' X percent E i . Type of completion: Slotted o I: 35 . x 10-inch—ID casing - 25 -| o o ‘I e 3 _ ....O. 9 on " _ a m... \ x I 40 -“ . Ar= 5.5. 30 (264 16 _ 7=—_,:_—;——m‘7.70009dwfl 00x00 _ x 0. on... o oo- ‘5 I I IIIIII I I IIIIII l I [Ll'l '° I ll||l||35 1 10 1M 1000 10,000 TIME I!) SINCE PUMPING STARTED, IN MINUTES FIGURE 27. — Semilog graphs of drawdown and residual drawdown of water level during pumping test in well 74—703, August 27—29, 1959. porosity occurs along joints in the quartzitic rocks and more rarely in sandstone, shale, and siltstone. All this secondary porosity is probably due to subaerial weathering. Clastic rocks differ from carbonate rocks in two impor- tant ways. First, secondary porosity rarely develops along bedding planes in any of the clastic rocks. The absence of such secondary porosity is probably due to the low solubility of the clastic strata, whose major con- stituents are quartz, mica, and clay minerals. Second, the mode of deformation of some of the clastic rocks differs significantly from that of the carbonate strata. Siltstone, shale, and sandstone commonly exhibit tight folding, slaty cleavage, and shearing in outcrop, whereas carbonate rocks and quartzite, though highly fractured, tend to form relatively broad folds. The difference in mode of deformation is probably due to differences in the strength of the rocks. The dense carbonate and quart- zitic rocks respond to stresses as a brittle solid, at least under relatively shallow overburden; the more porous finer grained rocks tend to deform plastically. The com- mon absence, in outcrop, of the Dunderberg Shale Member of the Nopah Formation is a good example of the squeezing out or marked thinning of an argillaceous rock in response to deformation. This difference in response to deformational forces has been specifically cited by Vincelette (1964) in his discussion of the geology of the northwestern Spring Mountains. The potential hydrologic significance of plastic defor- mation of siltstone and shale is twofold. First, in these rocks the fractures formed during folding or faulting tend to be sealed by the same process that formed them. Se- cond, wherever quartzites are interbedded with argillaceous strata, as is common in parts of the Stirling Quartzite and the Wood Canyon Formation, open frac- tures in the quartzites tend to be isolated or even sealed by the plastic deformation of the weaker strata. Figure 6 is a photograph of the flowage of shale into fractures in more competent rock. Micaceous partings and laminae are abundant within the quartzitic parts of the Johnnie and the Wood Canyon Formations. These partings are also likely to deform SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE C41 t/t'= TIME SINCE PUMPING STARTED/TIME SINCE PUMPING STOPPED 1 10 100 1000 10,000 0 I I I I I I I I I I I I | I I | I I I I I I I I I‘I' I I I 0 ":5 x x, % EXPLANATION — 55 “’8.“ _ _ — (264) (60) x T: coeffICIent of T= —? = 12,200 9pc! per ft x,‘ transmissibility 5 ' — 5 xx 0 = 60 gpm I— w _ x _ w u. ._ x 55 E m ‘0 Time since pumping stopped, _ 10 >- M- ° x in minutes I z a d ‘ " ' ‘ 0 Z Drawdown u _ m g 15 ° . x 15 g a Recovery 2 0 — ' _ I: z * 8 e Z 8 20 20 g o x 0 Z O a — - — z o . SE 5 ' o X I 25 . 25 a1 D .. 3 _ ., _ e '0 (264 so ‘” .0. T=—%i= 13,200 gpd per ft " g 30 — Formation tested: Valley fill (1,605- '... . 30 1,870 ft) unconfined A: :12 " Estimated penetration: 80—100 percent _ Type of completion: Machine slotted " M E 10 3/4—inch—OD casing 35 I | | | I I I I I l | | | | I I I I I I | I | I I | | I I I I | | 35 1 '10 100 1000 10,000 TIME (t) SINCE PUMPING STARTED, IN MINUTES FIGURE 28. — Semilog graphs of drawdown and residual drawdown of water level during pumping test in well 83—68, September 20—21, 1960. plastically and, thereby, seal or isolate the fractures in brittle quartzite. However, in those parts of the Stirling Quartzite and the Zabriskie Quartzite that contain few or no micaceous partings or argillaceous interbeds, inter- connected secondary fracture porosity is possible; and locally these rocks may be aquifers. Secondary porosity is generally absent in the subsur- face. About 3,500 feet of quartzite, siltstone, and argillite of the Stirling Quartzite and the Johnnie Formation were penetrated in well 89-68. About 10 percent of this footage was cored. All the cores were highly fractured and locally brecciated. The fractures were commonly lined with micaceous and chloritic selvage or were marked by slickensides. The brecciated zones were sealed with quartz or calcite veinlets. For each of 35 core runs the average number of major fractures per foot of core ranged from two to five; locally, the microfracturing was intense. In some quartzite, the shattering was so in- tense that almost every sand grain was bounded by a microfracture. Most of the fractures were tightly sealed either by selvage minerals or by quartz or calcite veinlets, or by virtue of their never having been opened. Only two cored intervals in the aquitard in well 89—68 contained joints that appeared to have significant secon- dary porosity. The interstitial porosity and the interstitial permeability of 43 cores from the lower clastic aquitard in test well 89—68 are given in table 4. Although restricted to cores from a single test hole, the data are considered TABLE 4. — Interstitial porosity and permeability of cores from the lower elastic aquitard, well 89—68, Yucca Flat [Total porosity, in percent, determined from grain and bulk densities; effective porosity, in percent, determined by mercury-injection or water-saturation methods; ermeability, in gallons per day per square foot, determined using Denver, Colo, tap water. nalyses by U.S. Geo]. Survey, Denver, Colo] Porosity or Number of permeability samples Range Median Mean Total porosity ________ 23 02—100 3.9 3.8 Effective orosity ______ 20 0.6—5.0 1.9 1.9 Permeabi ity _________ 18 0.0000007—00001 000002 .00001 Effective porosity of argillite and siltstone _ _ _ 10 0.7~3.6 2.0 2.0 Effective porosity of quartzite __________ 10 0.6—5.0 1.4 1.9 Permeability of argillite and siltstone ________ 9 0.0000007—0.000007 .000002 .000002 Permeability ofquartzite _ _ 9 0.000002—0.0001 .00001 .00002 C42 HYDROLOGY OF NUCLEAR TEST SITES 0 0'0 I | I I I | | I | | | I I I | I | | I I I | I I l I | I I l | I | z EXPLANATION E _ As 1 3 = . . . . . . _ a I— T: (264) (165) = 33 500 gpd p" ft T coefficrent of transmussubility fl 1-3 ' a = 165 gpm g "- 1.0 2 \ 8 - I 10 g ‘ _ 20 . . . _ < E + Time since pumping stopped, m m in minutes 0 > 10 _, O 2.0 x < 0 Recovery 2) Lu 0 I _ Formation tested: Valley fill (107-371 ft); unconfined _ 3 Estimated penetration: 100 percent 1: Type of completion: Slotted 10 3/4—inch-OD casing 30 I l l I I l | l | I | | l I I l I 1 | | | | l I I I I l l | | I | | 10 100 1000 10,000 100,000 t/t'= TIME SINCE PUMPING STARTED/TIME SINCE PUMPING STOPPED FIGURE 29. — Semilog graph of residual drawdown of water level during pumping test in well 91—74, November 23—25, 1959. representative of the lower elastic aquitard throughout the study area because the hole tapped 3,517 feet of aquitard (see below) and the cores resemble examined outcrop specimens. HYDRAULIC TESTS Pumping or bailing three test wells (well 66—69 in Mer- cury Valley, well 68—60 in southwestern Rock Valley, and well 89—68 in northern Yucca Flat) gave some measure of the fracture transmissibility of the lower elastic aquitard. The results indicate specific capacity ranging from 0.01 to 0.27 gpm per foot of drawdown for wells penetrating differing thicknesses of saturated rock. After normalizing to 1,000 feet of saturated rock, the specific capacities range from 0.04 gpm per foot of drawdown in well 68—60 to about 0.1 gpm per foot of drawdown in well 66—69. The pumping test of well 89—68 was most instructive because of the great thickness of elastic rocks penetrated. This well penetrated large thicknesses of Stirling Quartzite (587 ft), Johnnie Formation (2,930 ft), and Noonday(?) Dolomite (710 ft).2 The specific capacity of the well was 0.27 gpm per foot of drawdown at the end of a 36-hour pump test; the apparent fracture transmissibility derived from the second limb of the drawdown curve is 600 gpd per ft. Lost-circulation records and geophysical logs indicate that a fault zone (depth 5,290 ft) separating the Johnnie Formation from the Noonday(?) Dolomite is one of the major zones of transmissibility penetrated by the bore. Thus the specific capacity of the 3,517 feet of the Stirling Quart- zite and the Johnnie Formation penetrated is less than 0.27 gpm per foot of drawdown, and the transmissibility is less than 600 gpd per ft, because of the yield of the fault zone. Ignoring the yield of the fault zone and the -'The Nonnday(?) Dolomite. which underlies the Johnnie Formation (table 1). is considered part of the lower elastic aquitard; it is stratigraphically separated from the lower carbonate aquifer by as much as 10,000 feet of the lower elastic aquitard. possible yield of Noonday(?) Dolomite, the average specific capacity of the Stirling Quartzite and Johnnie Formation on a 1,000-foot basis can be no greater than 0.08 gpm per foot of drawdown; this suggests a coef- ficient of transmissibility on the order of 150 gpd per ft per 1,000 feet of aquitard. Hydraulic tests of two wells (67—68 in Mercury Valley and 67—73 in western Indian Springs Valley) which tap the Dunderberg Shale Member of the Nopah Formation (table 1) confirmed the results obtained for the lower elastic aquitard. The specific capacity of the Dunderberg Shale Member, on a 1,000-f00t basis, is less than 0.3 gpm per foot of drawdown in each well. Several thin strata within the Stirling Quartzite and the Zabriskie Quartzite of the lower elastic aquitard (table 1) are free of argillaceous laminae. The fracture transmissibility of these strata, not yet tested by drill- ing, might locally be as high as 10,000 gpd per ft. However, for the bulk of the aquitard, available data suggest that the coefficient of transmissibility probably does not exceed 1,000 gpd per ft locally. Regional flow through the aquitard may be governed principally by in- terstitial permeability for reasons suggested in the following section of the report. REGIONAL EVIDENCE OF LOW TRANSMISSIBILI’I‘Y The best comparative qualitative evidence on the gross fracture transmissibility of the lower elastic aquitard and the carbonate aquifers is derived from tabulations of spring discharge in that part of eastern Nevada underlain by miogeosynclinal rocks. Such a dis- charge tabulation was prepared by Maxey and Mifflin (1966) for springs discharging more than 450 gpm. These springs, yielding as much as 8,000 gpm per spring, dis- charge from carbonate rocks at valley level or from valley fill immediately adjacent to carbonate-rock outcrops; none of the major springs reportedly discharges from elastic rocks or valley fill adjacent to elastic rocks. SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE Individual springs associated with valley-level car- bonate rocks in the study area yield as much as 2,800 gpm, whereas those springs associated with elastic strata yield less than 25 gpm and are restricted to the northwestern Spring Mountains. The highest reported yield of a spring associated with clastic strata in the vicinity of the study area is 150 gpm at Resting Spring, Inyo County, Calif. (sec. 31, T. 21 N., R. 8 E.), about 10 miles south of the area of figure 1. This spring emerges from valley fill immediately east and topographically upgradient from a quartzite ridge. Hughes (1966) inter- preted the quartzite ridge as a barrier forcing water out of the valley fill, rather than as the source of the water. The apparent absence of high- or even moderate-yield springs in the elastic miogeosynclinal strata in Nevada probably reflects the absence of regionally integrated fracture transmissibility. Although the clastic strata were subjected to the same deformation as the carbonate rocks and consequently are as highly fractured, their ap- parent lack of integrated fracture porosity is probably due to (1) their low susceptibility to solution, (2) the tendency of the elastic argillaceous rocks to deform plastically, and (3) the tendency of micaceous partings and laminae to seal fractures in the brittle quartzite rocks. Thus, the regional movement of water through these rocks is probably governed by their very low in- terstitial permeability (table 4) rather than by fracture transmissibility (as measured locally by well tests). UPPER CLASTIC AQUITARD In this report, the upper clastic aquitard is syn- onymous with the Eleana Formation of Late Devonian to Late Mississippian age (table 1). The Eleana Formation consists predominantly of argillite, quartzite, and con- glomerate and is as much as 8,000 feet thick in western Yucca Flat; the argillite composes about two-thirds of the formation. The upper clastic aquitard (table 1) is of hydrologic importance only beneath western Yucca Flat and northern Jackass Flats. Beneath these areas, the Eleana Formation is thousands of feet thick and stratigraphically and hydraulically separates the upper and the lower carbonate aquifers (table 1); its outcrop pattern and probable subsurface distribution in the up- per part of the zone of saturation is shown on plate 1. Elsewhere at Nevada Test Site the Eleana Formation has been removed by erosion, occurs hundreds of feet above the regional water table, or is represented by equivalent rocks of carbonate lithology. Visual examination of selected outcrops and binocular~microscopic examination of hand specimens indicate that certain features of the Eleana Formation resemble those of the lower clastic aquitard. Although the argillites probably have moderate interstitial porosity, the interstitial permeability of those rocks and C43 of the quartzites and conglomerates appears negligible. Secondary porosity is poorly developed along fractures and, where present, appears entirely due to subaerial weathering. This formation as well as the lower clastic aquitard probably respond to deformation by shearing and tight folding; open fractures are unlikely to occur in the rock at depth. The tendency of the formation to deform plastically is evidenced by its behavior as the principal glide plane (or lower plate) for several major thrust faults at the Nevada Test Site. Laboratory analyses of 22 outcrop samples of the Eleana Formation indicate a moderate total porosity but a small effective porosity. The total porosity, ranging from 2.0 to 18.3 percent, has a median value of 6.1 and a mean value of 7.6 percent. The effective porosity, rang- ing from 0.6 to 15.1 percent, has a median value of 3.4 and a mean value of 4.2 percent (F. G. Poole, written commun., Sept. 1966). The samples consisted of 14 specimens of sandstone, 6 of conglomerate, and 2 of shale or argillite. No analyses of interstitial permeability of outcrop samples or cores were made; the interstitial permeability probably does not exceed the upper value (0.0001 gpd per sq ft) for the lower clastic aquitard (table 4). The Eleana Formation was hydraulically tested only in well 84—67, in central Yucca Flat. This well penetrated 84 feet of saturated dolomite and 133 feet of underlying argillite and minor dolomite. The results of this test by bailing indicate a specific capacity of about 0.18 gpd per foot of drawdown for 217 feet of saturated rock, or 0.83 gpd per foot of drawdown per 1,000 feet of saturated aquitard. The relatively high yield of the aquitard (in comparison with that of the lower clastic aquitard and the Dunderberg Shale Member) in this well is attributed to 84 feet of saturated dolomite in the upper part of the unit. By analogy with the lower clastic aquitard, the gross fracture transmissibility of the upper clastic aquitard is probably less than 500 gpd per ft; similarly, regional ground—water movement through the aquitard is probably controlled by interstitial permeability rather than fracture transmissibility. TUFF AQUITARD The tuff aquitard comprises all the tuffs and associated sedimentary rocks older than the Paintbrush Tuff (table 1). By definition, it includes the strata of the Wahmonie Formation, Salyer Formation, Tuff of Crater Flat, Rocks of Pavits Spring, Horse Spring Formation, Indian Trail Formation, and the rhyolite flows and tuf- faceous beds of Calico Hills. These formations consist of nonwelded to incipiently welded ash-flow tuff, ash-fall (or bedded) tuff, tuff breccia, breccia flow, tuffaceous sandstone, siltstone, mudstone, freshwater limestone, C44 and minor densely welded tuff. Despite their widely differing origins, these strata usually have one feature in common: matrices consisting of zeolite or clay minerals. The zeolitic or clayey matrices of these rocks are probably responsible for their very low interstitial permeability. The lenses of densely welded tuff in the In— dian Trail Formation and the freshwater limestone in the Horse Spring Formation and in the Rocks of Pavits Spring may not have zeolitic or clayey matrices, but they are believed to have negligible interstitial and fracture permeability and are therefore included as part of the tuff aquitard. The bedded tuff associated with the densely welded tuff of the Grouse Canyon Member of the Indian Trail Formation is usually vitric and permeable; this bedded tuff is assigned to the bedded-tuff aquifer. The tuff aquitard, by virtue of its stratigraphic position (table 1), generally separates the Tertiary welded and bedded-tuff aquifers from the underlying Paleozoic aquifers and aquitards. The tuff aquitard is as much as 2,000 feet thick in cen- tral Yucca Flat, where it consists of the Indian Trail For— mation. The aquitard is more than 2,000 feet thick in western Jackass Flats, where it consists of the informally designated rhyolite flows and tuffaceous beds of Calico Hills. In Frenchman Flat the aquitard may aggregate more than 4,500 feet in thickness and consists primarily of the Wahmonie Formation, the Salyer Formation, and the Rocks of Pavits Spring. In Rock and Mercury Val- leys the aquitard consists of the Rocks of Pavits Spring and the Horse Spring Formation. CHARACTER BASED ON OBSERVATIONS IN UNDERGROUND WORKINGS Geologic, geophysical, and hydrologic characteristics of zeolitized bedded tuff of the Indian Trail Formation were studied in more than 5 miles of tunnels, shafts, and drifts driven into these rocks beneath Rainier Mesa. These studies have been recorded in dozens of reports (Diment and others, 1958, 1959; McKeown and Dickey, 1961; Byers, 1961; Clebsch and Barker, 1960; Clebsch, 1960; and Thordarson, 1965). Thordarson (1965) observed that: 1. The average interstitial porosity of five beds of zeolitized (ash-fall) tuff ranged from 25 to 38 per- cent. The average interstitial permeability ranged from 0.0004 to 0.03 gpd per sq ft. 2. Except near the portals of the tunnels, the interstices of the tuff are 100 percent saturated. Interstitial saturation occurs as much as 400 feet above the top of the zone of saturation within open fractures. High bulk capillarity and very low interstitial permeability are probably responsible for the saturation. 3. The only free ground water observed in the un- HYDROLOGY OF NUCLEAR TEST SITES derground workings occurred in open fractures, mostly fault zones; however, not all open fractures were saturated. 4. The initial discharge of water from most of the frac- tures was less than 20 gpm, but the discharge from one fault zone was about 200 gpm. The discharge from all the fractures decreased rapidly, and within a few days it was a small fraction of the initial flow. 5. The water-bearing fractures were poorly connected or unconnected. The chief evidence for the poor con- nection was unsaturated fractures interspersed with saturated fractures at the same tunnel level; tunneling commonly intersected saturated frac- tures several hundred feet from other fractures which had been virtually dewatered several days earlier. The few static water levels available in wells or shafts indicated that the zone of fracture saturation had an irregular upper surface. 6. The nature of fractures was as follows: (a) Faults and joints have steep to nearly vertical dips. (b) Faults are scarce in comparison with joints. (0) Most of the faults have stratigraphic dis- placements measured in inChes, and many extend less than 300 feet. ((1) The widths of openings in faults and joints differ considerably. Some faults are open as much as 6 inches, whereas others are nearly sealed with fault gouge. The joints are generally closed but in places are open as much as several inches. Some fractures open several inches at one point are tightly closed within just a few feet along their strike. 7. Of the 110 faults and estimated 5,000 joints mapped in the U12e tunnel complex, 50 to 60 percent of the faults yielded most of the fracture water; a small percentage of the joints yielded a fraction of total water discharged. 8. Water in the fractures is perched above a buried ridge of unsaturated lower carbonate aquifer. The water is perched by the poor hydraulic connection of the fractures themselves. The poor hydraulic connec- tion may reflect the slight extent of the fractures. Several thin clayey strata within the otherwise zeolitized and relatively competent tuff sequence may also be partly responsible for the perching, if the water-bearing fractures close within these thin strata. 9. The specific capacity of test hole 88—63b, drilled into the aquitard from the floor of the U12e.03 drift, was less than 0.01 gpm per foot of drawdown. The hole had penetrated 614 feet of saturated aquitard at the time of the test. SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE Maps of the U12e tunnel complex by McKeown and Dickey (1961) indicate joint densities as great as four per 10 feet of tunnel, although many segments of tunnel 20 to 40 feet in length are unjointed. Thordarson’s observations were of water perched in the tuff aquitard hundreds of feet above the regional zone of saturation in the underlying lower carbonate aquifer. J. E. Weir, Jr. (written commun., Mar. 1967), on the other hand, documented water in two chambers mined in the aquitard beneath Pahute Mesa at depths of as much as 2,000 feet below the regional water table. In one chamber (1,000 ft below water table), according to Weir, “Most of the water seemed to be entering through that part of the chamber containing the most fractures. However, all the chamber walls were damp to wet, which suggests that some water was also moving through the rock, rather than entirely through the joint or fracture system.” The rate of water entry into this chamber, ex- cavated in zeolitized bedded tuff and tuffaceous sediments, was less than 4 gpm. In a deeper chamber (2,000 ft below water table), ground water flowed only from microfractures in one side of the room at a rate of about 1 gpm, and the walls of the remainder of the chamber were dry. Interstitial seepage, if any, must have been evaporated at the chamber face by the forced air ventilation. Weir also noted that the initial yield from microfractures decreased with time. Thordarson’s (1965) observations and Weir (written commun., Mar. 1967) indicate that the zeolitized ash- fall tuff and zeolitized tuffaceous sandstone should be classified as a fractured aquitard with high interstitial porosity but very low interstitial permeability. The frac- ture transmissibility of fault zones apparently ranges from nearly zero to a modest amount along some fault zones, but both the rapid drainage of these fractures and their interspersal with unsaturated fractures attests to their poor hydraulic connections. Accordingly, on a regional scale, ground-water movement through the tuff aquitard is probably controlled by the interstitial permeability of the rock rather than by fracture transmissibility. The strata composing the tuff aquitard in the un- derground workings are predominantly zeolitized ash- fall, or bedded, tuff and zeolitized tuffaceous sediments with minor clayey tuff. However, beneath Yucca Flat, Frenchman Flat, and Jackass Flats, nonwelded to par- tially welded ash-flow tuff, tuff breccia, siltstone, claystone, and freshwater limestone compose varying parts of the aquitard; these strata commonly have zeolitic or clayey matrices. Knowledge of their hydraulic characteristics was derived from several test holes in Yucca Flat, one hole (73—66) in northeastern Rock Valley, and one hole (74—57) in western Jackass Flats; these findings are described in the following two sec- tions. C45 CHARACTER BASED ON CORE AND ELECTRIC-LOG ANALYSES Interstitial porosity and permeability of the tuff aquitard beneath Yucca Flat and Rock Valley and the gross mineralogic character of the basal strata com- posing the aquitard in Yucca Flat were determined from core and electric-log analyses. Interstitial porosity and permeability of 72 cores of the tuff aquitard in Yucca Flat and Rock Valley are given in table 5. These cores were obtained from holes which completely penetrate the aquitard in Yucca Flat and in Rock Valley. TABLE 5 — Interstitial porosity and permeability of cores from the tuff aquitard, Nevada Test Site [Total porosity, in percent, determined from grain and bulk densities; effective porosity, in percent, determined by mercury-injection or water-saturation methods; ermeability, in gallons per day per square foot, determined using Denver, Colo., tap water. nalyses by US. Geol. Survey, Denver, Colo] Porosity or Number of permeability samples Range Median Mean Zeolitized tuff [Chiefly ash-flow and ash-fall tuff with zeolitic matrices] Total porosity ________ 34 19.8—48.3 38.8 37.7 Permeability _________ 34 000005—06 .006 .05 Clayey tuff and clayey sediments [Some clayey volcanic breccia] Effective orosity ______ 32 1.8—21.6 10.4 11.0 Permeabi ity _________ 38 0.000002~0.4 .00006 .02 The interstitial permeability is very small; median values of the clayey and zeolitized cores, respectively, are 0.00006 and 0.006 gpd per sq ft. Median values of porosity are 10 and 39 percent, respectively. In general, core analyses indicate that the interstitial porosity and permeability of the tuff aquitard beneath the valleys are similar to those of the tuff aquitard in the tunnels and shafts. Therefore, the hydraulic tests of the aquitard beneath Yucca Flat and other valleys will probably reflect principally the transmissibility of water-bearing fractures (fault zones?) that cut the rock. The basal 10 to 440 feet of the tuff aquitard beneath Yucca Flat commonly consists of clayey tuff and (or) clayey tuffaceous sediments, rather than of massively zeolitized rocks. Range in thickness of this clayey inter- val in 22 holes that reached the pre-Tertiary rock in Yuc- ca Flat is shown in the following tabulation: Number of wells Range in thickness of the clayey zone (feet) 10— 49 50— 99 100449 150—199 200—440 NAAoouh- In five additional holes, the basal clayey zone is absent or its presence is doubtful. The clayey basal part of the aquitard was verified through use of electric logs supplemented by X-ray analyses of cores and cuttings. R. D. Carroll, US. Geological Survey (written commun., July 1963), compared electrical resistivities in selected holes with X-ray analyses of cores and noted the follow- C46 ing general relationship: Tuffs and tuffaceous sediments with electrical resistivities of 20 ohm-meters or less have clay contents of 50 percent or more; resistivities of 10 ohm-meters or less indicate clay contents of 70 percent or more. X-ray analyses indicate that the clays consist chiefly of montmorillonite, illite, and mixed-layer minerals. HYDRAULIC TESTS The specific capacities for 32 intervals of the tuff aquitard in 9 test holes were determined by bailing, swabbing, or injection tests. Inflatable packers identical with those used to test the lower carbonate aquifer were used to isolate most of the intervals tested. Seven of these holes (83—69a, 84—68, 84—680, 87-62, 87—67a, ’ 88—63b, and 88~66) are in or adjacent to Yucca Flat; one (73—66) is at the head of Rock Valley, and one (74—57) is in western Jackass Flats. The values of specific capacity range from 0.01 to 3 gpm per foot of drawdown per 1,000 feet of penetration. The mean value is 0.5 gpm per foot of drawdown per 1,000 feet, and the median is 0.09 gpm per foot of draw— down per 1,000 feet. The higher specific capacities and the mean, which is disproportionately affected by high values, are above the arbitrary limit (0.1 gpm per ft of drawdown per 1,000 ft) established previously for aquitards. Short-term packer tests, however, affect a relatively small volume of rock. By analogy with the rapid drainage of fractures in underground openings in the tuff aquitard, the authors believe that the relatively high specific capacities are very localized. The median value is within the range defined for aquitards and suggests a coefficient of transmissibility of only 100 to 200 gpd per ft. Transmissibility of the tuff aquitard as measured in wells is probably controlled by fractures, because the in- terstitial permeability is usually too low to permit con- tinuous bailing or swabbing. Direct evidence of such con- trol is available from test well 87—62. The upper 2,000 feet of that hole w'as drilled with cable tools and was dry to a depth of 560 feet. During drilling through the tuff aquitard at that depth, water suddenly entered the hole and rose to a depth of 412 feet below the surface. The sudden entry of water into the bore probably reflects the intersection of a partially open fault zone or joint, similar to those observed in the underground workings. Some wells may eventually penetrate fault zones of high transmissibility, as in the tunnels. Highly fractured zones, if penetrated in wells, may also be poorly con- nected and readily dewatered, as were those penetrated by the underground workings. Because of the poor hydraulic connection of the water— bearing fractures observed in the tunnels, regional move- ment of ground water through the tuff aquitard is HYDROLOGY OF NUCLEAR TEST SITES probably controlled chiefly by the interstitial permeability of the tuff. LAVA-FLOW AND OTHER MINOR AQUITARDS The dacitic lava flows of the Wahmonie Formation constitute the lava-flow aquitard (table 1). These lava flows, aggregating as much as 4,000 feet in thickness, oc- cur chiefly northwest of the Cane Spring fault zone (fig. 31); individual flows are as much as 800 feet thick. The extent of these lavas is shown on maps by Poole, Elston, and Carr (1965) and by Ekren and Sargent (1965). Knowledge of the hydrologic properties of these rocks is limited to data from a few core holes and an un- derground chamber in Wahmonie Flat, about 11/2 miles northwest of Cane Spring (fig. 31). The core holes penetrated a perched zone of saturation at depths of about 80 to 166 feet below the surface (alt. 3,924 to 4,042 ft above mean sea level). The interstitial permeability of the dacitic lava flows is extremely low. The interstitial permeability to nitrogen gas was determined for five dacite core samples under confining pressures ranging from 14.7 to 2,000 psi (pounds per square inch) (Johnson and Ege, 1964). Under 1 atmosphere confinement (14.7 psi), four intact specimens had permeabilities less than 0.1 millidarcy, while one core with a small fracture had a permeability of 1.7 millidarcies. Under higher confining pressures the permeabilities of the intact cores were reduced by as much as three orders of magnitude, but the effect on the fractured core was slight. Based on these data the in- terstitial permeability to water should not exceed 0.0018 to 0.03 gpd per square ft (for the fractured core). Under the confining pressures at depths of several thousand feet, the permeability may be much lower. The in- terstitial porosity of the dacite samples ranges from 5.7 to 14.1 percent. Ground water is transmitted through the lava flows chiefly along fractures. From observations on the inflow of water from fractures into the chamber, C. E. Price (oral commun., 1963) concluded that the coefficient of transmissibility of the lava flow was probably less than 500 gpd per ft and could be a fraction of this quantity. Additional evidence that the gross transmissibility of the Wahmonie lava flows is very low is the perched water in these rocks. Two granitic stocks of probable Mesozoic age in north— central and northwestern Yucca Flat also constitute aquitards of minor areal extent. A detailed discussion of the occurrence of ground water in the Climax stock in north-central Yucca Flat was presented by Walker (1962). The hydraulic characteristics of the Gold Meadows stock in northwestern Yucca Flat are probably similar to the intrusive rocks of the Climax stock. SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE DISTRIBUTION AND SATURATED THICKNESS OF AQUIFERS AND AQUITARDS The complex structural and erosional history of Ter- tiary and pre-Tertiary rocks has resulted in a highly variable lateral and vertical subsurface distribution of hydrogeologic units. Because of Tertiary normal faulting and the pre—Tertiary large-scale folding, faulting, and erosion, the structural relief on many of the hydrogeologic units commonly ranges from 2,000 to 6,000 feet within a few miles and as much as 500 feet within 1,000 feet. Thus, a fully saturated hydrogeologic unit at depths of several thousand feet below the structurally deepest part of an intermontane valley may be only par- tially saturated near the margins of that valley. The same unit (unsaturated) may cap a mesa that rises 2,000 feet above the valley floor, or owing to erosion, it may be completely absent in outcrop. In addition to the complex structural disposition of the hydrogeologic units, the depth to water table also markedly influences the saturated thickness of most of the Cenozoic aquifers and aquitards beneath the valley floors. In valleys with relatively shallow water tables (less than 500 ft), all the hydrogeologic units except the uppermost few hundred feet of the valley-fill aquifer are usually saturated in the subsurface. However, in valleys with relatively deep water tables (500 to 2,000 ft), both the vertical disposition of the rocks and the depth to water influence the saturated extent and the thickness of the younger hydrogeologic units. In such valleys, the Cenozoic hydrogeologic units are commonly unsaturated beneath the margins of the valleys, and some units may be partially unsaturated even beneath the structurally deepest part of the valleys. , The effect of geologic structure and depth to water table on disposition and saturated thickness of the hydrogeologic units is best illustrated at Yucca Flat. YUCCA FLAT The general structural setting of Yucca Flat is shown on plate 2. The fence diagram (p1. 2A) shows areal and subsurface distribution of 8 of the 10 geohydrologic units (table 1). The major features illustrated are the following: 1. The lower clastic aquitard forms the hills bordering Yucca Flat on the northeast. These strata dip generally southwestward and form the southwest limb of the Halfpint Range anticline (structural feature 6 on pl. 1). 2. The lower carbonate aquifer occurs beneath the ridges bordering the valley on the east and the southeast and beneath the eastern two-thirds of the valley floor. The aquifer crops out or occurs at relatively shallow depths beneath ridges that rise 10. C47 as much as 1,300 feet above the valley floor; it is irregularly downfaulted to depths of 1,500 to 4,000 feet beneath the valley floor. .vThe upper clastic aquitard is the principal hydrogeologic unit beneath the western one-third of the valley and beneath the ridges bordering the valley on the west. These strata are bent into several large north-trending folds and have been thrust eastward over the lower carbonate aquifer along the relatively high angle Tippinip thrust fault. . The upper carbonate aquifer also occurs in the hills bordering the valley on the west, but its areal and subsurface extent is small. . Klippen of the lower carbonate aquifer locally overlie both the upper clastic aquitard and the up- per carbonate aquifer (pl. 2A, fences C—2 to C—3, D—2 to D—3, E—1 to E—3, and F—1 to F—3). . The general areal distribution of the pre-Tertiary rocks is controlled by pre-Tertiary structure, probably the Halfpint Range anticline (pl. 1), and by pre-mid-Tertiary erosion. The Tertiary block faulting markedly affected the vertical distribution of the. miogeosynclinal rocks but not their general areal distribution. . The tuff aquitard and the welded- and bedded-tuff aquifers, which crop out along the margins of the valleys at levels as much as 3,000 feet above the valley floor, are irregularly downfaulted to depths as much as 3,500 feet beneath the valley floor. The Tertiary block faulting is a major control on both the areal and the vertical distribution of these units. . The tuff aquitard generally separates welded—tuff, bedded-tuff, and valley-fill aquifers from the lower carbonate aquifer. In general, the structurally deepest part of Yucca Flat lies along the central to the eastern one-third of the valley; the western one-third of the valley is underlain by the upper clastic aquitard (or by klippen of the lower carbonate aquifer) at relatively shallow depth. The valley-fill aquifer occupies the troughs formed by the block faulting of the older rocks. However, the valley fill is not necessarily thickest in the structurally deepest part of the valley. Along fence E-2 to E-3 of plate 2A, a gravity survey indicates that the lower carbonate aquifer occurs nearly 3,500 feet below the valley floor at the site of well 81—69 where the valley fill is about 700 feet thick. On the other hand, beneath well 81—67 the valley fill is nearly twice as thick where the carbonate aquifer is probably buried only about 2,000 feet beneath the valley floor. A similar relationship is seen along fence D-2 to D-4. C48 The degree of folding and faulting of the hydrogeologic units is considerably greater than shown diagram- matically on plate 2A. Imbricate thrust faults of relatively minor displacement are common in outcrop within the lower and the upper clastic aquitards and may also be present within the lower carbonate aquifer. In addition, the subsurface contacts of the hydrogeologic units beneath the valley floor are depicted as relatively smooth surfaces on the fence panels, but the degree of stratigraphic offset and the frequency of faulting of the Cenozoic rocks beneath the valley floor are probably as great as those depicted along the fence panels near the margins of the valley. The difference in the detail shown in the panels is due to the sources used in their construc- tion. The panels near the margins of the valleys are based on areal mapping, whereas the panels across the valley floor are based only on well control and on structure-contour maps constructed from gravity sur- veys and well data. The influence of depth to water table on saturated thickness and extent of the hydrogeologic units is il- lustrated by plate 2A and B. Plate 23 is a generalized geologic map of Yucca Flat at an altitude of 2,400 feet. This level was chosen because it is the approximate altitude of the potentiometric surface in the lower car- bonate aquifer (and overlying Cenozoic hydrogeologic units) beneath the eastern two-thirds of the valley. The contacts may be in error by as much as 1 mile, but such an error does not detract from the major purpose of plate 2A and B, which show the following: I. The lower clastic aquitard is the chief hydrogeologic unit within and above the zone of saturation beneath the hills bordering Yucca Flat on the northeast. 2. The upper clastic aquitard is the chief hydrogeologic unit within and above the zone of saturation beneath the ridges bordering the valley on the west. 3. The lower carbonate aquifer lies within the zone of saturation and is under unconfined conditions beneath most of the low ridges bordering the valley on the east and southeast. Beneath much of the eastern two-thirds of the valley, on the other hand, the lower carbonate aquifer is also present within the zone of saturation but is under confined, arte- sian, conditions. Beneath this area of confinement, the tuff aquitard directly overlies the aquifer. The saturated thickness of the lower carbonate aquifer, ranging from a few tens to hundreds of feet in the immediate vicinity of the lower clastic aquitard (fence C—3 to C—4 and D—4 to D—5 of pl. 2A), probably increases to more than 10,000 feet toward the center of the valley. This significant increase in thickness reflects the stratigraphic sequence of these westward dipping rocks. The carbonate rocks at the water table along the east edge of fence D—3 HYDROLOGY OF NUCLEAR TEST SITES to D—5 of plate 2A, are probably of Middle Cam- brian age; those penetrated by well 83—69a are of Ordovician age (Antelope Valley Limestone); whereas those tapped by well 84—68d are of Devonian(?) age. Thus, near the east edge of this fence panel the saturated thickness of the lower carbonate aquifer probably ranges from a few hun- dred feet to 1,000 feet; at well 83—69a saturated car- bonate strata may aggregate as much as 10,000 feet in thickness, and beneath well 84—68d it possibly aggregates as much as 14,000 feet. 4. The upper carbonate aquifer is saturated below an altitude of 2,400 feet only beneath Syncline Ridge and a small area in the vicinity of well 83—66c. Because the potentiometric level in the western one-third of the valley ranges in altitude from 2,900 to more than 3,700 feet, the actual saturated extent of this aquifer beneath and adjacent to Syncline Ridge is probably twice that shown on the map; its saturated thickness may approach 2,000 feet. 5. The tuff aquitard is partly saturated over most of the eastern two-thirds of the valley; it is fully saturated only beneath the central part of the valley (and possibly in two small areas in the north-central part of the valley), where it is overlain by the saturated welded- and bedded-tuff aquifers. (Partly saturated zeolitized tuff also occurs beneath areas of high potentiometric surface within the upper clastic aquitard; these areas are not shown on plate ZB. The saturated thickness of the aquitard probably ranges from a few tens of feet to 1,500 feet. 6. The welded-tuff and bedded-tuff aquifers are fully saturated only where they are overlain by saturated valley fill; namely, in the center of the valley and in the immediate vicinity of well 86—67. Elsewhere they are partly saturated or unsaturated. Where fully saturated, these two hydrogeologic units may be as much as 1,000 feet thick. 7. Although the valley fill underlies most of the valley floor to depths of hundreds of feet, only in the cen- tral part of the valley and in the vicinity of well 86-67 is it thick enough to extend below the water table and be within the zone of saturation. The saturated thickness of the valley fill is at least 270 feet at well 83—68, but it probably does not exceed 400 feet at this site or elsewhere beneath the central part of the valley. Plate 2A and B are intended only as qualitative pic- tures of aquifer disposition and saturated thickness. Isopach maps of the valley fill and of the welded tuffs composing the welded-tuff aquifer have been prepared by Livingston Chase (written commun., May 1965) and by Harley Barnes (unpub. data) of the U.S. Geological Survey. These maps may be studied for more detailed SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE information on the saturated thickness of the Cenozoic aquifers. The scant distribution of saturated valley-fill, welded- tuff, and bedded-tuff aquifers beneath Yucca Flat is largely due to the great depth to water in this valley. Because of the relatively great thickness of the lower and upper clastic aquitards and the lower carbonate aquifer (in comparison with the younger hydrogeologic units), the depth to water table is usually not an important fac- tor in determining the extent or saturated volume of these Paleozoic rocks, as it is with the Cenozoic strata. Locally, where the lower carbonate aquifer occurs in thin klippen overlying the upper clastic aquitard, the depth to water plays an important role in controlling the saturated extent of these rocks. OTHER VALLEYS The surface and the subsurface extent of the hydrogeologic units and their thickness vary from valley to valley because of the complex structural and erosional history of these rocks. Gravity surveys of western Emigrant Valley, Frenchman Flat, Jackass Flats, southern Indian Springs Valley, Mercury Valley, and the Amargosa Desert were made by Messrs. D. L. Healey and C. H. Miller, US. Geological Survey (written com— mun., 1965), during the period 1961—64. These surveys, plus two deep-drill holes in Frenchman Flat (75—73 and 76—70) and one in Jackass Flats (74—57), indicate that the relief on the buried pre-Tertiary rocks is as much as several thousand feet. The gravity surveys and the out- crops of pre-Tertiary rock on the valley floors also in- dicate that the depth of burial of the pre-Tertiary rocks is not necessarily related to present topography; buried ridges of Tertiary or pre-Tertiary rocks commonly un- derlie parts of the valley floors at depths of a few tens to hundreds of feet. Therefore, by analogy with Yucca Flat, the structural relief on the Cenozoic hydrogeologic units beneath the valleys cited must also range from hundreds of feet to more than 2,000 feet beneath the valley floors and must reflect to a large degree the relief on the pre- Tertiary rocks. Thus, in those valleys with relatively deep water tables (Frenchman Flat, Jackass Flats, and Mercury Valley) the Cenozoic hydrogeologic units are probably saturated or partly saturated only beneath the structurally deepest parts of the valleys. In those valleys with relatively shallow water tables (western Emigrant Valley, southern Indian Springs Valley, and the Amargosa Desert), all the hydrogeologic units except the uppermost several tens to few hundreds of feet of the valley-fill aquifer are probably fully saturated. Details on the hydrogeologic setting of the valleys cited are presented in other parts of this report. The following generalizations about the regional dis- position of the hydrogeologic units appear valid: C49 1. The lower carbonate aquifer occurs alternately under confined (artesian) and unconfined (water-table) conditions. Beneath the deepest parts of most in- termontane basins, this aquifer is confined by the saturated tuff aquitard, whereas beneath ridges it is unconfined. Along valley margins (or beneath midvalley structural highs), it may be confined or unconfined depending upon the structural setting and the depth to water table. 2. The lower carbonate aquifer is saturated throughout the study area except beneath or in the vicinity of outcrops or buried structural highs of the lower clastic aquitard. 3. The tuff aquitard generally separates the welded-tuff and valley-fill aquifers from the lower carbonate aquifer, particularly in the structurally deep parts of the intermontane basins. However, in the vicinity of buried pre-Tertiary structural highs, the Cenozoic aquifers locally may be in direct contact with the lower carbonate aquifer; this would occur in areas where the tuff aquitard was never deposited or was eroded prior to the deposition of the strata comprising the Cenozoic aquifers. 4. In valleys with deep water tables (500 to 2,000 ft) the tuff aquitard may surround as well as underlie the Cenozoic aquifers at the altitude of the water table. 5. Because of the relatively great thickness of the lower and upper elastic aquitards and the lower car- bonate aquifer (in comparison with the younger hydrogeologic units), the depth to water table is usually not an important factor in determining the extent or saturated volume of these rocks, as it is with the Cenozoic strata. MOVEMENT OF GROUND WATER Ground-water movement within the study area may be classified as follows: (1) Movement of perched water; (2) intrabasin movement of water, including semiperched water; and (3) interbasin movement of water. The definition of perched ground water used in this report follows that of Meinzer (1923, p. 40), who stated: “Ground water is said to be perched if it is separated from an underlying body of ground water by unsaturated rock. Perched water belongs to a different zone of satura- tion from that occupied by the underlying ground water.” Perched ground water should not be confused with semiperched ground water, which Meinzer (1923, p. 41) defined: “Ground water may be said to be semiperched if it has greater pressure head than an un- derlying body of ground water, from which it is, however, not separated by any unsaturated rock * * *. Semiperched water, like perched water, is underlain by a C50 negative confining bed of either permeable or im- permeable type.” At Nevada Test Site and vicinity, semiperched water, as well as perched water commonly occurs within both aquitards (namely within Meinzer’s “negative confining bed”) and aquifers. Perched water commonly occurs in foothills and ridges flanking the basins and is water in transit to the regional water table. The tuff aquitard is the principal hydrogeologic unit in which the perched water occurs at Nevada Test Site. The movement of water between the Cenozoic and the Paleozoic aquifers and aquitards beneath a valley is called intrabasin movement of ground water. In Yucca and Frenchman Flats, ground water in the Cenozoic hydrogeologic units is semiperched and moves prin- cipally downward into the underlying lower carbonate aquifer. In the southern Amargosa Desert and in southern Indian Springs Valley, ground water in the Cenozoic rocks is derived through upward leakage of water from the underlying lower carbonate aquifer. In these areas, water in the lower carbonate aquifer has higher head than that in the Cenozoic rocks. At depths as much as several thousand feet beneath the valley floors and at shallower depths beneath the flanking ridges, ground water occurs within the pre- Tertiary aquifers and aquitards. This ground water generally moves laterally beneath the valleys and their bordering ridges. This lateral movement over wide areas is called interbasin movement of ground water. Such movement is possible in south-central Nevada, prin- cipally because of the widespread occurrence of the lower carbonate aquifer beneath most of the valleys and ridges within the study area. In test wells at Nevada Test Site, water levels either declined, rose, or remained unchanged as the holes were deepened. Such water-level changes can be interpreted to reflect more than one hydrogeologic setting, and such interpretations were evaluated during construction of the potentiometric maps. Declining heads with increasing depths are attributed to at least three causes: (1) penetration of an un— saturated zone beneath a perched zone; (2) energy losses in areas of dominantly vertical movement, as is common in recharge areas or areas with semiperched ground water; and (3) penetration of an aquifer of the same or greater transmissibility than an overlying aquifer but with a lower discharge point. Rises in water level with in- creases in depth are commonly due to penetration of zones of higher potential energy in discharge areas or on the upgradient side of hydraulic barriers. PERCHED GROUND WATER Perched ground water feeds several small springs (less than 5 gpm) at Nevada Test Site and numerous small- to moderate-yield springs (5 to more than 400 gpm) in the HYDROLOGY OF NUCLEAR TEST SITES Spring Mountains and the Sheep Range. The springs commonly emerge from consolidated rock within the mountains or ridges flanking valleys and are characterized by highly variable discharge and by variable temperature, usually less than 21.0°C. Perched ground water has also been observed and studied within some of the underground workings driven into Rainier Mesa in northwestern Yucca Flat and into the Climax stock of north-central Yucca Flat. The perched springs should be distinguished from springs that emerge from the valley-fill and the lower carbonate aquifers at low altitudes on the borders or floor of some valleys. These valley-level springs represent discharge points of a regional zone of saturation; they are characterized by high and uniform discharge and uniform temperatures that range from 24.0°C to 350°C. Some of the springs discharging at intermediate and low altitudes within the Spring Mountains may actually be semiperched at cer- tain times of the year, or even all year, rather than perched. For the purposes of this report these springs are all classified as perched. NEVADA TEST SITE Perched ground water at the Nevada Test Site occurs principally within the aquitards underlying ridges —— namely, within the widespread tuff aquitard and the lava-flow aquitard. Nine perched springs occur within Yucca Flat, Frenchman Flat, and Jackass Flats. Moore (1961) reported that only two, Whiterock and Cane Springs have discharges exceeding 1 gpm. Thordarson’s (1965) observation that all the springs in Yucca Flat discharge from the tuff aquitard is also true for the few springs in Frenchman Flat and Jackass Flats. Perched water oc- curs within the tuff aquitard and does not emerge from the bedded-tuff aquifer or the welded-tuff aquifer, which overlie the aquitard. Pavits Spring in southwestern Frenchman Flat (fig. 31) may be an exception. Five of six springs in Yucca Flat emerge from the tuff aquitard where it directly overlies the upper clastic aquitard; only at one spring (dry since 1958) does the aquitard immediately overlie the carbonate aquifer. The springs in Yucca Flat occur at altitudes of 5,050 to 6,250 feet above mean sea level. In the underground workings beneath Rainier Mesa, the perched water occurs only within the tuff aquitard. The tunnels penetrating the bedded-tuff aquifer are dry. Perched water in the tunnels occurs in poorly connected fractures, chiefly fault zones of minor stratigraphic offset. Figure 30, modified after Thordarson (1965, fig. 4), summarizes the occurrence of perched water in frac- tures within the tuff aquitard beneath Rainier Mesa. Beneath the eastern two-thirds of the mesa, the dolomite underlying the zeolitized tuff is unsaturated; the ground water within the fractures is perched. Beneath the SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE C51 8000"”SSt Eait <— Semiperched >“ Perched l— Rainier Mesa 7000'— Springs absent at — major contacts I, I U126 tunnel 6000’— P'- E — 8 .. f, o E 3: '- T: D o _ 5000‘— _ Regional water table _ _ _ __ 4000’— — 3000'— _ N “P yx on g 0 1000 2000 3000 4000 FEET ' I | l I l l I l 0 500 1000 METERS 2°00, DATUM IS MEAN SEA LEVEL Modified from Thordarso’n (1965, fig. 4) EXPLANATION Welded-tuft" aquifer V Test Hole L\\ Bedded-tuff aquifer FRACTURE TYPES 6) En echelon joints or faults, some contain water, others :1 "“P‘y @ Faults, closed above tuff aquitard—carbonate aquifer con— Tuff aquitard tact, fully and partly saturated 9 Fault, open at bottom, empty E 0 Fault, with pinch-and-swell structure, partly saturated Lower carbonate aquifer Perched and semiperched water is black FIGURE 30. — Diagrammatic section showing perched and semiperched ground water in the tuff aquitard of Rainier Mesa. Vertical exaggeration about X 2. C52 western one-third of the mesa, water in the fractures is semiperched, because the aquitard extends beneath the potentiometric surface of the lower carbonate aquifer. The top of the tuff aquitard at Rainier Mesa is about 6,600 feet above mean sea level at the east edge of the mesa, or about 800 feet beneath the top of the mesa. The perched water table in fractures within the aquitard is between 6,033 and 6,184 feet above mean sea level in the east-central part of the mesa (Thordarson, 1965, p. 34), or about 1,200 to 1,400 feet beneath the mesa surface. - This perched water table is in turn about 2,000 feet above the water table within the underlying lower car- bonate aquifer. Immediately south of the mesa at the site of well 87—62 (fig. 30), the top of the aquitard is about 5,931 feet above mean sea level and about 225 feet beneath the land surface. The perched water table here is about 5,746feet above mean sea level, or only 410 feet beneath the land surface (altitude 6,156 ft). The perched water level at this site is about 1,560 feet above the potentiometric surface within the lower carbonate aquifer and about 1,030 feet above the potentiometric surface within a welded tuff which is within the tuff aquitard (fig. 30). Perched water also occurs within the tuff aquitard in southwestern Frenchman Flat in the vicinity of Cane Spring, Pavits Spring, and test wells 73—66 and 73—68 (fig. 31). For example, well 73—66 began in the tuff. aquitard and was drilled through 3,137 feet of the aquitard. Perched ground water stood at a depth of 77 feet below the surface when the hole was 603 feet deep. The static water level within the lower carbonate aquifer (in the interval 3,137—3,400 ft) was 1,735 feet below the surface. At test hole 7 3—68 (fig. 31), water occurs within the tuff aquitard at a depth of 518i2 feet below the surface, or at an altitude about 600 feet higher than the regional potentiometric surface east and west of the site. Northwest of Cane Spring, perched ground water oc- curs within the lava-flow aquitard. The lava-flow aquitard crops out or occurs within a few tens of feet of the surface, and the perched water is within 80 to 166 feet of the surface, or at an altitude of 3,924 to 4,042 feet above mean sea level. A brief discussion of this perched water was presented by Johnson and Ege (1964). The occurrence of perched water in Climax stock and surrounding carbonate rocks in northern Yucca Flat was described by Walker (1962) and by Schoff and Winograd (1961). Perched water is not known to occur within the aquifers or aquitards beneath Yucca Flat, Frenchman Flat, or Jackass Flats. Occurrence of shallow ground water beneath these valleys reflects the presence of either the upper or the lower clastic aquitards at very shallow depth or the damming of water within a Cenozoic aquifer by the aquitards. HYDROLOGY OF NUCLEAR TEST SITES Based on the preceding described or cited examples, the occurrence of perched water at the Nevada Test Site may be summarized as follows: Where tuff, lava-flow, or granitic aquitards occur at or near the surface, perched water usually occurs at shallow depths — from about 80 feet at well 73—66 to as much as 410 feet at well 87—62. Where the aquitard lies deep beneath the surface, as at Rainier Mesa and at test well 73-68, the perched water table occurs at-depths of about 520 to 1,400 feet beneath the surface. Similarly, where the top of the aquitard is at a high altitude, as at Rainier Mesa, the perched water table is also at a high altitude (6,000—6,200 ft); whereas in western Frenchman Flat, where the aquitard crops out at low altitudes (3,900 to 4,100 ft), the perched water occurs at lower altitudes. Areal differences in precipitation probably do not cause these wide variations in vertical position of the perched water tables, because even at lower altitudes, where precipitation is at a minimum, fractures in the aquitards may be saturated nearly to the surface. Conversely, beneath Rainier Mesa, which receives the maximum recorded precipitation at Nevada Test Site (about 8 in. annually), the fractures in the upper 400 feet of the aquitard generally are unsaturated. The preferred explanation for the variation in depth to and altitude of the perched water beneath the ridges and hills flanking the valleys follows. The aquitards underlying the ridges or buried beneath the margins of the valley floor are blocks of poorly permeable rock surrounded and underlain by strata of much higher permeability. When drainage of recharge from the aquitard to the surrounding aquifers is retarded, perched ground-water mounds develop within the aquitards. The altitude of the mounds is largely a function of the vertical disposition of the aquitards. Perched ground water may be found locally throughout the Nevada Test Site wherever aquitards compose ridges or hills that lie above the regional zone of saturation. Thordarson’s work (1965) shows, however, that the occurrence of such water is erratic and depends largely upon the interconnection of the fractures within the aquitard and, in turn, their connection with the un- derlying aquifers. SPRING MOUNTAINS Occurrence, geologic control, and discharge of springs in the Spring Mountains (pl. 1) was described by Maxey and Jameson (1948) and by Hughes (1966). Maxey and Jameson presented data that illustrate the highly variable discharge of several major springs. Hughes prin- cipally discussed the geologic controls localizing the springs. Most springs within the Spring Mountains issue from the lower carbonate aquifer, which makes up most of the range and its highest part. Perhaps two dozen springs SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE also discharge from the lower clastic aquitard in the northwestern quarter of the range and from Mesozoic strata, which form part of the southeastern end of the range. Discharge of the more than 60 springs in the area ranged from a few gallons per minute to more than 16,000 gpm (Maxey and Jameson, 1948, fig. 8); however, only about eight of these springs discharged more than 100 gpm, and the majority discharged less than 10 gpm. Between spring and fall, discharges of several high-yield springs apparently differ by more than an order of magnitude. The 1964 summer water temperatures reported by Hughes ranged from 60° to 210°C and, in general, varied inversely with altitude. In the summer of 1964 Hughes measured a total dis- charge of about 1,430 gpm for the springs within or on the edge of the ipring Mountains. Of the total discharge, about 1,300 gpm discharged from the lower carbonate aquifer, which forms the highest part (6,000—12,000 ft) of the Spring Mountains. About 100 gpm discharged from sandstone (Aztec Sandstone) of probable Jurassic age, which crops out along the southeastern edge of the mountains at altitudes ranging from 4,000 to 6,000 feet. Only 30 gpm discharged from the lower clastic aquitard, which underlies the northwestern quarter of the moun- tain range at altitudes generally above 7,000 feet. Most of this discharge came from quartzite of the Stirling Quartzite and Johnnie Formation; little or none was dis- charged from shale or siltstone units. Geologic control of the springs varies. Most of the carbonate-rock springs described by Hughes are probably localized by faults or joints, although three of the high-yield springs (Deer Creek Spring, Lee Spring, and Trout Springs) reportedly emerge from small caverns. The springs discharging from the lower clastic aquitard are probably chiefly controlled by faults. The discharge from two of the major springs (Cold Creek Spring and Willow Spring) may be due to damming of the lower carbonate aquifer by the lower clastic aquitard, which crops out near the springs (fig. 33). Springs in the thick and crossbedded Aztec Sandstone discharge at the contact with the underlying relatively impermeable shales of the Triassic Chinle Formation. INTRABASIN MOVEMENT Ground water within the valley-fill aquifers of the in- termontane basins of the Southwest is commonly described as moving laterally from recharge areas in the flanking mountains toward discharge areas in playas, streams, or adjacent valleys. In addition, Tertiary and pre-Tertiary bedrock underlying and flanking the valley fill has commonly been considered relatively im- permeable in comparison with valley fill. At the Nevada Test Site, the valley-fill aquifers within Yucca and Frenchman Flats are surrounded by Tertiary and older C53 rocks; yet these valleys contain no “wet playas” — playas representing the intersection of the land surface and the water table — or perennial streams. On the con- trary, depth to the water table beneath these valleys ranges from 700 to 2,000 feet below the valley floors. The great depth to water in these structurally and topographically closed basins suggested to some geologists (in the early 1950’s) that the valley-fill and older aquifers had never filled with ground water. However, this early hypothesis is unreasonable because the valley-fill aquifer in adjacent basins — western Emigrant Valley and the Amargosa Desert — is nearly saturated. Moreover, widespread lake deposits indicate that most of the valleys in the study area contained lakes during the Pleistocene pluvial periods. Test drilling has shown that the deep water table in Yucca and Frenchman Flats is primarily due to the drainage of water from the valley-fill and older Cenozoic aquifers into the underlying and surrounding lower car- bonate aquifer. In Yucca and Frenchman Flats, this in- trabasin movement of ground water is downward from the younger into the older aquifers; that is, the water is semiperched. In other valleys, such as the Amargosa Desert, southern Indian Springs Valley, and perhaps eastern Jackass Flats, intrabasin movement is upward. YUCCA FLAT Ground water beneath Yucca Flat occurs within valley-fill, welded-tuff, bedded-tuff, and lower car- bonate aquifers and within tuff, upper elastic, and lower clastic aquitards (p1. 2A and B). The depth to water generally ranges from 535 to 1,915 feet beneath the surface, or from an altitude of 2,555 to 3,755 feet. Beneath the eastern two-thirds of the valley, the part underlain by the lower carbonate aquifer (pl. 23 and C), the range in depth to water and altitude of the water table is considerably less; here the depth to water generally ranges from 1,507 feet at well 81—69 near the south (and topographically lowest) end of the valley to 1,915 feet at well 88—66 near the north (and topographically highest) end of the valley, or from an altitude of 2,422 to 2,555 feet above mean sea level. Beneath the west-central and southwestern parts of the valley, the area underlain chiefly by the upper elastic aquitard (pl. 2B and C), the zone of saturation is con- siderably shallower and is more than a thousand feet higher than levels in the eastern two-thirds of the valley. Plate 23 shows that the tuff aquitard is the principal Cenozoic hydrogeologic unit within the zone of satura- tion in the eastern two-thirds of the valley and that it is unconfined through most of its saturated extent. The welded-tuff and the bedded-tuff aquifers are saturated beneath the central and northern parts of the valley and occur under both confined and unconfined conditions. C54 The valley—fill aquifer is saturated in an area of less than 10 square miles, chiefly in the central part of the valley, and it is unconfined. Data obtained during drilling of two holes penetrating the tuff aquitard provided the first clues on movement of ground water within the Cenozoic rocks. When well 88—66 was 2,045 feet deep, the static water level was 1,915 feet beneath the surface (J. E. Moore and others, 1963). After the well had been deepened to 2,535 feet and cased (but not cemented) to 2,121 feet, the water level was 1,959 feet beneath the surface. No further physical changes were made in the well for 5 months thereafter, but the water level gradually declined another 20 feet. After penetration of the lower carbonate aquifer (Pogonip Group) at a depth of 2,550 feet, the water level in the well fell to 2,05512 feet (the head Within the car- bonate aquifer). The water level within the well remained at 2,055i2 feet during subsequent penetration of the carbonate aquifer to a depth of 3,422 feet. A similar decline in head was documented in well 83—69a in east-central Yucca Flat (L. R. West and William Thordarson, written commun., July 1965). At well depths of 1,875 and 1,970 feet, the measured static water level was 1,716 feet beneath the land surface. After the hole was deepened to 2,430 feet (about 70 ft into the underlying lower carbonate aquifer), the water level dropped to 1,732 feet below the land surface. Later the hole was deepened to 2,620 feet, or 260 feet into the lower carbonate aquifer (Pogonip Group), and the static water level in the aquifer was 1,780 feet below the land surface. A comparison of the heads in wells tapping only the Cenozoic aquifers with the head in wells tapping the lower carbonate aquifer shows a head differential between Cenozoic and pre-Cenozoic rocks throughout the eastern two-thirds of Yucca Flat. Plate 2C shows that water-level altitudes in valley-fill and welded-tuff aquifers are higher than the head in the lower carbonate aquifer. Thus, ground water in the Cenozoic strata is semiperched with respect to water in the lower carbonate aquifer. Water- level altitudes in wells tapping the Cenozoic rocks indicate one, and possibly two, hydraulic sinks along the longitudinal axis of Yucca Flat. One apparent sink is in the vicinity of wells 83—68 and 84—68 in the cen- tral part of the valley, and the second is near well 87—67a in the north-central part of the valley (pl. 2C). The static water level in wells in both the valley-fill aquifer and the tuff aquitard beneath the southern sink ranges from 2,395 to 2,406 feet (wells 83—68, 84—68, 84—68a, and 84-680), whereas levels in surrounding wells tapping Cenozoic strata range from 2,422 to 2,455 feet (wells 81—67, 81—69, and 83—69a). In view of the documented head changes with depth, it is important to emphasize that the centrally located sink is probably not a reflec- tion of composite static levels of different depth inter- HYDROLOGY OF NUCLEAR TEST SITES vals. The levels in three key wells (81—69, 83—68, and 83—69a) were measured after penetration of only 25—170 feet of saturated rock, and these levels probably reflect accurately the top of the semiperched water table. The apparent sink in the north-central part of the valley, however, may reflect composite static levels. Some average hydraulic gradients toward the cen- trally located sink are given in table 6; also given are some average hydraulic gradients calculated for assumed predominantly vertical, rather than nearly horizontal water movement through the tuff aquitard. As expected, the lowest horizontal gradients occur between the pairs of wells in which at least one taps a Cenozoic aquifer. The computations of the average horizontal gradients are based on assumed uniform transmissibility and on the absence of hydraulic barriers between control wells. Availability of head measurements for both the tuff aquitard and the lower carbonate aquifer in the same well or in wells a few hun- dred feet apart permitted the estimate of possible average vertical hydraulic gradients. Both sets of gradients are used later to estimate the quantity of ground-water flow toward the sink or downward into the lower carbonate aquifer. Because of a relatively low hydraulic gradient within the lower carbonate aquifer (from less than 0.5 to 20 ft per mile) and the hydraulic sinks in the Cenozoic rocks, the difference in head between the Cenozoic hydrogeologic units and the lower carbonate aquifer is least near the central part of the valley and greatest along its periphery. Ground water in the Cenozoic water-bearing units probably cannot leave Yucca Flat without passing through the tuff aquitard to the underlying and sur- TABLE 6. —— Hydraulic gradients in Cenozoic hydrogeologic units, Yucca Flat Wells between, or at, which gradient was calculated Average hydraulic gradient Hydrogeolagic unit Feet per foot Feet per mile Near horizontal flow assumed 83— 69a ____________ Tuff aquitard ________ 84— 68a _______________ do ___________ } 0006 32 84—6811 _______________ do ___________ 84‘68 _______________ do ___________ } .002 11 837695! _______________ do --_____,__; 84—68c _______________ do ___________ } .006 32 844580 _______________ do ___________ 84768 _______________ do ___________ } 003 16 83 698 _______________ do ___________ 83— 68 ____________ Valley- fill aquifer _____ } '004 21 81 67 ____________ Bedded- tuff aquifer _ _ _ _ 83—68 ____________ Valley-fill aquifer _____ ~001 5 81-69 ____________ Welded-tuff aquifer _ _ _ _ 83— as ____________ Valley fill aquifer _____ } 0009 5 83— 68 _______________ do ___________ 84 68 ____________ Tuff aquitard ________ } ~0007 4 Near vertical flow assumed 83— 69a ____________ Tuff aquitard ________ 0.1 530 28— 23 _______________ go ___________ .2 1. 100 4- a _______________ o ___________ 84—6811 _______________ do ___________ } -02 “0 SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE rounding lower carbonate aquifer. It could move laterally out of the valley within the Cenozoic rocks only through a narrow strip about three-fourths of a mile wide at the south end of the valley (east of well 79—69a), where the tuff aquitard is not surrounded by the lower car- bonate aquifer at the altitude of the water table. (See pl. 23.) Beneath this strip, the tuff aquitard at the southern tip of Yucca Flat is hydraulically connected with the aquitard beneath northernmost Frenchman Flat. Significant lateral discharge through this strip is probably unlikely for four reasons: 1. The very low fracture transmissibility and interstitial permeability and the small cross-sectional area of the aquitard preclude movement of a significant quantity of water. 2. Underflow across this strip toward Frenchman Flat would be subjected to drainage into the lower car- bonate aquifer, which borders the strip on the east and west. 3. Water levels in the Cenozoic rocks beneath central and south-central Yucca Flat indicate a northerly, rather than a southerly, hydraulic gradient. 4. The 11-foot difference in water level between the southernmost well tapping the Cenozoic rocks in Yucca Flat (well 81—69) and the northernmost well (77—71a) tapping these rocks in Frenchman Flat (pl. 2C) does not support lateral flow. Both wells tap the welded-tuff aquifer, and the apparent hydraulic gradient between them is less than 2 feet per mile; the wells are about 7.5 miles apart. However, if it is assumed that they are connected hydraulically by lateral flow, which is chiefly through the intervening strip of tuff aquitard (pl. 2B), then greater differences in water level and hydraulic gradient are expectable between these wells. The data in table 6 suggest that horizontal components of hydraulic gradient in the aquitard range from 11 to 32 feet per mile; the lowest gradient indicates that the difference in water level between these wells should be several tens of feet. Both the apparent hydraulic sinks and the lower head in the carbonate aquifer as well as the disposition of the hydrogeologic units at the water table (pl. 28) indicate that ground water within the Cenozoic strata beneath the eastern two-thirds of the valley cannot leave Yucca Flat without entering the lower carbonate aquifer. However, whether the ground water in the Cenozoic rocks enters the carbonate aquifer primarily through radial flow toward the apparent hydraulic sinks or leaks vertically into the carbonate aquifer throughout the valley, or both, cannot be ascertained from our present data. The position and the nature of the centrally located hydraulic sink may be explained by one of two hypotheses or by a combination of both. The first C55 hypotheses is that ground-water flow is centripetal toward the sink, which represents an area of relatively good hydraulic connection between the Cenozoic and Paleozoic water-bearing strata. Hydraulic connection could occur in one of two ways. An isopach map of the valley fill and a structure-contour map of the pre- Tertiary surface prepared by Livingston Chase and W. J. Carr (written commun., Jan. and May 1965) indicate that valley-fill, welded-tuff, and bedded-tuff aquifers may locally be in direct contact with the lower carbonate aquifer near the west border of the saturated wedge of Cenozoic strata shown on plate 2B, particularly in the area between wells 81—67 and 84—68. This area is crossed by the Yucca fault, a major normal fault traversing the entire length of Yucca Flat. Vertical displacement of the lower carbonate aquifer (upthrown on the west) along this fault between wells 81-67 and 84—68 is more than 1,000 feet. The altitude of the top of the lower carbonate aquifer ranges from 1,000 to 1,700 feet above mean sea level immediately west of the fault. East of the fault the altitude of the base of the Cenozoic aquifers is about 1,000 to 1,500 feet. Thus, the base of the welded- or bedded-tuff aquifer could be faulted against the top of the lower carbonate aquifer. This condition could result in the low water level in well 83-68, which taps the valley-fill aquifer about 3,400 feet east of the fault. Similar faulting may also explain the apparent sink in the north-central part of the valley in the Vicinity of well 87—67a (pl. 2C). The structure-contour map shows a 1,000-foot displacement (upthrown on the west) of the carbonate aquifer along the Yucca fault 3,000 feet west of the well. If a fault-controlled hydraulic connection exists in the vicinity of the centrally located apparent hydraulic sink, it is a poor one, because the water-table altitude in well 83—68 is about 15 feet higher than the potentiometric level in the lower carbonate aquifer (alt 2,387 ft). Furthermore, the suggested direct-contact hydraulic connection via the Yucca fault does not explain the lowest Cenozoic water level in the valley, (alt 2,395 ft) in well 84—68; the well taps 600 feet of tuff aquitard. In the latitude of this well, displacement of the lower carbonate aquifer by the Yucca fault is minor (fence D-3—D-4, pl. 2A); another explanation must be sought for the water level, which is only 8 feet higher than the head in lower carbonate aquifer. Therefore, the 2,395-foot level in well 84—68 may represent a composite water level of several zones of decreasing head within the 600 feet of aquitard penetrated. The proximity of wells 83—68, 84—68, and 87—67a (the wells with the lowest water-table altitudes in the central and north-central parts of the valley) to Yucca fault suggests a mode of hydraulic connection other than direct juxtaposition of the Cenozoic and Paleozoic aquifers. The fracture transmissibility of the tuff C56 aquitard may have been greatly increased locally through fracturing that accompanied movement along Yucca fault. However, hydraulic tests of wells 84—68 and 87 —67a did not indicate above average transmissibility of the aquitard. Finally, neither of the two geologic conditions proposed to explain the hydraulic connection of the Cenozoic and Paleozoic strata satisfactorily explains the water level in well 81—67, which is about 3,500 feet west of the Yucca fault. This well probably taps the bedded- tuff aquifer. When first drilled, it reportedly had a water-level altitude of 2,439 feet. After deepening the hole from 1,575 to 1,800 feet, the driller reported a water- level altitude of 2,424 feet. Thus, the prepumping levels in this well stood 29 to 44 feet higher than the lowest levels least of the Yucca fault. Hydraulic isolation of this aquifer from the trough is difficult to explain (fence E—2 to E—3, pl. 2A) unless the Yucca fault is a hydraulic barrier. A second hypothesis for the position and the nature of the hydraulic sinks is that the water levels in the Cenozoic units reflect valleywide downward leakage into the lower carbonate aquifer after cessation of significant recharge at the end of the last pluvial period (roughly 9,000 yr ago). At the close of the pluvial period, the water table in both the Cenozoic rocks and the lower carbonate aquifer was probably hundreds of feet higher than pre- sent levels, and the volume of saturated Cenozoic rock was significantly greater. After reduction in recharge rate at the close of the pluvial period, two interrelated factors might have favored a more rapid decline of water table (within the Cenozoic hydrogeologic units) near the center than near the margins of the valley. First, the recharge from storm runoff or precipitation was probably greater along the margins of the valley than on the valley floor. Second, the tuff aquitard, structurally at successively higher altitudes with distance from the center of the valley, would particularly tend to retard lateral flow from the margins of the valley after the water table had dropped sufficiently to drain the overlying Cenozoic aquifers. Data supporting the second hypothesis are available from wells 83—69a and 88—66 in the east-central and northern parts of the valley, near the margin of the presently saturated wedge of the tuff aquitard (pl. 28). The top and the bottom of the tuff aquitard in wells 83—69a and 88—66 are several hundred feet higher than they are in more centrally located wells. The water levels in these two wells range from 31 to 131 feet higher than the water level in well 81—67, which has the next highest water level. By analogy with the occurrence of perched water in the aquitard, the high water levels in these two wells (83—69a and 88—66) may reflect extremely low transmissibility and structurally high position of the aquitard coupled with more postpluvial recharge to the HYDROLOGY OF NUCLEAR TEST SITES aquitard than that received by the centrally located wells. The water level in well 88—66, especially, illustrates the ability of the tuff aquitard to hold ground water above the level in the lower carbonate aquifer. Plate 2B shows that the aquitard tapped by this well is sur- rounded on three sides by the lower carbonate aquifer and that the aquitard is less than a mile wide. Despite the opportunity for drainage in three directions, the head in the aquitard is 140 feet higher than that in the sur- rounding carbonate aquifer. The second hypothesis does not necessarily rule out one or more geologically controlled hydraulic sinks, but it does not require hydraulic sinks to explain the con- figuration of the Cenozoic water levels. The water levels probably reflect variable vertical drainage of pluvial recharge into the lower carbonate aquifer via the tuff aquitard. The highest levels, near the margins of the present saturated zone, reflect the most recent recharge and the structurally high position of the tuff aquitard; the low levels reflect the oldest recharge and structurally lower positions of the aquitard. Leakage from the Cenozoic rocks into the lower car- bonate aquifer can be estimated using either the hypothesis of lateral flow toward a centrally located sink(s) or the hypothesis of vertical leakage throughout the valley. Assuming lateral flow through the tuff aquitard toward the central hydraulic sink, flow was es- timated using the equation Q = TI W. In the equation, T is coefficient of transmissibility, in gallons per day per foot; I is hydraulic gradient, in feet per mile; Wis length of the underflow strip, in miles; and Q is discharge, in gallons per day. The centripetal flow is assumed to be controlled by the tuff aquitard surrounding the Cenozoic aquifers. A value of 150 gpd per ft, calculated from hydraulic tests, was used for the coefficient of transmissibility of the tuff aquitard, and an average horizontal hydraulic gradient in the tuff aquitard of 25‘ feet per mile was suggested by data in table 6. The length of the underflow strip (W), about 15 miles, is the approximate length of the contact separating the welded- and bedded-tuff aquifers from the tuff aquitard on the east, north, and south (pl. 2B); little inflow from the west is likely. The calculated flow toward the sink is about 40 gpm, or about 65 acre-feet annually. If water also flows toward the questionable hydraulic sink in the north-central part of the valley, an additional few tens of acre-feet may discharge into the lower carbonate aquifer. However, if, as is likely, the fractures in the tuff aquitard are poorly connected, as noted in the underground workings, then the coefficient of transmissibility (150 gpd per ft) derived from the hydraulic tests of wells is much too high, and the actual flow toward the sink may be a fraction of 65 acre—feet annually. Assuming that leakage from the tuff aquitard to the SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE lower carbonate aquifer throughout the valley is vertical, the volume of water in transit can be estimated using the formula Q = PIA, where Q is discharge, in gallons per day; P is coefficient of permeability, in gallons per day per square foot; I is vertical component of hydraulic gradient, in feet per foot; and A is cross-sectional flow area, in square feet. The median coefficient of permeability (0.00006 gpd per sq ft) of the 38 analyses of clayey tuff and clayey sediments listed in table 5 is used for P. This value of the coefficient of permeability was used on the assumption that the vertical leakage is con- trolled principally by the clayey tuff and sediments at the base of the tuff aquitard. A maximum vertical com- ponent of hydraulic gradient (I) of 0.2 foot per foot is suggested by the data of table 6. The area (A) occupied by the tuff aquitard at an altitude of 2,400 feet is about 66 square miles, or about 1,850X 106 square feet . Using the given values of P, I, and A, a downwardleakage of only about 15 gpm, or about 25 acre-feet per year, is calculated. Hydraulic gradients one or more times greater than 0.2 might occur within the basal clayey tuff because its permeability is lower than that of zeolitized tuff, but even gradients several times larger than 0.2 do not increase the leakage to a significant magnitude. Potentiometric contouring of the water table in Cenozoic aquifers and aquitards in Yucca Flat is un- warranted for two reasons. First, if the movement of water is primarily downward throughout the valley, then contours of such a system are misleading. Second, of the 10 water levels in Cenozoic rock in the eastern two-thirds of the valley (pl. 2C), three are probably composite levels, and a fourth is a reported level. A single poten- tiometric contour, hachured to suggest downward move- ment of water, is shown for the Cenozoic strata on the regional potentiometric map (pl. 1). The preceding discussion of the ground water within the Cenozoic strata applies only to the eastern two- thirds of the valley, where the potentiometric level in all hydrogeologic units ranges from about 2,380 to 2,550 feet. In the vicinity of wells 83-66 to 83—66g, water was found in both the tuff aquitard and the underlying elastic aquitard at an altitude about 500 to 1,300 feet higher than the water table in the Cenozoic rocks in the eastern two-thirds of the valley. The high water table in the tuff aquitard reflects the elevated potentiometric level within the upper elastic aquitard; reasons for the higher head within the upper clastic aquitard are dis- cussed under “Interbasin Movement.” Similarly, because the potentiometric surface within the lower clastic aquitard is hundreds of feet above that in the lower carbonate aquifer, ground water within the Cenozoic strata beneath the northeastern part of the valley probably also occurs hundreds of feet above that semiperched in the same rocks underlying the eastern two-thirds of the valley. Most of the ground water in C57 these areas should lie within the tuff aquitard, because water in the overlying bedded-tuff, welded-tuff, and valley-fill aquifers probably has drained to lower levels in the central part of the valley. FRENCHMAN FLAT Knowledge of the occurrence and the movement of ground water within Cenozoic strata of Frenchman Flat is based on data from 14 wells or test holes within the valley and 4 wells along the periphery of the drainage basin. The location of these wells, the altitude of the water level in them, and the strata tapped by each are shown in figure 31. The figure also shows the areal dis- tribution of the hydrogeologic units; subsurface data (Jan. 1967) are insufficient for construction of fence diagrams across the valley or isopach maps and structure-contour maps of the strata underlying this valley. The regional water table occurs in the major Cenozoic hydrogeologic units at depths ranging from 518 to 1,176 feet beneath the valley floor, but perched water is found at depths as shallow as 70 feet within the tuff and lava- flow aquitards that crop out in the southwestern part of the drainage basin (fig. 31). In general, the depth to the water table is shallowest (about 700 ft) in wells on the playa and increases to nearly 1,200 feet near the margins of the valley; but exceptions occur in the northwest and the southwest corners of the valley. The altitudes of the static water levels of the wells show several patterns: 1. Water-table altitudes in the Cenozoic rocks in the eastern two-thirds of the valley (the area east of the Mercury Highway) range from 2,386 to 2,411 feet above mean sea level in nine wells. Of these levels, the three lowest are in wells along the southeastern half of the playa (Frenchman Lake), whereas the water levels in six other wells, as much as 6 miles north of the playa, range from only 2,409 to 2,411 feet above mean sea level (fig. 31). 2. Water levels in the two wells (79—69 and 75—73) tap- ping the lower carbonate aquifer along the north and the east peripheries of the valley are 10 to 30 feet lower than the levels in the Cenozoic hydrogeologic units; the water level in well 73—66, which taps the lower carbonate aquifer along the southwestern edge of the basin, is also lower than all but two of the wells tapping the Cenozoic rocks. Thus, ground water in the Cenozoic rocks is semiperched with respect to that in the lower car- bonate aquifer. 3. Water-level altitude in two wells west of the Mercury Highway (fig. 31) —- wells 77—68 in the northwestern part of the valley (the CP Basin) and well 73—68 in C58 110°15’ E. 600,000 E. 680,000 36°55’ — N. 780,000 — 2784 t 5 Q (flTal; Tmalg QTal _ 70—60‘D _ *' <3233 (wa) N. 760,000 N. 740,000 _ 73—68 2982 P (.7) 03 lthb or Ts) 73459 a) <2772 (pr at 3143) O 30°45’ N. 720.000 — 116°00' E. 700,000 Yuccal l ‘ , ' ® I \ \ . I \{F . . \ ~ 0377—7” C: 77—70‘D 2411 (Tm; Ts) l , \\ 2410 (Trnr) I QTal \i \ 77-71 0 93 77-70: \ m 2410 (0Tal; Trna) \ I >| ‘2: 76-70 :3 $2410i2(02a|;wa) o LII‘ic_0LN_C__OUN_TY_ o CLARK COUNTY LL} | _ A >- FHENCHMAN FLAT i/ fl 2' )lfj’L/ \7 I // 75-72 / \\ /// 2335 a mnnw I 237351—33 ) \\ { Frenchman l p . \ \ I \ / Lake \ 74—70a o / 2409 (OTall/ (Playa) / \ / 74-700 / l \ \ 2392 (OTal) ~\\ \ V / \\\ L / \\ 73-70 §\\\ 2397 (T03?) \Q‘q} _\\\\.\‘ Q\\\\ QTal , » R: \\\\ \\\T.. Q1“ \ \\\ \§\ §\ \\\\ \\\\ \\\\\ \\\\\ \\\\\ \\\\\ \\\\\— \\\\\ §:\\\ \\R\\ HYDROLOGY OF NUCLEAR TEST SITES E. 720,000 115°45’ E. 740,000 20,000—foot grid based on Nevada coordinate system, central zone 0 l Hydrogeology by I. J. Winograd, 1965 Geology modified from Poole, Elston, and Carr (1965); Harley Barnes (unpub. data) 4 MILES 3 4 KILDMETERS FIGURE 31. — Hydrogeology of Frenchman Flat. the southwest corner — are nearly 400 and 600 feet higher, respectively, than the water levels in wells east of the highway. The anomalously high water level in well 73—68 probably represents a perched zone of saturation within the tuff aquitard. This well penetrated the tuff aquitard at a depth of 165 feet and bottomed in the aquitard at a depth of 1,504 feet. Drillers first reported water at a depth of about 660 feet. Several days after completion of drilling, the water level stood 518 feet below land surface (water-level alt 2,982i2 ft). Perched water at three other nearby places within the tuff aquitard — Pavits Spring, Cane Spring, and well 73—66 (fig. 31) — indicates that the water level in well 73—68 is probably perched or possibly semiperched. The altitudes of the perched water at Cane Spring, Pavits Spring, and well 73—66 are 4,060, 3,940, and 4,066 feet, respectively, or roughly 1,000 to 1,100 feet higher than the level in the well 73—68; but the aquitard, which crops out at these three sites, is as much as 600 to 700 feet higher than the top of the aquitard (alt 3,335 ft) beneath the site of well 73-68. The hills and the flanking valley floor in the SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE C59 EXPLANATION D QTal i >-' Valley-fill aquifer 9; Alluvial fan, fluvirl, fanglomerate, Iakebed, and mud/low deposits ['21 fl is: (a Welded-tuff and bedded-tuff aquifers Densely to semi-welded tuff of Piapi Canyon Group; interbedded vitric ash-fall tuff is bedded-tuff aquifer Tuff and lava-flow aquitards Ash-fall and ash-flow tuff mam'vely altered to zeolite or clay; lava flours; also in- clude tuffaceous and clayey lake 1 r ' a, yr " ' 'y rocks of ‘ ' L ' , Salyer, and Horse Spring Formations and Rocks of Pavits Spring Upper elastic aquitard Argillite, quartzite, conglomerate, and minor limestone of Elana Formation,- overlain at shallow depth by upper carbonate aquifer and by klippen of lower carbonate aquifer and lower elastic aquitard Lower carbonate aquifer Dolomite and limestone; minor shale and quartzite Contact _L __—————— Fault Arrows indicate direction of relative movement; dashed where approximately located HYDROGEOLOGIC AND GEOLOGIC UNITS SYMBOL GEOLOGIC UNIT HYDROGEOLOGIC UNIT QTal Valley fill Valley-fill aquifer Tma Ammonia Tanks Member of Timber ‘ Mountain Tuff Tmr Rainier Mesa Member of Timber Mountain Tuff Tpt Topopah Spring Member of Paint- brush Tuff Tph Sandstone and tuff of Hampel Hill (lower part of Piapi Canyon Group) pr Tuff and sandstone of Piapi Canyon Group, and tuff, sandstone, and Tuff aquifers and aquitard lithic tuff of Wahmonie Forma- > and lava-flow a uita d tion undifferentiated q I Tw Wahmonie Formation undifferen- tiated wa Dacite and rhyodacite lava flows of Wahmonie Formation thb Tuff, sandstone, and lithic tuff breccia of Wahmonie Forma- tion Ts Salyer Formation undifferentiated th Tuff of Crater Flat Tpa Rocks of Pavits Spring J 12c Paleozoic carbonate rock 0p Pogonip Group } Lower carbonate aquifer Ccu Carrara Formation, upper half FIGURE 31. — Continued. TERTIARY CAMBRIAN DEVONIAN AND TO DEVONIAN MISSISSIPPIAN QUATERNARY HYDRAULIC SYMBOLS NOTE: All altitudes in feet; datum is mean sea level; potentiometrie contours not drawn for reasons out- lined in text 75-73 . 2381 (09) Test well Well ,r '1, lower our ‘ 1 'f , upper ‘ is well number; lower number is altitude of static water level; symbol in parentheses is formation tapped 13-66 4066 P (Tub) 3612 P (Trc; Tm) .233 (Re) Test well Well tapping tuffaquitard and lower carbonate aquifer; upper number is well number; lower numbers are altitude of static water level; P, perched water; symbols in parentheses are formations tapped 73—E < 2772 wlpr "3143i Test well Well tapping Tertiary h ydrogeologic units; upper number is well number; lower number is altitude of static water level; symbol and number in parentheses are formation tapped and its altitude; symbol < denotes well was dry 75-72 02386 R “Hall Test well or water well Well tapping valley-fill aquifer; upper number is well number; lower number is altitude of static water level; R, reported water level; symbol in parentheses is formation tapped 77-71!) @2410 (0Ta|;Tmal Test well Well tapping both valley-fill aquifer and Tertiary hy- drogeologic units; upper number is well number; lower number is altitude of static water level; sym- bols in parentheses are formations tapped fl) #068 F (Twl Spring Number is altitude; P, perched or semiperched; symbol in parentheses is formation supplying spring 3:218]: Inferred ground-water barrier Width of symbol not intended to represent width of barrier, which may range from several tens to a few thousand feet 060 southwestern part of Frenchman Flat (fig 31), probably contain perched or possibly semiperched ground water at altitudes hundreds of feet above the water table beneath the eastern two-thirds of the valley. The existence of such a perched ground-water mound is likely because of the extremely low gross transmissibility of the aquitards and their structurally high position near the flanks of the valley. The anomalously high water level in well 77—68 in the northwestern part of Frenchman Flat, an area called the CP Basin (fig. 31), is due, at least in part, to low gross transmissibility of the lava-flow aquitard. Well 77—68 penetrated 1,093 feet of the valley-fill aquifer and 150 feet of the welded-tuff aquifer. The water level in well 77—68 stands at a depth of 7 963:5 feet, or at an alittude of 2,784 feet above mean sea level; this level is ap- proximately 370 feet higher than water levels in either the valley-fill or the welded-tuff aquifers east of the Mercury Highway (fig. 31). Because of the moderate to high transmissibility of the valley-fill and the welded-tuff aquifers (table 3) and because of the very low hydraulic gradient within these aquifers east of the Mercury Highway, the anomalously high water level in well 77—68 cannot be ascribed to perching or semiperching of water within strata of low gross transmissibility or to the ex- istence of steep hydraulic gradients within the aquifers. A hydraulic discontinuity must exist between the valley- fill and the welded-tuff aquifers beneath the CP Basin and the same aquifers beneath the eastern two-thirds of the valley, as shown in figure 31. The Cane Spring fault zone may extend northeastward and form this barrier, either by offsetting permeable units or by forming a curtain of fault gouge. The outcrop pattern and the data from test hole 76—68 suggest that the lava-flow aquitard occurs at relatively shallow depth beneath the arm of the valley connecting the CP Basin to the remainder of the valley. This inter- pretation is supported by a gravity map of the area (D. L. Healey, oral commun., 1965), which shows a northeast-trending gravity high connecting the unnamed hills northeast and southwest of the highway. In marked contrast to the two anomalous water levels in the northwest and the southwest corners of the valley, the water levels in nine wells tapping the valley—fill and the welded-tuff aquifers east of the Mercury Highway differ by only 25 feet; moreover, water levels in six of these wells differ by only 2 feet. The static water levels in five test holes in the northern half of the valley probably represent the top of the zone of saturation. The static levels in test holes 77—70, 77—71, and 77—71a were measured only after completion of the drilling; hence, any head changes with increasing depth would be reflected in the levels obtained. The water level in well 77—71b was measured after penetration of about 720 feet of saturated valley fill, yet its water—level HYDROLOGY OF NUCLEAR TEST SITES altitude is the same as that measured in the three nearby wells that penetrated only 112 to 272 feet of the valley- fill or the welded-tuff aquifer. The similarity of water- level altitude in these wells suggests either little downward movement in the Cenozoic aquifers in this part of the valley or very small head losses accom- panying vertical flow through the aquifers. The static water level in well 76—70 (alt 2,410i2 ft) was measured during a shutdown period before completion of drilling but after penetration of about 452 feet of valley-fill aquifer. This level is also considered representative of the head at the top of the zone of saturation because of its similarity to the other water-level altitudes within the valley-fill aquifer. However, after this well had been cased to 1,682 feet and drilled to 2,682 feet, about 1,330 feet into the lava-flow aquitard, the static water level had dropped to an alittude of 2,392 feet, or 18 feet lower than the level in the valley-fill aquifer. This drop in head is further evidence that water in the Cenozoic strata in Frenchman Flat is semiperched compared with that in the lower carbonate aquifer, which underlies most of the valley. The key question on the measured water levels in the three water-supply wells (73—70, 74—70b, and 74—70a) concerns the amount of dewatering since pumping began in the early 1950’s. Hood (1961, p. 48—52) concluded that the dewatering as a result of pumping from 1951 through 1960 could have been as much as 2 feet in well 73~70 and as much as 6 feet in well 74—70a. A reevaluation of Hood’s conclusions using more recent water-level data suggests that dewatering in wells 74—70b and 74—70a (as of 1961) was less than 4 feet. The water-level altitudes for these wells (fig. 31) represent levels measured after well 74-70a had been shut down for 6 days in 1959 and after a 133-day shut-down of well 74—70b in 1960—61. These water levels are probably within 4 feet of the static water level under virgin conditions. There is no detectable horizontal component of hydraulic gradient among the test wells in the north half of the valley or between these wells and water-supply well 74—70a in the south half. If a hydraulic gradient ex- ists, it is a fraction of a foot per mile. In contrast, hydraulic gradients of up to 16 feet per mile seemingly exist between the three wells along the west edge of the playa. Reasons for the contrast between hydraulic gradients in the north and south halves of the valley are not clear, but the contrast appears real. Water levels in the three watersupply wells and destroyed well 75—72 suggest that a hydraulic sink may exist in the Vicinity of well 74—70b, and possibly near well 75—72. But the ex- istence of a sink in the valley-fill aquifer tapped by these two wells seems incompatible with the apparent absence of a gradient in this aquifer northwest of the playa. Available subsurface geologic data are insufficient to explain the water-level patterns. By analogy with con- SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE ditions in Yucca Flat, the water levels in Frenchman Flat may be interpreted as follows: the lower water levels in wells 74-70b and 75—72 represent a hydraulic sink formed by geologic conditions favoring a direct hydraulic connection between the valley-fill aquifer and the un- derlying lower carbonate aquifer. The apparent absence of a hydraulic gradient in the valley-fill aquifer in the northern half of the valley in turn suggests that a northeast-trending hydraulic barrier that retards water flow exists near the northwest end of the playa (Frenchman Lake). The suggested barrier parallels the trend of the major faults cutting both the Cenozoic and the Paleozoic strata, including the Cane Springs fault zone mentioned above as a possible barrier isolating the CP Basin. The low water-level altitudes in two of the three wells (75—72 and 73-70) in the southeastern half of the playa may reflect a relatively shallow depth to Paleozoic rocks beneath the bottom of these wells. A gravity map suggests that Paleozoic rocks may be only about 700 feet beneath the bottom of well 75—72 and about 1,200 feet beneath well 73—70. Proximity of the bottom of the two wells (75—72 and 73-70) to the underlying carbonate aquifer might facilitate downward drainage of the Cenozoic strata if the tuff aquitard was fra *tured. However, the low potentiometric level in production well 74—70b cannot readily be attributed to the proximity of the pre-Tertiary rocks, which the gravity map indicates are roughly 2,400 feet beneath the bottom of this well. The very low gradient in the valley fill in the northern half of the valley may reflect extremely high transmissibility, the lateral movement of very small quantities of water through the aquifer, or a combination of high transmissibility and small volume of water in transit. The hydraulic tests of the valley-fill aquifers at Nevada Test Site indicate that those aquifers have only low to moderate transmissibility (table 3). Thus, the low gradient must reflect the lateral movement of small quantities of water. An alternative interpretation is that there is little or no lateral movement within the valley— fill or the welded-tuff aquifer in northern Frenchman Flat; that is, the water is ponded above the lower car- bonate aquifer by the surrounding and the underlying older aquitards and by the postulated hydraulic barrier. Ground water semiperched in the Cenozoic aquifers in Frenchman Flat probably cannot leave the valley without entering either the tuff or the lava-flow aquitard or the lower carbonate aquifer. Geologic maps and a gravity map of the Frenchman Flat area indicate that the valley fill is enclosed on all sides by older rocks and that the welded-tuff aquifer is probably surrounded and underlain by either the lava-flow aquitard or the tuff aquitard. That is, neither the valley-fill nor the welded- tuff aquifer occurs beneath the 2,400-foot level along the periphery of the valley. Therefore, if ground water in the C61 Cenozoic aquifers is to leave the valley, it can do so either by lateral flow toward an adjacent valley through the surrounding and underlying Tertiary aquitards or by downward movement through the tuff. aquitard into the underlying lower carbonate aquifer. It could also move laterally through the aquitard — for example, toward the Ranger Mountains — and then enter the lower car- bonate aquifer. That ground water in the Cenozoic aquifers leaves the valley by entering the lower carbonate aquifer rather than by lateral movement into another valley via the surrounding aquitards is suggested by the following: 1. Water-level altitudes in the Cenozoic strata in Yucca Flat and Jackass Flats are Virtually the same as those in Frenchman Flat. This condition is incon- sistent with lateral movement of significant quan- tities of water through the aquitards because such movement would require locally steep hydraulic gradients and in turn would result in:markedly dif- ferent altitudes of the water tables in thesevalleys. 2. The small (9-ft) range between the lowest water levels (in Cenozoic rocks) in Yucca Flat, Frenchman Flat, and Jackass Flats (pl. 1) suggests a common base level to which the semiperched ground water in these valleys drains, the head within the widespread and highly transmissive lower car- bonate aquifer. 3. The scant well data for the valley can be interpreted to suggest internal drainage into the underlying lower carbonate aquifer beneath the playa. 4. A structure-contour map of the Frenchman Flat area suggests that the lower carbonate aquifer rises above the 2,400-foot level on all sides of the valley except possibly the southwestern side (C. H. Miller and D. L. Healey, written commun., Feb. 1965). Thus, even if ground-water movement through the aquitard is lateral or if the aquitard is missing, water would still enter the lower carbonate aquifer within the drainage area of the valley. The authors believe that the semiperched ground water in the Cenozoic aquifers and aquitards leaves the valley by downward leakage into the underlying car- bonate aquifers. The water can move through struc- turally controlled sinks as may exist beneath Frenchman Lake, through vertical drainage throughout the valley, or through both mechanisms operative in different parts of the valley. By analogy with Yucca Flat, the magnitude of such leakage should be no greater than 70 acre-feet an- nually. Available data do not justify construction of poten- tiometric contours for the Cenozoic aquifers in the valley. A single hachured contour, labeled 2,400 feet and inserted on the small-scale regional potentiometric map (pl. 1), indicates that drainage of ground water within Frenchman Flat is probably internal. C62 Because the water table in Frenchman Flat is con- siderably shallower than that beneath Yucca Flat, the saturated extent of the valley-fill aquifer in Frenchman Flat is considerably greater than that in Yucca Flat. The valley fill is saturated beneath an area at least 6 miles long and at least 41/2 miles wide and thus has a minimum saturated area of about 27 square miles. In contrast, the valley fill in Yucca Flat is saturated beneath an area of less than 10 square miles. The saturated thickness of the valley fill in Frenchman Flat is probably at least 500 feet within the polygon formed by lines connecting wells 74—70b, 74—70a, 76—70, 77—71, 77—71b, and 75—72. Thus the valley-fill aquifer is of major importance in this valley. The welded-tuff aquifer is probably of secondary importance in Frenchman Flat: first, because it is buried more deeply than valley fill, and second, because it is ab- sent or very thin beneath the southern half of the valley. OTHER VALLEYS By analogy with Yucca and Frenchman Flats, leakage of semiperched water is also probable beneath northern Indian Springs Valley and northern Three Lakes Valley (north of US. Highway 95), eastern Emigrant Valley, and Desert Valley (fig. 1). The basis for the analogy is: (1) The water table in the Cenozoic rocks beneath these valleys is relatively deep (it generally ranges from 300 to 700 ft); (2) the basal Tertiary rocks (Horse Spring For- mation or equivalent) are aquitards; (3) the Tertiary rocks are underlain principally by the lower carbonate aquifer; and (4) all these valleys, except Desert Valley, are remote from major recharge areas; thus, the probability of higher heads in the lower carbonate aquifer than in the Cenozoic strata is unlikely. In southern Indian Springs Valley, southern Three Lakes Valley (south of US. Highway 95), east-central Amargosa Desert, and possibly also eastern Jackass Flats (fig. 1), the Cenozoic aquifers are recharged prin- cipally by upward leakage from the lower carbonate aquifer. This upward movement is related in part to prominent hydraulic discontinuities within the lower carbonate aquifer. INTERBASIN MOVEMENT Regional movement of ground water through the lower carbonate aquifer flanking and underlying the valleys at Nevada Test Site and vicinity is called interbasin move- ment in this report. Such movement of ground water is not significantly influenced by the topographic boun- daries of the individual valleys. One of the major con- trols of such movement is rather the disposition of the lower carbonate aquifer and of the lower and upper clastic aquitards (table 1). The lateral movement of ground water through the carbonate aquifer integrates seVeral intermontane valleys into a single large ground- water basin, the Ash Meadows ground-water basin. HYDROLOGY OF NUCLEAR TEST SITES EVIDENCE FOR INTERBASIN MOVEMENT Water-level altitudes in wells tapping the lower car- bonate aquifer in Yucca and Frenchman Flats, in Mer- cury Valley, and in east-central Amargosa Desert offer the most direct evidence of interbasin movement within the lower carbonate aquifer (pl. 1). Water levels indicate a hydraulic gradient from northwestern Yucca Flat and from eastern Frenchman Flat toward the Ash Meadows discharge area; the gradient generally ranges from 0.3 to 5.9 feet per mile to the south and the southwest. The water-level altitudes between well 88—66 in northern Yucca Flat and Devils Hole, a cavern in the discharge area, differ by only 56 feet; these control points are 58 miles apart. In the 53 miles between well 85—68 in north- central Yucca Flat and Devils Hole, the difference in water-level altitude is only about 28 feet; the apparent hydraulic gradient is only 0.5 foot per mile. In this dis- tance, the land-surface altitude of the valley floors drops about 2,000 feet. Depth to the static water level in the lower carbonate aquifer decreases from about 2,055 feet below land surface to at least several feet above land sur- face. Plate 1 illustrates these relations. The cited hydraulic gradients may be step-like rather than smooth, as fault zones may locally compartmen- talize the lower carbonate aquifer. Whether or not com- partmentalization (discussed later in this report) exists does not change the fact of decreasing potential energy in a south and southwesterly direction from Yucca Flat toward the Amargosa Desert. Additional evidence for interbasin movement of ground water is based on (1) the wide subsurface dis- tribution of the lower carbonate aquifer, (2) the similarity in water-level altitudes in the Cenozoic strata in several valleys, (3) the chemistry of the ground water, and (4) the anomalous relationship of the spring dis- charge at Ash Meadows to the size of the apparent catch- ment area for this discharge. These are discussed briefly in the following paragraphs. 1. The lower carbonate aquifer occurs within the upper several thousand feet of the zone of saturation throughout most of the study area. It underlies both the ridges and the saturated Cenozoic aquifers and aquitards beneath the valley floors. The saturated thickness of the lower carbonate aquifer generally ranges from only a few tens of feet in the vicinity of the areas where the lower clastic aquitard is close to the surface (pl. 1) to possibly as much as 10,000 feet beneath central Yucca Flat; projections of mapped areal geology suggest that saturated thickness of the lower carbonate aquifer is probably at least 4,000 feet thick beneath most of the study area. Because of the widespread distribu- tion of the lower carbonate aquifer, interbasin movement of ground water through this aquifer is both possible and probable. SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE 2. The water-level altitudes in the Cenozoic aquifers and aquitards in Yucca Flat, Frenchman Flat, Jackass Flats, and northern Indian Springs Valley (north of US Highway 95) differ by less than 170 feet; and the lowest water levels in the three flats differ by only 9 feet. By contrast, the water table in Emigrant Valley, Kawich Valley, Gold Flat, southern Indian Springs Valley (south of US. Highway 95), and northern Three Lakes Valley (north of US. Highway 95) range from 370 to 2,600 feet higher than the lowest water levels in the three flats. The similarity in water-level altitudes in the three flats and in northern Indian Springs Valley suggests that these valleys are graded to a common discharge area. 3. The chemical quality of water from the lower car- bonate aquifer beneath Yucca and Frenchman Flats closely resembles that of water emerging from springs in the major discharge area at Ash Meadows. In contrast, the chemical quality of ground water from the lower carbonate aquifer in other areas and of ground water in the Cenozoic aquifers of Yucca and Frenchman Flats differs markedly from that of the water emerging in the discharge area. (See discussion under “Ground Water Chemistry, Hydrochemical Facies, and Regional Movement of Ground Water.”) 4. The marked anomaly between the large measured spring discharge at Ash Meadows (about 10,600 gpm, or 17,100 acre-ft per yr) and the small size (a few hundred square miles) and aridity of the precipitation catchment area for this discharge is suggestive of interbasin movement. This basic hypothesis was first discussed by Omar Loeltz, US. Geological Survey (1960). The evidence for interbasin movement of ground water through the lower carbonate aquifer at Nevada Test Site and vicinity is strong. Since completion of test drilling at Nevada Test Site, other hydrologists using the preceding and other criteria have postulated that interbasin move- ment of ground Water occurs in other parts of the miogeosyncline in the eastern one-third of Nevada. Eakin (1965, 1966) stated, for example, that 13 inter- montane valleys within or adjacent to the White River drainage basin in east-central Nevada are hydraulically integrated into a single ground-water basin by move- ment of water through the Paleozoic carbonate rocks. Maxey and Mifflin (1966) inferred that interbasin move- ment of ground water best explains the uniform yield of most of the highyield springs in the miogeosyncline. Malmberg (1967) stated that approximately 12,000 acre- feet annually leaves Pahrump Valley by subsurface out- flow through the Paleozoic carbonate aquifers toward California. C63 INFLUENCE OF CLASTIC AQUITARDS AND MAJOR SHEAR ZONES ON INTERBASIN MOVEMENT Interbasin movement of ground water within the lower carbonate aquifer is greatly influenced by major geologic structures, particularly by folds that bring the lower clastic aquitard close to the surface or by faults that jux- tapose the lower or the upper clastic aquitards and the lower carbonate aquifer. In some areas, such geologic structures result in water levels differing as much as 2,000 feet between adjacent valleys or as much as 500 feet in carbonate aquifers within a single valley. In other areas, where the clastic aquitards are absent, some ma- jor faults within the carbonate rocks may also act as hydraulic barriers, or ground-water dams, and may com- partmentalize (but not necessarily totally isolate) the carbonate aquifer. In still other areas, hydraulic barriers may be totally absent; or more likely, they may not be discernible because of the low hydraulic gradients. Areas illustrating the effect of geologic structure on water movement in the lower carbonate aquifer are discussed in the following sections of the report. NORTHEASTERN YUCCA FLAT—WESTERN EMIGRANT VALLEY The alluvial apron west of Groom Lake playa in Emigrant Valley (fig. 32) is bordered on the east, south, and southwest by the lower clastic aquitard. Except along the west edge of Groom Lake playa, the lower clastic aquitard is exposed continuously on the east in the Groom and Papoose Ranges for a distance of about 25 miles; the strata dip eastward. On the south and southwest the lower clastic aquitard is discontinuously exposed over a distance of 10 miles in the Halfpint Range. Data from well 89—68 and geologic mapping in- dicate that the lower clastic aquitard extends at least another 2 miles northwest of the northernmost outcrop in the Halfpint Range and that the northwestward- trending clastic rocks are truncated by the Climax stock (fence diagram of Yucca Flat, pl. 2A). A gravity survey indicates that the pre-Tertiary rocks are now buried as much as 4,000 feet beneath the floor of western Emigrant Valley (D. L. Healy and C. H. Miller, written commun., 1965). The geologic and hydraulic section extending from northeastern Yucca Flat to Groom Lake playa (fig. 32) is approximately at right angles to the major structural trends. Water levels in three widely spaced wells tapping the valley-fill aquifer in western Emigrant Valley range from 4,340 to 4,371 feet above mean sea level and in- dicate a gentle hydraulic gradient sloping eastward toward the playa at about 4 feet per mile. Water levels in wells tapping older rocks immediately east and west of western Emigrant Valley are considerably lower than levels in wells tapping the valley-fill aquifier. The high 064 HYDROLOGY OF NUCLEAR TEST SITES 5mm iis°oo' 5.720.011) ' E760 non 115°4s' l , ., It-.. — IA“- — I | l \ CQUNTY COUNTY E\ LINEOLN 1-1.: QB] _ 6 é" A38 < z 0 / / / / 9368 / ___' 6 4385(Trk))D/ § ‘36 l / E! /’ 15:" /\\ 5 93-70 4,9140%. ,. / A Groom : _ ’_ ”71-2mm /——4m(oTu) pl I I'Lake j E 7 / / I [—1-(Playa) g ' [8me EMIGRANT 2 C W HALFPINT 5 0 Lu ' RANGE ‘ , —‘ " P \ I / \TPfipoosle Lake( Ilaya) Base from US. Geological Survey l:250,000 Modified from Winograd and Thordarson Caliente and Goldfield, 1954 (1968, fig. 2). Geology compiled from unpub- lished maps by Harley Barnes, E. N. Hinrichs, 20,000—foot grid based on Nevada P. P. Orkild, and F. A. McKeown, and coordinate system, central zone Tschanz and Pampeyan (1961) A A' “ 5 = 5 = .E g .5 . 13% 90-70 ‘g 23 1g .3 g as a 8 8 9‘ 7‘8 3 3 88-67 H _ _ T i “9‘63 92 7° 91-14.\ 1 ‘ /<170 h per mile n 9" mil- Poterllgrlnetric . /Z per mile ‘1300 it PM mill I'MJE mile Vertical exaggeration x10 HYDRAULIC SECTION .5 r: 5 i: E = E = ‘5 33.. 'u .9. u .2 1:: g § § 5, § EMIGBANT VALLEY 3- § a, g A YUCCA FLAT 91-7 Groom Lake (Playa) A’ - 91-74. 90-14 /90-75 .— QTal QTal __ 1" Tpi P; _ T ‘2 1? C L. Pie? N CFC m I - Modified from Winograd and Vemcal exaggeration x2 GEOLOGIC SECTION Thordarson (1968, fig. 3) 0 2 4 6 8 MILES (I 2 4 6 BKILOMETERS FIGURE 32. — Hydrogeology of Emigrant Valley. SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE 065 EXPLANATION Lower elastic aquitard Mostly Stirling Quartzite and Johnnie Formation but also include: lower part of Carrara Formation, Zabn'skie Quartzite, and Wood Canyon Formation Upland area Contact __"—-—L___ _,. Fault Dashed where inferred. Bar and ball on downthrown side. Arrows indicate direc- tion of relative movement HYDROGEOLOGIC AND GEOLOGIC UNITS SYMBOL GEOLOGIC UNIT HYDROGEOLOGIC UNIT QTal Quaternary and Tertiary valley fill Valley—fill aquifer Tpi Piapi Canyon Group and Indian Trail Formation undifferen- tiated T ff . . Tp Ash-fall(?) tuff of Piapi Canyon(?) “ mm“ and “mm“ Group Trk Rhyolite of Kawich Valley PLc Paleozoic carbonate rocks > L we b n t 'f Cb Bonanza King Formation o I car 0 a e aqui er €p€l Lower Cambrian and Precambrian clastic rocks pCs Stirling Quartzite and Precambrian Lower elastic aquitard elastic and carbonate rocks an Noonday(?) Dolomite HYDRAULIC SYMBOLS Note: All altitudes and contours in feet; datum is mean sea level 89-68 . 3918 (p65) Test well Well tapping pre-Tertiary hydrogeologic units; upper number is well number; lower number is altitude of static water level; symbol in parentheses is formation tapped PRECAMBRIAN TERTIARY AND ANDCAMBRIAN QUATERNARY 90-74 0 3951 (Tn?) Test well Well tapping Tertiary tuff aquifer; upper number is well number; lower number is altitude of static Mater level; symbol in parentheses i: for- mation tapped 93-71 0 4368 :2 (are) Test well Well tapping Quaternary and Tertiary valley-fill aquifer; upper number is well number; lower number is altitude of static water level; symbol in parentheses 1‘: formation tapped. am——-— Potentiometric contour Shows altitude of potentiometric surface in Cenozoic hydmgeologic units; dashed where inferred; contour interval variable -— —4500— — Potentiometric contour Show: altitude of potentiometric surface in pre-i‘ertiary hydrogeologic units; dashed where inferred; contour interval variable 90-70 Tul— Test well, dashed where projected FIGURE 32. — Continued. and virtually flat potentiometric surface in the valley-fill aquifer within western Emigrant Valley and the steep hydraulic gradients developed in the older rocks in the flanking areas probably are caused by a higher outlet for discharge from the valley-fill aquifer than for the older rocks and by a lower gross transmissibility of and a lower hydraulic outlet for the clastic rocks that surround and underlie western Emigrant Valley. The steep hydraulic gradients on both ends of the cross section probably reflect the movement of water through the thick lower elastic aquitard or tuff aquitard toward points of lower head in Yucca Flat and in eastern Emigrant Valley. The high water level within the valley-fill reservoir probably is due to impoundment by the impermeable clastic rocks. Along the west edge of Groom Lake playa, about 1,000 feet northeast of well 91—74, an outlier of Precambrian quartzite (Stirling Quartzite) rises a few feet above the playa surface (fig. 32). This outlier and a gravity survey indicate that the Precambrian and Lower Cambrian clastic rocks composing the Groom and Papoose Ranges form a continuous buried ridge at very shallow depth along the west side of the playa and under well 91—74. The gravity survey'also indicates that the depth to bedrock increases steeply in both directions from this buried ridge (D. L. Healey and C. H. Miller, written commun., 1965). The buried ridge is probably acting as the spillway for the valley-fill reservoir, which is saturated to within 100 feet of the surface. That the clastic rocks act as a hydraulic barrier or dam was first hypothesized in 1957 by Omar Loeltz (written commun., 1959). The saturation of the valley fill to within 107 feet of the land surface at well 91—74 in Emigrant Valley and the slope of the potentiometric surface in that aquifer toward the playa (where the elastic aquitard is largely buried) suggest that very little water leaves the valley by C66 movement through the clastic aquitard. The lower clastic aquitard thus acts as an effective hydraulic dis- continuity between the lower carbonate aquifer down- faulted beneath western Emigrant Valley (fig. 32) and the same aquifer beneath northeastern Yucca Flat and beneath eastern Emigrant Valley (east of the Groom and Papoose Ranges). Two qualifications of the preceding arguments are necessary. First, an objection may be raised that water levels in the Tertiary and Quaternary aquifers should not be utilized as a reflection of heads within the lower clastic aquitard. However, in Yucca and Frenchman Flats, Indian Springs Valley, and Amargosa Desert, water levels in the Cenozoic aquifers and aquitards beneath the valley floors are usually within 100 feet of levels in the underlying Paleozoic aquifers and aquitards, except where true perched conditions exist along the margins of the valleys. This fact, coupled with the large differences in water levels between wells along the east and west ends of the western Emigrant Valley area, makes the use of water levels in Cenozoic rocks as indicators of heads in the clastic aquitard appear valid. Moreover, in both Emigrant Valley and the other areas to be discussed, at least one control point in the pre~ Tertiary rocks is available. The second qualification is that the hydraulic gradients within the lower clastic aquitard may not be continuous; the water levels may instead reflect discon- tinuous levels within blocks of aquitard separated by faults. Whether the gradients are continuous or steplike does not materially change the principal interpretation, namely that the lower clastic aquitard is an effective hydraulic barrier. NORTHWESTERN AND WEST-CENTRAL YUCCA FLAT Water levels in wells of northwestern and west-central Yucca Flat (pl. 20) offer another example of the com— partmentalization of the lower carbonate aquifer, but in this case by the upper clastic aquitard. Northwestern and west-central Yucca Flat is bordered on the west by the upper clastic aquitard (Eleana Formation), which forms Quartzite Mountain, the Eleana Range, and several smaller unnamed ridges. These ridges extend nearly 18 miles in a north-northeasterly direction and form a nearly unbroken trend of the upper clastic aquitard. The aquitard, which ranges from 4,000 to 8,000 feet in thickness, also underlies the western one-third of the valley floor. The approximate eastern limit of the up- per clastic aquitard at the 2,400-foot level is shown by plate 2B and C. The difference of water-level altitude within the lower carbonate aquifer east and west of the area underlain chiefly by the upper clastic aquitard is about 1,800 feet. The water-level altitude within the lower carbonate aquifer in central Yucca Flat ranges from 2,381 feet at HYDROLOGY OF NUCLEAR TEST SITES well 79—69a at the south end of the valley to 2,415 feet in well 88—66 at the northwest end. About 7 miles west of well 88—66, well 87—62 (south of Rainier Mesa) also taps the lower carbonate aquifer (p1. 2C); but here the poten- tiometric surface of the aquifer is 4,189 feet above mean sea level. The lower carbonate aquifer at this site has been thrust over the upper clastic aquitard, as shown in fence C-1 to C—2 of plate 2A and in the section on plate 1. The difference in water-level altitude is attributed directly to the low gross transmissibility of the interven- ing aquitard. The apparent hydraulic gradient within the aquitard between the two wells is about 330 feet per mile (pl. 1). Data from several wells in the upper clastic aquitard and one in the upper carbonate aquifer in west-central Yucca Flat suggest that an apparent hydraulic gradient as steep or steeper than the one cited also exists within the upper clastic aquitard south of the wells cited in the preceding paragraph. The gross stratigraphy and the potentiometric surface at these well sites is given in the cross section of plate 20. The section also illustrates another example of compartmentalization of the car— bonate aquifers by the upper clastic aquitard. Well 83—66c bottomed within the basal part of the upper car- bonate aquifer, the Tippipah Limestone. The poten- tiometric level within the limestone is, however, 500 feet higher than levels within the lower carbonate aquifer or the Cenozoic rocks about 21/2 miles east of the well. The marked difference in potentiometric level is again at- tributed to the upper clastic aquitard being between the two carbonate aquifers. Beneath central and northern Yucca Flat, the lower and the upper clastic aquitards (pl. 23 and C) isolate the lower carbonate aquifer from adjacent valleys. Thus, any interbasin movement of ground water into the lower car- bonate aquifer would have to pass through and be con- trolled by the transmissibility of the clastic aquitards. However, an important distinction exists between the upper elastic aquitard bordering the valley on the west and the lower clastic aquitard bordering it on the northeast. In western Yucca Flat the upper clastic aquitard probably is underlain by thousands of feet of lower carbonate aquifer, whereas the lower clastic aquitard beneath northeastern Yucca Flat is probably underlain at depth principally by Precambrian clastic or crystalline basement rocks. Thus, the upper clastic aquitard need not necessarily retard the movement of some ground water into Yucca Flat from the west or northwest, because such movement could occur at depths of several thousand feet through the underlying carbonate aquifers. The effectiveness of the aquitard may depend in part on the nature of the Tippinip thrust fault (pl. 2A and C). If the thrust dips steeply for several thousand feet, it could partly isolate the lower carbonate aquifer beneath the central part of the valley from SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE equivalent strata to the west. If it does not dip steeply, then the ground water in the upper clastic aquitard may be semiperched above the underlying carbonate rocks. SOUTHERN INDIAN SPRINGS AND THREE LAKES VALLEYS Well data from southern Indian Springs Valley in- dicate a marked compartmentalization of the lower car- bonate aquifer within a single valley and within dis- tances of a few miles. Potentiometric contours (fig. 33) suggest two prominent west-trending hydraulic barriers between the Nye—Clark County line and Indian Springs, a distance of about 12 miles. The water-level altitude becomes lower northward (fig. 33). The position of the southerly hydraulic barrier almost coincides with the in- ferred position of the Las Vegas Valley shear zone (Longwell, 1960; Longwell and others, 1965) and may be created by gouge developed along this shear zone (struc- tural feature 13 on pl. 1). The northerly barrier may result from the presence of the lower clastic aquitard within the zone of saturation north of wells 66—75 and 67—73 (fig. 33). Observations by W. E. Hale (written commun., 1963) suggest that the difference in water- level altitude in this area is not due to steep gradients developed within the lower carbonate aquifer. First, water levels in the carbonate aquifer indicate a gentle gradient of about 4 feet per mile to the west. Second,’ pumping tests indicate coefficients of transmissibility of about 11,000 and 20,000 gpd per ft (table 3); steep gradients are unlikely to develop in such rocks. In other words, the lower carbonate aquifer tapped by these wells is part of a permeable block bounded on both north and south by hydraulic barriers that separate it from other aquifer blocks. Although the gross potentiometric con- tours of figure 33 suggest a regional northerly movement of water, the principal movement within the lower car- bonate aquifer between the barriers may actually be to the west, as is the regional movement of ground water north of the barriers. Water levels in three wells in southern Three Lakes Valley (fig. 33) also suggest a possible major hydraulic barrier parallel to the Las Vegas Valley shear zone. Wells 65—82, 65—83, and 69—84 tap the valley-fill aquifer. The average hydraulic gradient between the southerly two wells is about 85 feet per mile, whereas the average gradient between the two northerly wells is about 30 feet per mile. Hydraulic gradients in the valley-fill aquifer in surrounding valleys are commonly less than 60 feet per mile and are usually less than 30 feet per mile. The average gradient of 85 feet per mile in southern Three Lakes Valley may reflect below-average transmissibility of the valley fill, above-average quantity of water in transit through the aquifer, or a hydraulic barrier within either the valley fill or the underlying lower carbonate aquifer. An above-average quantity of water in transit through the valley fill might be due to upward leakage C67 from the carbonate aquifer into the valley fill along a hydraulic barrier. A short-term pumping test indicates that the specific capacity of well 65—82 is less than 1 gpm per foot of draw- down (L. R. West, written commun., 1963). If this test is representative of the valley-fill aquifer, it might explain the rather steep hydraulic gradient; but if it does explain the gradient, equally steep gradients are also expectable between wells 65—83 and 69—84, which are farther from the principal source of detritus, the Spring Mountains. The authors tentatively conclude that the anomalous hydraulic gradient within the valley-fill aquifer reflects a hydraulic barrier in the lower carbonate aquifer ap- proximately parallel to and between the 3,000- and 3,100-foot potentiometric contours of figure 33. A major hydraulic barrier may also exist in the vicinity of the village of Indian Springs. The valley-fill aquifer supplies Indian Springs, the major spring in the area. This spring, which discharges about 435 gpm, is about 0.5 mile north of a ridge of the lower carbonate aquifer (fig. 33). The spring is probably fed directly by upward leakage of ground water from the flanking and probably underlying lower carbonate aquifer, for the following reasons. (1) The extremely small catchment area immediately upslope from the spring can hardly yield the nearly constant discharge cited; conversely, contribution from a larger catchment area, such as the alluvial fans bordering the Spring Mountains, is precluded by the intervening unnamed carbonate-rock ridge 0.5 mile south of the spring, unless such recharge passes through the carbonate rocks; (2) water-level data presented by Maxey and Jameson (1948, app. I, p. 20—21) suggest an increase in head with increase in depth of well, which in turn suggests that the valley-fill aquifer is fed by an underlying aquifer. (3) The spring has a high-level outlet near the carbonate-rock ridge. The topographic setting of the spring suggests either that a brimful carbonate aquifer spills over at a low point near the outcrop or that the carbonate aquifer is dammed north of but close to the spring and, hence, the water is forced to the surface. The absence of springs in topographically lower carbonate-rock outcrops north of US. Highway 95 probably rules out the first possibility, whereas the water level in destroyed well 68—79, (fig. 33) about 4 miles north of US. Highway 95, further supports the idea of a hydraulic barrier north of Indian Springs and probably north of the highway. The reported water level in this well is at least 500 feet lower than levels in the valley-fill reservoir adjacent to the highway (Carpenter, 1915). In summary, hydraulic data in southern Indian Springs Valley indicate that the carbonate aquifer is compartmentalized by two ground-water dams or barriers; the northern barrier appears to be due to jux— taposition of the lower clastic aquitard and the lower car- 068 HYDROLOGY OF NUCLEAR TEST SITES 116°00' E. 700,000 E. 740,000 45’ E. 780,000 115°30' E. 860,000 0 I _ \\\ V ‘\ \\ l 30 50 \\\\\\§‘\\\r0€ _ \C was)?“ \' < FRENCHMAN Rxhkgv N. 700,000 T.153.\ & 0‘0“”) {73 \22000 n (Tr) ~l “239‘“ \ (gift? P€B:75\\\ \ 6) \‘ (en;cb?I,// \2742(€n)\\\§ / .. sees — C N 660 000 ~ <3060 (CITall‘ 333:; T. 16 5. ”V0, \\ Indian S\prings 3175 :5 (PPM) 30°30 — T. 17 S. N. 620,000 , — P4,)» Willow Spring 047,0 flannel : 50 (PIPMb) V . a 4 Cold Creek Spring \ \ x \ T. 18 s. (15’, \ 175200250020 k X/ \.:‘\\\\ \\\‘\ l I \\\ I \.\ 3 \ \\ I \ 0.53 E. . . R.55 E. WHEELER PASS H.546 E. E.57 E. Fl.58 E. Base from US. Geological Survey 1:250,000 THRUST FAULT Modified from Winograd and Thordarson (1968, Death Valley and Las Vegas, 1954 0 2 6 8 10 MILES fig. 6). Rock type distribution from Longwell, 40,000—foot grid based on Nevada I I I I I J Pampeyan, BOWYeL and Roberts (1965) and coordinate system, central zone f ' | I I | R; L. Chnstlansen, R. H. Moench, and 0 2 4 8 10 KILOMETERS M. W. Reynolds (written commun, Mar. 1965) A Inferred ground- A' 5000’ water barriers INDIAN SPRINGS 3300-0101 VALLEY I LAs VEGAS VALLEY 4000 A 8:200?) SHEAR ZONE I 3000. 5000’ QTal 55—75 QTal 65-76 — I u 2400 foot levelf SEA LEygEWO All 7 he __ 2000 2000 / €pCl'.’ Vertical exaggeration x 5.7 HYDRAULIC SECTION Modified from Winngrad and Thordarson (1968, Fig. 7) GEOLOGIC SECTION FIGURE 33. — Hydrogeology of southern Indian Springs Valley. bonate aquifer (fig. 33), whereas the southern barrier appears to be related to the Las Vegas Valley shear zone and may be due to gouge developed along the major fault - zone. Well data near the village of Indian Springs and in southern Three Lakes Valley suggest additional hydraulic barriers which approximately parallel the Las Vegas Valley shear zone (Longwell, 1960; Longwell and others, 1965). Whether these barriers are due to the jux- taposition of the elastic aquitard and the carbonate aquifers, to the development of gouge along the shear zone, to the dragging of the elastic strata into the shear zone, or a combination of these mechanisms is unknown. SOUTHWESTERN MERCURY VALLEY Two wells in southwestern Mercury Valley (fig. 33) provide another example of the hydrologic effect of jux- taposition of the lower clastic aquitard and the lower car- bonate aquifer. Well 66—69 was drilled into the lower clastic aquitard (Johnnie Formation). The water-level altitude in this well is 2,415 feet above mean sea level. SOUTH—CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE C69 EXPLANATION \ \ I-ss 11h} {‘2 z a) 2mm 8 m) \ \ Q\ \ Z < 5 5 Test well Lower carbonate aquifer g a: Well n ‘ v tuff ‘ ‘ 1; upper ‘ is well ‘ ; lower ‘ Dolomite and limestone; minor shale and quartzite; chiefly Middle Cambrian : E is altitude of static water level; R, water level reported; symbol in through Devonian age; in Spotted Range includes thin elastic strata, possible 5 o parentheses is formation tapped; water level reported for well 68-69 equivalents of Eleana Formation; in southeastern part of map includes Monte l" is probably inaccurate but given for completeness Cristo Limestone and Bird Spring Formation Z i 5 fl 2 g o <3060 (CIT-ll E 2 Test well . _ 5 5 Well tapping valley-fill aquifer; upper number is well number; lower _ lower elastic aqunard . u: G number is altitude of static water level; symbol < denotes dry well; Quartztte, siltstone, shale, and mmor limestone E g symbol in parentheses is formation tapped 65-76 Contact 6 3329 (OTaI; Tr) —_L_‘ _____ Test well ‘— . Well tapping both valley-fill aquifer and tuffaquitard; upper number is Fault well number; lower number is altitude of static water level; symbols Dashed where inferred. Bar and ball on downthrown side. Arrows indicate direction of relative movement A—J—A—A—A—L Thrust fault Sawteeth on upper plate; dashed where inferred HYDROGEOLOGIC AND GEOLOGIC UNITS SYMBOL GEOLOGIC UNIT HYDROGEOLOGIC UNIT QTal Valley fill Valley-fill aquifer Tt Tertiary tuff, probably pre-Piapi(?) Tuff aquitard Canyon Group PLc Paleozoic carbonate rocks PPMb Bird Spring Formation C—O Cambrian and Ordovician . carbonate rocks Lower carbonate aquifer €n Nopah Formation Cb Bonanza King Formation CpCl Early Cambrian and Precambrian elastic rocks Lower clastic aquitard pCJ' Johnnie Formation HYDRAULIC SYMBOLS NOTE: All altitudes and contours in feet;'datum is mean sea level 66-69 . 2415 (psi) Test well Well tapping pre-Terturry h ydrogeologic units; upper number is well number; lower number is altitude of static water level; symbol in parentheses is formation tapped 86-79 . 3084 (OTII; he) Water well Well tapping valley-fill and lower carbonate aquifer; upper number is well number; lower number is altitude of composite static water level of both hydrogeologic units; symbols in parentheses are forma- tions tapped in parentheses are formations tapped Indian Springs . m) Spring Spring in, or near outcrop of, lower carbonate aquifer; above line, name of spring; below line, altitude of land surface; symbol in parentheses is formation supplying spring Cactus Springs _/0 3238 :5 (OT-I) Spring Spring in valley-fill aquifer; above line, name of spring; below line, altitude of land surface; symbol in parentheses is formation supply- ing spring —2700— — — — Potentiometric contour Shows altitude of potentiometric surface in lower carbonate aquifer; contour interval variable; dashed where inferred —33w____ Potentiometric contour Shows altitude of potentiometric surface in valley-fill aquifer; contour interval variable; dashed where inferred ====l=l= Inferred ground water barrier along concealed fault zone Width of symbol not intended to represent width of hydraulic barrier which may range from several tens to a few thousand feet Dry lakebed 66—75 T Test well Number is well number FIGURE 33. — Continued. Well 67—68, about 1.5 miles northwest of well 66—69, was drilled into the lower carbonate aquifer (Nopah Forma- tion and Bonanza King(?) Formation). The water-level altitude in this well is 2,370 feet above mean sea level, or about 45 feet lower than the level in the adjacent well. The difference in levels does not appear striking until the water level in the lower carbonate aquifer, about 30 miles away, in central Yucca Flat is noted to be only 17 feet higher (alt 2,387 ft) than in well 67—68. The authors believe that the higher water level in well 66—69 reflects the low gross transmissibility of the lower elastic aquitard. C70 ASH MEADOWS The most striking example of structural control of ground-water movement within the lower carbonate aquifer is the spring line at Ash Meadows in east-central Amargosa Desert (pl. 1). These springs aline for about 10 miles in a northwesterly direction. This trend parallels the strike of the lower carbonate aquifer as well as a ma— jor fault zone delineated by a gravity survey of the region (D. L. Healey and C. H. Miller, written commun., 1965). The spring line and the details of the individual springs are discussed in the section “Character and Geologic Control of Spring Discharge.” GENERAL SIGNIFICANCE OF THE HYDRAULIC BARRIERS The hydraulic barriers cited suggest the presence of ground-water dams not shown by the potentiometric levels and contours. Hydraulic compartmentalization of the lower carbonate aquifer is expectable throughout the study area owing to the complex geologic structure. Some areas, for example the area between eastern Frenchman Flat and Ash Meadows (pl. 1), are seemingly free of compartmentalization of the lower carbonate aquifer. However, the absence of hydraulic barriers in this area is probably only apparent. The examples of compartmentalization discussed in this chapter were chosen specifically because adequate well and geologic control was available to demonstrate clearly the role of the clastic rocks and perhaps that of the major shear zones in controlling regional ground-water movement through the lower carbonate aquifer. Actually, the hydraulic barriers cited could have been detected from the water-level data alone, without the aid of areal geologic mapping or subsurface stratigraphic control. But, the examples cited were chosen from areas close to the margins of the flow system, where significant differences in water level occur across the hydraulic barriers. In other areas, such as Yucca and Frenchman Flats and the region between Frenchman Flat and Ash Meadows, the total drop in head within the lower car- bonate aquifer is only a few feet. Consequently, detec- tion of clastic-rock or fault barriers in such areas will probably be impossible without considerable subsurface control. For example, the head within a ridge of clastic rocks buried beneath a valley but surrounded by car- bonate rocks would probably be controlled by and con- sequently be the same or nearly the same as that within the adjacent carbonate aquifer. In this situation, water- level data alone might not permit identification of the clastic barrier. Similarly, major fault zones, in- dependently of clastic strata, may locally compartmen— talize the carbonate aquifer in the area between central Yucca Flat and the Amargosa Desert; but evidence of such fault barriers would not be apparent on the poten- tiometric map. Water levels in four wells tapping the HYDROLOGY OF NUCLEAR TEST SITES lower carbonate aquifer in eastern Amargosa Desert suggest that compartmentalization may also occur in areas of low hydraulic gradient. All four wells are in the NW% sec. 27, T. 16 S., R. 51 E., and only 400 feet separates the two most widely spaced wells (the location of one of these wells is shown on pl. 1 and fig. 34). Depth to water is about 42 feet. Static levels in the three westernmost wells are within 0.2 foot of each other, but the level in the easternmost well is 0.5 to 0.7 foot lower than in the other wells (R. H. Johnston, written com- mun., May 1967). Flowmeter surveys indicating no flow in the bore of the anomalous well preclude downward movement as an explanation for the discrepancy. A possible reason for the discrepancy is a hydraulic barrier(s) between the easternmost well and the three other wells. 7 . Awareness of the probable occurrence of numerous hydraulic barriers in the lower carbonate aquifer is ex- tremely important for a realistic interpretation of the regional potentiometric map (pl. 1). First, the change in head between wells plotted on the potentiometric map may locally be steplike, rather than smooth, because the lower carbonate aquifer is probably compartmentalized locally by fault or elastic-rock barriers throughout the study area; the actual hydraulic gradients, therefore, may be considerably smaller between barriers, but larger across the barriers, than gradients shown on plate 1. Second, because of the known, and probably numerous unknown, hydraulic barriers cutting the lower carbonate aquifer, the potentiometric contours may best be regarded as illustrating the direction of decrease in head within a compartmentalized aquifer system rather than the exact direction of ground-water movement. Although the regional movement of water in the aquifer is probably roughly at right angles to the potentiometric contours as drawn, local movement may depart greatly from the regional average owing to either the anisotropy and heterogeneity of the aquifer, the presence of hydraulic barriers, or both. In the vicinity of some major hydraulic barriers, ground water may move parallel to, rather than across, the barriers. However, because of the prominent difference in water level on opposite sides of a barrier and because of the sparsity of well data, the potentiometric contours drawn for such areas may suggest flow at right angles to the barrier. For example, in southern Indian Springs Valley (fig. 33) ground-water flow between the two hydraulic barriers may actually be principally to the west rather than to the north as suggested by the 500-foot potentiometric contours. The tightness of the hydraulic barriers is probably highly variable. Where the lower carbonate aquifer is completely surrounded by the lower clastic aquitard, as in western Emigrant Valley, it is probable that little water moves across the barrier despite large differences (in places as great as 2,000 ft) in head. On the other SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE hand, where the lower carbonate aquifer is only partly juxtaposed against a clastic (or tuff) aquitard or contains a barrier formed chiefly by gouge developed along a ma- jor shear zone within the aquifer, the situation differs; here a prominent difference in head need not necessarily preclude the movement of important quantities of water across the barrier. The author’s judgment on the tightness of specific barriers is discussed at appropriate places in the text. CONSTRUCTION AND INTERPRETATION OF THE POTENTIOMETRIC MAP OF THE LOWER CARBONATE AQUIFER NOTES ON CONSTRUCTION Two assumptions are generally made during construc- tion of the potentiometric map of an aquifer: (1) the water levels used represent only head in the aquifer of in- terest; and (2) in the vicinity of the control wells, water flows nearly horizontally through the aquifer. These assumptions are generally fulfilled in the lower car- bonate aquifer. First, because the key wells used in con- structing the potentiometric map were drilled specifically for the hydrologic study program, they per- mitted careful control on the measurement of poten- tiometric levels. Second, drill-stem tests and flowmeter surveys indicate that flow in the lower carbonate aquifer is probably horizontal in the vicinity of the key wells. The potentiometric maps of the lower carbonate aquifer for Yucca Flat (pl. 2C) and for the entire area (pl. 1) represent a synthesis of both hydraulic and geologic data. Briefly, these maps were constructed as follows. First, water-level contour maps were drawn using only water-level data for the lower carbonate aquifer. On another pair of maps, outcrops of both the upper and the lower clastic aquitards were drawn; in addition, areas where the clastic aquitards were likely to be the major or only pre-Tertiary rock within the zone of saturation were delineated. Next, on the assumptions that the clastic aquitards are considerably less permeable than the car— bonate aquifers and that the lower clastic aquitard is the effective hydraulic basement for ground-water flow, the contours were modified as follows: Where the contours crossed into areas underlain by elastic rocks the contours were sharply bent (refracted) to indicate the probable steeper hydraulic gradients within the elastic aquitards. In areas where major water-level anomalies and geologic mapping or gravity surveys suggested major discon- tinuities, hydraulic barriers were shown. Thus, the poten- tiometric maps represent a synthesis of hydraulic, geologic, and geophysical data. Such a synthesis was both reasonable and essential because contouring of the water-level data without regard for geologic structure led to several improbable local hydrologic conditions. In any event, the major features (discussed in the next section) C71 of the initial water-level contour map surprisingly were not greatly affected by the synthesis with the geologic and geophysical data. Contours of the potentiometric surface in the Cenozoic aquifers are also drawn on the regional potentiometric map (pl. 1) for areas surrounding Nevada Test Site. In some areas, these contours probably reflect the head in the underlying lower carbonate aquifer; in other areas they offer clues to the position of ground-water divides within the clastic aquitards. Several mechanical matters evaluated during the con- struction of the contour map were (1) accuracy of measurement of deep water levels, (2) effects of hole crookedness and water-column density on water-level measurement, and (3) effects of previous pumpage on the static water levels measured. A discussion of the first two items is presented by Garber and Koopman (1968) and Winograd (1970). The accuracy of the water-level measurements was satisfactory for potentiometric contouring at the 10- and 20-foot intervals used on plates 1 and 2. INTERPRETATION OF MAJOR FEATURES OF POTENTIOMETRIC MAP Major features shown on the regional potentiometric map (pl. 1) are the trough in the potentiometric surface in Yucca Flat and the major trough that extends from eastern Frenchman Flat to the Ash Meadows discharge. area in east-central Amargosa Desert. The potentiometric surface within the lower carbonate aquifer in Yucca Flat is marked by a prominent north- northwest trending trough about 20 miles long and 2 to 8 miles wide (pl. 2C). The apparent hydraulic gradient along the axis of the trough ranges from a fraction of a foot to 5.9 feet per mile. (See section on pl. 1.) The ap- parent gradient along the flanks of the trough is as much as 20 feet per mile. At its south end, the trough merges with the major southwest-trending trough. The water levels in wells 84-68d and 85—68 suggest a possible rever- sal of hydraulic gradient within the trough in Yucca Flat. Neglecting correction of the water levels for temperature, the level in the southerly well (84—68d) is 1 foot higher than that in the northerly well (85—68). This condition suggests a very low northerly gradient. However, the water-level altitude calculated after cor- recting the water column in well 84—68d for density is the same as that in well 85—68. The water level in destroyed well 85-68 was not corrected for temperature, because temperature data were not obtained; if such a correction had been made, it would probably have resulted in a water-level altitude several feet higher than that in well 84—68d. In any event, a reversal in gradient in Yucca Flat seems unlikely because of other water levels in the valley. The trough within the lower carbonate aquifer in Yuc- C72 ca Flat may reflect variations in the transmissibility of the lower carbonate aquifer, or it may be due to a series of subparallel northward-trending hydraulic barriers. The saturated thickness of the carbonate aquifer in- creases westward between the east margin and the cen- tral part of Yucca Flat (pl. 2A). Near the east margin of the valley, only the lowest part of the aquifer (the Bonanza King Formation) is saturated, whereas toward the west, successively overlying formations are saturated; that is, westward from the east margin of saturated carbonate rocks, the saturated thickness probably increases from a few hundred feet to possibly more than 10,000 feet. If this increase in thickness is ac- companied by even a moderate increase in transmissibility, it could explain the general position of the trough. Alternately, because the trough crudely parallels the Yucca fault, movement along this major fault may have resulted in above-average fracture transmissibility within the lower carbonate aquifer beneath the center of the valley. However, available well data are too scant to support or refute the relationship of aquifer transmissibility to the Yucca fault or to any other fault zone. The extremely low hydraulic gradients along the center of the trough do not necessarily reflect high transmissibility. Low hydraulic gradients can be due to high transmissibility or a small quantity of water moving through the aquifer, or to a combination of both factors. In Yucca Flat the quantity of water moving through the system is small, probably less than 350 acre-feet an- nually. (Estimates of the quantity of water moving through the aquifer in Yucca Flat are developed in the section “Sources of Recharge to the Lower Carbonate Aquifer.”) Thus the very low gradient in the center of the trough need not reflect high transmissibility, only a transmissibility possibly greater than that along the margins of the trough. The trough also might reflect hydraulic barriers due to faults within the lower carbonate aquifer. The regional strike of major block faults cutting the lower carbonate and of bedding in the blocks is northward and northwestward. Underflow to the lower carbonate aquifer is from both the east and the west, via the clastic aquitards, and also from the overlying Cenozoic rocks. If the northward-striking faults, possibly aided by the thin elastic rocks within the aquifer, deflect only part of the underflow southward (or parallel to the fault planes), ‘ another explanation for the trough is possible. Namely, southward diversion of some of the recharge by the block faults, which dropped the aquifer as much as 4,000 feet below the valley floor, would result in only a fraction of the total recharge entering and moving southward within the structurally deepest blocks of aquifer. The reduction in quantity of flow carried by the carbonate rocks beneath the center of the valley would of necessity be ac- HYDROLOGY OF NUCLEAR TEST SITES companied by a lower potentiometric level if transmissibility were constant.~ Subsurface information and transmissibility data available in 1965 (table 3) are insufficient to indicate which of the conditions, or what combination of con- ditions, is causing the trough within the lower carbonate aquifer beneath Yucca Flat. The authors favor the last of the three explanations. The second major feature of the potentiometric map is the trough that runs from east-central Frenchman Flat to Ash Meadows (pl. 1), a distance of about 40 miles. This trough is roughly outlined on the northwest and east by the 2,380-foot potentiometric contour (pl. 1) and on the south by the 2,400-foot contour. It is about 15 miles wide at the Ash Meadows discharge area; about 5 miles wide beneath the Specter Range; and possibly as much as 20 miles wide within the Nevada Test Site, where its width is not well defined. The hydraulic gradients within this trough range from 0.3 to about 1.5 feet per mile. The trough indicates that ground water within the lower carbonate aquifer beneath Yucca, Frenchman, and eastern Jackass Flats and beneath a vast area east, northeast, and southeast of the Nevada Test Site is moving toward a prominent spring-discharge area (Ash Meadows) in the east-central Amargosa Desert. That the part of the trough between the Specter Range and Ash Meadows (pl. 1) represents a region of very high transmissibility (1 to several million gpd per ft) is dis- cussed in the next section of the report. The high transmissibility of the carbonate rocks beneath and southwest of the Specter Range may be due to a greater thickness of aquifer, to a higher degree of fracturing than found elsewhere in the study area, or to a combination of both. Several cross sections through the Specter Range (R. L. Christiansen, R. H. Moench, and M. W. Reynolds, written commun., Mar. 1965) indicate that the saturated thickness of the lower carbonate aquifer beneath the Specter Range probably ranges from 5,000 to 8,000 feet. This thickness is more than double that at some well sites; however, thickness alone cannot account for the difference in transmissibility which, beneath the Specter Range, is two to three orders of magnitude greater than that at 8 of 10 well sites. The in- crease in transmissibility must, therefore, be due to above-average fracture transmissibility. Geologic mapping suggests that the Specter Range may, in general, be structurally more complex than most of the ridges and hills within the study area (Burchfiel, 1965; Christiansen, Moench, and Reynolds, written com- mun., Mar. 1965). Most of the fault-block ridges of pre- Tertiary rocks at the Nevada Test Site are of simple structure, even though the rocks themselves are highly fractured. The Specter Range, by contrast, appears to be a mosaic of fault blocks. Possible reasons for the intense SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE deformation of the Specter Range strata were presented by Burchfiel (1965), Stewart (1967), and Albers (1967). They noted that the Las Vegas Valley shear zone, seemingly dies out east of the Specter Range quadrangle (pl. 1), and they attributed the unexpected dis- appearance of this major shear zone to large-scale bend- ing of the miogeosynclinal rocks. That is, southeast of the Specter Range, the shear zone, which is probably a deep-seated strike-slip structure, cuts and displaces the miogeosynclinal strata 25 to 40 miles; but in the longitude of the Specter Range, the strike-slip move- ment is taken up in the sedimentary cover by large-scale bending (Albers, 1967). Evidence for this bending northeast of the Specter Range is shown by the change in strike of the axes of major folds in the pre-Tertiary rocks (pl. 1). Figure 6 of Stewart (1967) and figure 4 of Albers (1967) suggest that the axis of the Spotted Range syn- cline (structural feature 7 on pl. 1) swings sharply to the southwest and the south after passing through the Specter Range. If their interpretation is correct, intense fracturing of the pre-Tertiary rocks would be expectable in the Specter Range area, because fracturing is com- monly most intense near the crest of flexures. A possible related explanation for the intense frac- turing is suggested by the axes of the Spotted Range syn- cline and the Pintwater Range anticline (structural features 7 and 8 on pl. 1). The axes of these folds con- verge as they approach the Specter Range. If both axes pass through the Specter Range area, the carbonate rocks now forming the range were subjected to relatively tight folding with probable attendant intense fracturing. Regardless of the interpretation favored, the Specter Range lies at the intersection of two major structural trends — the Las Vegas Valley shear zone and the Spotted Range syncline (pl. 1). The high fracture transmissibility of the lower carbonate aquifer beneath the Specter Range is probably directly related to the complex deformation of the aquifer in this area. The very high transmissibility of that part of the ma- jor potentiometric trough between the Specter Range and the Ash Meadows discharge area may be due in part to downfaulted and buried segments of the upper plate of the Specter Range thrust (structural feature 19 on pl. 1) and (or) the presence of solution channels, par- ticularly near the discharge area. Four wells drilled in the NW% sec. 27, T. 16 S., R. 51 E., penetrated a possi- ble thin thrust plate of Bonanza King Formation overly- ing the Carrara Formation. This thrust plate may be part of the upper plate of the Specter Range thrust fault (pl. 1). If this upper plate is widespread elsewhere beneath the trough and consists principally of carbonate rocks, it could significantly increase the gross transmissibility of the lower carbonate aquifer in this area. Northeast of the Specter Range the trough is poorly C73 defined. The trough appears to widen greatly from about 5 miles beneath the Specter Range to perhaps as much as 20 miles beneath eastern Frenchman’Flat.Prediction of the possible effects of particular geologic structures on the distribution of transmissibility within the trough in this area would be difficult. LOWER CARBONATE AQUIFER TRANSMISSIBILITY DERIVED FROM MAP The potentiometric map can be used to calculate the gross fracture transmissibility of the lower carbonate aquifer underlying the Specter Range and other areas. The Specter Range area is particularly well suited for such a determination because both the width and the depth of the aquifer, the hydraulic gradient, and the volume of water moving through the system are known within reasonable limits. Geologic sections through the Specter Range, prepared by R. L. Christiansen, R. H. Moench, and M. W. Reynolds (written commun., Mar. 1965), indicate that the average width of saturated lower carbonate aquifer beneath the range generally ranges from 4.5 to 7 miles; the saturated thickness generally ranges from 5,000 to 8,000 feet. The hydraulic gradient between well 67—68, northeast of the Specter Range, and well 65—62, southwest of the range (pl. 1), is 0.9 foot per mile. This gradient assumes the absence of any hydraulic barriers between the two wells. The volume of water flowing through the Specter Range along the lines of cross section is considered equal to the average measured spring discharge at Ash Meadows, about 10,600 gpm, or about 17,100 acre-feet annually (table 7). That the discharge cited is probably a minimum and that practically all the measured discharge passes through this underflow strip will be established in the section “Ash Meadows Ground-Water Basin.” The gross transmissibility of the lower carbonate aquifer beneath the Specter Range is calculated using the above information and the equation Q=TIW, or T: ,where Q is discharge, in gallons per day; I is hydraulic gradient, in feet per mile; Wis width of the un- derflow strip, in miles; and T is coefficient of transmissibility, gallons per day per foot. Using an average width of 5.8 miles, the gross transmissibility is about 3><106 gpd per ft. This value is probably a max- imum because the hydraulic gradient beneath the Specter Range (where the trough is narrowest) may be two or three times that used. The gross transmissibility of the aquifer within the trough between the Specter Range and the Ash Meadows discharge area is also in the millions. The apparent hydraulic gradient in this area is about 0.3 foot per mile (pl. 1). Using this gradient, underflow strips 8 to 14 miles wide, and the same discharge, the calculated coefficient C74 of transmissibility roughly ranges from 4X106 to 6X106 gpd per ft. Based on the geologic mapping of R. L. Christiansen, R. H. Moench, and M. W. Reynolds (written commun., Mar. 1965), the thickness of car- bonate rocks beneath this segment of the trough may be expected to range from a few hundred feet near the lower clastic aquitard, on the southeast border (pl. 1) of the trough, to more than 8,000 feet near the lower clastic aquitard, on the northwest border of the trough. The coefficient of transmissibility of the aquifer within the trough between the Specter Range and Mercury Ridge (fig. 1), beneath Mercury Valley, cannot readily be calculated, because the exact width of the trough is poorly defined and the trough receives some recharge from the southeast and the northwest in this reach. Nevertheless, the trough’s position within the regional flow system and its low hydraulic gradient suggest that the lower carbonate aquifer beneath at least parts of the area probably has a gross coefficient of transmissibility of greater than a million gallons per day per foot. Northeast of Mercury Ridge the major trough is poorly defined and probably receives recharge from the north, the northeast, the east, and the southeast. The coef- ficient of transmissibility cannot be calculated because neither the volume of water entering from each direction nor the place of entry is well known. The coefficient of transmissibility of the lower car- bonate aquifer at the sites of wells 67—68 and 75—73 (figs. 33 and 31) is unexpectedly low (table 3) in comparison with the coefficients of transmissibility inferred for the aquifer beneath the major trough in the potentiometric surface. Transmissibility of the aquifer in well 67—68 is about 39,000 gpd per ft, and that in well 75—73 is about 3,800 gpd per ft (table 3). Both wells are in or along the margin of the potentiometric trough. The discrepancy may reflect the presence of aquifer blocks of low to moderate transmissibility interspersed with highly permeable blocks which control the regional poten- tiometric levels within the trough. The time-drawdown plot for well 67—68 shows a hydraulic barrier (fig. 11), as does an expanded-scale plot of data for well 75—73. These barriers could preclude sampling of the surrounding aquifer blocks of high transmissibility. In contrast to data from these wells, data from wells 79—69 and 79—69a at the southern tip of Yucca Flat and one of a group of four wells southwest of the Specter Range in the NW% sec. 27, T. 16 S., R. 51 E. (fig. 34), also in the trough, suggest minimum coefficients of transmissibility of 200,000 to 900,000 gpd per ft. The coefficient of transmissibility of the lower car- bonate aquifer beneath central Yucca Flat probably averages less than 10,000 gpd per ft. In this valley, the low hydraulic gradient indicates relatively minor un- derflow (less than 350 acre-ft per yr) moving through the system rather than high transmissibility of the aquifer. HYDROLOGY OF NUCLEAR TEST SITES DEPTH 0F GROUND-WATER CIRCULATION Saturated thickness of the lower carbonate aquifer is several thousand feet throughout much of the study area and probably exceeds 10,000 feet in parts of the region. Depth of ground-water circulation in the lower carbonate aquifer is in part a function of the variation in fracture transmissibility with depth and the degree of continuity of the relatively thin clastic aquitards (Dunderberg Shale Member of the Nopah Formation, Ninemile For- mation, and Eureka Quartzite) interbedded within the aquifer (table 1). Drill-stem test data suggest that at least the upper 1,500 feet of the lower carbonate aquifer contains open and interconnected fractures and that there is no ap- parent decrease in fracture yield with depth. (See section “Lower Carbonate Aquifer.”) Data from other areas in the miogeosyncline indicate that water-bearing fractures are open to depths far in excess of 1,500 feet below the top of a Paleozoic carbonate-rock sequence equivalent in age to the strata composing the lower carbonate aquifer. In the Eureka mining district, Stuart (1955) reported that the entry of large quantities of water from carbonate rocks (equivalent to the lower carbonate aquifer) at the 2,250-foot level flooded the Fad shaft. Drill-stem tests of the Paleozoic carbonate rocks in oil-test wells in western White Pine County indicate open fractures and fresh ground water at depths of as much as 9,400 feet below the top of the carbonate rock sequence (McJannett and Clark, 1960). In some areas where carbonate aquifers are nearly un- deformed as in parts of the Appalachian Plateau, cir- culation of ground water is retarded at the top of the first interbedded clastic stratum that lies below the altitude of the deepest surface drainage. This condition does not prevail at Nevada Test Site and vicinity because the car- bonate rocks are highly fractured and faulted. Detailed geologic mapping (1224,000 scale) at Nevada Test Site shows that the relatively thin clastic strata included within the lower carbonate aquifer are commonly offset by normal faults and minor strike-slip faults of far greater displacement than the thickness of these units. Therefore, these clastic strata probably do not significantly influence the depth of circulation in the aquifer on a regional scale. Several intuitive reasons suggest that fracture transmissibility should decrease markedly with depth. Moderate to high transmissibility of dense carbonate rocks is commonly accepted as being due largely to secondary porosity developed along fractures by solu- tion. Solution of the carbonate rock within the zone of saturation is principally a function of initial carbon diox- ide content at the water table, rate of flow, pressure, and temperature. However, because the primary source of carbon dioxide is the soil zone, the concentration of car- SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE bonic acid in ground water normally may be expected to decrease both with depth in the aquifer and with dis- tance from major recharge areas. In addition, owing to the relatively great depth to water within the lower car- bonate aquifer, one may argue that little vadose water laden with carbon dioxide reaches the aquifer within the zone of saturation. First, precipitation on ridges would have to pass through several hundred to more than a thousand feet of unsaturated carbonate rock; solution of carbonate rock during passage through this vadose zone might deplete some or most of the carbon dioxide. Sec- ond, recharge to the valley floors would have to pass through hundreds of feet of Cenozoic aquifers and aquitards before it could enter the carbonate aquifer, and the rate of such movement would be extremely slow. The preceding reasons for possible decrease in fracture transmissibility with depth neglected several important factors. First, climate, topography, and structural posi- tion of the Paleozoic carbonate rocks varied greatly between Permian and Pliocene time. For example, the absence of Mesozoic sedimentary rocks and the moderate relief developed on the Paleozoic rocks before deposition of the Tertiary tuffs and lake beds might have been conducive to moderately deep circulation of ground water in late Mesozoic and early Tertiary time, par- ticularly if the climate was subhumid to humid. (Ad- mittedly, the absence of evidence for karstification beneath the Tertiary-Paleozoic unconformity does not support the postulated climatic conditions.) Second, the arguments treated the carbonate aquifer as an isotropic reservoir and neglected the possible effect of anisotropy on depth of circulation. For example, vadose water may move vertically, more rapidly, and to greater depth in the carbonate aquifer that is highly shattered, as beneath the Specter Range, than in the aquifer elsewhere in the study area. Resolution of the question of depth of circulation within the lower carbonate aquifer must await drilling through and detailed drill-stem testing of the aquifer. Locally, the entire thickness of aquifer may have signifi— cant fracture transmissibility; thus, in the lower car- bonate aquifer, water may be flowing to depths of thousands of feet beneath the top of the aquifer, or down to the lower clastic aquitard, the “hydraulic basement.” ASH MEADOWS GROUND-WATER BASIN The regional potentiometric map (pl. 1) indicates that ground water beneath Nevada Test Site is tributary to three major discharge areas: (1) Ash Meadows, in the east-central part of the Amargosa Desert; (2) Alkali Flat, near Death Valley Junction in southern Amargosa Desert; and (3) Oasis Valley, between Beatty and Springdale. C The Ash Meadows ground-water basin is defined as C75 that area contributing ground water to the springs at Ash Meadows (fig. 1). DISCHARGE AREA A prominent spring line in the southeastern and east- central part of the Amargosa Desert (Tps. 16—18 8., Rs. 50—51 E.), an area called Ash Meadows, is the dominant feature of the discharge area of the Ash Meadows ground-water basin. The hydrogeologic setting is given in figure 34. The discharge area consists of the spring line and an unnamed valley northeast of the spring line. The valley contains a playa saturated to within a few feet of the surface (T. 17 S., R. 51 E). The playa is drained to the west by a tributary of Carson Slough (fig. 34). The unnamed valley is bordered on the north by the Specter Range and the Skeleton Hills, on the east by the northwest end of the Spring Mountains, and on the south and the southwest by several low ridges of pre- Tertiary rocks. On the west, in the vicinity of the spring line, the valley merges with the main part of the Amargosa Desert. The altitude of the playa is about 2,331 feet, and that of the surrounding ridges generally ranges from 3,000 to 6,000 feet above mean sea level. The valley northeast of the spring line is bordered by prominent structural features as shown on plate 1 and in figure 34. The Specter Range thrust fault, which brings tightly folded Middle Cambrian and Precambrian car- bonate and clastic rocks over Ordovician through Devo- nian carbonate rocks, borders the discharge area on the northwest (fig. 34). The Montgomery thrust (Hamill, 1966), which brings Lower Cambrian and Precambrian clastic rocks over Ordovician to Devonian carbonate rocks, borders the area on the southeast. On the southwest, the discharge area is bordered by a major nor- mal fault that was identified by the gravity survey of the area. The normal fault (pl. 1) extends from Big Spring on the southeast to a point about 5 miles north-northeast of Lathrop Wells. D. L. Healey and C. H. Miller (written commun., Mar. 1965) estimated that the displacement along this fault or fault zone (downthrown on the west) ranges from 500 to 1,500 feet in the vicinity of Lathrop Wells and is several thousand feet near Big Spring. The hydraulic barrier, the boundary of the Ash Meadows basin, and the normal fault shown on plate 1 (Ash Meadows area) are actually coincident but are separated on the plate for clarity. The actual position of the normal fault is best approximated by the trend of the hydraulic barrier. The gravity survey indicates that Quaternary and Ter- tiary rocks beneath the unnamed valley northeast of the spring line may be as much as 3,500 to 5,000 thick and that the structurally deepest part of this valley is im- mediately northwest of the northwest edge of the playa. The survey also indicates, as do the outcrops, that the CV76 R. 49 E. 116°25’ HYDROLOGY OF NUCLEAR TEST SITES R. 50 E. E. 600,000 116°15’ 11. 51 E. E. 650,000 R, 52 E. 116°05’ N.700,000 T. 15 s. T. 15 s. r a 16152-8c1 023171 22131229 <21;27 cv , ”A 02365 e ° m1 \2283 7’0“ QTal F99 4102311 [7 {o / 0 {ms 22111 E [22—9 // 65-66 ‘T. 165. 22640 ? 10 l ’, // 23154 221110 \ \ \\ >’ _65—62 16/51 351 // ‘ . - a N.650,000 — fi%\: \\ Q PLC 2361 19mm). —’2344 //b €p€1 @121qu 2 :Q\ x 16/51481110 \\3 / ‘h 5 —— \ — I’fm zsu2\ \ 231m 11 \ / 2250 2256 2245 ‘ \ \ \ 4/ “ 9.», a , 2230/ 3591 EM A M l. V‘“ _ 2343 36 30 2224 7 Q 223" RngersSpring // '3: “ ,\ ‘4}, 2280 ,/ ' / 3 “ 15A ’ \ " I ll 22130 / \ Lg":rj:;“%m 17/50—15a1 I 17/5243“ ‘i‘ / — 1.17s. 1‘0 2310—.AO >229"‘”‘""’ ' 2362(0TaI;Cb?)\l T.17s. 27’ : W \ / / O V 1750—29111 2345 Ash £9 >2170 (5 ETD-11k ‘ Tree Spring 5)? O .2330\ \ —2200 ( Dewls Hale fl V [CA L U 9 _ 2359 \\ V ‘ 26M/5-5b1‘ ’ ' av ""5 3 m0 ‘ 2172 7 \ (2:92:21) 1311,5221) goal X table) (1 \ ,z PAHRUMP HILLS N. 600,000 g M Ham ANTlCLINE ‘ §§ % Spring > 126 N. \ : \2270 3 T. 18 s. ,' %4’ % BoleSnring g . , 224s QTal ( . (““0 “16402270 _ .‘ 0.94;! / ,0:st ; / / \\ '1 l// \ S/:pevin>§o \O‘ Q; Springs - ._ —':;:::: 025N/e—1aa1 k. >203: ‘L\ Death 3 T. 25 N. Valley 3 g T. 19 s. Junction 025N/6-20c1 5 >201!) / BM / / N.550,000 — 2°27 ) K’e\ _ 36°15' — / Alkali Flat/I QTal — A @ \\ // Sixmileswing T14 N_ I. ,\ EAGLE MOUNTAIN 255315 QT“ T.2os. ] [\fizc 1 3‘ I “.5 E. R.BE H.7E. H.52‘E. .R.53 E. Base from US. Geological Survey l:250,000 l-lydrogeology Compiled by I. J. WmOgrad Death Valley and Las Vegas, 1954 m 1965 50,000—foot grid based on Nevada 0 2 4 5 ””5 coordinate system, central zone 2 4 6 KILOMETERS Geology generalized from R. L. Christiansen, R. H. Moench, and M. W. Reynolds (written commun., Mar. 1965). Contours for valley- fill aquifer modified from Walker and Eakin (1963). Selected well data for Pahrump‘and Stewart Valleys from Malmberg (1967) FIGURE 34. — Hydrogeology of southeastern Amargosa Desert. SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE C77 EXPLANATION : I‘I/524el QTal : < .2352 (11mm Z E E a Water well Valley-fill aquifer a: < [- Well tapping valley-fill aquifer and lower carbonate aquifer; upper num- Alluvial firn, fluvial, fanglomerate, mudflow, playa, like, and spring deposits; an E 5 bet is well number; lower number is altitude of composite static water aquitard in Ash Meadows 0 level of both hydrogealogic units; symbols in parentheses are forma- tions tapped >- fi mun—29m - >217 Tut‘f aquitard I; 0 ° ‘5 m" Thffaceous, fluvial, and lacustrine strata; Rocks afPavits Sprig and the Horse {-2 Water well Spring Formation in north-northeastern part of map; sandstone, claystone, Well upping valley-fill aquifer; upper number '3 well number; lower COMIOMCMIC, and tuff in southern W! of WP number is altitude of static water level; R, reported water level; Cv, caved well; symbol > denotes well was flowing; flow rate isin \ \\ \ \‘ \\ z z parentheses \ \ PLC 1 \ S S \ \ \\ \ fl oz an H O . Lower carbonate aquifer E z % Dolomite and limestone; minor shale and qurtzite U D J. z 5 Spring or cavern S. E Swim or cavern in bwa u. ‘ _., ‘f (8 King Fm ' ,'; 9.5 g number is altitude of spring or of water level in cavern ' ' E 4‘. Lower elastic aquitard 5 o thzite, siltstone, shale, and minor limestone a % Crystal Pool “‘ <1 )0 2200 Spring Contact Spring in Quaternary lake beds and travertine deposits; number is alti- tude of spring Anticline —2380— Potentiometric contour, inferred hH— ‘ - ‘ - ‘ Shows altitude of potentiametric surface in lower carbonate aquifer and Thrust fault lower clastic aquitard; contour interval, 10 feet Sawteeth on upper plate; dashed where inferred HYDROGEOLOGIC AND GEOLOGIC UNITS SYMBOL GEOLOGIC UNIT HYDROGEOLOGIC UNIT Cn Nopah Formation Cb Bomnza King Formation Lower carbonate aquifer Ccu Carrara Formation, upper part, Ccl Camra Formation, lower part lower elastic aquitard HYDRAULIC SYMBOLS NOTE: All altitudes and contours in feet; datum is mean sea level. Areas of significant evapotranspiration are at Ash Meadows and at Alkali Flat and vicinity 87-68 .2370 (em-cm) Test well ‘Well tapping pre-Tertiary hydmgeologic units; upper number is well number; lower number is altitude of static water level; symbols in parentheses are formations tapped —2330— — — — Potentiometric contour Shows altitude of potentiometric surface in valley-fill aquifer; dashed where inferred; contour interval, 20 feet; contours for southwest quadrant given on plate I :1 i=1 1:: i=1 :1 1:1 lnfened ground-water barrier Width of symbol not intended to represent width of hydraulic barrier, which may r-ge from several tens to a few thousand feet Dry lakebed BM X 2017 BM, bench mark, showing altitude of land surface FIGURE 34. — Continued. pre-Tertiary rocks form a continuous partly buried ridge extending northward from Rogers Spring toward the Skeleton Hills. The areal distribution of the lower carbonate aquifer and the lower elastic aquitard in the discharge area and the relation of their distribution to the Specter Range and Montgomery thrusts are shown by figure 34; the in- : ferred distribution of the elastic aquitard within the zone of saturation is shown on plate 1. The hydrologic character of the Tertiary rocks beneath the unnamed valley northeast of the spring line and beneath the region immediately southwest of the spring line are not well known, but available information suggests that these rocks are probably related to C78 and are as impermeable as the tuff aquitard beneath the Nevada Test Site. Along the north border of the un- named valley the Tertiary rocks consist of the Rocks of Pavits Spring and the Horse Spring Formation, which are part of the tuff aquitard (table 1). Near the center of the unnamed valley, rocks directly overlying the lower carbonate aquifer in four holes drilled in NW 1A: sec. 27, T. 16 S., R. 51 E., consist of 250 to 310 feet of highly im- permeable silty and clayey tuffaceous sandstone overlain by 10 to 80 feet of basalt (Johnston, 1968). Both geologic mapping (Denny and Drewes, 1965) and the configuration of potentiometric contours of the valley-fill aquifer suggest that the Tertiary rocks south and southwest of the spring line are aquitards. Near the north end of the Resting Spring Range (fig. 34), Denny and Drewes described 800 feet of “fairly cemented boulder fanglomerate” resting with angular unconfor- mity on Precambrian quartzite. Overlying the fanglomerate is 2,000 feet of fine-grained clastic rocks consisting of sandstone, claystone, and subordinate amounts of conglomerate, siltstone, tuff, and freshwater limestone. Their work suggests that the Tertiary rocks at the south end of the spring line are probably chiefly aquitards. That the Tertiary rocks southwest of the spring line are aquitards is also suggested by geologic and potentiometric maps for the area near the intersec- tion of State Highway 29 and the Nevada-California line (pl. 1). The maps show a significant tightening of poten- tiometric contours for the valley-fill aquifer (pl. 1) near the northwesternmost outcrop of Tertiary rocks shown on the geologic maps (Denny and Drewes, 1965; R. H. Moench, written commun., Mar. 19, 1965) and in figure 34. The steepening of hydraulic gradient may reflect low gross transmissibility of the Tertiary rocks, coupled with a thinning of the valley-fill aquifer. Thus, available evidence indicates that the Tertiary rocks northeast and southwest of the spring line are the hydrogeologic equivalents of the tuff aquitard beneath Nevada Test Site. Tertiary tuffs equivalent to the bedded-tuff and the welded-tuff aquifers are apparently absent in the discharge area. Data from seven wells in Tps. 17—18 S., R. 50 E., suggest that permeability of the valley-fill aquifer im- mediately southwest of the spring line is also generally low. The specific capacity of these wells ranges from 0.4 to 10 gpm per foot of drawdown and has a median value of 0.5 gpm per foot of drawdown (R. H. Johnston, written commun., Mar. 1967). These wells penetrate about 450 to 840 feet of saturated valley fill. Lithologic logs based on cuttings (furnished by Messrs. E. L. Reed and Chester Skrabacz) indicate that the bulk of the valley- fill deposits at these well sites are lake beds. The water tapped by the wells probably comes principally from dis- continuous gravel lenses interbedded with the lake beds. HYDROLOGY OF NUCLEAR TEST SITES GROUND-WATER DISCHARGE Ground water discharges from the Ash Meadows area in east-central Amargosa Desert through springs, eva- potranspiration, and underflow. The average spring dis- charge amounts to about 17,100 acre-feet annually. (about 10,600 gpm). The underflow and the eva- potranspiration cannot be readily estimated. CHARACTER AND GEOLOGIC CONTROL OF SPRING DISCHARGE Thirty springs lie along a line trending N. 20°—25° W. that extends from the north end of Resting Springs Range to lat 36°30’ N., a distance of about 10 miles (fig. 34). A polygon enclosing the springs wouldube about half a mile wide on its north end and about 3%, ‘miles wide at the south end; 20 of the springs lie within a rectangle 10 miles long by 1 mile wide. The spring lineament closely parallels the trend of the ridges that border most of the springs on the east. The altitude of the springs ranges from about 2,200 to 2,345 feet, with the highest orifices closest. to the ridges of Paleozoic carbonate rock (Bonanza King Formation). The highest water level in the discharge area (2,359 ft above sea level) is that of the pool in Devils Hole, a cavern in the Bonanza King For- mation near the center of the spring line (fig. 34). The springs are generally of similar geologic setting and shape and are variously referred to as pool springs, tubular springs, or ojos de agua (eyes of water). All but one of the springs (a minor spring at Point of Rocks) emerge from relatively flat-lying P1eistocene(?) lake beds (clays and marls) and local travertine deposits. At the surface the spring pools have a roughly circular out- line and generally range from a few feet to 30 feet in diameter (frontispiece). Some of the springs are 15 or more feet deep. The pools narrow irregularly with depth to relatively small orifices a fraction of the pool’s surface diameter. Some orifices extend vertically downward, whereas others trend diagonally downward from the bot- tom of the pool. Where the walls of the pools are clearly visible to depths of several feet, lake beds and travertine are the only rock types exposed; no permeable gravel beds are visible. Lips, overhanging ledges composed of lake sediments or of travertine, are common on the 'deepest side of the pool (frontispiece). The water moving up toward the surface of the pools is usually crystal clear but carries fine silt in suspension. Reflection of light by the silt particles gives the water a boiling appearance, an aid in spotting the narrow orifices at the bottom of some pools. The spring discharge area is overgrown with a variety of phreatophytes: copper rush (Juncus cooperi), saltgrass (Distichlis spicata), saltbush (Atriplex canescens), saltcedar (Tamarisk gallica), arrow weed (Pluchea sericea), fat-hen saltbush (Atriplex hastata), SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST‘SITE C79 E 530,000 n.50E. E an 5.620.000 ”6°15", , I I 36 30 Soil 171° 17/5240: // / 231 / Roy's 17/5945. 28.!) QTal /9 / // Lm‘n / "ISO-221 283° ‘ / // / ’ /// //n/so-230 T‘ ‘7 s. / // 345° / / _ // N - N. 620,!!!) EXPLANATION \ HYDROGEOLOCHC AND GEOLKHC UNITS \s \ Q 2 >‘ QTal 17/50-350 QT"ll ‘t E 20.5" . . E E 17/50-350 Valley-fill aim-fer 5 E 4033.5 Lake bat: with Mabddedgwnd i- < // rem . S D / I7/50-35d 1- ° / 34.539 21123; 2 //l£vils Hole \\ \, t S. /P Crystal Pool / (cavern) _ . g 18/50—3a // 315° __ 3503' Lower carbonate aquifer 2 333° / Dolomite and limestone of Bonanza < // King Formation 0 Davis —___. 18/524 M 25.0 Contact // £1. _ / Pix/sn-izc // Point of Rocks — 1000000 its 26.5" 18/51-7d 32.0°-34.0° HYDRAULIC SYMBOLS King and Jack Rabbit Point 0! Books 18/5141“: T 18 S 18/51- 7d 233° . . ”320K341? Spring or cavern in lower Big Spring . 18/5149: carbonate aquifer o _ 20.5 Name, spring or cavern number, and /JQ ~ - / temperature rn degrees Celsius / a I. QT al 0 07:12! Pool 18/514010 18/51-29b 18 50—3: 2213 220° 1033.1? 1° ,9 Spring-m valley-fill aquifer [0 Last chance Name, spring number, and remper- 13/51 40¢ azure in degrees Celsius 20 no | ‘ l 20,000—f00t grid based on Nevada Discharge and specific conduct- coordinate system, central zone I) ‘ 2 3 MILES 3:322:13“ from Walker and Eakirn l 1 l I I ‘ ) l ' | l | Note. — Specific conductance} 0 1 Z 3 KILOMETERS measurements for Devils Hole determined in laboratory are 10—40 micromhos higher than field measurements reported by Walker and Eakin (1963) FIGURE 35. — Major springs at Ash Meadows; interrelationships among discharge, specific conductance, temperature and outcrops of the lower carbonate aquifer. Torrey seepweed (Suaeda torreyana), and mesquite (Prosopis sp.). One of a group of springs at Point of Rocks (fig. 34) emerges directly from the lower carbonate aquifer. This spring discharges less than 20 gpm and is the only one emerging directly from the lower carbonate aquifer in the Ash Meadows discharge area. However, ground water also occurs in the lower carbonate aquifer in Devils 080 HYDROLOGY OF NUCLEAR TEST SITES Hole, where the water table is about 50 feet below land surface; this cavern is described in the section “Lower Carbonate Aquifer.” Discharge, water temperature, and specific conduc- tance of 11 of the highest hielding springs and water temperature of 6 minor springs are summarized in figure 35. The total measured discharge of 24 springs discharg- ing more than 1 gpm was about 10,300 gpm (about 16,600 acre-ft annually) in the summer of 1962 (Walker and Eakin, 1963, table 8). The distribution of discharge along the 10-mile spring line is greatest near the center of the line (at Crystal Pool), but it is otherwise surprisingly uniform. Discharge measurements presented by Walker and Eakin (1963) for a few of the 11 major springs extend back to 1910. These measurements show a substantial decrease in the discharge at some springs but no signifi- cant decrease at others. No firm conclusions can be drawn from the apparent variation in discharge because of the uncertain accuracy of the older measurements and possible variations in discharge caused by alteration of the spring orifices by man. However, the similarity of the discharge measurements at several springs suggests that there has been no significant variation in total discharge since the turn of the century. Measurements at 17 com- mon springs in 1953 and 1962 suggest no significant change in discharge in this period of time. This conclu- sion is supported by measurements of water-level fluc- tuation in Devils Hole between 1945 and 1960. These measurements indicate that changes in water level of the pool are short term and due only to changes in barometric pressure, earthquakes, and earth tides (O. J. Loeltz, written commun., 1960); measurements made between 1963 and 1967 substantiate Loeltz’s evaluation. TABLE 7. — Spring discharge at Ash Meadows in 1953 and 1962 [Data from Walker and Eakin (1963)] Number of Total Total annual springs Month and year discharge discharge measured (gpm) (acre-ft per yr) 17 Jan. and Feb. 1953 _ _ - 10,900 17,600 17 July 1962 ________ 10,300 16,600 Head gradients, temperature of water, and chemical quality of water suggest that water emerging from the pool springs is derived 'iy upward leakage from the lower carbonate aquifer, which flanks and underlies the Quaternary strata at the spring line. The potentiometric surface in the carbonate aquifer at Devils Hole is 14 to 159 feet higher than the orifice altitudes at the spring pools (fig. 34). Thus a positive head gradient exists for moving water upward from the carbonate aquifer into the overlying sedimentary deposits and to the land surface, provided an avenue of permeability is available. The possible nature of the flow path is discussed elsewhere in this chapter. A comparison of water temperature in the lower car- bonate aquifer, at the spring pools, and in the Quater- nary strata suggests that the springs are fed by the car- bonate aquifer. The water temperature in Devils Hole and at Point of Rocks Springs (where water discharges directly from the carbonate aquifer) ranges from 33.5° to 340°C (fig. 35). The temperature of water from springs whose discharge exceeds 1,000 gpm ranges from 27 .0° to 330°C; water temperature of lower yield springs varies from 230° to 34.5°C. These temperatures are 45° to 160°C higher than the mean annual air temperature at Lathrop Wells (185°C). Periodic temperature measurements reported by Walker and Eakin (1963, table 8) and Miller (1948, table 41) and made by the senior author indicate no seasonal or long-term variation (since 1930) in water temperature at the major springs. In contrast to the temperatures of the springs, temperatures of water from deep wells tapping the valley-fill deposits at, and west of, the spring line range from 195° to 28.5°C but are usually less than 245°C. These data suggest, in a general way, that the spring dis- charge, though emerging from Quaternary lake beds, is fed by direct leakage from the lower carbonate aquifer. Areal variations of spring-water temperatures at Ash Meadows offer further evidence that the pool springs are fed by the carbonate aquifer. Springs within a half mile of the carbonate-rock ridges that border the discharge area on the northeast (fig. 35) have water temperatures above 320°C, regardless of their discharge rate, whereas springs at greater distances from the ridges have water temperatures that are generally lower than 320°C and that appear to be related to discharge rate (compare, for example, Fairbanks and Soda Springs). This pattern is explainable as follows. Water in the carbonate aquifer, as mentioned above, is 33.5° to 34.0°C, and the mean an- nual temperature in the region is about 185°C. Water in the upper several hundred feet of the valley fill at Ash Meadows, on the other hand, is about 195°C, as in- dicated by temperature of water flowing from wells 17/50-15a1 and 17/50—29d1 (fig. 34); these wells are respectively 464 and 471 feet deep (Walker and Eakin, 1963, table 3). Thus, a temperature gradient is present for transporting heat from the carbonate rocks to the land surface. Near the carbonate-rock ridges, where the buried carbonate aquifer is shallowest, water moving to the surface along a funnel or fault zone (see below) has little time to come into temperature equilibrium with the cooler surrounding Quaternary and Tertiary sediments; here, even springs with discharges as low as 100 gpm have water temperatures as high as 33.5°C. However, at greater distances from the ridges (where the aquifer is buried at depths of hundreds to probably more than 1,000 ft), the traveltime of water from the car- bonate aquifer to the surface is generally 1 nger, permit- ting greater loss of heat to the surroundi g sediments. Furthermore, if depth to the aquifer at ad acent springs SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE is equal, the spring with the higher discharge should reach the surface at a higher temperature because the heat content of the springs is directly proportional to dis- charge, but the heat-dissipation rate in the surrounding sediments is roughly independent of discharge. The specific conductance of water from 11 major springs (discharge about 100 gpm or more) emerging from the lake beds ranges from 640 to 750 micromhos per cm at 25°C (fig. 35); the specific conductance of water from 8 of these springs ranges from 640 to 675 micromhos per cm at 25°C. The specific conductance of water from Devils Hole and Point of Rocks Springs ranges from 645 to 686 micromhos per cm at 25°C. By contrast, the specific conductance of water from wells tapping the valley-fill aquifer in the central Amargosa Desert generally ranges from 300 to more than 1,000 micromhos per cm at 25°C (Walker and Eakin, 1963, tables 3 and 9). The consistent similarity between the specific conduc- tance of the ground water from the pool springs and that of water in the lower carbonate aquifer is a third indica- tion that the springs are fed by upward leakage from the flanking and underlying lower carbonate aquifer. Complete chemical analyses of these waters, discussed in another chapter, also support this conclusion. In summary, head differentials, water temperature, and water chemistry all indicate that water emerging from the spring pools (developed in the Quaternary strata) originates from the underlying and flanking lower carbonate aquifer. Two questions now merit brief ex- amination. First, what geomorphic or structural- feature(s) is responsible for forcing water in the car- bonate aquifer to the surface at Ash Meadows? Second, how were the orifices developed in the relatively im- permeable lakebeds and other Cenozoic sediments, and what is their character at depth? The spring line could be caused by topographic, stratigraphic, or structural geologic factors, or by a com- bination of factors. The spring line might simply be at- tributed to an intersection of the water table in the lower carbonate aquifer and the land surface. However, the lower carbonate aquifer crops out in the southwestern and southern parts of the Amargosa Desert (NWM sec. 35, T. 27 N., R. 4 E.) and at the north end of Eagle Mountain (SEM: sec. 7 and SW% sec. 8, T. 24 N., R. 6 E.) at altitudes as much as 120 to 160 feet lower than the water level in Devils Hole. If the topographic setting is a factor controlling the spring line — that is, the carbonate reservoir is brimful and overflows near Ash Meadows — then spring discharge should also occur near the topographically lower outcrops of the lower carbonate aquifer; no such discharge occurs. Moreover, topography does not explain the large pool springs, such as Fair- banks Spring, Crystal Pool, and Big Spring, which emerge from lake sediments at distances of as much as 2 miles from outcrops of the lower carbonate aquifer. C81 Several factors suggesting that the spring line is fault controlled follow: 1. Linearity of the spring line for a distance of 10 miles. 2. Recent(?) faults that cut the lake beds parallel the spring line and lie immediately west of several springs. 3. Parallelism between the spring line and the strike of both bedding and the faults cutting the carbonate bedrock. 4. A major displacement in the pre-Tertiary rocks —- the inferred gravity fault (pl. 1) —— parallels and nearly coincides with the spring line (D. L. Healey and C. H. Miller, written commun., Mar. 1965). More than a single fault zone may control the spring discharge. The inferred gravity fault (shown as an hydraulic barrier in figure 34) lies as much as a mile southwest of a line connecting Fairbanks, Rogers, Longstreet, and Point of Rocks Springs, and it may serve as the barrier for these springs (fig. 34). However, Crystal Pool and Big Spring, which together contribute about 40 percent of the measured discharge of the line of springs, are as much as 1 mile west of the inferred gravity fault and are over lows on the gravity map beneath which the pre-Tertiary rocks are probably buried several thousand feet (D. L. Healey and C. H. Miller, written commun., Mar. 1965). One or more other faults may be needed to explain these springs, although the springs could be fed by eastward dipping funnels (see discussion below) originating east of the gravity fault. Several hypotheses may explain the geological character of the fault-controlled hydraulic barrier(s). The simplest hypothesis is that the lower carbonate aquifer, cropping out in the ridges east of the spring line, is completely or partially dammed by downfaulted Ter- tiary and Quaternary aquitards. R. L. Christiansen (oral commun., Mar. 1966) suggested this hypothesis to the authors. Christiansen’s hypothesis is strengthened by consideration of the stratigraphic position of the car- bonate rocks composing the ridges immediately east of the spring line. These ridges consist of the Bonanza King Formation (Middle Cambrian age), which, except for several hundred feet of carbonate rocks at the top of the Carrara Formation, constitutes the oldest and stratigraphically the lowest formation within the lower carbonate aquifer (table 1). Thus, downfaulting of the relatively impermeable Cenozoic rocks, which may aggregate more than 3,000 feet, against the lower car- bonate aquifer and the underlying lower elastic aquitard could effectively seal the lower carbonate aquifer and force the water to discharge from it east of the fault zone. The gravity map indicates that the pre-Tertiary surface is downdropped 2,000 to several thousand feet (along the inferred gravity fault) immediately west of a line con- necting Longstreet and Point of Rocks Springs. Thus, partial juxtaposition of the Tertiary rocks and the lower 082 elastic aquitard is possible with attendant damming of the lower carbonate aquifer. The presence of Tertiary rocks in discontinuous northwest-trending outcrops between Ash Tree Spring and Grapevine Springs (fig. 34) does not preclude them at depth beneath the spring line, because a northwest-trending gravity low occurs between the two trends. Normal faulting may satisfactorily explain the geologic control of the springs in the southern half of the spring line, but it may not offer adequate explanation for the springs northwest of Longstreet Spring. Geologic mapping indicates that the buried lower carbonate aquifer is probably much thicker east of Fairbanks, Longstreet, and Rogers Springs than south of Longstreet Spring (Denny and Drewes, 1965, and R. H. Moench, written commun., Mar. 1965). In addition, the displace- ment on the inferred gravity fault decreases northward; near Crystal Pool the displacement of several thousand feet is indicated by the gravity survey, whereas at the northwest end of the spring line the displacement may amount to only 1,000 to 2,000 feet. Accordingly, although juxtaposition of the Tertiary and Quaternary aquitards against the lower carbonate aquifer could markedly reduce the cross-sectional area of flow through the car- bonate rocks, complete blockage of the aquifer in this area is less likely unless the fault zone itself is relatively impermeable. Thus, some ground water in the lower car- bonate aquifer (northwest of Longstreet Spring) might move directly into the central Amargosa Desert without being forced upward into the Quaternary and Tertiary valley fill. A second hypothesis to explain the spring line requires curtailment of the lower carbonate aquifer through either (1) fault juxtaposition of the aquifer and the lower elastic aquitard or (2) folding of the elastic aquitard into a structurally high position. The bulk of the north- northwest trending Resting Springs Range consists of the lower elastic aquitard, specifically rocks of the Wood Canyon Formation and the Stirling Quartzite (fig. 34). At the north end of the Resting Springs Range the lower ' elastic aquitard is overlain by Tertiary elastic rocks that trend northwestward to Ash Tree Spring, a distance of about 12 miles from the northernmost outcrop of the lower elastic aquitard in the range (fig. 34). The Tertiary outcrops indicate that the pre-Tertiary rocks are close to the surface in this area, and the gravity map suggests that they may be buried at depths as shallow as 2,000 feet below the surface just west of the spring line (D. L. Healey and C. H. Miller, written eommun., Mar. 1965). If the Tertiary rockswest of the spring line are underlain principally by the lower elastic aquitard, as they are at the north end of the Resting Springs Range, the lower elastic aquitard could serve as a principal or secondary hydraulic barrier responsible for the spring line. The areal dispositions of the lower elastic aquitard and the HYDROLOGY OF NUCLEAR TEST SITES lower carbonate aquifer could presumably have been set during pre-Tertiary deformation, and subsequent Ter- tiary block faulting would have created the gravity anomaly reported by Messrs. Healey and Miller (written eommun., Mar. 1965). A third hypothesis involves a major strike-slip fault zone mapped in Stewart Valley by R. L. Christiansen, R. H. Moench, and M. W. Reynolds (written eommun., 1964) and shown on plate 1 (structural feature 17). This postulated major fault zone trends almost parallel to the spring line. If the strike-slip fault zone continues in the subsurface northwestward beyond Stewart Valley, the mapped area, it might create the hydraulic barrier described either by virtue of gouge developed along it or through juxtaposition of Paleozoic carbonate and elastic rocks. The authors favor the idea that the ground-water barrier is caused by normal faulting of the poorly permeable Cenozoic rocks against the lower carbonate aquifer. Of course, a combination of two or all three of the hypotheses may explain the hydraulic barrier. On plate 1(sec.A—A’),the barrier is diagrammatically shown as a combination of the first and second hypothesis. Granting that the spring line is structurally controlled and that the spring water originates within the lower car- bonate aquifer, how does the water move upward through the relatively impermeable Cenozoic strata to become concentrated in the funnel-shaped spring pools? If the hydraulic barrier(s) is caused principally by pre— Tertiary structural curtailment of the lower carbonate aquifer, then it is possible that the major springs may reflect the approximate position of pre-Tertiary springs, some of which, owing to hydrostatic head, continued to flow during deposition of both the Tertiary and Quater- nary sediments. If this occurred, each spring is probably fed by a sand- and silt-filled funnel which extends irregularly downward to the lower carbonate aquifer and which is surrounded by generally impermeable Cenozoic strata. Some leakage from the postulated funnels to the gravel lenses within the Cenozoic sequence probably oc- curs, but the rate of such leakage is retarded by the probable discontinuous nature of these lenses. If the spring line was created in middle to late Tertiary time by fault juxtaposition of the Tertiary aquitard against the lower carbonate aquifer, then the position of the orifices may be controlled in part by fault zones, most of which have since been obscured by deposition of the youngest Quaternary lake deposits. Hydrostatic head, again, might have enabled major springs to maintain themselves by piping during deposition of the Quater- nary lake and other valley-fill deposits. Evidence that a major spring once discharged from Devils Hole follows. Devils Hole, with a reported minimum depth of 365 feet, is about 70 feet long and 30 to 40 feet wide; at the water surface, 50 feet below land SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE CS3 surface, the pool is about 40 feet long and 10 feet wide (Worts, 1963). Although the water level in Devils Hole has probably been constant for the last 15 and possibly 50 years, several bits of evidence indicate that it fluc- tuated widely in the geologic past and that a large spring once emerged from the hole. Water-level fluctuations of possibly as much as 20 feet are recorded on the walls of the sink itself. Figures 36 and 37, photographs taken in the sinkhole, show former stands of the water. Geologic and zoological evidence suggests that the water level in Devils Hole was once at the surface. Denny and Drewes (1965, p. L30—L31) reported a lobate sheet of spring deposits adjacent to Devils Hole. They stated: Just west of Devils Hole a sheet of spring deposits * * * measures roughly 500 by 1,500 feet and mantles ridges and shallow gullies. The edge of the sheet is lobate and the fingers point downwash. The deposits are at least 2 feet thick, thinning to a few inches near their borders; on the ridges they overlie a desert pavement. An east-trending narrow band of slightly more massive deposits suggests seepage along a fissure. The sheet is draped over the landscape and is perhaps the relic of a wet meadow which surrounded a flowing spring. Fish in Devils Hole and in major spring pools (Miller, 1946, 1948; Hubbs and Miller, 1948) provide possible zoological evidence for interconnected pluvial lakes in FIGURE 36. — Solution notches (bracket) marking water levels as much as 4 feet above 1966 water level of pool at Devils Hole. FIGURE 37. — Possible former stand of water (horizontal line in middle ground) about 20 feet above 1966 water level on south wall of Devils Hole. Bonanza King Formation dips 45° to the right, or west. both Amargosa Desert and Death Valley. The small fish‘ (Cyprinodon) presumably survived the dessication of lakes at the end of the last pluvial period by adapting to living in the spring pools. Presumably with decline of the water level, some of the fish population retreated into the depth of Devils Hole, where they live today. Eleven of 30 springs discharge more than 98 percent of the water at Ash Meadows, and 5 of them (Fairbanks, Longstreet, Crystal Pool, King, and Big Springs) dis- charge about 80 percent of all the water at Ash Meadows (fig. 35). When considered together with the preceding discussion of Devils Hole, these large discharges from five widely spaced springs may be evidence that all the major pool springs originate as flow concentrated by caverns developed within the lower carbonate aquifer. If cavernous control were lacking, the spring line might be more continuous, and the discharge from individual springs would be much smaller. However, hydraulic tests of the lower carbonate aquifer indicate that frac- ture transmissibility of the aquifer can be as great as 900,000 gpd per ft and that the transmissibility is highly‘ variable from well to well or at depth within a single bore. Therefore, the pool springs might also originate from highly fractured and transmissive areas within the lower carbonate aquifer as well as from caverns. For ex- ample, a block of highly fractured carbonate rock even as small as 40><40><40 feet could serve as a highly efficient natural infiltration gallery capable of discharging hun- dreds of gallons per minute under small head differen- tials; solution caverns need not be invoked as the sole source of the high-yield pool springs. The authors favor solution-cavity control of the discharge pattern. C84 UPWARD LEAKAGE FROM LOWER CARBONATE AQUIFER BENEATH. VALLEY NORTHEAST OF SPRING LINE Discharge from the lower carbonate aquifer is upward into the Cenozoic rocks underlying the unnamed alluvial valley and playa northeast of the spring line (fig. 34). Stands of mesquite and other phreatophytes fringing the east side of the playa (pl. 1) indicate a shallow water table in the vicinity of the playa. The fluffy texture of surface materials underlying parts of the playa and the high moisture content of the sediments within a foot or two of the surface are additional indications of a shallow water table. In the southwestern corner of the playa, several small pits excavated during mining of clay in- tersect the water table. The altitude of the floor of the playa is 2,331 feet above mean sea level, or about 30 feet lower than the head within the lower carbonate aquifer at Devils Hole and at well 65—62 north of the playa. With this head differential, some upward leakage of water from the lower carbonate aquifer into the Quaternary and Tertiary rocks is certain. Altitude of the water level in wells 16/51—28d1, 16/51-36a1, and 17/51—1a1, which tap the valley-fill aquifer, indicate that some leakage also is probably up- ward in areas beyond the playa; these levels are 17 to 61 feet lower than the head in well 65—62, which taps the lower carbonate aquifer. The quantity of upward leakage from the lower car— bonate aquifer into the Cenozoic rocks is difficult to es- timate. In Yucca Flat an estimate of downward leakage could be made because, owing to the deep water table, virtually all the crossflow had to move through the tuff aquitard. However, in the unnamed valley the water table ranges from 454 feet below the surface at well 65—66 to a few tens of feet below the surface near the playa (fig. 34). Because of the shallow water table, the valley-fill aquifer and the lower carbonate aquifer may locally be in direct contact where the carbonate aquifer forms shallow buried ridges if the relatively impermeable Tertiary strata are absent (owing to nondeposition or erosion). Conversely, beneath the structurally deepest parts of the valley, crossflow between aquifers would be greatly retarded (as in Yucca Flat) by the probable thick section of Tertiary strata between the lower carbonate aquifer and the valley fill. By analogy with Yucca Flat, where the leakage probably amounts to less than 100 acre-feet annually, the upward leakage in this area is believed to be less than 1,000 acre-feet annually because Of the low gross transmissibility of both the basal valley fill and the Tertiary rocks. The upward leakage into the valley-fill aquifer may leave the valley in two ways. The principal ground-water flow is probably toward the playa to replenish the dis- charge by evapotranspiration. Some water may flow westward toward the main part of the Amargosa Desert through the area between Longstreet Spring and the hills HYDROLOGY OF NUCLEAR TEST SITES of carbonate rock in sec. 28, T. 16 S., R. 50 E. Such flow is probably retarded by (1) the low permeability of the valley fill between Longstreet and Fairbanks Springs and (2) possible upward leakage from carbonate rocks buried at relatively shallow depths between Fairbanks Spring and the hills north of the springs. ESTIMATES OF EVAPOTRANSPIRATION Ground water is also discharged from the region by evaporation from playa surfaces and through transpira- tion by phreatophytes. The major areas of evapotranspiration are shown on plate 1. Estimates of evapotranspiration made by Walker and Eakin (1963, p. 21—24, and table 7) suggest that the water discharged by phreatophytes at Ash Meadows may be derived entirely from recycled spring discharge. They estimated a total evapotranspiration in the Amargosa Desert of about 24,000 acre-feet annually, 10,500 acre-feet of which oc- curs at Ash Meadows; the remainder represents dis- charge from playa surfaces, chiefly Alkali Flat (southeast of Death Valley Junction; pl. 1 and fig. 34), and an area of very shallow water table (1—3 ft) extend- ing several miles north and northeast of this playa. Nevertheless, the possibility that some of the phreatophytes at Ash Meadows are fed by water which has leaked up from the carbonate aquifer, rather than by recycled spring discharge, cannot be dismissed until a detailed study of evapotranspiration and the spring runoff regimen is made. Until such a study is done, the spring discharge (about 17,000 acre-ft annually) is con— sidered the most accurate available measure of discharge from the lower carbonate aquifer at Ash Meadows. POSSIBLE UPWARD LEAKAGE SOUTH OF LATHROP WELLS Upward flow from pre-Tertiary rocks into the valley- fill aquifer may also occur along a 10-mile strip between Lathrop Wells and well 16/49—26d1 (fig. 34). The con- figuration of potentiometric contours for the valley-fill aquifer in this area suggests a hydraulic discontinuity that parallels the inferred gravity fault (fig. 34). The dis- continuity, indicated by apparent change in direction of ground-water flow and the steepening of hydraulic gradient, may reflect upward crossflow. The geologic map and the sections of R. L. Christiansen, R. H. Moench and M. W. Reynolds (written commun., Mar. 1965) suggest that pre-Tertiary rocks within the zone of saturation between the inferred gravity fault on the west, the Specter Range thrust on the south, and the northern one-third of the Specter Range on the east (fig. 34) consist principally of lower clastic aquitard overlain locally at shallow depth by the lowest part of the lower carbonate aquifer; these rocks form the bulk of the upper plate of the Specter Range thrust fault (pl. 1 and fig. 34). Their interpretation that SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE the lower carbonate aquifer is underlain by the clastic aquitard at shallow depth is supported by data for well 68—60, where the water table is 674 feet below land sur- face. Well 68-60 penetrated the lower carbonate aquifer in the interval 160 to 400 feet and the lower clastic aquitard in the interval 400 to 910 feet. The inferred gravity fault affects the distribution of the pre-Tertiary rocks. At a point about 1 mile east of Lathrop Wells, the gravity survey suggests a displace- ment (downthrown on the west), of the Tertiary—pre- Tertiary surface of 500 to 1,500 feet (D. L. Healey and C. H. Miller, written commun., Mar. 1965). At a point about 1 mile southeast of the intersection of the inferred gravity fault with the trace of the Specter Range thrust, the displacement is about 1,200 to 2,200 feet. The configuration of the potentiometric contours for the valley-fill aquifer probably reflects westward drainage of a ground-water ridge possibly formed by up- ward leakage along one or more fault zones paralleling and including the inferred gravity fault (fig. 34) and drainage of poorly permeable valley fill toward more transmissible valley fill west of the fault zone. These possibilities are supported by analogy with hydrologic gmdjtions ”along the inferred gravity fault adjacent to. the spring line and by‘drillers’ log of wells 16/49-14a1 and 16/49—23a1 (Walker and Eakin, 1963); these logs indicate that the valley fill tapped by these wells is composed chiefly of clay. The quantity of upward leakage in this area is probably small, because such leakage probably originates principally from the lower clastic aquitard. MAGNITUDE 0F POSSIBLE UNDERFLOW ACROSS THE DISCHARGE AREA In addition to discharge at the spring line (about 17,000 acre-ft per yr), discharge through upward leakage within the unnamed valley northeast of the spring line (1,000 acre-ft per yr, estimated), and possible minor dis- charge along the inferred gravity fault south of Lathrop Wells, some ground water probably also moves southwestward across the Ash Meadows discharge area through the lower carbonate aquifer. Between Longstreet Spring and the southwest corner of T. 16 S., R. 50 E., the carbonate aquifer may be imcompletely blocked by Tertiary aquitards, and some underflow may move westward through the lower carbonate aquifer and Cenozoic aquitards. (See section “Character and Geologic Control of- Spring Discharge”) The magnitude of the hypothesized underflow across the spring line into the central Amargosa Desert cannot be approximated despite the availability of crude es- timates of discharge from central and southern Amargosa Desert. Ground water can leave central and southern Amargosa Desert either by underflow and evaporation in the vicinity of Eagle Mountain (fig. 34) at CS5 the south end of the valley or by underflow through the lower carbonate aquifer toward Death Valley. Walker and Eakin (1963, p. 22) estimated underflow toward the south to be about 500 acre-feet annually. The evapora- tion from Alkali Flat and vicinity north of Eagle Moun- tain may amount to more than 10,000 acre-feet annually. Westward movement of ground water from the Amargosa Desert to the Furnace Creek area in Death Valley was suggested by Hunt and Robinson (1960) and by Hunt, Robinson, Bowles, and Washburn (1966) on the basis of a similarity of the chemistry of spring water in both areas; they further hypothesized that ground-water flow between the two areas is through fault zones. The total discharge from the Furnace Creek area, about 30 miles west of Devils Hole, is reported to be 3,150 gpm (Hunt and others, 1966, table 25), or about 5,100 acre- feet annually. The regional potentiometric map (pl. 1) and water chemistry (pl. 3) show that the ground water in central and southern Amargosa Desert is derived from three sources: (1) possible underflow across the spring line plus recycled spring discharge, (2) flow from Jackass Flats, and (3) flow from northwestern Amargosa Desert. Thus, until the magnitude of the contribution from each of these sources is known, as well as the head relationships between the lower carbonate and the valley-fill aquifers in central and southern Amargosa Desert, no reliable es- timate of underflow across the spring line at Ash Meadows is possible, even were precise data on discharge in and underflow out of the central and the southern Amargosa Desert available. In summary, in view of possible underflow beneath the spring line, the measured spring discharge of about 17,000 acre-feet annually must be considered the minimum quantity of ground water in transit through the lower carbonate aquifer at Ash Meadows. AREAL EXTENT OF THE GROUND-WATER BASIN Definition of the hydrologic boundary of the Ash Meadows ground-water basin is greatly hindered by the complexity of the geologic structure, the limited poten- tiometric data, and, most critically, the interbasin movement of ground water through the thick and areally extensive lower carbonate aquifer. Interbvasin movement precludes straightforward use of topographic and isohyetal maps to pinpoint possible hydrologic divides. An estimate of the minimum area of the Ash Meadows ground-water basin, based on isohyetal, potentiometric, and geologic evidence discussed below, is shown on plate 1. The boundary should be considered a first approxima- tion at best. INFERRED FROM ISOHYETAL MAP In a region of known or inferred interbasin movement of ground water, use of an isohyetal map to estimate the C86 boundaries of a basin is severely limited. Yet, certain patterns evident on the isohyetal map of the study area. (fig. 3) may provide clues for at least a part of the bound- ary of the Ash Meadows ground-water basin. The mean annual precipitation in Amargosa Desert, Yucca Flat, Frenchman Flat, Jackass Flats, Indian Springs Valley (except the southernmost part), and Three Lakes Valley (except the southernmost part) ranges from 3 to 8 inches. On the Spring Mountains and the Sheep Range the mean annual precipitation generally ranges from 10 to 25 inches; and on the Belted, Timpahute, and Pahranagat Ranges it ranges from 10 to 14 inches. This distribution of precipitation suggests that the Spring Mountains and the Sheep Range may form the southern and eastern border of the Ash Meadows ground-water basin and that the Belted, Timpahute, and Pahranagat Ranges may form the northwestern, northern, and northeastern borders. On the southwest, specifically in the area between the south end of Shoshone Mountain and the northwest end of the Spring Mountains, the mean an- nual precipitation amounts to about 6 inches. Thus, the isohyetal map suggests that the regional drainage of ground water is toward Ash Meadows and that possibly eight intermontane valleys — Three Lakes, Desert, In- dian Springs, Emigrant, and Mercury Valleys and Yuc- ca, Frenchman, and Jackass Flats —— might be tributary to the discharge area. Two important considerations limit use of the isohyetal map: (1) areal variations in rock type and soil cover, and (2) interbasin movement of water. For exam— ple, less infiltration may occur in welded-tuff than in carbonate-rock terrane because of the greater density and solution alteration of fractures in the carbonate rocks and because of clayey soil developed on the tuff. Thus, as much or more recharge may result from 6 inches of annual precipitation on the fractured carbonate rock of the Spotted Range than from 12 inches on the welded tuffs capping the highest parts of the Belted Range. If recharge could somehow be accurately estimated from the isohyetal map, taking differences of rock type, soil development, precipitation type, and slope into ac- count, the map would still not serve as a positive in- dicator of the hydrologic boundary of the Ash Meadows ground-water basin. Near the margin of a ground-water basin, recharge of as little as a few tenths of an inch of water annually to the principal aquifer influences the position of the ground-water divide in that aquifer. However, the addition of several times this recharge near the center of the system, where the volume of ground water in transit may be several hundred times the recharge cited, may do no more than create a minor mound in the potentiometric surface; such a mound need not significantly affect regional flow patterns in thick aquifers (Toth, 1963). Thus, the possibility of a HYDROLOGY OF NUCLEAR TEST SITES shallow ground-water mound beneath a ridge of car- bonate rock receiving relatively moderate precipitation — for example, the northern Spotted Range (8 to 9 in.) or the Pahranagat Range (10 to 12 in.) — cannot be cited as proof that the mound forms a hydrologic boundary of the basin even should the quantity of recharge and the posi- tion of the mound within the basin be known. Despite these qualifications, the isohyetal map suggests that the Spring Mountains and the southern half of the Sheep Range — the highest ranges in southern Nevada — could serve as the southern and the eastern hydrologic boundaries of the Ash Meadows basin. The highest parts of both ranges are composed chiefly of the lower carbonate aquifer, which should be conducive to maximum infiltration. If these ranges mark the ap- proximate southern and eastern boundaries of the basin, then both Three Lakes and Indian Springs Valleys are probably within the Ash Meadows ground-water basin. Use of the isohyetal map (fig. 3) as an indicator of the basin boundary may also be justifiable for areas of relatively low precipitation (10 to 16 in.), such as the northwestern Spring Mountains, which are underlain chiefly by the lower clastic aquitard. But, use of this map to establish hydrologic divides in areas of similar precipitation that are underlain by the lower carbonate aquifer is risky. INFERRED FROM POTENTIOMETRIC MAP The regional potentiometric map (pl. 1) indicates that most or all of Yucca Flat, Frenchman Flat, eastern Jackass Flats, southern Indian Springs Valley (south of US. Highway 95), Mercury Valley, and the unnamed valley northeast of the spring line in east-central Amargosa Desert are tributary to the Ash Meadows dis- charge area. The contours for the lower carbonate aquifer also indicate that ground water flows into Frenchman Flat from the east and the northeast. This condition suggests that northern Indian Springs Valley (north of US. Highway 95), northern Three Lakes Valley (north of US. Highway 95), and possibly Desert Valley may also be tributary to the basin. The potentiometric contours for the valley-fill aquifer and for the Tertiary aquifer and aquitards also provide information on segments of the hydrologic boundary of the Ash Meadows ground-water basin. The poten- tiometric contours for the valley-fill aquifer in northwestern Las Vegas Valley indicate that water in this aquifer is moving southeastward, whereas the con- tours for southern Three Lakes Valley indicate that this water is moving northward (pl. 1). A divide is indicated in an area near the intersection of US. 95 and Nevada State Highway 52, but its exact position cannot be iden- tified with existing well control. This ground-water divide in the alluvium probably reflects a divide in the SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE lower carbonate aquifer, because elsewhere in the study area head changes in the Cenozoic rocks closely reflect head changes in the underlying pre-Tertiary rocks. The approximate position of a divide on the northwest side of the Ash Meadows basin is also shown by the potentiometric contours for Cenozoic rocks beneath Emigrant Valley and Pahute Mesa (pl. 1). Southeastward movement of ground water is indicated beneath western Emigrant Valley and southwestward movement beneath Pahute Mesa. The ground-water divide between the basins may lie beneath the Belted Range, which separates the two areas contoured. Two areas that are not tributary to the Ash Meadows ground-water basin are also delineated by the poten- tiometric contours for the valley-fill aquifer. The poten- tiometric contours, the hydraulic barrier(s) extending from Lathrop Wells to Big Spring, and the disposition of the lower clastic aquitard east of Lathrop Wells (pl. 1) indicate that ground water beneath central Amargosa Desert and southwestern Jackass Flats does not dis- charge at Ash Meadows and is not part of the ground- water basin. This is the same conclusion reached by O. J. Loeltz (written commun., 1960) on the basis of water- level data alone. Contours also suggest that ground- water in Pahrump Valley is not moving toward Ash Meadows. Because water levels in the valley-fill and welded-tuff aquifers of northwestern Jackass Flats and northwestern Amargosa Desert are higher than the water level in Devils Hole (alt 2,359 ft), it may still be argued that water can move from these areas toward Ash Meadows (pl. 1). Such postulated flow might occur not through the Cenozoic aquifers, which appear to contain no areally ex- tensive aquitards to maintain the head needed, but rather through the lower carbonate aquifer, which could maintain the necessary head due to regional confine- ment by the tuff aquitard. Unfortunately, this argument fails to explain the absence of spring discharge from several outcrops of the lower carbonate aquifer in the central Amargosa Desert; these outcrops (location given previously in the section “Character and Geologic Con- trol of Spring Discharge”) are lower than the water level in Devils Hole and should discharge some water if the source of the Ash Meadows discharge is north or northwest of the spring line. The proposed origin from carbonate rocks beneath the northwestern Amargosa Desert also ignores the widespread distribution of the lower clastic aquitard beneath this part of the Amargosa Desert. That the clastic aquitard, rather than the car- bonate aquifer, probably underlies the Cenozoic strata is suggested by outcrops of the aquitard (pl. 1), the Roses Well anticline, and well 2506 (pl. 1), which penetrated the lower Clastic aquitard immediately beneath the Cenozoic rocks. C87 INFERRED FROM MAJOR GEOLOGIC FEATURES The areal and inferred subsurface disposition of the lower Clastic aquitard, the hydraulic basement, permits an estimate of the position of parts of the boundary shown on plate 1. Geologic character and extent of the Gass Peak thrust fault (structural feature 12 on pl. 1) suggest that most of the recharge to the Sheep Range, particularly to the highest part of the range, may be diverted westward toward Three Lakes and southern Desert Valleys. The thrust fault brings Cambrian and late Precambrian carbonate and clastic rocks over Mississippian to Permian carbonate rocks (Bird Spring Formation). Where exposed, the base of the upper plate consists of the lower Clastic aquitard, specifically the lower part of the Carrara Formation, the Wood Canyon Formation, Stirling Quartzite, and Johnnie Formation. Although the lower clastic aquitard is exposed only adja- cent to the southern 25 miles of the fault trace, it is probably close to the surface beneath the inferred trace of the fault in northernmost Clark and southernmost Lincoln Counties. There, westward-dipping Middle and Upper Cambrian rocks crop out west of the inferred fault trace, and overturned Mississippian and younger rocks crop out east of the fault trace (Longwell and others, 1965, p. 76 and plate I; Tschanz and Pampeyan, 1961). The argillaceous clastic rocks at the base of the upper plate are missing from outcrop possibly because they are more susceptible to erosion in an arid climate than are carbonate rocks. Clastic strata are inferred to underlie the carbonate strata for some distance west of the fault trace for three reasons. (1) Both the thrust surface and the bedding planes dip westward. (2) Other westward-dipping major thrust faults in the region (Tippinip, Wheeler Pass, and Montgomery thrust faults) contain westward dipping Clastic strata at the base of the upper plate, which suggests that the clastic strata served as the glide plane for the thrust-fault movement, and (3) the fault plane generally dips at a steeper angle than the bedding in the upper plate and hence displaces older clastic rocks with depth; that is, the thrust surface does not cut out the clastic strata but rather increases their thickness (in the upper plate) downdip from the fault trace. (See cross section B—B’, pl. 1 of Longwell and others, 1965.) Because of the low gross transmissibility of the elastic rocks, their nearly continuous exposure east of the highest part of the Sheep Range, and their westward dip, they probably form a highly efficient barrier to the eastward movement of ground water within the car- bonate rocks. If there were no clastic aquitard or if the aquitard were horizontal or were deeply buried, one might assume that approximately half the recharge to the Sheep Range moves eastward and half moves C88 westward; but because of the described disposition of the aquitard, most of the recharge to carbonate rocks in the’ Sheep Range probably moves to the west toward southern Desert and Three Lakes Valleys. However, the phreatophyte lineament and minor spring discharge (about 125 gpm) at Corn Creek Ranch and the poten- tiometrie contours for the valley-fill aquifer in northwestern Las Vegas Valley (pl. 1) suggest that some recharge probably also moves southward and thence southeastward into Las Vegas Valley through the valley- fill aquifer. The movement of significant quantities of ground water eastward across the Gass Peak thrust where it is covered by valley fill opposite the northern half of the Sheep Range (pl. 1) is possible but unlikely. Opposite the southern and by far the loftiest part of the range, major springs are absent at the contact of the lower car- bonate aquifer with the lower elastic aquitard (altitude of contact generally ranges from 4,000 to 6,000 ft). The only significant low-level spring — one at Corn Creek Ranch —- emerges at an altitude of about 3,000 feet. This suggests that the carbonate reservoir beneath the Sheep Range is not full of water even opposite the highest and wettest part of the range. By analogy, opposite the northern and much lower half of the range, the water table in the carbonate aquifer may also be lower than its buried contact with the elastic aquitard. If the lower car- bonate aquifer is saturated above the altitude of its buried contact with the elastic aquitard, eastward move- ment would occur only if the tuff aquitard were absent ab0ve the buried pre-Tertiary-Tertiary unconformity. In summary, some ground water undoubtedly moves eastward through, or over, the elastic aquitard, but the quantity is probably insignificant compared with that moving westward (and possibly southward) through the carbonate aquifer. If most of the recharge to the. Sheep Range is deflected westward by the subsurface disposition of the elastic and carbonate rocks, then both Three Lakes and southern Desert Valleys are within the Ash Meadows ground- water basin. The structural disposition of the lower carbonate aquifer and the lower elastic aquitard also suggests that recharge to the Spring Mountains in the central part of Tps. 18—19 S. and Rs. 54—55 E. is tributary to southern Indian Springs Valley, but not to Pahrump Valley as in- dicated by position of topographic divide (pl. 1). In this area the aquifer lies within the center of a major northward-plunging syncline (structural feature 15 on pl. 1) and is surrounded by the lower elastic aquitard on the southeast, south, and southwest. Thus, unless the aquifer is brimful, a condition not supported by the oc- currence of major contact springs (at the contact" of the carbonate aquifer and the lower elastic aquitard), HYDROLOGY OF NUCLEAR TEST SITES recharge should be deflected north into the Ash Meadows ground-water basin. Disposition of the elastic aquitard was also used to es- timate a part of the northwestern and southwestern boundary of the ground-water basin shown on plate 1. Further discussion of the southwestern boundary (specifically the region between Ash Meadows and Pahrump and Stewart Valleys) is given in the section “Relation to Pahrump Valley Ground-Water Basin.” EXTENT OF BASIN A synthesis of the preceding isohyetal, potentiometric, and geologic evidence permits a first approximation of some boundaries of the Ash Meadows ground-water basin, except at the north and northeast (pl. 1). The preliminary basin boundaries encompass an area of about 4,500 square miles; included in the basin are 10 in- termontane valleys (Desert Valley, Three Lakes Valley, Indian Springs Valley, Emigrant Valley, Yucca Flat, Frenchman Flat, eastern Jackass Flats, Mercury Valley, Rock Valley, and the unnamed valley in east-central Amargosa Desert). A northeast boundary for the Ash Meadows ground- water basin cannot be defined on the basis of available geologic or hydrologic evidence. The geologic map of Lin- coln County (Tschanz and Pampeyan, 1961) was ex- amined for possible geologic structures which could serve as hydraulic barriers between the north end of the Sheep Range and the north end of the Groom Range; however, no outcrops of the lower elastic aquitard occur in the hills and ridges surrounding Desert Valley and Pahranagat Valley or in the hills separating these valleys from Coal, Garden, and Penoyer (Sand Spring) Valleys. Moreover, the ages of the carbonate rocks shown by the map suggest that the lower elastic aquitard is probably buried thousands of feet beneath the base of the ridges and hills and at still greater depths beneath the valley floors. Outcrops equivalent to the upper elastic aquitard (the Eleana Formation) are scattered throughout the area, but these elastic strata aggregate only a fraction of the thickness of the Eleana Formation at the Nevada Test Site (7,900 ft); the Devonian and Mississippian elastic rocks in the Desert Valley—Pahranagat Valley area aggregate less than 1,500 feet in thickness. Available water-level data also suggest that ground water may enter the Ash Meadows ground-water basin from Pahranagat, Coal, or Garden Valleys to the northeast. The lowest water-level altitudes in those valleys are 700 to more than 2,000 feet higher than levels within the central Ash Meadows basin (Eakin, 1966). Eakin (1966) suggested a geologic mechanism that may in part control southwesterly movement of water from southern Pahranagat Valley toward Desert Valley, although he did not suggest such movement. At the SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE south end of Pahranagat Valley, near Maynard Lake (pl. 1), the hydraulic gradient shown by Eakin (1966) in- dicates a barrier within the lower carbonate aquifer. Eakin suggested that the barrier at the south end of Pahranagat Valley is formed by a major fault zone tren- ding southwestward for about 16 miles (structural feature 10 of pl. 1). If this fault (or a nearby major strike- slip fault zone; feature 9 of p. 1) is responsible for dam- ming the lower carbonate aquifer, it may also deflect some ground water in the aquifer to the southwest toward Nevada Test Site. Thus, the northeastern extent of the basin cannot be precisely defined from available data; however, the Ash Meadows ground-water basin probably extends at least to the Pahranagat and Timpahute ranges and may possibly include areas further north and east. Eakin ( 1966) included Pahranagat, Coal, and Garden Valleys within another vast interbasin ground-water system. He considered the western and the southwestern topographic divides of these valleys as equivalent to a ground-water divide that separates them from Desert Valley. However, he presented no specific hydrologic or geologic evidence in support of the coincidence of the ground water and topographic divides. Precipitation on the Pahranagat Range, which separates Pahranagat Valley from northeastern Desert Valley, ranges from 8 to 12 inches but is mostly less than 10 inches (fig. 3). In ad- dition, a 10-mile-long area between the north end of the Sheep Range and the south end of the Pahranagat Range receives only 7 to 8 inches precipitation (fig. 3). The Timpahute Range, which separates northern Desert Valley from Garden and Coal Valleys, receives 8 to 14 in- ches precipitation. Because of the low precipitation on these ranges, there need not be a ground-water divide beneath these ranges significant enough to affect regional flow patterns in the lower carbonate aquifer. In summary, hydraulic connection between the Ash Meadows ground-water basin as delineated on plate 1 and the areas northeast of the basin is probable because (1) the occurrence of interbasin movement in the lower carbonate aquifer in both regions, coupled with the absence of the lower clastic aquitard with the upper part of the zone of saturation between the regions; (2) signifi- cant head differences; and (3) the low precipitation (7 to 14 in.) on the carbonate-rock ridges separating the regions. An estimate of the magnitude of underflow into the Ash Meadows basin from the northeast is given in the section “Underflow from the Northeast.” Underflow southward from the Penoyer Valley, north of Emigrant Valley, may also occur. Such underflow, through either the Cenozoic or the Paleozoic aquifer, would have to pass through or over the lower clastic aquitard, which surrounds western Emigrant Valley (pl. 1), before entering the central part of the Ash Meadows basin. C89 RELATION TO LAS VEGAS GROUND-WATER BASIN The approximate boundary of the Ash Meadows ground-water basin (pl. 1) suggests the need for revision of the boundaries of the Las Vegas ground-water basin. Recharge, regional movement, and discharge of ground water from and the boundaries of the Las Vegas Valley were discussed by George Hardman (written com- mun., 1931), Maxey and Jameson (1948), and Malmberg (1961 and 1965). All four hydrologists considered the Spring Mountains to be the principal area contributing recharge to the valley-fill reservoir of the Las Vegas ground-water basin, although some recharge from the Sheep Range was inferred. The regional direction of ground-water movement in the valley-fill aquifer was shown to be eastward and northeastward from the Spring Mountains toward the valley and then southeastward toward discharge areas in and east of the city of Las Vegas. Southeastward movement in the northwestern part of the Las Vegas Valley is shown by the potentiometric contours of plate 1. The contours are modified from Malmberg (1965, pl. 3). These four hydrologists considered the ridges and mountains flanking the basins to be relatively im- _ permeable, and, in effect, they used topographic divides in outlining the boundaries of the Las Vegas ground- water basin. The boundaries assigned by each to the ground-water basin differed somewhat, however. George Hardman (written commun., 1931) concluded that ground water in Indian Springs Valley, Three Lakes Valley (which he called Owens Dry Lake and Mormon Gulch), and Ivanpah Valley (about 40 miles south- southwest of Las Vegas) is tributary to the Las Vegas basin. Maxey and Jameson ( 1948) did not mention Ivan- pah Valley and excluded Indian Springs Valley. Their plate 2 shows a south-west trending topographic divide that crosses U.S. Highway 95 about 4 miles east of In- dian Springs. North of the highway, the divide follows the crest of the Pintwater Range; south of the highway, it follows Indian Ridge (fig. 1) and continues south to the Spring Mountains. Maxey and J ameson (1948) included at least the southern part of Three Lakes Valley within the Las Vegas basin. Malmberg ( 1961, pl. 1), like Hard- man, considered Ivanpah Valley and Three Lakes Valley to be tributary to the Las Vegas ground-water basin, but he included only the southern part of Indian Springs Valley. Until recently, the hydrology of the Indian Springs—Three Lakes Valley area was virtually un- known. Few wells had been drilled within the region, and they penetrated only the valley-fill aquifer to shallow depths. Sparsity of data probably accounts for the differ- ing ground-water boundaries that George Hardman (written commun., 1931), Maxey and Jameson (1948), and Malmberg (1961 and 1965) suggested for the Las Vegas ground-water basin. Hydrologic and geologic data 090 available since the completion of their work indicates that Indian Springs Valley and most, if not all, of Three Lakes Valley are within the Ash Meadows ground-water basin (pl. 1). However, the exact position of the divide between the Ash Meadows and Las Vegas Valley ground- water basins is still open to question. Corn Creek Springs and the northwest—trending phreatophyte lineament at the south end of the Sheep Range (pl. 1) indicate that some of the recharge to the Sheep Range probably flows into Las Vegas Valley, rather than toward Three Lakes Valley. The phreatophyte lineament is controlled by faults (Haynes, 1965); the lineament also nearly coincides with the posi- tion of the Las Vegas Valley shear zone (structural feature 13, pl. 1). That this recharge to Las Vegas Valley probably does not exceed a few thousand acre-feet an- nually is suggested by the following:(1) Geologic struc- ture suggests that the lower elastic aquitard may be pre- sent along the Las Vegas Valley shear zone; and (2) the discharge of Corn Creek Springs is only 125 gpm (Malmberg, 1965, table 8), and that of other springs in the area is but a fraction of this amount. RELATION TO PAHRUMP VALLEY GROUND-WATER BASIN Plate 1 indicates that ground water in Pahrump Valley and in Stewart Valley is not tributary to the Ash Meadows ground-water basin. However, a wide divergence of opinion exists on the degree of hydraulic connection of these areas. Loeltz (written commun., 1960) noted that recharge to the Pahrump Valley ground-water basin, estimated by Maxey and Jameson (1948), greatly exceeded estimates of natural discharge from Pahrump Valley. He therefore concluded that some of the discharge at Ash Meadows probably came from the southwest slope of the Spring Mountains via Pahrump Valley. Also favoring such flow, according to Loeltz, are the following factors: (1) The westward slope of the potentiometric surface in Pahrump Valley; (2) the altitude of the water surface in Pahrump Valley, which is several hundred feet higher than that in Devils Hole; (3) possible hydraulic conduits through the highly deformed carbonate rocks that form part of the topographic boundary between the two areas; and (4) the possibility that the chemical quality of the ground water in Pahrump Valley might change between the two areas and attain the same chemical quality as that discharged by the springs. Loeltz also recognized, however, that some of the spring discharge might have come from areas to the north and northeast of Ash ~Meadows. On the basis of Maxey and J ameson’s work (1948) and estimates of natural discharge in Pahrump Valley, Walker and Eakin (1963, p. 21) suggested that possibly 13,000 of the 17,000 acre-feet of annual spring discharge HYDROLOGY OF NUCLEAR TEST SITES at Ash Meadows is derived from Pahrump Valley and that the remaining 4,000 acre-feet of discharge is derived annually from carbonate rocks northeast of Ash Meadows. Winograd (1963), on the basis of an oral com- munication from G. T. Malmberg (1962), stated “that about 11,000 acre-feet of the spring discharge at Ash Meadows represents underflow from Pahrump Valley via carbonate aquifers.” Hunt, Robinson, Bowles, and Washburn (1966) concluded that “Pahrump Valley seems to be the source of the ground-water discharge at the large springs at Ash Meadows and possibly at some of the other large warm ones farther south in the Amargosa River drainage.” Their conclusion was pri- marily based on the arguments stated by Loeltz (1960) and supplementary geomorphic evidence. They placed greater emphasis on the similarity of the chemistry of water in the Pahrump V alley—Ash Meadows area than did Loeltz, but, like Loeltz, they did not assign a value for the percentage of the total Ash Meadows discharge presumably derived from Pahrump Valley. Nor did they discuss possible underflow toward Ash Meadows from the north or northeast. Since the completion of the fieldwork associated with the preceding studies, Malmberg (1967, p. 32—33) com- pleted a study of the ground-water hydrology of Pahrump Valley, and R. H. Moench (written commun., Mar. 1965) mapped the geology of the hills between Pahrump Valley and Ash Meadows. On the basis of his own work and the geologic mapping of Moench, Malmberg concluded that 12,000 acre-feet per year of ground-water underflow leaves Pahrump Valley through both the Paleozoic carbonate rocks (10,000 acre-ft per yr) and the valley-fill aquifer (2,000 acre-ft per yr). He believed that most of this underflow is toward the Nopah and Resting Spring Ranges, which border Pahrump Valley on the southwest (pl. 1), but said “part may move northwestward through a thin section of carbonate rocks or along major fault zones to the springs at Ash Meadows.” The evidence Malmberg (1967, p. 24) cited in favor of a southwestward movement of the bulk of the underflow is the following: (1) “Except for a small wedge of carbonate rocks north of Stewart Valley, the reservoir is terminated by structural deformation along the northwest side of Pahrump Valley, where quartzite and other poorly permeable rocks crop out”; (2) the poten- tiometric contours for the valley fill indicate southwestward movement of water across the valley (Malmberg presumed that the direction of flow in the carbonate rock reservoir also is southwestward); (3) the configuration of the potentiometric contours and the absence of a shallow water table beneath southwestern Pahrump Valley suggests that ground water is moving into rather than being dammed by the Nopah and Resting Spring Ranges (the Nopah Range is composed predominantly of carbonate rocks, which favor such movement; the Resting Spring Range, on the other SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE hand, has a Clastic-rock core); and (4) water quality does not support movement of water from Stewart Valley toward Ash Meadows. Malmberg (1967, p. 25—26) did not discount the possibility of some movement to the northwest because ( 1) “The small wedge of carbonate rocks north of Stewart Valley provides a potential avenue of flow northwestward to the springs in Ash Meadows —— Last Chance, Bole, Big, and Jack Rabbit Springs, which issue at depth from carbonate rocks” and (2) the head differential between northern Stewart Valley and Ash Meadows (about 80 ft) favors northwestward movement. He concluded, “until the head distribution in the car- bonate rocks in Pahrump and Stewart Valleys and in the intervening area to the northwest is known, the possibility of northwestward flow to the springs must re- main unresolved.” Hughes (1966) inferred in his study of springs in the Spring Mountains that as much as 3,000 acre-feet of water may move annually toward Ash Meadows from the Spring Mountains via Pahrump Valley and that the remainder of the Ash Meadows discharge comes from the north and northeast. In summary, these cited studies suggest that as much as 13,000 or as little as 3,000 acre-feet of the yearly dis- charge at Ash Meadows (roughly 75 and 20 percent of the spring flow) is derived from Pahrump Valley via un- derflow through Paleozoic carbonate rocks. The authors of this report conclude that at most only a small percentage of the Ash Meadows discharge originates in Pahrump Valley. Evidence for this follows: 1. The lower Clastic aquitard crops out in a nearly con- tinuous band between Pahrump and Stewart Valleys and the east-central Amargosa Desert (pl. 1). Geologic sections by R. H. Moench (written commun., Mar. 1965) suggest further that the lower Clastic aquitard is probably also the principal pre-Tertiary rock within the zone of saturation beneath the small valleys and arroyos separating the ridges of pre-Tertiary rocks (pl. 1). Minor car- bonate strata also occur locally within the zone of saturation, as suggested by Malmberg (1967), but the cross sections indicate that they are not con- tinuous between Pahrump and Stewart Valleys and the Amargosa Desert; if the cross sections are in error, the carbonate rocks still do not constitute an important fraction of the pre-Tertiary rocks within the zone of saturation. The distribution of the elastic rocks between the north end of Stewart Valley and the Johnnie mining district is controlled by the Montgomery thrust fault (structural feature 16 of pl. 1), which resembles the Gass Peak and the Wheeler Pass thrust faults. The fault plane dips westward at an angle in excess of 25°. The base of the upper plate consists of late Precambrian clastic C91 rocks that dip westward and northwestward but generally at a gentler angle than the fault plane. The overridden rocks consist of Devonian to Mississippian carbonate rocks. Thus, this thrust probably isolates ground water in the lower car- bonate aquifer and in the Cenozoic aquifers in northwestern Pahrump Valley from the same aquifers in the east-central Amargosa Desert. Data on interstitial permeability and fracture transmissibility in similar rocks in Yucca Flat were used to estimate the potential maximum underflow through a 10-mile-long strip of the lower elastic aquitard (between the northern Stewart Valley and a point near Johnnie). The lower Clastic aquitard was assumed to be 10,000 feet thick (full thickness); the hydraulic gradient was assumed to be 200 feet per mile. (This represents a probable maximum gradient, because the difference in water level between northwestern Pahrump Valley and Devils Hole is about 200 ft.) A permeability value of 0.0001 gpd per sq ft was taken from table 4; this value is the maximum listed for interstitial permeability. The calculated underflow amounts to 2,000 gpd, or about 1 gpm. Evidence that regional flow through the clastic aquitard is probably controlled by flow through interstices rather than through fractures was presented in the section “Lower Clastic Aquitard.” Nevertheless, assuming an improbable average coefficient of transmissibility of 1,000 gpd per ft between the two areas and a hydraulic gradient of 200 feet per mile, an underflow rate of about 1,400 gpm, or about 13 percent of the Ash Meadows spring discharge, is computed. 2. Comparison of the chemical quality of the water in Pahrump and Stewart Valleys with that at Ash Meadows precludes movement of significant quan- tities of water from these valleys toward Ash Meadows. Such a comparison is discussed under “Ground-Water Chemistry, Hydrochemical Facies, and Regional Movement of Ground Water.” 3. Depth to and altitude of the water table in Stewart Valley and in northwestern Pahrump Valley also suggest that these valleys are separated from Ash Meadows by a relatively impermeable barrier. The water table beneath the playas in northern Stewart and northwestern Pahrump Valleys is only a few feet below the surface, or at an altitude of about 2,440 and 2,550 feet, respectively (Malmberg, 1967; pl. 1). These water-level altitudes are about 80 and 190 feet higher than the water-level altitude (2,359 ft above mean sea level) in the lower carbonate aquifer at Devils Hole. In contrast, the water table beneath the playa in southwestern Pahrump Valley is at an altitude of about 2,420 feet (Malmberg, 1967; pl. 1), but it is fully 100 feet below the playa C92 level. Thus, the water table beneath this playa is 20 feet lower than the water table beneath the playa in northern Stewart Valley and is 130 feet lower than that beneath the playa in northwestern Pahrump Valley. The water table in the valley-fill aquifer in northern Stewart Valley and in northwestern Pahrump Valley thus stands well above levels in the same aquifer to the south-southeast and to the northwest. This difference in altitude and satura- tion of the valley fill nearly to the surface beneath the playas in Stewart and western Pahrump Valleys suggest that the ground water in these areas is ponded by some impermeable boundary, namely, the lower clastic aquitard. Such ponding does not preclude underflow of small magnitude. In contrast, the playa in southwestern Pahrump Valley is bordered on the southwest by the Nopah Range, which is composed predominantly of the lower carbonate aquifer. Malmberg’s (1967) poten- tiometric contours for the valley-fill aquifer reproduced on plate 1 of this report indicate that ground water in this aquifer is moving toward (and into) the Nopah Range. In summary, the preceding evidence indicates that at most only a few percent of the Ash Meadows discharge can be derived from either Stewart Valley or western and northwestern Pahrump Valley. SOURCES OF RECHARGE TO THE LOWER CARBONATE AQUIFER Within the basin boundary delineated on plate 1, the lower carbonate aquifer is recharged principally by precipitation in areas of high rainfall and favorable rock type and secondarily by downward leakage of water from the Cenozoic hydrogeologic units. Underflow into the basin from the northeast may also constitute a major source of recharge. PRECIPITATION Recharge from precipitation is probable beneath and immediately adjacent to the highly fractured Paleozoic carbonate rocks of the Sheep Range, northwestern Spring Mountains, southern Pahranagat Range (south of State Highway 25; fig. 1), and, to a lesser extent, beneath the Pintwater, Desert, and Spotted Ranges. The approximate average annual precipitation within the Ash Meadows basin is about 320,000 acre-feet on the Sheep. Range, about 100,000 acre-feet on the northwestern Spring Mountains, and about 90,000 acre- feet on the southern Pahranagat Range (fig. 3). For these mountains, the 8-inch isohyetal contour roughly cor- responds with the lowest outcrop of Paleozoic bedrock, Precipitation on the lower Desert, Pintwater, and Spotted Ranges was estimated only for those parts of the ranges receiving 8 inches or more rainfall. This amounted to about 60,000 acre-feet. HYDROLOGY OF NUCLEAR TEST SITES Thus, a total of about 570,000 acre-feet of precipita-V tion falls annually within the basin on prominent ridges and mountains that are composed principally of the lower carbonate aquifer. This quantity is an approxima- tion at best: precipitation that falls on carbonate-rock outcrops at low altitudes in the Spotted, Pintwater, or Desert Ranges or on the other minor hills and ridges in the region was not included; conversely, some of the precipitation included in the tabulation falls on the valley fill bordering the mountains, or on clastic rock, and not on the lower carbonate aquifer; it should be sub- tracted from the total. The preceding estimate could have been refined by planimetering the area of carbonate-rock outcrop for select altitude zones and by applying Quiring’s (1965) altitude-precipitation curves of the region; however, such precision is unwarranted because of the approximate nature of the basin boun- dary. Precipitation falling on the valley floors underlain by carbonate rocks was not estimated because recharge to either the lower carbonate aquifer or the younger aquifers beneath such areas seems improbable under present climatic conditions. Moreover, recharge to car- bonate rocks beneath the valleys is controlled by the tuff aquitard. Assuming that the spring discharge at Ash Meadows is derived principally from precipitation falling on carbonate-rock uplands within the boundaries of the Ash Meadows basin (pl. 1) and that steady-state conditions exist in the ground-water basin, the percentage of rain- fall that infilitrates to the carbonate aquifer beneath the ranges can be estimated. Using the 17,100 acre-feet of measured spring discharge (average of two values given in table 7) and the precipitation estimate of roughly 570,000 acre-feet, about 3 percent of the rainfall falling on areas of carbonate-rock outcrop may infiltrate to the zone of saturation. The cited percentage of infiltration is in error in proportion to (1) the magnitude of underflow into the basin from the northeast, (2) underflow out of the basin at Ash Meadows, and (3) evapotranspiration in Ash Meadows discharge area in excess of that supported ‘by recycled spring discharge. UNDERFLOW FROM THE NORTHEAST Geologic and hydrologic evidence presented in the sec- tion “Areal Extent of the Ground-Water Basin” in- dicates that the Ash Meadows ground-water basin may receive underflow from the northeast, but this evidence does not permit estimation of the quantity of underflow. A comparison of the deuterium content of ground water in Pahranagat Valley, along the flanks of the Spring Mountains and Sheep Range, and at Ash Meadows in- dicates that possibly as much as 35 percent (about 6,000 acre-ft annually) of the Ash Meadows discharge may enter the basin from the northeast. The deuterium data SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE are discussed in the section “Underflow from Pahranagat Valley.” DOWNWARD LEAKAGE FROM CENOZOIC ROCKS A minor source of recharge to the lower carbonate aquifer is downward leakage of ground water semiperched in the Cenozoic rocks. In Yucca Flat, the magnitude of downward leakage was estimated, on the basis of hydraulic data, to be in the range of 25—65 acre- feet per year. Downward leakage of similar magnitude is probable also in Frenchman Flat. By analogy with Yucca and Frenchman Flats, downward leakage of water probably also occurs beneath Desert Valley, eastern Emigrant Valley, the northern two-thirds of Three Lakes Valley, and the northern two- thirds cf Indian Springs Valley. These four valleys have hydrogeologic settings similar to those of Yucca and Frenchman Flats. (See section “Intrabasin Movement”) The vertical leakage of the semiperched water in these valleys may also be on the same order. The aggregate leakage beneath the six valleys is es- timated, then, to be between 150 and 400 acre-feet per year, or less than 3 percent of the discharge at Ash Meadows. Calculations of leakage based on hydrochemical evidence are given below in the section “Estimates of Downward Crossflow from the Tuff Aquitard into the Lower Carbonate Aquifer.” QUANTITY DERIVED FROM NORTHWEST SIDE OF BASIN The quantity of recharge entering the lower carbonate aquifer from the northwest side of the basin is probably only a few percent of the measured discharge at Ash Meadows. The northwestern border of the Ash Meadows basin is defined approximately by the crest of Belted Range, Rainier Mesa, and Eleana Range uplands, which receive 8 to 14 inches of annual rainfall. An estimate of the quantity of this precipitation that eventually reaches the lower carbonate aquifer within the central and southwestern parts of the basin may be obtained by calculating the eastward and southward underflow across the nearly continuous trend of elastic rocks ex- tending from the north end of the Groom Range to western Jackass Flats (pl. 1); this trend is as much as 15 miles east of the basin boundary (pl. 1). The annual quantity of basinward underflow through the lower clastic aquitard and into the lower carbonate aquifer between the northeast end of Groom Lake playa and northern Yucca Flat is probably less than 40 acre- feet. This value was determined by applying the follow- ing values of T, I, and L to the underflow equation (Q=TIL). The coefficient of transmissibility, T, assumed to be about 1 gpd per ft, was determined by multiplying the highest value of coefficient of interstitial permeability measured in the laboratory (0.0001 gpd per sq ft; table 4) by 10,000 feet, a maximum probable C93 thickness of aquitard. The apparent hydraulic gradient, I, within the lower clastic aquitard ranges from 250 to 1,300 feet per mile (fig. 32); a gradient of 1,000 feet per mile was used to maximize the underflow. The length of underflow strip L, is about 35 miles; it is the distance measured along the inferred contact between carbonate and clastic rock between the northeast end of the playa and the northern tip of Yucca Flat (pl. 1). Eastward underflow into the lower carbonate aquifer (east of the Groom Range) between northeastern Groom Lake playa and the north end of Groom Range may amount to about 20 acre-feet annually if hydraulic gradients as steep as those shown in figure 32 occur beneath clastic rocks of the Groom Range. Eastward underflow through the valley-fill aquifer between the Groom and the Papoose Ranges (fig. 32) also probably reaches the lower carbonate aquifer beneath eastern Emigrant Valley after passage through the tuff aquifers and aquitards. The coefficient of transmissibility of the valley-fill aquifer is not less than 30,000 gpd per ft in two wells (91—74 and 91—74a) in western Emigrant Valley (table 3), and the hydraulic gradient may be as much as 4 feet per mile toward the playa (fig. 32). The playa is about 3 miles wide, but the clastic aquitard is above the zone of saturation locally (fig. 32). Assuming a coefficient of transmissibility of 30,000 gpd per ft, a hydraulic gradient of 4 feet per mile, and an underflow strip 1.5 mile wide, the flow across the barrier via the valley-fill aquifer is about 200 acre-feet annually. This figure is probably a maximum value because thinning of the valley-fill aquifer near the buried aquitard would probably result in a reduction in transmissibility. Underflow through the Cenozoic aquifers elsewhere along the boundary between carbonate and clastic rock between northern Groom Range and northern Yucca Flat appears negligible. The valley-fill aquifer west of Groom Lake is nearly fully saturated only because of the damming effect of the elastic aquitard, which nearly sur- rounds western Emigrant Valley. However, in general, the Cenozoic aquifers are probably unsaturated because of their structurally high position; in Yucca Flat, for example, the Cenozoic aquifers are unsaturated along the borders of that valley. The tuff aquitard, however, may be saturated in a structurally high posi- tion because of its very low transmissibility; but owing to its thinness, underflow through the tuff aquitard is probably a fraction of that through the thick lower clastic aquitard. Eastward underflow into the lower carbonate aquifer beneath central Yucca Flat can follow two routes: (1) flow through the upper clastic aquitard (pl. 2),,and (2) flow through the lower carbonate aquifer underlying the upper clastic aquitard. Underflow through the Cenozoic aquifers overlying the upper clastic aquitard is unlikely C94 for reasons outlined in the preceding paragraph; minor underflow through the tuff aquitard is possible. Maximum flow through the upper clastic aquitard along the west side of Yucca Flat is about 30 acre-feet annually when computed with the same values for transmissibility and hydraulic gradient used’to calculate flow through the lower clastic aquitard. The quantity of eastward underflow into Yucca Flat through the lower carbonate aquifer underlying the up— per clastic aquitard is difficult to estimate and probably depends in part on the subsurface nature of the Tippinip thrust fault. (See discussion in section “Northwestern and West-Central Yucca Flat.”) Available data suggest such underflow is probably minor. First, the high water- level altitudes (3,800 ft) in the upper clastic aquitard“ beneath the western half of Yucca Flat (pl. 2, C) suggest that the head within the underlying lower carbonate aquifer may be considerably higher than that in the same aquifer east of the Tippinip thrust fault (pl. 2C); this in turn suggests a hydraulic barrier between the car- bonate rocks east and west of the fault. However, the water in the aquitard may be semiperched above the lower carbonate aquifer, whose head is no higher than beneath central Yucca Flat (alt about 2,400 ft). Second, the CP thrust fault (structural feature 2 on pl. 1), believed to extend as far southwest as northwestern Jackass Flats, may be rooted in Precambrian clastic rocks (Barnes and Poole, 1968); this fault may thus structurally and hydraulically isolate carbonate rocks of‘ western Yucca Flat from similar rocks west of the fault. Third, the Eleana Range and Shoshone Mountain are bordered on the west by the eastern rim of two calderas, the Timber Mountain caldera (Carr, 1964; Christiansen and others, 1965) and the Silent Canyon caldera (Noble and others, 1968; Orkild and others, 1968). The ap- proximate position of the rim of the Timber Mountain caldera is shown on plate 1; this caldera generally ranges from 15 to 20 miles in diameter. The Silent Canyon caldera is centered beneath Pahute Mesa immediately north of the Timber Mountain caldera; this caldera is roughly elliptical in plan and measures 10 by 14 miles (Orkild and others, 1968). Test drilling has shown that at least 13,500 feet of volcanic rocks underlie the Silent Canyon caldera. Gravity surveys suggest that the Silent Canyon caldera may be underlain by as much as 15,000 to 16,000 feet of Tertiary volcanic rocks and the Timber Mountain caldera by as much as 12,000 feet of volcanic rocks (D. L. Healey and C. H. Miller, written commun., 1966). Whether these volcanic rocks are underlain directly by pre-Tertiary miogeosynclinal rocks or are un- derlain directly by a magma chamber or whether the miogeosynclinal rocks are present, but largely dis- membered by stoping, will probably never be known. However, limited outcrops of silicic intrusive rocks within the Timber Mountain caldera suggest that a HYDROLOGY OF NUCLEAR TEST SITES magma chamber may underlie the volcanic rocks filling the caldera. In summary, because of the two major volcano-tectonic features briefly described and possibly the CP thrust fault, the lower carbonate aquifer un- derlying the upper clastic aquitard in western Yucca Flat and northern Jackass Flats is probably not struc- turally or hydraulically continuous with the carbonate aquifer (if any) beneath the two calderas or beneath areas west and north of the calderas. Therefore, the quantity of eastward underflow through the lower car- bonate aquifer into Yucca Flat may be restricted to downward leakage from the overlying upper clastic aquitard and possible lateral movement eastward from the tuff and lava-flow aquitards beneath the calderas. Such underflow is unlikely to exceed 200 acre-feet an- nually. Similarly, the flow into the basin across the east- west—trending contact of the upper clastic aquitard and the lower carbonate aquifer, between northwestern Frenchman Flat and northeastern Jackass Flats (pl. 1), is probably less than 100 acre-feet per year. The total underflow of water into the lower carbonate aquifer from the northwest side of the Ash Meadows ground-water basin probably amounts to less than 600 acre-feet per year, or about 4 percent of the measured discharge at Ash Meadows. The estimated quantity flowing into the lower car- bonate aquifer beneath Yucca Flat, from both northeast and west, is about 250 acre-feet per year, or about 1 per- cent of the discharge at Ash Meadows. The lower car— bonate aquifer beneath Yucca Flat is also recharged by downward leakage of water semiperched in the Cenozoic hydrogeologic units; this leakage probably ranges from 25 to 65 acre-feet per year. (See section “Intrabasin Movement.”) Precipitation falling directly on the low carbonate-rock ridges bordering Yucca Flat on the east probably contributes minor recharge. The total flow within the lower carbonate aquifer beneath Yucca Flat probably is less than 350 acre—feet per year. OASIS VALLEY—FORTYMILE CANYON GROUND-WATER BASIN Ground water beneath the western half of Nevada Test Site —— the Pahute Mesa—Timber Mountain area —— is not tributary to the Ash Meadows ground-water basin. This ground water, predominantly in Tertiary tuff and rhyolite, moves southwestward toward discharge areas in Oasis Valley (pl. 1) and probably also moves southward toward the Amargosa Desert through western Jackass Flats. This area is tentatively considered part of a single ground-water basin, informally designated the Oasis Valley—Fortymile Canyon basin, which is tributary to the central and northwestern Amargosa Desert. The hydrologic boundaries of the Oasis Valley—For- tymile Canyon ground-water basin have not yet been fully defined. Malmberg and Eakin (1962) recognized SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE that the boundaries of the area contributing ground water to this basin need not correspond with the surface drainage basin. Test drilling on Pahute Mesa in 1963—65 substantiated their prediction (R. K. Blankennagel and J. E. Weir, written c0mmun., 1966). The generalized potentiometric contours for the Pahute Mesa area (pl. 1) suggest that some ground water flows from Pahute Mesa toward Oasis Valley and that the water beneath Pahute Mesa may, in turn, be derived from Kawich Valley to the northeast. Thus, interbasin movement of water also is through the volcanic rocks (at least the younger volcanic rocks) within the Oasis Valley—Fortymile Canyon basin. A tentative position of the ground-water divide between the Ash Meadows ground-water basin and the Oasis Valley—Fortymile Canyon ground-water basin is shown on plate 1. The ground-water hydrology of the Oasis Valley—For- tymile Canyon ground-water basin was described by Blankennagel and Weir (1973). PROBABLE HYDRAULIC CONNECTION BETWEEN CENTRAL AMARGOSA DESERT AND FURNACE CREEK WASH—NEVARES SPRINGS AREA IN DEATH VALLEY The Furnace Creek Wash—Nevares Springs discharge area lies in east-central Death Valley (fig. 1 and pl. 1). Altitudes in the area range from 200 feet below to about 1,000 feet above sea level. The area is the center of tourism in the Death Valley National Monument and in- cludes the following wellknown attractions: Furnace Creek Inn, Furnace Creek Ranch, Texas Spring campground, and Zabriskie Point. Furnace Creek Wash (pl. 1), a major northwestward draining arroyo, marks the southwestern border of the area, and the Funeral Mountains mark the northeastern border; the Death Valley salt pan borders the area on the west. Annual precipitation at Furnace Creek Ranch (alt 168 ft below sea level) is 1.66 inches (47-yr record), and pan evaporation at Cow Creek (alt about 160 ft below sea level) is 149 inches (3-yr record). The average annual temperature at Furnace Creek Ranch was 240°C for the period 1911—52; for the same period the average July temperature was 390°C, and the average January temperature was 10.5°C. Temperatures of 490°C or higher are not uncommon during May through September; a high of 56.5°C has been recorded in July (Hunt and others, 1966, p. B7—B9). Water for the area is derived principally from three groups of springs — Travertine Springs, Texas Spring, and Nevares Springs (pl. 1). The hydrogeologic setting at these springs was described briefly by Pistrang and Kunkel (1964) and by Hunt, Robinson, Bowles, and Washburn (1966). Travertine and Texas Springs issue from Quaternary gravels underlain at shallow depth and C95 partly surrounded by Tertiary lacustrine deposits. These springs discharge abOut 850 and 225 gpm, respectively; the water temperature is about 33.5°C. Travertine Springs are about 400 feet and Texas Spring 380 feet above sea level. The main spring at Nevares Springs emerges from a travertine mound about 100 feet from an outcrop of the lower carbonate aquifer (Bonanza King Formation). Discharge of this spring is 270 gpm; the temperature of the water is 400°C, and the altitude of the spring is 937 feet above sea level. Ground water is also discharged by small springs, evapotranspiration, and seepage into tunnels and tile fields. The estimated total discharge in the area is about 2,500 gpm, or about 4,100 acre-feet per year (Pistrang and Kunkel, 1964, table 4). Hunt, Robinson, Bowles, and Washburn (1966) estimated a total discharge of about 3,200 gpm (about 5,100 acre-ft) for a larger dis- charge area than that considered by Pistrang and Kunkel. Pistrang and Kunkel (1964) considered the discharge to be derived from precipitation on the highlands border- ing Death Valley on the east. The estimated size of the catchment area was between 30 and 150 square miles. They suggested that the spring discharge is fed by a com- bination of ground water moving along faults in pre- Tertiary and Tertiary rocks, through travertine con- duits, and through permeable gravel lenses. In contrast, Hunt, Robinson, Bowles, and Washburn (1966, p. B40) suggested that the spring discharge “is derived directly from Pahrump Valley, by movement along faults in the bedrock under the valley fill.” Elsewhere in the same report (p. B1) they suggested that the discharge is derived from Pahrump Valley “by way of Ash Meadows.” The major springs in the Furnace Creek Wash—Nevares Springs area — Travertine Springs, Tex- as Spring, and Nevares Springs — are probably fed by upward leakage from the lower carbonate aquifer, as are major springs at Ash Meadows, Pahranagat Valley, and many other places in eastern Nevada. Hunt and Robin- son (1960) suggested that the major springs in the Fur- nace Creek Wash—Nevares Springs area, though they emerge from Quaternary deposits, represent discharge from Paleozoic carbonate rocks, but they offered no sup- porting evidence. Evidence in support of this belief follows: (1). Nevares Springs, at the foot of the Funeral Moun- tains (pl. 1), are about 100 feet from the topographically lowest outcrop of the lower car- bonate aquifer (Bonanza King Formation) in the area (Hunt and Mabey, 1966, pl. 1). Minor seeps emerge directly from the carbonate aquifer a few hundred feet south of the main spring. The setting of these springs is generally similar to that of major springs at Ash Meadows, in Indian Springs Valley, C96 and in Pahranagat Valley; in those valleys (and elsewhere in eastern Nevada) springs also emerge from Paleozoic carbonate rocks, or from valley fill adjacent to carbonate rocks, at topographically low outcrops in, or along the margins of, the intermon- tane valleys. (2). At Ash Meadows the springs closest to the outcrop of the lower carbonate aquifer generally had the highest temperature. (See section “Character and Geologic Control of Spring Discharge”) This same relation exists among the major springs in Death Valley. The temperature of water from Nevares Springs is about 6.5°C higher than that of water from Texas or Travertine Springs, even though Nevares Springs are about 540 feet higher in altitude. The higher temperature of Nevares Springs is explained by the direct hydraulic con- nection between the lower carbonate aquifer and the spring orifices. In contrast, pre-Tertiary car- bonate rocks are at least hundreds and possibly more than 2,000 feet below the surface at Texas and Travertine Springs (Mabey, 1963); accordingly, lower temperature would be expected at these springs due to loss of heat to the Tertiary rocks dur- ing movement of water from the carbonate aquifer to the surface. (3). As noted by Pistrang and Kunkel (1964, p. Y32) and by Hunt, Robinson, Bowles, and Washbum (1966, p. B38), the chemical quality of the water from the three major springs is nearly identical. Water chemistry thus provides a further clue that water from Travertine and Texas Springs, like that at Nevares Springs, is derived from the lower car- bonate aquifer. Notions regarding the nature of the hydraulic barrier(s) forcing water out of the carbonate aquifer at Nevares Springs and the mode of hydraulic connection ‘ between the spring orifices and the deeply buried car- bonate aquifer at Texas and Travertine Springs are not discussed here. Several models can be visualized through the geologic maps, cross sections, and block diagrams presented by Hunt and Mabey (1966) and by Pistrang and Kunkel (1964). Underflow toward Death Valley through the lower car- bonate aquifer requires that three conditions be met. First, the lower carbonate aquifer must extend from the Furnace Creek Wash—Nevares Springs area to the cen- tral or south-central Amargosa Desert; second, a favorable hydraulic potential must exist for movement westward through this aquifer; and third, a source(s) of recharge to the lower carbonate aquifer must be available. The lower carbonate aquifer crops out in a nearly con- tinuous band between the south-central Amargosa Desert and the Furnace Creek Wash—Nevares Springs HYDROLOGY OF NUCLEAR TEST SITES area (Jennings, 1958). The Death Valley sheet of the geologic map of California (Jennings, 1958) and the geologic map of Death Valley by Hunt and Mabey (1966) show that carbonate rocks, Cambrian through Devonian in age, crop out across the south end of the Funeral Mountains; the strata extend from T. 26 N., R. 5 E., and T. 27 N., R. 4 E., on the east (where they border south- central Amargosa Desert) to Nevares Springs on the west, a distance of about 20 miles. The approximately wedgelike outcrop pattern tapers from about 12 miles wide on the northeast and east to less than 1 mile wide at Nevares Springs. Paleozoic carbonate rocks are absent in the parts of the Funeral Mountains and the Black Moun- tains that border this carbonate-rock wedge on the north and south (pl. 1). Thus, assuming that the carbonate rocks also are within the zone of saturation, a possible route exists for interbasin movement of ground water from the south-central Amargosa Desert to the area of major spring discharge in Death Valley. The assumption that the carbonate rocks are thick enough to lie within the zone of saturation appears safe. The geologic map of the area by Jennings (1958) shows that the bulk of the carbonate rocks cropping out between south-central Amargosa Desert and Furnace Creek are of Ordovician age or younger; hence, thousands of feet of carbonate rocks probably lie within the zone of saturation, the top of which ranges from 900 to 2,200 feet in altitude. Therefore, the lower clastic aquitard is unlikely to lie above the zone of saturation in a continuous band between the two valleys. The lower carbonate aquifer between the south-central Amargosa Desert and the Furnace Creek Wash—Nevares Springs area thus afford an avenue for ground water to move from the Amargosa Desert into Death Valley. A hydraulic gradient with a westward component should exist within the lower carbonate aquifer because the difference in the land-surface altitude between the south-central Amargosa Desert (2,400 ft) and the Nevares Springs area (937 ft) is nearly 1,500 feet. The water level in the valley-fill aquifer in south-central Amargosa Desert ranges from about 2,100 to 2,200 feet above mean sea level, or about 1,200 feet above the orifices of Nevares Springs. The distance between south-central Amargosa Desert and Nevares Springs ranges from 10 to 20 miles and suggests an average hydraulic gradient of 60 to 120 feet per mile between the two areas. This gradient, which is anomalously high for the lower carbonate aquifer, suggests that one or more hydraulic barriers probably exist within the lower car- bonate aquifer in the area. Ground water in the lower carbonate aquifer beneath the sOuth-central Amargosa Desert may be derived from two sources: direct southwestward underflow from the Ash Meadows ground-water basin through the lower car- bonate aquifer (assuming the aquifer is extensive SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE beneath the central Amargosa Desert), and downward leakage from the valley-fill aquifer beneath the central and south-central Amargosa Desert. Water in the valley- fill aquifer may have been derived from spring runoff at Ash Meadows, from Jackass Flats, or from northwestern Amargosa Desert. Important quantities of water from the valley-fill aquifer could leak downward only if the head in the valley fill beneath the central and south- central Amargosa Desert were higher than that in the underlying carbonate aquifer and if the lower carbonate aquifer and valley fill were in direct hydraulic continuity near buried structural highs where the Tertiary aquitard may not have been deposited or may have been removed by erosion prior to deposition of the valley fill. Data on the head relation between the two aquifers beneath the central and south-central Amargosa Desert are not available. The minimum area of the Ash Meadows basin is about 4,500 square miles and the minimum discharge is about 17,000 acre-feet. In contrast, the superficial watershed tributary to the springs in Death Valley is 150 square miles (Pistrang and Kunkel, 1964), and the discharge ex- ceeds 4,000 acre-feet. In addition, the Ash Meadows ground-water basin encompasses two of the highest mountain ranges in southern Nevada, whereas the Death Valley catchment area is the most arid in the Nation. Because of this relation and the foregoing hydrogeologic information, the authors suggest that most of the spring discharge in the very arid Furnace Creek Wash—Nevares Springs area (possibly more than 95 percent) originates outside of Death Valley. Hunt, Robinson, Bowles, and Washburn (1966) suggested that the spring discharge in Death Valley comes principally from Pahrump Valley, either directly or through Ash Meadows. The present authors have previously considered that movement of significant quantities of ground water from Pahrump or Stewart Valleys to Ash Meadows is unlikely. (See section, “Rela- tion to Pahrump Valley Ground-Water Basin.”) Direct movement to Death Valley from Pahrump Valley also appears unlikely because the Resting Spring Range, which borders Stewart Valley and Chicago Valley on the west, is composed chiefly of the lower clastic aquitard (pl. 1). GROUND-WATER CHEMISTRY, HYDROCHEMICAL FACIES, AND REGIONAL MOVEMENT OF GROUND WATER Chemical analyses are available for ground water from 147 sources: 74 wells, 49 springs, and 24 water-bearing fractures in underground workings. Forty of the wells are in the immediate vicinity of or are within Nevada Test Site, and the aquifer or aquitard sampled is known beyond a reasonable doubt. Many of these 40 wells were sampled 2 or more times. In several of the test holes C97 drilled specifically for hydrologic data, water samples were obtained from more than one aquifer or from two or more depths within a single aquifer. The authors use water chemistry to (1) define parts of the boundary of the Ash Meadows ground-water basin, (2) determine the direction of ground-water movement in the lower car- bonate aquifer in the basin, (3) estimate the magnitude of downward leakage of semiperched ground water from the Cenozoic rocks into the lower carbonate aquifer, and (4) speculate on the depth of circulation within the lower carbonate aquifer. The chemical analyses used are chiefly from the following sources: Maxey and Jameson (1948), Clebsch and Barker (1960), J. E. Moore (1961), Malmberg and Eakin (1962), Walker and Eakin (1963), Schoff and Moore (1964), and Pistrang and Kunkel (1964). In addi- tion, analyses for the Pahrump Valley were obtained from the files of the US Geological Survey in Carson City, Nev. Post-1963 analyses of ground water by W. A. Beetem and his associates, though not yet published, are on file at the US. Geological Survey offices in Denver, Colo. PREVIOUS INTERPRETATION OF GROUND-WATER CHEMISTRY Schoff and Moore (1964) presented the following observations and conclusions on the regional flow of ground water at the Nevada Test Site: 1. They recognized three types of ground water at Nevada Test Site and vicinity: (a) sodium and potassium bicarbonate; (b) calcium and magnesium bicarbonate; and (c) mixed. The sodium and potassium bicarbonate type is found in tuff aquifers and aquitards, and in the valley-fill aquifer in Emigrant Valley, Yucca Flat, Frenchman Flat, and Jackass Flats. The calcium and magnesium bicarbonate type is found in Paleozoic carbonate aquifers, as well as in valley- fill aquifers that are composed chiefly of carbonate- rock detritus. Schoff and Moore (1964) recognized such water only in southern Indian Springs Valley. They (1964, p. 62) defined mixed water as water having characteristics of both the preceding types. They believed that such water may have formed in one of three ways: (a) movement of water from tuf- faceous into carbonate rocks (or alluvium with carbonate-rock detritus), followed by dissolution of carbonate minerals; (b) movement of water from carbonate rocks into tuff (or tuffaceous alluvium), followed byeacquisitionof sodium either by solution or by ion exchange of calcium for sodium; or (c) mixing of calcium and magnesium bicarbonate water with sodium and potassium bicarbonate water. They noted further that water of mixed chemical type is found in some of the carbonate C98 rocks within the Nevada Test Site and that such water predominates in the Amargosa Desert. 2. From dissolved-solids content Schoff and Moore (1964, p. 56 and 57) concluded that appreciable ground water in the Cenozoic aquifers beneath Emigrant Valley is not moving into Cenozoic aquifers in Yucca Flat, becuase the dissolved-solids content of water in both valleys is similar. The average dissolved-solids content of water from the Amargosa Desert is “twice that for water in Indian Springs Valley and substantially greater than the dissolved solids in most water of the Test Site. The maximum for the Amargosa Desert is the greatest in the region. The dissolved solids point to the Amargosa Desert, therefore, as the destination to which ground water may be going, not as the place from which it comes.” 3. From the sodium content Schoff and Moore (1964, p. 60) concluded that: [a] The water in the Paleozoic carbonate rocks underlying the Test Site is in part recharged by percolation downward through tuff or through alluvium containing detrital tuff, or both. The water entering the carbonate rocks in this manner is generally a sodium potassium type, which when added to the calcium magnesium type already in the rocks yields a water of mixed chemical character. [b] The water in the carbonate rocks of the Test Site may be : moving toward the Amargosa Desert, where the water generally is of mixed chemical character, [has] a generous amount of sodium, and [is] more concentrated than [water] within the Test Site. Not all the water reaching the Amargosa Desert, however, need come from the Test Site. [c] The water of Indian Spring Valley has had little oppor- tunity for contact with tuff or tuffaceous alluvium, or with another rock material containing much soluble sodium. This water probably entered the rocks as recharge on the upper slopes of the Spring Mountains, which lie to the south. The mountains contain extensive outcrops of carbonate rocks, from which calcium and magnesium could be dissolved. [d] The water in the carbonate rocks is not moving southward from the Test Site to Indian Spring Valley. If it did so, the waters of Indian Spring Valley would contain more sodium, and also would probably be higher in dissolved solids. The cited observations and conclusions of Schoff and Moore (1964) generally appear sound. Hunt, Robinson, Bowles, and Washburn (1966, p. B40) compared the chemistry of ground water at Ash Meadows with that at Furnace Creek Wash (in Death Valley) and with that in Pahrump Valley. They con- cluded that Pahrump Valley is the source of the spring discharge at Ash Meadows. They argued further that the chemical quality of the major springs in the Furnace Creek Wash area resembles that of water from Ash Meadows and that Pahrump Valley is also its source. HYDROLOGY OF NUCLEAR'TEST SITES HYDROCHEMICAL FACIES The chemical character of ground water is influenced by many variables. Several cited by Back (1966) include: (1) Chemical character of the water as it enters the zone of saturation; (2) distribution, solubility, and adsorption capacity of minerals in the rocks; (3) porosity and permeability of the rocks; and (4) the flow path of the water. The flow path introduces variables such as mix- ture of water from two sources, changes in pressure and temperature with depth, and rate of flow. Within geographically restricted areas, ground water from a single aquifer or related group of aquifers may have a relatively fixed chemical character imposed by the listed variables. This chemical character is referred to in some recent literature as hydrochemical facies (Back, 1966; Seaber, 1965). Plate 3 shows regional patterns of hydrochemical facies at Nevada Test Site and vicinity. Pie diagrams represent the equivalents per million of major cations and anions. On plate 3, Nevada Test Site and vicinity have been divided on the basis of geography, known hydrologic setting, and hydrochemical character. The several areas and the generalized chemical characteris- tics of their waters are summarized in table 8. The me- dian character of sampled ground water for each area is depicted graphically in figure 38. Whereas space precluded plotting of pie diagrams for each available analysis on plate 3, all representative analyses, except as noted hereafter, are included in the statistical summary in table 8. Plate 3 and table 8, confirm the three types (or facies) of water defined by Schoff and Moore (1964), suggest two other facies, and identify an area where three facies mix. Four of the five facies are defined in table 9. The calcium magnesium bicarbonate facies, which Schoff and Moore identified in southern Indian Springs Valley (area IB, pl. 3), also is found in southern Three Lakes Valley (1B), northwestern Las Vegas Valley (IB), and Pahrump Valley (IC), as well as within the Spring Mountains (IA). It is also found in Pahranagat Valley (ID), where prominent valley-level springs dis- charge from the lower carbonate aquifer. Water of this facies occurs in the cited areas in wells tapping either the lower carbonate aquifer or the valley-fill aquifer rich in carbonate-rock detritus; water from perched or valley- level carbonate rock springs in these areas is also of this facies. The sodium potassium bicarbonate facies, noted by Schoff and Moore (1964) for ground water from western Emigrant Valley (area IIB), Yucca Flat (IIC), Frenchman Flat (IID), and Jackass Flats (IIE), also is found in water beneath Pahute Mesa (IIF) and Oasis Valley (IIG), northwest and west of Nevada Test Site. This water is found in tuff, rhyolite, and valley-fill aquifers rich in volcanic detritus; rarely, as in well 84—67 SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE 100 )c 0; {3, e 0° l Medians of dissolved solids o 1000 mg/l Li-LLI-J Diameter scale 100 FIGURE 38. — Chemical types of the ground water at Nevada Test Site and vicinity. Roman numerals refer to map numbers and areas listed in table 8 and shown on pl. 3; circles with crosses represent pop- ulations of perched water; patterns in circles next to map numbers facilitate visual grouping of the hydrochemical facies. (in central Yucca Flat), it is also found in thin carbonate , strata within the upper clastic aquitard. Water of mixed chemical character, noted by Schoff and Moore (1964) in the Nevada Test Site (area IIIC) and the east-central Amargosa Desert (IIIA and IIIB), is designated the calcium magnesium sodium bicarbonate facies in this report. This water occurs within the lower carbonate aquifer between Ash Meadows and eastern Nevada Test Site. As noted by Schoff and Moore (1964), water from two wells tapping Cenozoic rocks in Yucca Flat (well 83—68 tapping the valley-fill aquifer and well 81—67 tapping the bedded-tuff aquifer) also is of mixed character (pl. 3). The dissolved-solids content of water C99 from these wells is, however, 100 mg/l (milligrams per liter) less than that of the mixed water in the lower car- bonate aquifer. Schoff and Moore’s explanation of the anomalous water in well 81—67 appears reasonable; namely, that water tapped by this well is derived from Paleozoic strata, which occur about 1 mile west of the well (pl. 28). However, this explanation is not applicable to the anomalous water from well 83—68; nor, for that matter, is it consistent with the sodium potassium bicar- bonate water from well 84—67, which taps thin carbonate strata within the upper clastic aquitard. Two other facies are suggested by plate 3: a playa facies (area V), which appears restricted to the “wet” playas (playas from which ground water is discharged by evapotranspiration) or to shallow wells in discharge areas; and a sodium sulfate bicarbonate facies, which appears to be restricted to the springs in the Furnace Creek Wash—Nevares Springs area (area VI) and to a few wells in the west-'central Amargosa Desert. The chemistry of the playa facies is highly variable, depen- dent in part on the depth of the sampling well; a formal definition of this facies is not attempted in this report. Analyses of water from selected wells in western Pahrump and Stewart Valleys were excluded from the statistical summary presented in table 8. These wells are less than 100 feet deep and are generally on or along the periphery of the playas in northwestern Pahrump and Stewart Valleys, where the water table is shallowest, less than 20 feet below the surface. Most of the excluded wells are less than 40 feet deep. The water from some of these wells is more highly mineralized than the water from most deeper wells in Pahrump Valley. Choice of the 100-foot depth limitation was arbitrary. Generally, the highest mineralization was found in wells drilled to depths of 35 feet or less on or along the playa in Stewart Valley. The generally higher mineralization of water from these shallow wells is probably due to accumulation of solutes in water within the fine-grained salt-incrusted sediments that characterize valley-fill deposits in the vicinity of wet playas. Ground water in such sediments in areas of upward movement of water is neither hydrologically nor chemically similar to that of the deeper wells tapping the valley—fill aquifer in Pahrump Valley. Water from the deeper wells is of the calcium magnesium bicarbonate facies (pl. 3), whereas water from many of the shallow wells belongs to the playa facies. Water of the playa facies is also found in shallow wells along the periphery of Alkali Flat at the south end of the Amargosa Desert (north of Eagle Mountain, fig. 34). Along the margins of the study area, in Pahranagat Valley and at Furnace Creek Wash in Death Valley, only analyses from major springs discharging from the lower carbonate aquifer at valley level were used in the tabula- 0100 HYDROLOGY OF NUCLEAR TEST SITES TABLE 8. — Chemical constituents of ground water [All constituents reported in milliequivalents Ca+Mg Na+K HC03+COu Map number and area (pl. 3) Hydrogeologic setting 1:33;? Range Median Mean Range Median Mean Range Median Mean Calcium magnesium bicarbonate facies IA Spring Mountains2 ______ Major recharge area; highest 4 4.6—5.5 4.9 5.0 0.05—0.7 0.10 0.3 4.4—5.5 4.7 4.8 Earts underlain principally y Paleozoic carbonate rocks. Water sampled from perched springs. 13 Northwest Las Vegas Valley; Piedmont alluvial plain 10 3.1—6.3 4.0 4.2 .08—.71 .30 .34 3.0—4.7 3.7 3.7 southern Three Lakes bordering Spring Moun- Valley; southern Indian tains on the northeast. Springs Valley. Water sampled from both valley-fill and lower car- bonate aquifers. IC Pahrump Valley:l ______ Piedmont alluvial plain 26 3.2—12 4.5 5.2 .22—2.0 .57(25) .86 3.3—8.5 3.9 4.2 bordering Spring Moun- tains on the southwest. Water sampled from valley<fill aquifer only. ID Pahranagat Valley ______ Major discharge area for 3 3.4—4.2 4.1 3.9 1.1—1.6 1.4 1.4 3.8-4.5 4.3 4.2 lower carbonate aquifer; high-yield springs sampled. Sodium potassium bicarbonate facies IIA—l Rainier Mesa _________ Minor recharge area; ground 24 0.01—1 .1 .3 0.72—43 1.4 1.7 0.79—2.3 1.2(23) 1.3 water perched in tuff aquitard sampled. IIA-2 Hills west of Yucca and Minor recharge area; ground 9 .16—2.4 .87 .93 1.0—4.5 1.7 1.9 .63—3.3 1.6 1.8 Frenchman Flats. water perched in tuff and ‘ lava-flow aquitards sampled. IIA—8 Hills west of Oasis Valley. Minor recharge area; ground 5 .48—1.4 .72 .88 2.5—3.3 2.8 2.8 1.9—2.5 2.3 2.2 water perched in tuff aguitard sampled. 11B Emigrant Valley _______ Hy raulically closed basin 3 .39—1.0 .42 .60 2.5—4.0 3.2 3.2 2.8—3.6 2.9 3.0 with minor discharge to east. Water sampled from valley-fill and tuff aquifers(?) and aquitards. 11C Yucca Flat __________ Ground water semiperched in 5 .08—2.0 1.1 1.0 1.9—4.0 3.4 3.1 2.5—3.4 3.2(4) 3.1 valley-fill and tuff aquifers and aquitards above un- derlying lower carbonate aquifer; only Cenozoic strata sampled. IID Frenchman Flat _______ Ground water semiperched in 3 .15—.52 .16 .28 4.5—7.2 5.7 5.8 2.9—6.3 4.9 4.7 valley—fill and tuff aquifers and aquitards above un- derlying lower carbonate aquifer; only Cenozoic strata sampled. [IE Jackass Flats _________ Ground water in welded-tuff 3 .88—5.3 .88 2.4 1.9—7.0 2.2 3.7 1.7—2.1 2.0 1.9 aquifer; water is discharged southward into Amargosa Desert. IIF Pahute Mesa _________ Ground water in tuff and 10 .02—2 .4 .7 1.3—6.5 2.8 3.5 1.1—5.2 2.2 2.3 rhyolite a uifers and aquitards in ilent Canyon caldera. IIG Oasis Valley _________ Major discharge area for 17 .28—3.8 1.4 1.5 3.8—9.8 5.5 6.1 2.6—8.7 4.5 4.7 water in welded-tuff aquifer of Oasis Val- ley—Fortymile Canyon ground-water basin. Calcium magnesium sodium bicarbonate facies IIIA Ash Meadows ________ Principal discharge area for 6 3.5-4.2 4.0 3.9 3.1—4.8 3.8 3.8 5.W5.2 5.0 5.0 water in lower carbonate aquifer of Ash Meadows ground-water basin. IIIB East-central Amargosa Area of upward leakage of 3 3.2—3.6 3.5 3.4 2.8—5.6 3.3 3.9 3.4—5.7 4.5 4.5 Desert. water from lower carbonate aquifer into valley-fill aquifer. IIIC Eastern Nevada Test Site‘. Area of interbasin movement 6 3.1-5.9 4.2 4.3 1.7-5.9 3.6 3.7 4.2—8.6 5.0 5.5 of ground water through the (a) lower carbonate aquifer. Sodium sulfate bicarbonate facies VI Furnace Creek Wash— Major discharge area for 3 3.3—3.9 3.4 3.5 6.3—7.2 7.0 6.9 5.2—5.3 5.7 5.6 Nevares Springs area. water in lower carbonate Death Valley. aquifer. 'Number in parentheses after select constituents indicates number of samples when less than shown in number of samples column. ‘Excludes Grapevine Spring, in mineralized zone in northwest Spring Mountains. ”Excludes anal ses of water from wells less than 100 ft deep in wertem Pahrump and Stewart Valleys; such wells are prin- cipally along perip cry of playas. SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE C101 in the Nevada Test Site and vicinity per liter, except as indicated] . D‘ l d l‘ SOWC‘ CHME HCOWCO“ 5'02 (mg/1) (Residuésiivgvaggigtsiohmgtfliso°C) D ata sources ‘ + O; + ; + +0] . , Range Median Mean C; +10Mfégcaenlf) HCX 1'06j gxercsegé) Range Median Mean Range Median Mean Calcium magnesium bicarbonate facies — Continued 0.15-0.60 0.38 0.38 98 92 6.5—33 8.4 14 232-351 248 270 Maxey and Jameson (1948); Geo]. Survey files, Denver, Colo. .4047 48 .77 93 88 6.6—25 15(9) 15 200-340 216 235 Do. 1.1 1.8 89 80 8—38 20(21) 20 208—822 290(25) 354 Maxey and Jameson (1948); U.S. Geol. Survey files, Carson City, Nev. .79—1.1 1.0 1.0 75 81 31—33 31 32 _ _ _ 277 (1) _ _ _ Eakin (1963) Sodium potassium bicarbonate facies — Continued 0.23~1.1 0.48(23) 0.50 7 71 34—126 52 54 91— 424 192 220 Clebsch and Barker (1960). 40—19 .67 .90 35 70 32—66 50 51 166- 330 190 228 J. E. Moore (1961); Schoff and Moore (1964). .91—1.8 1.1 1.2 20 68 52-55 54(2) 54 171— 266 224 217 Malmberg and Eakin (1962). .58—.66 .62 .62 11 83 77—86 85 83 268- 810 279 286 J. E. Moore (1961); Schoff and Moore (1964); U.S. Geol. Survey files, Denver, Colo. 5042.3 .62 1.1 24 84 61—107 74 78 274— 370 296 317 Do. .77—1.8 .83 1.1 3 86 55—60 56 57 337- 451 369 386 Do. .68—10 .72 4,1 29 74 55—67 58 60 211— 886 236 444 Do. .1746.0 .94 1.7 10 71 41—50 44 45 117— 583 242 297 U.S. Geol. Survey files. Denver, Colo. 1.6—5.9 2.2 2.7 20 67 54—68 65(3) 62 330—l,071 532 580 Malmberg and Eakin (1962). Calcium magnesium sodium bicarbonate facies — Continued 2.2—3.1 2.2 2.5 51 70 20—33 22 24 413-500 420 441 Walker and Eakin (1963); Schoff and Moore (1964); U.S. Geol. Survey files, Denver, Colo. 1.3—5.0 1.9 2.7 51 70 18—20 18 19 342-548 372 421 0. 1.6—4.1 2.4 2.4 54 68 13—40 27 26 323—606 437 455 J. H. Moore (1961); Schoff and Moore (1964); US. Geol. Survey Files, Denver, Colo. Sodium sulfate bicarbonate facies — Continued 4.5—4.6 4.6 4.6 32 56 _ _ _ 25(1) 616—716 625 652 Pistrang and Kunkel (1964). ‘Excludes 3 wells tapping the lower carbonate aquifer in northwestern Yucca Flat. The dissolved-solids content of 2 of those wells (87~62 and 88—66) is abnormally low. This property and the hydrogeologic setting of the wells suggest only local recharge; the third well (84—67) contains water apparently derived only from tuff. -5 Data for north-central and central Amargosa Desert (area IV) and for “wet” playas (area V) omitted because hydrogeologic setting of these areas precludes meaningful statistical summary; see text discussion. C102 tion. Chemical quality of discharge from the major springs should be an average of the chemical quality of water in the lower carbonate aquifer, whereas water from low-yield springs (for example, Daylight and Keane Wonder Springs in the Funeral Mountains) or from wells of unknown construction may represent local recharge, recycled water, or water from several aquifers. TABLE 9. — Classification of hydrochemical facies at the Nevada Test Site and vicinity Percentage range of milliequivalents per . liter of major constituents‘ Hydrochemical facies Ca+Mg Na+K HC03+C03 S0.+Cl Calcium magnesium bicarbonate ______ 75—100 0—25 80—90 10—20 Sodium potassium bicarbonate ______ 5— 35 65—95 65—85 15—35 Calcium magnesium sodium bicarbonate ______ 50— 55 45—50 70 30 Sodium sulfate bicarbonate ______ 30 70 60 40 lMinor constituents such as Li, Sr, N03, and F are not included in cation-anion percentages; percentages are taken from median values in table 8 and rounded to nearest 5 percent. VARIATIONS OF DISSOLVED-SOLIDS CONTENT WITH DEPTH IN THE LOWER CARBONATE AQUIFER Qualitative information on the vertical variation of A dissolved solids in the lower carbonate aquifer is derived from several wells at Nevada, Test Site, three oil-test wells drilled northeast of Nevada Test Site, and a com- parison of the water from wells at Nevada Test Site with discharge from the springs at Ash Meadows. Well 89—68 was drilled to a depth of 6,000 feet in northern Yucca Flat (pl. 2). Between 1,773 and 5,290 feet, the bore penetrated the lower clastic aquitard (Stirling Quartzite, 1,773—2,360 ft; Johnnie Formation, 2,360-5,290 ft; and the Noonday(?) Dolomite, 5,290—6,000 ft). A major permeable fault zone was found between the Johnnie Formation and the Noonday(?) Dolomite. Analyses of water samples from two intervals (1,785—1,940 and 1,785—6,000 ft) are given in table 10. The two analyses indicate little change in chemical quality with depth. The upper interval was sampled when the well was 1,940 feet deep. The drill-stem test data indicate that most of the water of the second sam- ple came from depths greater than 2,170 feet and that a significant quantity of it, probably more than half, may have come from the permeable fault zone at a depth of about 5,290 feet. Heads measured during drill-stem testing indicate that the second sample does not reflect water that moved downward along the bore from upper to lower zone during or after drilling. The analyses suggest that the water quality in the lower clastic aquitard is relatively uniform to depths of several thou- sand feet. The absence of a significant change in the chemistry of water within the lower clastic aquitard suggests that the water in the lower carbonate aquifer may also be relatively uniform chemically to depths of several thousand feet. HYDROLOGY OF NUCLEAR TEST SITES TABLE 10. — Chemical analyses of water from test wells 89—68 and 67—68, Yucca Flat and Mercury Valley, Nye County [Values for chemical constituents are in milligrams per liter. Analyses by U.S. Geol. Survey, Denver, Colo] Well 89—68 Well 67—68 Depth interval (ft) - _ _ _ l,785—1,940 1,785—6,000 786-1,050 1,333—1,946 Silica (SiOz) _______ 8.5 17 20 21 Calcium (Ca) _______ 45 41 46 47 Magnesium (Mg) _____ 11 13 21 21 Sodium (Na) _______ 98 96 38 37 Potassium (K) ______ 16 15 4.9 5.2 Bicarbonate (HCOa) _ _ _ 357 384 254 256 Carbonate (COa) _____ 0 0 0 0 Sulfate ($04) _______ 65 54 58 53 Chloride (Cl) _______ 17 11 17 16 Specific conductance (umhos per cm at 25°C) 715 720 483 554 pH _____________ 7.7 7.8 7.5 7.1 Test hole 67-68 was drilled to a depth of 1,946 feet in southern Mercury Valley (fig. 33). In the zone of satura- tion, carbonate rocks of the Nopah Formation (table 1) are between 786 and 1,168 feet, the Dunderberg Shale Member of the Nopah Formation between 1,168 and 1,333 feet, and the carbonate rocks of the Bonanza King (7) Formation between 1,333 and 1,946 feet. Through use of two strings of cemented casing and packers, the Nopah Formation and the Bonanza King (‘?) Formation were test pumped separately. At the conclusion of each test, a water sample was collected and analyzed. The results of these analyses are shown in table 10. Drill-stem tests of the Nopah Formation indicated that the sample for this formation actually came from the interval 786—1,050 feet. The drill-stem tests also indicated that the head in the Bonanza King (?) Formation was possibly as much as 4 feet higher than that in the upper aquifer. A comparison of the chemical analyses of the water from the Nopah and Bonanza King(?) Formations in- dicates that the water in the two formations is prac- tically identical. The near identical nature of the chemical quality of water from both aquifers may reflect natural upward crossflow through the Dunderberg Shale Member, which separates the two carbonate aquifers at the well site. Such crossflow is possible because of the 4- foot head differential that may exist and because the Dunderberg, a thin clastic unit, is rarely continuous areally, owing to normal faulting and its tendency to be pinched out along major faults and tight folds. If crossflow in the region of well 67—68 is upward and is principally responsible for the similarity in water chemistry noted for the two sampled intervals, then water of relatively low mineralization probably occurs in the lower carbonate aquifer at depths even greater than that penetrated by the well. Specific conductance of water swabbed from two other test holes, wells 88—66 in Yucca Flat (pl. 2) and 66—75 (fig. 33) in southern Indian Springs Valley, suggests no increase in dissolved-solids content in the upper 800 feet of the lower carbonate aquifer. However, these data, ob- SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE tained from additive drill-stem tests, are not as reliable as the data reported for wells 89—68 and 67—68. Data from three deep oil tests in White Pine County in east-central Nevada indicate that rocks stratigraphically equivalent to the lower and upper carbonate aquifers contain water of low dissolved-solids content to depths as great as 10,000 feet. McJannett and Clark (1960, p. 248—250) summarized the results of drill-stem tests of three oil-test wells in White Pine County. In one well (16N/56—31), the Lone Mountain Dolomite (Silurian) reportedly contained fresh water in the depth interval 9,405—9,431 feet. A second well (15N/59—17), tested the Chainman Shale (Mississippian), the Joana Limestone (Mississippian), and the Nevada Formation (Devonian). Shaly limestone of the Chainman Shale reportedly con- tained either fresh water or drilling fluid in the interval 3,770—4,053 feet. The top 39 feet of the Joana Limestone reportedly includes cavernous zones ranging from 2 to 9 feet in thickness, into which circulation was repeatedly lost. A drill-stem test of the cavernous interval, from 4,065 to 4,098 feet, reportedly recovered 1,890 feet of fresh water. Drill-stem tests of the Nevada Formation were made in the intervals 4,437—4,865, 4,867—5,020, and 5,023—5,117 feet; each zone reportedly yielded fresh water containing only 1 grain per gallon (or 17 mg/l) of chloride (McJannett and Clark, 1960, p. 250). By com- parison, the chloride content of water from the lower car- bonate aquifer at Nevada Test Site and vicinity is 15—30 mg/l. In a third oil-test well (20N/60—32), nine drill-stem tests were made of Permian carbonate rocks in the inter- val 4,000—8,600 feet; only drilling fluid or fresh water was reportedly recovered during the tests. A comparison of the spring water at Ash Meadows with that within the lower carbonate aquifer beneath Nevada Test Site also suggests that the chemical- constituent content in the lower carbonate aquifer does ~ not vary markedly with depth. Data in table 8 and in figure 38 indicate that the chemical quality of the water within the lower carbonate aquifer beneath Nevada Test Site (area IHC) is remarkably similar to that of the water discharging from the major springs at Ash Meadows (area IIIA). The principal source of spring discharge at Ash Meadows is probably ground water moving southwestward within the lower carbonate aquifer (pl. 1); the chemical similarity between the springflow and the waters of area IIIC certainly does not contradict this assumption. In fact, Schoff and Moore (1964) considered it as proof of the southwestward movement of ground water. The samples of area IIIC represent only the upper 1,000 feet of the lower carbonate aquifer, which locally is several thousand feet thick. The chemical character of Ash Meadows springs, on the other hand, must repre- sent, by virtue of the large discharge and its structural control, a chemical integration of water from the entire C103 thickness of the lower carbonate aquifer within the Ash Meadows ground-water basin. Hence, the chemical similarity suggests that dissolved-solids content does not change significantly with depth in the carbonate aquifer at Nevada Test Site and vicinity. An alternate explana- tion is that water quality may deteriorate with depth but that such deterioration is not reflected at Ash Meadows because of an accompanying reduction in fracture transmissibility of the lower carbonate aquifer with depth. In summary, chemical data from four wells at Nevada Test Site and three deep oil tests in east-central Nevada, as well as a comparison of the chemical quality of spring water at Ash Meadows with that of water in the lower carbonate aquifer beneath Nevada Test Site and vicinity, indicate no significant increase in the dissolved- solids content of water in the lower carbonate aquifer to depths of several thousand feet.3 SOURCES OF SODIUM AND SULFATE IONS IN WATER OF THE LOWER CARBONATE AQUIFER Of the five hydrochemical facies, the calcium magnesium sodium bicarbonate facies is of major impor- tance for mapping the movement of ground water in the Ash Meadows basin. The origin of this facies, as suggested by the sodium and sulfate content of ground water, is discussed in this chapter, and the use of the facies for determination of the origin of the Ash Meadows spring discharge is the subject of the next chapter. Ground water within the lower carbonate aquifer beneath Nevada Test Site (area IIIC of table 8) and at Ash Meadows (area IIIA) differs significantly from water in the lower carbonate aquifer elsewhere in the study area. The principal difference is in the milliequivalents per liter (meq/l) of sodium plus potassium and of sulfate plus chloride. For the water in the lower carbonate aquifer northeast (area ID) and southeast (areas IA, IB, and IC) of Nevada Test Site (pl. 3; table 8), median values of sodium and potassium range from 0.1 to 1.4 meq/l; whereas for water in the same aquifer beneath Ash Meadows and at Nevada Test Site (areas IIIA and IIIC), median values range from 3.6 to 3.8 meq/l. The sulfate and chloride content of water in the lower carbonate aquifer southeast and northeast of Nevada Test Site ranges from 0.38 to 1.1 meq/l; whereas beneath Ash Meadows and at Nevada Test Site, it ranges from 2.2 to 2.4 meq/l. Similarly, the median dissolved-solids content ranges from 216 to 290 mg/l southeast and northeast of . the site and from 420 to 437 mg/l beneath the site and at Ash Meadows. These variations are summarized in table 8. "In the Las Vegas area and elsewhere in eastern Nevada, miogeosynclinal carbonate rocks (chiefly of Permian age) are interbedded with sedimentary gypsum or with elastic rocks con~ taining detrital gypsum. In such areas, ground water in the Paleozoic carbonate aquifers may be highly mineralized (even at relatively shallow depths) in comparison with water in the gypsum-free Paleozoic carbonate rocks of the Ash Meadows ground-water basin. C104 In the ion pairs (sodium plus potassium and sulfate plus chloride), sodium and sulfate are the principal ions. Potassium typically ranges from only 5 to 10 percent of the sum of sodium and potassium, whereas chloride typically ranges from 25 to 35 percent of the sum of sul- fate and chloride. These percentages persist in all the areas in table 8. The dichssion that follows pertains in general only to sodium and sulfate ions. For convenience, the median values of the ion pairs sodium plus potassium and sul- fate plus chloride for areas IA—IIIC (table 8) are taken as representative of sodium and sulfate, respectively. This shortcut seems justifiable because of the cited dominance of Na (90—95 percent) and 804 (65—75 per— cent) in the ion pairs for each group and the magnitude . of the difference, between select areas. The sodium or the sulfate content of water from individually cited wells, on the other hand, will always pertain only to a single cation or anion. SODIUM The principal source of sodium ions within the lower carbonate aquifer beneath Nevada Test Site is ground water that originated in or passed through the Tertiary tuff aquifers and aquitards. Median values of sodium for ground water in areas IA, IB, and 10 (pl. 3; table 8) are, respectively, 0.1, 0.3, and 0.6 meq/l (roughly 2—15 mg/l). The water from these areas was derived directly from either the lower car- bonate aquifer or the valley-fill aquifer rich in carbonate-rock detritus. The low sodium content is ex- pected of water from these aquifers in the areas represented by areas IA to 10 because minerals (for ex- ample, plagioclase feldspar and halite and other evaporites) that normally contribute sodium to ground water are either absent or sparse in the aquifers of these areas. The median value of sodium for ground water in areas IIB to IIG ranges from 2.2 to 5.7 meq/l (roughly 50—130 mg/l). This water comes directly from either Tertiary tuff aquifers and aquitards of rhyolitic or quartz latitic composition or valley fill rich in tuff detritus. Sodium comes chiefly from the alteration of rhyolitic glass (shards and pumice), plagioclase feldspar (this and cristobalite are the chief devitrification products of rhyolitic glass), and perhaps zeolite minerals, which together constitute the bulk of these volcanic rocks. Leaching of sodium from glassy and crystalline tuffs at the Nevada Test Site was discussed at length by Lipman (1965) and Hoover (1968). The low sodium content of water from the lower car- bonate and valley-fill aquifers bordering the Spring Mountains contrasts markedly with the higher sodium content of ground water derived from the volcanic terrane of Nevada Test Site and vicinity. This contrast HYDROLOGY OF NUCLEAR TEST SITES and the absence of any obvious important source of sodium in the carbonate rocks beneath or flanking Nevada Test Site suggest that much, probably most, of the sodium within the lower carbonate aquifer beneath Nevada Test Site (area IIIC) comes from ground water that has moved through tuffaceous aquifers and aquitards of rhyolitic or quartz latitic composition. Sodium ions may enter the lower carbonate aquifer beneath Nevada Test Site in three ways. Some may originate in semiperched ground water in the tuff aquitard and enter the carbonate aquifer by downward crossflow in Yucca and Frenchman Flats and possibly also in the valleys east of Nevada Test Site (northern In- dian Springs, northern Three Lakes, eastern Emigrant, ’and Desert Valleys). A second mechanism involves the movement of ground water upward from the lower car- bonate aquifer into the tuff aquitard and then back down into the carbonate aquifer. Upward movement might oc- cur, for example, in the vicinity of ground-water barriers within the carbonate aquifer. Once the water enters the tuff aquitard, sodium might be picked up by the ion ex- change of calcium (in the carbonate-aquifer water) for sodium during contact of the water with the zeolitic and clayey minerals common within the aquitard. A third source of some (possibly one-third) of the sodium is im- portation, through the carbonate aquifer, from the region northeast of Nevada Test Site, mostly from Pahranagat Valley (area ID). The water emerging from the lower carbonate aquifer in Pahranagat Valley con- tains about five times as much sodium as that in area IB — namely in southern Indian Springs, southern Three Lakes, and northwest Las Vegas Valleys — but this amount of sodium is only about one-third that in the lower carbonate aquifer at Nevada Test Site (area IIIC) or at Ash Meadows (area IIIA). The high sodium content of the water from the upper carbonate aquifer in Pahranagat Valley is not unexpected. As in the Nevada Test Site area and in much of the area between Pahranagat Valley and Nevada Test Site, the Paleozoic strata in the ridges flanking Pahranagat Valley and sur- rounding valleys are commonly overlain by rhyolitic volcanic rocks, chiefly ash-flow tuffs and associated tuf- faceous sedimentary rocks. Therefore, these rocks probably also occur locally within the zone of saturation and probably contribute sodium to the lower carbonate aquifer in areas of downward crossflow. Of the three possible sources, the second appears least likely for two reasons. First, the zeolitized and clayey tuff aquitard at the base of the Tertiary strata both at and east of Nevada Test Site would tend to retard up- ward crossflow from the lower carbonate aquifer into the Tertiary rocks and also the return flow. Second, table 8 7 and figure 39 show no apparent reduction in calcium and magnesium content between areas IE or ID and areas IIIC and IIIA; such a reduction would be expected if ion SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE 8 II 7 3 E 6 t .1 c: E 5 f2 2 LL! .1 < Z 4 8 of: A E m B m C :I © 2 3 E g. + 5 Z 2 @ID 1 @lC 1.318 I o A@ 0 1 2 3 4 5 Ca+Mg, IN MILLIEQUIVALENTS PER LITER FIGURE 39. — Regional variations in Na+K and Ca+Mg within the lower carbonate aquifer. Roman numerals refer to map numbers and areas listed in table 8; media: values are from the same table. exchange were a significant factor in the pickup of sodium. Although ground water within Tertiary aquifers and aquitards is probably a logical principal source for the sodium in water within the lower carbonate aquifer, a major problem remains. The sodium content of water in the carbonate rocks beneath Nevada Test Site (area IIIC) is slightly greater than that of water in the Tertiary rocks of Emigrant Valley (area IIB) and Yucca Flat (area IIC) and in the lower carbonate aquifer in Pahranagat Valley (area ID). Only the sodium content of ground water from the Tertiary and Quaternary aquifers of Frenchman Flat (area IID) is greater than that of water in the carbonate roe-ks. This indicates (1) still another source of sodium in addition to those outlined, (2) that downward leakage of water from Cenozoic rock in Frenchman Flat and perhaps other valleys constitutes a significant part of the Ash Meadows discharge (an assumption not supported by hydraulic data; see section “Intrabasin Movement,”) or (3) that the sodium content of ground water in the Tertiary rocks increases markedly C105 with depth or with stratum tapped. Evidence presented in the section on sulfate suggests that the sodium con- tent of the ground water in the basal strata of the tuff aquitard, in the southern part of Nevada Test Site, is, at least locally, significantly greater than that of water sampled from the upper part of the zone of saturation in areas IIB, HO, and IID. Because the chemical quality of water in the lower car- bonate aquifer in eastern Frenchman Flat (well 75—73) is almost identical with that of water at Ash Meadows (pl. 3; table 8), most of the sodium may have entered the lower carbonate aquifer (from the overlying tuff aquitard) principally in the valleys east or northeast of Frenchman Flat (namely, northern Indian Springs, northern Three Lakes, eastern Emigrant, and Desert Valleys). By analogy with hydrologic conditions in Yuc- ca and Frenchman Flats, such downward leakage is possible in the cited valleys for reasons outlined in the section “Intrabasin Movement.” SULFATE The increase in sulfate content of water in the lower carbonate aquifer between areas northeast and southeast of Nevada Test Site and the Nevada Test Site—Ash Meadows area is significant, although it not as marked as the increase in sodium content. (See table 8.) The sul- fate content of water in the carbonate aquifers increases 120 percent between Pahranagat Valley (area ID) and Ash Meadows (table 8) and about 360 percent between the northeast flank of the Spring Mountains (area IB) and Ash Meadows. Moreover, the sulfate content within the lower carbonate aquifer beneath Nevada Test Site and Ash Meadows is three to four times that within Cenozoic aquifers or aquitards sampled in Emigrant Valley, Yucca Flat, and Frenchman Flat (areas IIB, HC, and IID) (table 8). Three sources of sulfate ion are possible: (1) sulfide and sulfate minerals in granitic stocks, altered carbonate rocks, and altered volcanic rocks; (2) evaporite deposits within the Paleozoic car- bonate strata; and (3) evaporite deposits within the basal Tertiary volcanic(?) strata in areas of downward crossflow. Granitic stocks and altered carbonate and volcanic rocks occur locally within Nevada Test Site and may also occur within the zone of saturation east of Nevada Test Site. Oxidation of sulfide minerals such as pyrite or possible solution of sulfate minerals such as alunite —— KA13(OH)6(SO4)2 — might serve as a source of sulfate ions. For example, in ground water perched in the Climax stock in northern Yucca Flat, sulfate content (exclusive of chloride) ranges from about 7 to 21 meq/l (Walker, 1962). The high sulfate is presumably due to oxidation of pyrite, which is common along fractures in the rock. Similarly, the high sulfate content (about 9 meq/l) of water from well 74—61, in central Jackass Flats, C106 may be due to movement of ground water through rocks similar to those exposed in the Calico Hills, 5 miles north of the well. The Calico Hills consist of hydrothermally altered volcanic strata that locally contain abundant pyrite and alunite. However, the sulfate content of two other wells in Jackass Flats (74—57 and 73—58) — one of them no farther from the Calico Hills than well 74—61 — is less than 0.5 meq/l. Although altered volcanic and carbonate rocks, or stocks, may locally influence the sulfate content of water within the carbonate aquifers, they are not considered an important regional source of sulfate because (1) their distribution is limited, and their size is relatively small; (2) the transmissibility of stocks and altered volcanic rocks is usually very low; and (3) pyrite and other sul- fides presumably are oxidized chiefly in the vadose zone and perhaps also in the uppermost part of the zone of saturation but the aridity of most of the region precludes movement of much (if any) water through the vadose zone. These factors tend to prevent the contact of a significant volume of ground water with these sources of the sulfate. A second potential source of sulfate in the ground water within the lower carbonate aquifer is evaporite deposits, specifically gypsumlaminae or strata within the Paleozoic rocks east of Nevada Test Site. Sedimen- ‘tary gypsum occurs in the Permian and Triassic rocks in the southeastern one-third of the Spring Mountains and in several mountain ranges east of the study area, but no gypsum or other evaporites have been reported in the Paleozoic rocks older than Permian in the study area. Furthermore, the geologic maps of Clark and Lincoln Counties indicate that no Permian or Triassic rocks oc- cur in the ridges flanking Indian Springs, Three Lakes, or Desert Valleys. Permian rocks are probably absent also in the subsurface, because most of the exposed rocks are Devonian or older, which indicates generally deep erosion of the Paleozoic sequence in the area east of Nevada Test Site. Permian carbonate rocks composing the upper carbonate aquifer are present in western Yuc- ca Flat, but these strata include no evaporites. Actually, the chemical quality of water in the area east of Nevada Test Site indicates that neither gypsum nor other sulfate-bearing evaporites occur in significant amount within the Paleozoic strata there. The chemical in- fluence of any evaporites along the margins of the Ash Meadows ground-water basin is presumably already reflected by the water quality of areas IA, IB, and ID (table 8); the sulfate in these waters is negligible in com- parision with quantities in ground water from terrane containing sedimentary gypsum. The most likely source of the additional sulfate within the water of the lower carbonate aquifer beneath areas IIIA—IIIC is the solution of gypsum from the basal strata HYDROLOGY OF NUCLEAR TEST SITES composing the tuff aquitard. In the southern half of Nevada Test Site, the oldest Tertiary rocks consist primarily of tuffaceous sedimentary rocks, claystone, and freshwater limestone of the Rocks of Pavits Spring and the Horse Spring Formation (table 1). Laminae of gypsum have been reported in the Rocks of Pavits Spring near Mercury, Nev. (E. N. Hinrichs, oral commun., 1966). Longwell, Pampeyan, Bowyer, and Roberts (1965, - p. 45—48) reported gypsum in the Horse Spring Forma- tion in eastern Clark County, and Denny and Drewes (1965, p. L18) mentioned sparse gypsum in the Tertiary claystone south of Ash Meadows. Thus, in areas of downward crossflow beneath Nevada Test Site, solution of the gypsum in these basal Tertiary strata could add important quantities of sulfate to water in the lower car- bonate aquifer. Because the chemical quality of water in the lower car- bonate aquifer beneath eastern Frenchman Flat (well 75—73) is almost identical with that of water discharging at Ash Meadows (pl. 3), sulfate-rich water may leak downward principally east or northeast of Frenchman Flat, in northern Indian Springs and Three Lakes Valleys. The Horse Spring Formation crops out in ridges bordering these valleys and probably also underlies these valleys in the zone of saturation. Gypsum within this for- mation probably constitutes a source of sulfate, if downward crossflow occurs. Direct evidence for very high sulfate and sodium con- tents in ground water from the basal Tertiary rocks com- posing the tuff aquitard is. derived from well 68—69, and evidence of moderate increases in sulfate and sodium with depth is suggested by well 73—66. Well 68—69, in central Mercury Valley (fig. 1), bottomed at a reported depth of 1,220 feet in saturated Tertiary sedimentary rocks (fig. 33). The interval from 500 to 1,220 feet con- sists of siltstone, mudstone, and shale (B. D. Jorgensen, written commun., 1951); these strata tentatively cor- relate with the Rocks of Pavits Spring, although they might also be in the Horse Spring Formation (table 1). Jorgenson reported that the chief aquifer is a 5-foot-thick pinkish medium—grained sandstone at a depth of about 1,133 feet; the yield of this sandstone was reported as less than 5 gpm. A chemical analysis of water bailed from well 68—69 reported by Schoff and Moore (1964, p. 27) is presented in table 11. The water is unlike other waters described in this report. Sodium constitutes about 75 percent of the total cations, and sulfate about 95 percent of the anions. A pie diagram for this water is not shown on plate 3 because, even if drawn to half scale, it would obscure large parts of the map. In a discussion with Mr. S. R. McKinney of Las Vegas, Nev., the driller of well '68—69, the senior author learned that water from well 74—70a (in Frenchman Flat) and some aqua gel were used in drilling the well with cable tools, but no gypsum SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE TABLE 11. — Chemical analysis of water from test well 68—69, Mercury Valley, Nye County Milli rams Milliequivalents per iter1 per liter Silica (SiOg) ______________________ 23 _ _ - Calcium(Ca) ______________________ 281 14.02 Magnesium (Mg) ____________________ 90 7.40 Sodium (Na) ______________________ 1,290 56.12 Bicarbonate (HCOa) __________________ 98 1.61 Carbonate (003) ____________________ Tr _ _. _ Sulfate (SOi) ______________________ 3,600 74.99 Chloride (Cl) ______________________ 135 .99 Dissolved solids (sum) _________________ 5,420 _ _ _. Hardness as CaCOa (total) ______________ 1,070 _ _ _ pH ____________________________ 8.1 _ _ _ ‘Analysis by Smith-Emery Co. of Los Angeles, Calif, 1951. Sodium, sulfate, dissolved solids, and hardness have been rounded to Survey standards. 2Erroneously reported as 98 by Schoff and Moore (1964, p. 27). cement was used. The chemical analysis should be representative of water from the Rocks of Pavits Springs or, perhaps, from the Horse Spring Formation. According to Schoff and Moore (1964, p. 28): the unusual character of the water from well 68—69 suggests that the confining layer under the zone of saturation provides a relatively tight seal. The mineralized water seems not to appear in wells tapping other aquifers. The well is less than 3 miles “upstream” from well 67—68, which taps carbonate rocks and has water containing only 330 mg/l dis- solved solids and 38 mg/l sodium (28 percent of total cations). No more than a trickle of the mineralized water can be reaching the carbonate rock aquifer at well 67—68. This observation has merit because nowhere in the lower carbonate aquifer is the sodium content more than 100 mg/l or the sulfate content more than 200 mg/l. The slow rate of leakage from the Cenozoic aquifers, which Schoff and Moore (1964) inferred, is supported by the hydraulic tests of the tuff aquitard in general and of the Rocks of Pavits Spring in particular. Chemical analyses of perched (or semiperched) ground water tapped by well 73—66, in Rock Valley, southeast of Skull Mountain (fig. 1), indicate an increase in quantities of sulfate and sodium with depth in water within the tuff aquitard (Wahmonie Formation, Salyer Formation, Tuff of Crater Flat, and Rocks of Pavits Spring). Laboratory analyses of water swabbed from the hole during drill-stem tests of the tuff aquitard (depth intervals 77—693 and 1,565—1,695 ft) and pumped from the lower carbonate aquifer (depth in interval 3,140—3,400 ft) are given in table 12. The first two analyses suggest a marked increase in sulfate and sodium to a depth of about 1,700 feet in the tuff aquitard. The content of these ions in the carbonate aquifer (third analysis) is presented for comparison. The anomalously high sulfate content of water from well 68—69 and the threefold increase in sulfate with depth in the tuff aquitard in well 73—66 is believed due to solution of gypsum within the basal strata of the tuff aquitard. An explanation for the much higher sodium content in water from wells 68-69 and 73—66 than that in water from Tertiary and Quaternary strata in areas IIB C107 TABLE 12. — Chemical analyses of water from three depth intervals in test well 73—66, Rock Valley, Nye County [Analyses by U.S. Geo]. Survey, Denver, Colo] Depth interval (ft) ______________ 77-693 1,565-1,695 3,140—3,400 Major constituents, in milligrams per liter Silica (SiOz) _________________ 32 14 31 Calcium (Ca) _________________ 13 4.0 68 Magnesium (Mg) _______________ 1.0 .0 30 Sodium (Na) _________________ 99 424 63 Potassium (K) ________________ 6.4 4.4 9.6 Bicarbonate (HCOg) _____________ 199 719 273 Carbonate (C03) _______________ 0 68 0 Sulfate (SO‘) _________________ 34 110 181 Chloride (Cl) _________________ 32 35 11 Physical characteristics and computed values Dissolved solids, in milligrams per liter calculated _________________ 327 981 534 Hardness as milligrams per liter CBCOa: Total ___________________ 37 10 293 Noncarbonate ______________ 0 0 65 Specific conductance (umhos per cm at 25°C) __________ 492 1,640 751 p _______________________ 7.3 8.8 7.3 Temperature (°C) ______________ 22.0 33.5 64.5 to IID, and for the increase in sodium with depth in well 73—66, is difficult because of the absence of information ‘ on the zeolite, clay, and evaporite mineralogy of the aquitard at these well sites. The sodium content of these well waters is tentatively attributed to one or more of the following: (1) Ion exchange of calcium, derived from solution of gypsum, for sodium in clays within the . aquitard; (2) possible presence of sodium-bearing evaporite minerals, such as trona or nahcolite, in the basal tuff aquitard; and (3) possible continued altera- tion, by downward-moving water, of the sodium-rich zeolite minerals (clinoptilolite, mordenite, and . analcime) that compose the bulk of certain strata within . the aquitard. The strong tendency of sodium to remain in solution (Hem, 1959, p. 84—85) would enhance any of these mechanisms. Regional relations among absolute quantities of Na+K, SO4+Cl, and HCO3+COg in the lower carbonate aquifer are shown in figure 40. In summary, ground water of the calcium magnesium bicarbonate facies found in the lower carbonate aquifer along the periphery of the Ash Meadows ground-water basin (areas IA, IB, and ID) is converted to the water of the calcium magnesium sodium bicarbonate facies, found in the aquifer beneath the central and southwestern parts of the basin (areas IIIA—IIIC), by means of downward leakage of sodium- and sulfate-rich Water from the tuff aquitard. The higher sodium and sul- fate contents of water in the carbonate aquifer beneath areas IIIA—IIIC than in water in the overlying Cenozoic aquifers and aquitards of areas IIB—IID, the proposed source of the sodium and sulfate, reflects sampling posi- tion. Wells in areas IIB-IID mostly tap water from the upper part of the zone of saturation within the Cenozoic hydrogeologic units and therefore do not reflect increases ‘ in sodium and sulfate believed to occur within the tuff aquitard with depth. 0108 Na+K' IN MILLIEOUIVALENTS PER LITER 5 IIIA—(9 /, A[IIIA ©mc “111C 1]] B@ / III BA 3 / / / l/ 2 / o ‘10 1 1/ ID / / @lC lC /@IB [8‘ o @lA lAA 0 1 2 3 4 5 6 SO.+CI OR HC03+C03, IN MILLIEQUIVALENTS PER LITER FIGURE 40. — Regional variations in Na+K, HCOa+C03, and SO4+Cl within the lower carbonate aquifer. Roman numerals refer to map numbers and areas listed in table 8; median values are from the same table. HYDROCHEMICAL EVIDENCE FOR REGIONAL GROUND-WATER FLOW Hydrochemical facies provide some quasi- independent evidence on the direction of ground-water movement in the lower carbonate aquifer, the magnitude of the intrabasin movement of ground water, and the boundaries of the Ash Meadows basin. HYDRAULIC CONNECTION BETWEEN PAHRUMP VALLEY AND ASH MEADOWS On the basis of water chemistry and head relation, Hunt, Robinson, Bowles and Washburn (1966, p. B28) suggested that the spring-water discharge at Ash Meadows comes from Pahrump Valley. They stated: “Water in Pahrump Valley is a bicarbonate water very similar to that at Ash Meadows * * * .” They were cor- rect in noting that water in both areas is rich in calcium, magnesium, and bicarbonate, but they did not note the significantly larger amounts of sodium, potassium, sul- fate, chloride, and dissolved solids in the water of Ash HYDROLOGY OF NUCLEAR TEST SITES Meadows. This is shown by position of populations IC (Pahrump Valley) and IIIA (Ash Meadows springs) in figure 38 and by the data in table 8. First, sodium and potassium, which constitute only about 10 percent of the cations in ground water from Pahrump Valley, con- stitute about 50 percent of the cations in the spring dis- charge at Ash Meadows. Second, sulfate and chloride, which constitute about 20 percent of the anions in Pahrump Valley, constitute about 30 percent at Ash Meadows. And last, the median dissolved-solids content of the Pahrump water is 290 mg/l, in contrast to 420 mg/l at Ash Meadows. If Hunt, Robinson, Bowles, and Washburn (1966) could have seen all the analyses used by the authors of this report, particularly analyses of water from the lower carbonate aquifer, their conclusion on similarity of the waters would probably have been different. The differences between the waters do not in themselves rule out the possibility of northwestward movement of ground water from western Pahrump Valley (or from Stewart Valley) into Ash Meadows. Could the chemical quality of the ground water change during movement between the two areas through either the valley-fill or the lower carbonate aquifer? This possibility appears slim for two reasons. First, analyses of water from the valley-fill aquifer in Pahrump Valley and data presented by Malmberg (1967, pl. 5) fail to reveal a progressive westward increase in sodium and potassium, sulfate and chloride, or dissolved-solids con- tent of water in this aquifer. Only water from the very shallow wells (less than 100 ft and usually less than 40 ft deep) on and along the periphery of the playa in Stewart Valley has a similar or greater content of the ions in- dicated and of dissolved solids; however, this water is of the playa facies and, in any event, does not resemble that discharging from the major springs at Ash Meadows (pl. 3; table 8). Thus, water in the valley-fill aquifer is not a likely source of the water discharging at Ash Meadows. Second, the water in the lower carbonate aquifer beneath Pahrump Valley, although not sampled in wells, is also unlikely to contain the necessary quan- tities of sodium and potassium for reasons outlined below. The only sodium-rich ground water within the study area, except the playa facies, is in or has passed through rhyolitic volcanic aquifers or aquitards. If such strata underlie the valley fill beneath Pahrump Valley and if ground-water movement were downward from the Cenozoic rocks into the Paleozoic carbonate strata (as in Yucca or Frenchman Flats, for example), then the water in the carbonate aquifers beneath Pahrump Valley might contain quantities of sodium and potassium equivalent to those in the water at Ash Meadows. However, the direction of crossflow in northwestern Pahrump Valley is upward from older to younger rocks, SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE as indicated by flowing wells, springs, and areas of phreatophyte discharge. Therefore, the sodium and the potassium contents of water in the lower carbonate aquifer beneath PahrumpValley are probably similar to those of water in the carbonate aquifer in the Spring Mountains (area IA) and in Indian Springs Valley (area IB). (See table 8 and pl. 3.) The remarkable chemical similarity of the discharge at all the major springs at Ash Meadows (pl. 3,; table 8) also suggests that little of the water discharging at Ash Meadows comes from Pahrump Valley or from Stewart Valley. If significant inflow were derived from these valleys, the spring discharge at the southeast end of the spring line (for example, at Big Spring or at Jack Rabbit Spring, fig. 34) would probably more closely resemble the ground water in Pahrump or Stewart Valleys than springs at the north end of the spring line 9 miles dis- tant; such a resemblance does not exist. Some interbasin movement between Pahrump and Stewart Valleys and Ash Meadows must occur because of the head difference between the two areas, but the quantity of such move- ment probably doés not exceed a few percent of the Ash Meadows discharge at most. (See calculation of un- derflow in the section “Relation to Pahrump Valley Ground-Water Basin”) DIRECTION OF GROUND-WATER MOVEMENT WI'IHIN THE LOWER CARBONATE AQUIFER BENEATH NEVADA TEST SITE Schoff and Moore (1964) suggested that ground water within the carbonate aquifers at Nevada Test Site (pl. 3, area IIIC) must be moving southwestward toward Ash Meadows. They noted that water from the carbonate and valley-fill aquifers in southern Indian Springs Valley (area IB) contained little sodium (with minor potassium) and less dissolved solids than water from the lower carbonate aquifer at Nevada Test Site; they therefore ruled out the possibility of southeastward movement from the test site to southern Indian Springs Valley. They did not consider eastward movement into northern Indian Springs Valley. Inherent in their use of sodium as a chemical tracer is the fact that sodium, once in solution, tends to stay in solution (Hem, 1959, p. 84—85). Additional evidence in support of Schoff and Moore’s conclusions is provided by figure 38 and table 8. The water of area IIIC closely resembles that discharging at, and in the unnamed valley northeast of, Ash Meadows (areas IIIA and IIIB), but it differs significantly from water in Indian Springs, Three Lakes, and northwest Las Vegas Valleys (area IB) and Pahranagat Valley (area ID). Not only does the water in the lower carbonate aquifer of Nevada Test Site contain markedly more sodium and potassium than does the water in Indian C109 Springs, Three Lakes, northwest Las Vegas Valleys and Pahranagat Valley, but also it contains more sulfate and' chloride. In contrast, the very close similarity of water in carbonate aquifers beneath the Nevada Test Site to that in Ash Meadows suggests that water of the Nevada Test Site is probably moving southwestward. However, southeastward or eastward movement of water into In- dian Springs Valley probably cannot be ruled out on the basis of chemical evidence alone, because of the possible dilution of water derived from the Nevada Test Site by a significantly larger volume of water derived from the Spring Mountains. ESTIMATES OF DOWNWARD CROSSFLOW FROM THE TUFF AQUITARD INTO THE LOWER CARBONATE AQUIFER The variations in chemical quality of the ground water within the lower carbonate aquifer beneath and east of Nevada Test Site (area IB, ID, and IIIC, pl. 3) and the chemical difference between these waters and those in the tuff aquitard suggest that the chemical data can be used to compute the approximate magnitude of downward crossflow of semiperched water from the aquitard (underlying the valleys in and east of Nevada Test Site) into the lower carbonate aquifer. In the discussion that follows, milliequivalents-per- liter values of the ionic pairs sodium plus potassium and sulfate plus chloride for areas IA—IIIC (table 8) are taken as representative of sodium and sulfate,,respectively. Justification for this shortcut was given in the section “Sources of Sodium and Sulfate Ions in Water of the Lower Carbonate Aquifer.” The sodium or sulfate con- tent of water from two cited wells, on the other hand, pertains to a single cation or anion. Computation of crossflow is based on the following equations (Hem, 1959, p. 231): QICI+Q202=Q303, and Q1+Q2=Q3, where Q1 is the discharge rate of the more saline water, Q2 is the discharge rate of the less saline water, Q3 is the discharge rate of the mixture (that is, Q3=Q1+Q2), and the constants C1, Cz, and Ca represent the concentrations of the tracer constituent (sodium, for example) in the three discharges. Use of this formula at Nevada Test Site rests upon the following assumptions: (1) Sodium and sulfate in water of the lower carbonate aquifer are derived principally by flow from the overlying Tertiary rocks; (2) the cited chemical quality of the ground water from wells 68-69 and 73—66 (tables 11 and 12) is representative of water in the basal part of the tuff aquitard beneath the valleys east and northeast of the Nevada Test Site; (3) sodium and sulfate remain in solu- tion once introduced into the lower carbonate aquifer; and (4) mixing of water from the tuff aquitard with water in the lower carbonate aquifer is complete at Ash Meadows. 0110 In applying the computation to the study area, equations above are combined as follows: = Q3(Ca—Cz) _ (Cl—CZ) Q1 is the downward leakage of semiperched water from the tuff aquitard (the unknown), and Q3 is the measured spring discharge at Ash Meadows, roughly 10,000 gpm. Similarly, C3 represents the tracer concentration in the spring water at Ash Meadows (see table 8); C2 is the same parameter for water within the carbonate aquifer along the periphery of the Ash Meadows basin (an average of the median values listed in table 8 for areas IB and ID); and C1 applies to ground water in the basal part of the tuff aquitard (as represented by water from wells 68~69 and 73—66) (tables 11 and 12). Using sodium as the tracer, with values of 56 (well 68—69), 0.8, and 3.8 meq/l, respectively, for C1, C2, and C3, the computed downward leakage (Q1) is about 550 gpm. The maximum sodium content of the water in well 73—66 is about one-third that in well 68—69 (18 versus 56 meq/l); the lower concentration indicates leakage of about 1,700 gpm. Because the water from well 73—66 is perched and was collected about 1,500 feet above the lower carbonate aquifer, it may not be as representative of water in the basal strata of the tuff aquitard as that collected from well 68—69. Using sulfate as the tracer, with values of 75 (well 68—69), 0.75, and 2.2 meq/l respectively for C1, C2, and Cs, the computed downward leakage is 200 gpm. Water from the tuff aquitard tapped by well 73—66 cannot be used to determine leakage because the sulfate content of this water is about the same as that in the lower car- bonate aquifer ,at Ash Meadows. Finally, using dissolved-solids content as a measure of leakage and 5,420 (well 68—69), 245, and 420 mg/l for C1, C2, and C3, respectively, Q1 was found to be about 350 gpm. If the maximum dissolved-solids content of tuff water from well 73—66 (about 1,000 mg/l) is used instead for C1, then Q1 amounts to about 2,300 gpm. The preceding computations suggest that downward leakage from the tuff aquitard within the intermontane basins of the Ash Meadows ground-water basin into the lower carbonate aquifer is only a few percent to perhaps as much as 20 percent of the discharge at Ash Meadows. Estimates of downward leakage computed using only the analysis of water from well 68—69 are of the same magnitude as estimates obtained from hydraulic data. The aggregate leakage of semiperched water beneath the six valleys is estimated roughly as 150 to 400 acre-feet per year, on the basis of hydraulic data. (See section “Sources of Recharge to the Lower Carbonate Aquifer.”) Leakage computed using the chemical analyses and the salt-dilution formula ranges from 200 to 550 gpm(about Q1 HYDROLOGY OF NUCLEAR TEST SITES 300—900 acre-ft per yr). Thus, both methods, though crude and subject to one or more assumptions, indicate that the semiperched water may contribute only a few percent (1—5) of the ground water in the lower carbonate aquifer. UNDERFLOW FROM PAHRANAGAT VALLEY Underflow from Pahranagat Valley (ID) southwestward into the Ash Meadows ground-water basin through the lower carbonate aquifer is compatible with available chemical data. The spring water in Pahranagat Valley has about one-third as much sodium and potassium as that at Nevada Test Site and Ash Meadows. Similarly, the Pahranagat water has about one-half as much sulfate and chloride as that at Nevada Test Site and Ash Meadows. If movement is southwestward from Pahranagat Valley toward Nevada Test Site, downward crossflow from the tuff aquitard into the carbonate aquifer beneath Desert, northern Three Lakes, and northern Indian Springs Valleys could readily transform the chemical quality of Pahranagat water into that of water in the lower carbonate aquifer beneath eastern Frenchman Flat. A comparison of the deuterium content of ground water in Pahranagat Valley, along the flanks of the Spring Mountains, and at Ash Meadows indicates that about 35 percent of the Ash Meadows discharge could originate from Pahranagat Valley (and possibly Garden and Coal Valleys). This conclusion is from the work of I. J. Winograd and Irving Friedman (1969, 1972). A sum- mary of their work follows.” Winograd and Friedman sampled major low-level springs in Pahranagat Valley, at Ash Meadows, and along the flanks of the Spring Mountains and the Sheep Range. The springs sampled discharged either directly from the lower carbonate aquifer or from valley fill believed to be fed by the carbonate aquifer. Their data are given in table 13. TABLE 13. — Summary of deuterium content of water from major springs, southern Great Basin, Nevada-California [Analyses by US. Geol. Survey, Denver, Colo] Number of Number 6D (permil deviation from S.M.O.W.) Area sources of sampled samples Range Mean Median Standard deviation Pahranagat Valley _____ 3 9 v110 to —115 —113 —113 1 Spring Mountains— Sheep Range - 6 12 —97 to — 106 — 102 _ 102 3 Ash Meadows _ _ 4 15 —99to 7110 ~106 —107 3 The deuterium content of these waters is reported as permil deviations (61)) from S.M.O.W. (Standard Mean Ocean Water). Winograd and Friedman showed that the differences between the means of the three areas is statistically significant at the conservative 0.01 level. Comparing the means, they argued that about 35 per- SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE cent of the Ash Meadows discharge comes from the lower carbonate aquifer beneath Pahranagat Valley rather than from recharge to the Spring Mountains or Sheep Range. However, they pointed out that other inter- pretations of the deuterium data are possible. At this writing, it appears safe only to say that the deuterium data are consistent with the hypothesis (for- mulated on the basis of geologic, hydrologic, and hydrochemical data) that water enters the Ash Meadows ground-water basin from the northeast. GROUND-WATER MOVEMENT IN AMARGOSA DESERT Examination of data for Amargosa Desert on plate 3 provides some evidence on ground-water movement in that valley. This evidence pertains to directions of crossflow in the east-central Amargosa Desert, position and possible effectiveness of the hydraulic barrier responsible for the spring discharge, and sources of water within the central part of the Amargosa Desert. UPWARD CROSSFLOW IN THE EAST-CENTRAL AMARGOSA DESERT The chemical quality of water from wells 17/52—801 and 17/51—1a1 (fig. 34 and pl. 3) in the east-central. Amargosa Desert, in the unnamed valley northeast of the Ash Meadows spring line, suggests direct upward crossflow from the lower carbonate aquifer into the valley fill. Well 17/52—8c1 is 400 feet deep and may tap the lower carbonate aquifer as well as the valley-fill aquifer; well 17/51—1a1 is 135 feet deep and penetrates only valley fill (Walker and Eakin, 1963, table 3). A gravity map of the area indicates that the Paleozoic rocks are probably more than 1,500 feet below land sur- face at well 17/51—1a1. The chemical character of the water from these wells (see area IIIB on pl. 3 and in table 8) closely resembles that of water discharging from the springs at Ash Meadows, but it differs from that of water in valley-fill aquifers in other valleys within the study area. However, the median concentrations of principal ions in the well waters are 10 to 20 percent lower than median values for the same ions in the major springs at Ash Meadows, and the median dissolved-solids content is about 10 percent lower. The difference in absolute ionic content suggests that the upward leakage from the lower carbonate aquifer may have been diluted by ground water of lower dissolved-solids content. Such ground water might well have originated from local recharge; local recharge from runoff is likely in this part of the Amargosa Desert because the depth to water table is very shallow — only 33 feet at well 17/52-8c1 and 60 feet at well 17/51—1a1. High sodium, sulfate, and dissolved-solids contents and low calcium and magnesium contents of water from well 65—66 (pl. 3), relative to wells 17/51—1a1 and 17/52-8c1, are puzzling; they may result from upward C111 leakage, which was forced to pass through a great thickness of tuff aquitard enroute to the valley-fill(?) aquifer. SOURCES OF WATER IN THE CENTRAL AMARGOSA DESERT. The chemical quality of ground water in the valley-fill aquifer varies greatly from place to place in the central Amargosa Desert (area IV, pl. 3) and thereby contrasts with the more uniform chemical quality of water in the aquifer in surrounding areas. Water belonging to three of the four hydrochemical facies of table 8, as well as to the playa facies, is found in this area (pl. 3); only the calcium magnesium bicarbonate facies is absent. Some of the water in the area west of the Ash Meadows spring line is of the calcium magnesium sodium bicarbonate facies. Water in the area between well 74—57 in western Jackass Flats and well 16/48—23b1, northwest of the T and T Ranch, is of the sodium and potassium bicar- bonate facies. Pie diagrams of water from two wells in the west-central and the northwestern parts of the valley (wells 13/47—35a and 16/48—17a1) resemble the pie diagrams of the sodium sulfate bicarbonate facies found in Death Valley (area VI). Finally, water from shallow wells in the Death Valley Junction area is of the playa facies; these wells are along the periphery of Alkali Flat, where the depth to water ranges from 0 to 5 feet. Water from one spring and one well does not fit this general geographic distribution. Water from Ash Tree Spring (17/49—35d1), for example, is of the sodium and potassium bicarbonate facies, which differs from sur- rounding sources. Water from well 17/50—29d1, west of the inferred hydraulic barrier (fig. 34), is of the playa facies. The pattern just described indicates that ground water in the central Amargosa Desert is probably derived from at least three sources. Water of the calcium magnesium sodium bicarbonate facies most likely comes from flow across the hydraulic barrier responsible for the spring line at Ash Meadows. Water of the sodium potassium bicarbonate facies southwest of Lathrop Wells probably comes from western Jackass Flats, and water in the west-central and northwestern Amargosa Desert probably comes from Oasis Valley. Thus, the pie diagrams of plate 3 suggest that water enters the central Amargosa Desert from the east, north, and northwest. Because of the diversity of source areas for the ground water and uncertainties about the aquifers tapped by the deeper wells in the central Amargosa Desert, no attempt was made to define this water statistically, as was done for the other populations in table 8. Several other hydrologic inferences regarding the cen- tral Amargosa Desert may be made from the available chemical analyses, however: 1. The dissolved-solids content of water from wells C112 74—57, 73—58, 15/49—22a1, 16/49—901, 16/48-15a1, and 16/48—23b1 is significantly less than that of water from other wells in the desert (pl. 3). In fact, the first four wells yield the most dilute water sampled in the study area, except for southeastern Pahute Mesa. The six wells cited are along For- tymile Wash or its distributaries. Timber Moun- tain and Pahute Mesa highlands are tributary to this prominent arroyo via Fortymile Canyon (fig. 1). The location of the cited wells along this drainage suggests that the low dissolved-solids con- tent may reflect recharge primarily via infiltration along the arroyo bed rather than underflow from areas north of Jackass Flats. Such infiltration may not have been a significant source of recharge since the last pluvial. 2. The low mineralization of the water tapped by wells 15/49—22a1 and 16/49—901 (west and southwest of Lathrop Wells) contrasts sharply with the mineralization of the water from wells 15/50—1805 and 16/50—7c1 (at and south of Lathrop Wells). This contrast, suggesting that little ground water is moving westward through the valley-fill deposits in the area, is supported by water-level and geologic data presented in the section “Possible Upward Leakage South of Lathrop Wells.” 3. Chemical quality of water from well 17/50—29d1 resembles that of the playa facies despite the well’s proximity to the spring line, about 3 miles, and its relatively great depth, 530 feet. Water closely resembling the spring discharge would have been expected from a well of this depth and location. The well was flowing about 5 gpm when sampled in 1962 (Walker and Eakin, 1963, table 3); at time of sampling the bore was open to a depth of 471 feet. The driller’s log of this hole indicates alternating clay and limestone strata to a depth of 245 feet, and sand, gravel, and clay between 245 and 530 feet. Evidence that the aquifer(s) tapped by well 17/50—29d1 is poorly connected to the lower car- bonate aquifer was provided by a short pumping test. Throughout a 4-hour pumping test the water temperature remained at 19.5°C (R. H. Johnston, written commun., Mar. 1967), about 8.3°C lower than that in the lower carbonate aquifer, 3.5°C lower than that of the coolest major spring (Soda Spring; see fig. 35), and only 1.0°C above the mean annual temperature at Lathrop Wells. The test also showed that the gravel aquifer(s) was cut by at least one negative hydraulic boundary (R. H. Johnston, written commun., Mar. 1967). The low temperature, in particular, indicates that the water tapped by the well does not come directly from the lower carbonate aquifer. The anomalous chemical HYDROLOGY OF NUCLEAR TEST SITES character of water from well 17/50—29d1 may reflect slow movement of water through Cenozoic aquitards. POSSIBLE SOURCE OF SPRING DISCHARGE AT FURNACE CREEK WASH—NEVARES SPRINGS AREA, DEATH VALLEY Hunt, Robinson, Bowles, and Washburn (1966) con- cluded that the water discharging from the major springs in the Furnace Creek Wash-Nevares Springs area is derived from Pahrump Valley. They stated (p. B40): However, the composition of the water discharging in Death Valley differs in detail from that in the valley fill above Eagle Mountain * * * but it is much like the water at the Ash Meadows springs, which suggests that the water discharging in Death Valley has had the same history as that discharging at Ash Meadows. Probably the water dis- charging at the springs in Death Valley, like that at Ash Meadows, is derived directly from Pahrump Valley by movement along faults in the bedrock under _the valley fill. Plate 3, and table 8 indicate that the spring discharge in the Furnace Creek Wash—Nevares Springs area differs markedly in chemical quality from the water in Pahrump Valley and to a lesser, but significant, extent from spring discharge at Ash Meadows. Such differences alone do not preclude the movement postulated by Hunt, Robinson, Bowles, and Washburn (1966), because the chemical quality of the water may change enroute to Death Valley. However, the available chemical analyses suggest a more likely source for the Furnace Creek Wash-Nevares Springs water than ground water in Pahrump Valley. Figure 38 suggests that the Death Valley water may be a mixture (with addition of sulfate and chloride) of water from Oasis Valley (area IIG) and Ash Meadows (area IIIA). Plate 3 suggests that the water may be closely related to that in valley fill beneath the west side of the Amargosa Desert. (See pie diagrams for wells 13/47—35a, 16/48—17a1, and 27N/4—27b2.) Therefore, on the basis of chemical quality of water, the spring discharge at the Furnace Creek Wash—Nevares Springs area seems to come from water in the valley-fill aquifer of the central and northwestern Amargosa Desert rather than directly from Pahrump Valley or from Pahrump Valley via Ash Meadows. Whether water in the valley-fill aquifer can enter the lower carbonate aquifer, and thereby reach Death Valley, cannot, however, be evaluated until head relations between these aquifers are determined for the central and south-central Amargosa Desert. SUMMARY OF HYDROCHEMICAL EVIDENCE ON REGIONAL MOVEMENT OF GROUND WATER Major inferences pertinent to the ground-water regimen, made largely on the basis of hydrochemical variations, are as follows: 1. Ground water beneath Nevada Test Site moves towards the Ash Meadows area. SOUTH—CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE 2. Chemical quality of the water within the lower car- bonate aquifer may not change markedly with depth. Potable water may be present to depths as great as 10,000 feet in the Ash Meadows ground- water basin. 3. Sulfate and sodium contents in ground water in the tuff aquitard beneath the Nevada Test Site in- crease with depth, at least locally. 4. Ground water within the tuff aquitard drains into the underlying lower carbonate aquifer beneath Nevada Test Site and possibly also beneath the valleys east and northeast of Nevada Test Site. 5. Leakage of water from the tuff aquitard into the lower carbonate aquifer is probably less than 5 percent of the water discharged at Ash Meadows. 6. Ground water may move into the Ash Meadows basin from Pahranagat Valley and if so, may constitute as much as 35 percent of the spring discharge at Ash Meadows. 7. Ground-water movement from Pahrump or Stewart Valleys into the Ash Meadows area is minor. 8. Ground water within the central Amargosa Desert comes from the east, the north, and the northwest. 9. Flow from the central Amargosa Desert into Death Valley is the most likely source of the major spring discharge in east-central Death Valley. GROUND-WATER VELOCITY A major objective of the study of the hydrogeology of Nevada Test Site and vicinity was a determination of the ground-water velocity in the principal aquifers and aquitards. Velocity was determined for ground water in the tuff aquitard in Yucca Flat and for water in the lower carbonate aquifer in central Yucca Flat and beneath the Specter Range. Velocity was not estimated for ground water in the welded-tuff and valley-fill aquifers because in Yucca and Frenchman Flats the movement of water through these rocks is controlled by the surrounding and underlying tuff aquitard. Ground-water velocity in the tuff aquitard was deter- mined using the equation PI V= —, 7.48p (1) where V is velocity, in feet per day; P is interstitial permeability, in gallons per day per square foot; I is the hydraulic gradient, in feet per foot; and p is interstitial effective porosity expressed as a decimal. (A correction for the effect of temperature on permeability may also be incorporated into the formula but is not necessary for es- timating velocity at Nevada Test Site. The possible variations in P, I, and p far exceed corrections for "0113 temperature.) Values for P, p, and I were obtained from tables 5 and 6. Ground-water velocity in the lower carbonate aquifer was determined using the equation Q=Apv, (2) where Q is underflow, in cubic feet per day; A is the cross-sectional area of flow, in square feet; p is effective fracture porosity, expressed as a decimal; and v is velocity, in feet per day. Values for Q, A, and p were ob- tained from data in this report or were estimated. The velocity determinations are crude in that they vary by two to three orders of magnitude at each of the three places where velocity was computed. Nevertheless, the determinations are still valuable (for example, for first approximation of the movement of certain radionuclides) because of the major differences between the velocity in the tuff aquitard beneath Yucca Flat, in the lower carbonate aquifer beneath central Yucca Flat, and in the lower carbonate aquifer beneath the Specter Range. VELOCITY 0F MOVEMENT FROM THE TUFF AQUITARD INTO THE LOWER CARBONATE AQUIFER IN YUCCA FLAT Ground water in the tuff aquitard beneath Yucca Flat is semiperched above the lower carbonate aquifer and is slowly draining into the deeper aquifer through the clayey tuff at the base of the aquitard or by movement . toward a centrally located hydraulic sink. Movement of water in the aquitard by either method is probably con- trolled by interstitial permeability. , The estimated average velocities are given in table 14, which shows flow rates for two limiting conditions and for an intermediate condition. For first condition, where P=0.00005 gpd per sq ft and p=10 percent (or 0.10 for computations), the interstitial permeability of the clayey tuff exerts principal control on the rate of downward movement. This condition results in a minimum value of velocity. For the third condition, Where P=0.005 gpd per sq ft and p=30 percent, the in- terstitial permeability of zeolitized tuff controls downward movement of water and results in maximum velocity values. These permeability and porosity values approximate the median values determined from laboratory analyses of cores of zeolitized and clayey tuff and clayey sediments. (See table 5.) The limiting values of the vertical hydraulic gradient for each condition are taken from table 6. The average hydraulic gradient within the clayey tuff at the base of the aquitard may be greater than the 0.2 foot per foot, but even a threefold in- crease in gradient does not produce a significant in- crease in average velocity. The significance of the computations in table 14 is that the average velocities of downward movement are 0114 HYDROLOGY OF NUCLEAR TEST SITES TABLE 14. —— Estimated ground-water velocity in tuff aquitard, Yucca Flat, Nye County Interstitial Effective Hydraulic Average vertical Years for Volume of permeability interstitial gradient velocity water downward (gpd per porosity (ft to move leakage sq ft) (percent) per ft) Feet Feet 1 000 (acre-ft (P) (p) (I) per day per year ft per yr)l 0.00005 10 0.02 1><10‘6 5X10“ 2X106 2 .2 1><10‘5 5><10‘3 2><105 20 .0005 20 .02 7><10‘6 2X10“3 4X105 20 .2 7X10'5 2><10‘2 4x104 200 .005 30 .02 4><10‘5 2><10’2 6X10“ 200 .2 4X10“ 2><10‘1 6X103 2,000 Assumes vertical crossflow through area of 66 sq mi (about 18><103 sq ft). extremely small regardless of input data. Estimated ver- tical velocities range from 5X10‘4 to 0.2 foot per year. Most of the data in the table represent permeability measured approximately parallel to bedding, which is generally greater than vertical permeability. If data on vertical permeability were available, flow rates in the table would probably be somewhat smaller than shown. The average vertical velocities in table 14 are reduced significantly if the water in the tuff aquitard is assumed to move laterally toward a sink in the center of the valley from which it then moves vertically into the lower car- bonate aquifer. (See section “Intrabasin Movement.”) The average horizontal component of hydraulic gradient in the tuff aquitard ranges from 1/300 . to 1/§ of the ver- tical gradient. (See table 6.) The average velocities given in the table would be reduced proportionately if move- ment were predominantly horizontal. Velocities in table 14 may be used to obtain rough values of the age of ground water within the tuff aquitard. The saturated thickness of the tuff aquitard ranges from a few to as much as 1,500 feet. Assuming an average saturated thickness of 1,000 feet, the average time needed for a water particle to move from the top to the bottom of the tuff aquitard is about 6,000 to 2,000,000 years; if lateral movement to a centrally located hydraulic sink is assumed,.considerably older ages are indicated, at least for the water in the lower parts of the aquitard. Because the head differential between the tuff aquitard and the lower carbonate aquifer probably was greater during and immediately after the close of the last pluvial period (about 9,000 yr ago), ground water near the base of the tuff aquitard to- day probably moved through higher parts of the aquitard in less time than present hydraulic gradients suggest. A carbon-14 date of water from the valley—fill aquifer in Frenchman Flat suggests that the age of the ground water in the tuff aquitard is probably in the range of several tens of thousands of years. This water, from well 74—70a (fig. 31), is about 13,000 years old (Grove and others, 1969). Accordingly, water in the tuff aquitard beneath this well site is probably at least several tens of thousands of years o1d; an age of several hundred thou- sand years is not beyond imagination for waters near the base of the aquitard in Yucca or Frenchman Flats. VELOCITY WITHIN THE LOWER CARBONATE AQUIFER BENEATH CENTRAL YUCCA FLAT Average ground-water velocity within the lower car- bonate aquifer beneath central Yucca Flat was es- timated through use of the formula Q=Apv. The formula requires an estimate of the flow through the aquifer (Q, in cubic feet per day), the cross-sectional area of flow (A, in square feet), and the effective fracture porosity (p, as a decimal). Of the variables, the volume (Q) and the cross-sectional area (A) of flow are reasonably well known. However, the magnitude of the effective fracture porosity (p) is difficult to estimate. It may be as large as 1 or 2 percent, a value suggested by examination of out- crops of the carbonate aquifer, or conceivably as low as 0.01 percent, suggested by the sparseness of water- bearing fractures penetrated in test holes. Thus, a varia- tion of two orders of magnitude in effective fracture porosity is possible. In contrast, estimates of the volume of flow and the cross-sectional area of flow are probably in error by less than a factbr of 2 or 3. The area of underflow (A) used in the computation of velocity across central Yucca Flat was assumed to be 10 ’miles long and 5,000 feet thick. The fence diagram for Yucca Flat (pl. 2A). and cross sections by Harley Barnes (unpub. data) suggest that the underflow area may be as much as 11/2 to 2 times as large as the assumed area but refinement of the area of underflow is not warranted because of the potentially large variation in effective porosity. The quantity of water moving through the lower car- bonate aquifer beneath central Yucca Flat probably is less than 350 acre-feet per year (see discussion in section “Quantity Derived From Northwest Side of Basin,”) and is considered equivalent to the sum of (1) downward leakage from the tuff aquitard; (2) underflow into the aquifer from east and west through the lower and upper clastic aquitards; and (3) possible underflow into the aquifer from the west through the lower carbonate aquifer, which underlies the upper clastic aquitard. Estimates of the velocity in the lower carbonate aquifer beneath central Yucca Flat for the cited values for Q and A are summarized in table 15. The estimated velocity ranges from 0.02 to 2.0 feet per day, or from 6 to 600 feet per year. The variation of two orders of SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE magnitude reflects the uncertainty of the magnitude of effective fracture porosity. The velocity estimates for each assumed porosity are maximum values, because the cross-sectional area used was a minimum area and because only half of the 350 acre-feet is likely to have originated north of the centrally located section of un- derflow. The velocities cited in the table are average velocities, and locally the velocity may exceed the average by several times. Marked areal variations in fracture transmissibility of the aquifer are expected, as is in- dicated by the spread of three orders of magnitude in values of fracture transmissibility (table 3). Discussion of the possibility of above-average velocities on a regional scale is presented in a following section of the report. VELOCITY WITHIN THE LOWER CARBONATE AQUIFER BENEATH SPECTER RANGE An estimate of probable upper limits of ground-water velocity Within the lower carbonate aquifer in the study area is obtained by computing the average flow rate through the Specter Range. Such flow rates are of value in evaluating those estimated for the lower carbonate aquifer in Yucca Flat. The cross-sectional area of flow is assumed to be 5 miles long and 5,000 feet thick. Flow through the cross section is considered equivalent to the measured dis- charge at Ash Meadows, about 17,000 acre-feet per year. The average porosity is assumed to range from 0.01 to 1 percent; these values probably satisfactorily bracket the possible range in effective fracture porosity of this aquifer. Ground-water velocity ranges from 2 to 200 feet per day. (See table 15.) The spread of two orders of magnitude in values of average velocity is due to uncer- tainty of the average fracture porosity. The average-velocity estimates for the Specter Range area are considered to be probable upper limits of velocities for the lower carbonate aquifer, because beneath the Specter Range virtually the entire discharge of the Ash Meadows basin passes through a minimum underflow area. The possible errors in Q and A are probably less than 100 percent. Elsewhere in the flow system, the ground-water velocity must be considerably smaller either because Q is markedly smaller, as in Yuc- ca Flat, or because A is much larger, as it probably is beneath Frenchman Flat. The estimated average velocities beneath the Specter Range are about 100 times larger than those for the lower carbonate aquifer beneath central Yucca Flat. This difference principally reflects the differences in values of Q. The velocity within the lower carbonate aquifer beneath Yucca Flat is about 103 to 104 times greater than the estimated velocity within the tuff aquitard (table 14). C115 TABLE 15. —— Estimated ground-water velocity in lower carbonate aquifer beneath central Yucca Flat and Specter Range, Nye County Assumed average Average velocity Area effective Years for water fracture Feet Feet to travel 1 mile porosity per day per year (percent) Central 1.0 0.02 6 900 Yucca .5 .03 10 500 Flat. .1 .2 60 90 .05 .3 100 50 .01 2.0 600 9 Specter 1 .O 2 600 9 Range. .5 3 1,000 5 .1 20 6,000 .9 .05 30 10,000 .5 .01 200 60,000 .09 EVIDENCE BEARING ON POSSIBLE FLOW THROUGH INTEGRATED SOLUTION CHANNELS OR HIGHLY PERMEABLE FRACTURE ZONES OF REGIONAL EXTENT Three prominent solution caverns in or near the study area — Devils Hole, Gypsum Cave, and Worthington Cave — and the widespread development of caverns beneath other carbonate—rock terrane raise the possibility that a regionally integrated cavern system may exist beneath the study area. Such a system could readily result in flow velocities an order of magnitude higher than the average velocities cited for the Specter Range. Similarly, if zones of above-average fracture . transmissibility are well connected regionally, the average flow rates cited would not be of value for current and future radiologic-safety appraisals of Nevada Test Site. These two possibilities are examined below. Outcrop observations and drill-hole data for the Nevada Test Site and vicinity, presented in the section “Lower Carbonate Aquifer,” indicate that solution caverns probably are not important controls of the regional movement of ground water. Furthermore, if an undetected integrated system of solution channels ex- ists, it must have formed after the cessation of the block faulting (since late(?) Pliocene or early Pleistocene), which would have dismembered channels formed earlier and probably brought some to view. The following suggest that development of extensive solution networks in the study area since the late Pliocene or early Pleistocene is unlikely: 1. The climate of most of the study area may have been semiarid during part or all of the Pleistocene. Much of the present relief of the Sierra Nevada developed during Pleistocene time, but some may have developed before early Pliocene (Christiansen, ‘ 1966, p. 177). Therefore, the study area probably ' was in the rain shadow of the Sierra Nevada at least during a part of the Pleistocene. Development of soils rich in organic matter, the major source of carbon dioxide in ground water, was probably marginal during this time except on the Spring Mountains and Sheep Range. C116 2. If major cavern development is restricted to the zone immediately beneath a water table, as suggested by recent work (Davies, 1960; White, 1960; G. W. Moore, 1966a,b; and Bedinger, 1966), then solution is significant only beneath those areas where the lower carbonate aquifer is under water-table con- ditions — namely, beneath the ridges. Solution would be unlikely where the aquifer is buried by tens to hundreds of feet of saturated Tertiary rocks. In the Allegheny Plateau, Davies (1960) noted that “numerous deep oil wells that have penetrated the otherwise cavernous Mississippian and Devonian limestones have not encountered any significant cavern openings where the cover over the limestone is more than 100 feet thick and the site of the well is away from major valleys.” Solution in the zone of saturation may not even occur in southern Nevada, even beneath areas where the aquifer is uncon- fined, because the available carbon dioxide might be completely used in dissolving carbonate rocks in the vadose zone, hundreds to more than 1,500 feet thick. 3. Except near major discharge areas, solution caverns formed at or near the water table during any of the pluvials would now be several feet to possibly tens of feet above the water table. 4. Finally, even in humid areas most explored caverns are only a few thousand feet long. The passages in the largest of the caverns aggregate several miles in length, but most caverns are areally small features. Moreover, most of the major caverns occur along' large valleys; the passages in the major caverns decrease in size, though they are more numerous, in the part of the cavern away from major valleys (Moore, 1960, 1966a). In conclusion, some solution caverns are found within the carbonate aquifer, and, undoubtedly, others will be discovered by drilling, particularly near major discharge areas. However, the probability for regionally integrated solution channels through which ground water flows at high velocities over distances of miles appears low, ex- cept in the immediate vicinity of the major discharge area and possibly in the vicinity of major hydraulic barriers. If regionally integrated zones of above-average frac- ture transmissibility exist, they also probably formed after the middle to late Tertiary block faulting. Outcrop, drill-hole, and potentiometric data and geologic map- ping suggest that klippen (erosional remnants of overthrust sheets, or gravity slump blocks) and rocks in areas of intense structural deformation (that is, the Specter Range) have above-average fracture transmissibility. Most of the deformation of the Paleozoic carbonate rocks occurred during the Late Cretaceous and early Tertiary orogeny. The overthrust HYDROLOGY OF NUCLEAR TEST SITES sheets and other structures in the Paleozoic rocks poten- tially favoring high transmissibility are now discon- tinuous due to (1) the deep erosion of the carbonate rocks postdating the orogeny and (2) the repeated offset of these rocks by middle to late(?) Tertiary block faulting. Thus, if zones of above-average fracture transmissibility are to be continuous over distances of miles, avenues of above-average transmissibility must exist along the Ter- tiary fault planes. Possible evidence against such hydraulic connection is suggested by the presence of one or more hydraulic barriers in two-thirds of the wells test pumped. In summary, the authors are not claiming that all zones of above-average fracture transmissibility are isolated from each other, that individual or combined zones may not extend for thousands of feet, or that the rocks in some areas, such as the Specter Range, are not more transmissible than rocks in other areas, such as Yucca Flat. They believe that zones of above-average fracture transmissibility are probably separated by zones of average and below-average transmissibility and, therefore, that the continuity of zones of above-average transmissibility over distances of tens of miles is im- probable. RECOMMENDATIONS FOR FURTHER STUDY Because of the geologic complexity of the region, the drilling of tens of additional deep test holes, although it would increase the qualitative knowledge of the system, might not result in a quantitative understanding of the hydrogeology of the area. However, the following three steps are suggested as a modest means of testing and adding to existing knowledge when and if a reevaluation of the hydrogeology of the area is required. 1. Study of the isotopic content of water from all wells and major springs in the study area, including the large springs in Pahranagat, Pahrump, and Death Valleys. The isotopes of importance are C“, C13, H3, D, 018, and, possibly, S34. Water from the base of the tuff aquitard should also be sampled and dated wherever possible, in order to evaluate the in- fluence of the downward leakage on the isotopic content of the water in the lower carbonate aquifer. 2. Drilling of three additional test holes through the en- tire thickness of the lower carbonate aquifer to determine changes in character, frequency, permeability, head, and water chemistry of the water-bearing fractures with depth. One hole should be drilled near the center of the major trough in the potentiometric surface (pl. 1) either immediately northeast or southwest of the Specter Range. A second hole should be drilled in northern Desert Valley in the vicinity of the southernmost dry hole shown on plate 1. The third well should be SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE drilled in the narrow valley between southern Sheep Range and southern Desert Range, at about the latitude of Sheep Peak. Downhole photography in the first hole might aid in an evaluation of the fracture porosity and therefore in the computation of average velocity beneath the Specter Range, whereas the head and hydrochemical data from the second and third holes might help to better define the eastern and northern boundaries of the Ash Meadows ground-water basin. Hydraulic characteristics and water chemistry of the Cenozoic aquifers and aquitard should also be determined in the three proposed holes, if they are penetrated within the zone of saturation. Represen- tative samples of water from the base of the tuff aquitard will be difficult to obtain because of the very low permeability of the rocks, but such samples are of major importance for estimating downward crossflow from the tuff aquitard into the carbonate aquifer. 3. Cuttings and water samples should be collected, and water level and depth of well should be recorded routinely for all new wells drilled along the periphery of Nevada Test Site. Collection of such data is relatively inexpensive, and some of it may help to better define the regional hydrogeology. SUMMARY The Nevada Test Site, a US. Atomic Energy Com- mission nuclear testing facility encompassing an area of about 1,400 square miles, has been the site of a detailed study of ground-water geology and hydrology. The test site lies within the miogeosynclinal belt of the COI- dilleran geosyncline, where 37,000 feet of marine sediments accumulated during the Precambrian and Paleozoic Eras, and within a Tertiary volcanic province where as much as 13,000 feet of rocks were erupted from caldera centers. Except for a few small intrusive masses, Mesozoic rocks are absent. Quaternary and Tertiary detritus as much as 2,000 feet thick underlies the valleys. The region has experienced two major periods of defor- mation. The first, in late Mesozoic and early Cenozoic time, resulted in both broad and tight folds and thrust faults in Precambrian and Paleozoic rocks. During mid- dle to late Cenozoic time, block faulting produced the Basin and Range structure of the region. Displacements along major strike-slip faults measured several miles during both periods of deformation. Precambrian to Middle Cambrian strata are predominantly quartzite and siltstone 10,000 feet thick. The Middle Cambrian to Upper Devonian strata are chiefly limestone and dolomite 15,000 feet thick. Upper Devonian and Mississippian rocks are chiefly argillite and quartzite about 8,000 feet thick. Pennsylvanian and C117 Permian rocks are chiefly limestone about 4,000 feet thick. No major unconformities or disconformities marked by deep subaerial erosion of underlying rocks oc- cur within this miogeosynclinal section. The Tertiary volcanic rocks are ash-flow tuff, ash-fall tuff, rhyolite, rhyodacite, and basalt; the tuffs are com- monly of rhyolitic and quartz-latitic composition. Sedimentary rocks associated with the volcanic strata include conglomerate, tuffaceous sandstone and siltstone, calcareous lacustrine tuff, claystone, and freshwater limestone. The Tertiary rocks are largely of Miocene and Pliocene age, although Oligocene rocks are present. Extent, thickness, and physical properties of the Tertiary rocks vary widely within and between the intermontane valleys. Precambrian and Paleozoic miogeosynclinal rocks, Tertiary volcanic and sedimentary rocks, and Quater- nary and Tertiary valley fill are grouped into 10 hydrogeologic units. The grouping is based on similar hydraulic properties, lithologic character, and stratigraphic position. The hydrogeologic units, in order of decreasing age, are: Lower clastic aquitard, lower car- bonate aquifer, upper clastic aquitard, upper carbonate aquifer, tuff aquitard, lava-flow aquitard, bedded-tuff aquifer, welded-tuff aquifer, lava-flow aquifer, and valley-fill aquifer. The lower clastic aquitard, the lower carbonate aquifer, and the tuff aquitard control the regional movement of ground water. The coefficient of transmissibility of the lower clastic aquitard is less than 1,000 gpd per ft. The effective in- terstitial porosity of 20 cores ranges from 0.6 to 5 percent and has a median value of 1.9 percent. The coefficient of permeability of 18 cores ranges from 0.0000007 to 0.0001 gpd per sq ft and has a median value of 0.000002. Although the elastic strata are highly fractured throughout the study area, regional movement of water through these rocks is probably controlled principally by interstitial permeability rather than fracture transmissibility because (1) the argillaceous strata have a tendency to deform plastically, (2) fractures in the brittle quartzite sequences tend to be sealed by in- terbedded micaceous partings or argillaceous laminae, and (3) the clastic rocks have low solubility. The highly fractured to locally brecciated lower car- bonate aquifer consists of limestone and dolomite. On the basis of pumping tests of 10 wells and regional flow analysis, the coefficient of transmissibility of the aquifer ranges from 600 to several million gallons per day per foot. Core examination indicates a fracture porosity of a fraction of 1 percent. The effective intercrystalline porosity of 25 cores ranges from 0.0 to 9.0 per- cent and has a median value of 1.1 percent. The coef- ficient of permeability of 13 cores ranges from 0.00002 to 0.1 gpd per sq ft and has a median value of 0.00008. The water-bearing fractures are probably solution- 0118 modified joints, fault zones, and breccia. Drill-stem tests in eight holes suggest that the water-bearing fractures are few and widely spaced, are present to depths of at least 1,500 feet beneath the top of the aquifer and as much as 4,200 feet below land surface, do not increase or decrease with depth, and are no more abundant or permeable immediately beneath the Tertiary—pre- Tertiary unconformity than elsewhere in the aquifer. The lower carbonate aquifer contains several solution caverns in outcrop. One of the caverns, Devils Hole, reportedly extends at least 300 feet vertically into the zone of saturation. The caverns probably do not con- stitute a hydraulically integrated network of solution openings, except possibly near major discharge areas; variations in fracture transmissibility control the regional movement of ground water through the aquifer. The u'pper clastic aquitard consists principally of argillite (about two-thirds of unit) and quartzite (about one-third of unit). Unlike the lower clastic aquitard and the lower carbonate aquifer, which are thousands of feet thick, of relatively uniform lithology, and widely dis- tributed, the upper clastic aquitard is of hydrologic significance only beneath western Yucca Flat and northern Jackass Flats. In much of the area, it is represented by time-equivalent carbonate rocks, has been removed by erosion, or occurs well above the water table. The coefficient of transmissibility of this aquitard is probably less than 500 gpd per ft; interstitial permeability is negligible. The tuff aquitard consists primarily of nonwelded ash- flow tuff, ash-fall (or bedded) tuff, tuff breccia, tuf- faceous sandstone and siltstone, claystone, and fresh- water limestone. Despite the widely differing origins of these strata, they generally have one feature in common: their matrices consist principally of zeolite or clay minerals, which are responsible in part for the very low interstitial permeability of these relatively porous rocks. Strata composing the aquitard have moderate to high in- terstitial effective porosity (median values ranging from 10 to 39 percent), negligible coefficient of permeability (median values ranging from 0.00006 to 0.006 gpd per sq ft), and very low coefficient of transmissibility (less than 200 gpd per ft). Evidence from several miles of tunnels indicates that the regional movement of ground water through the tuff aquitard is probably controlled by in- terstitial permeability rather than by fracture transmissibility. The welded-tuff aquifer consists of moderately to densely welded ash-flow tuff. The coefficient of transmissibility of the aquifer at four well sites ranges from 200 to more than 100,000 gpd per ft and is probably controlled principally by interconnected primary (cool- ing) and secondary joints; interstitial permeability is negligible. The valley-fill aquifer consists of alluvial-fan, HYDROLOGY OF NUCLEAR TEST SITES mudflow, fluvial deposits, and lake beds. The coefficient of transmissibility of the valley-fill aquifer at six well sites ranges from 800 to about 34,000 gpd per ft; average interstitial permeabilities range from 5 to 70 gpd per sq ft. Owing to the complex structural and erosional history of .the area, the subsurface distribution and the saturated thickness of the hydrogeologic units differ from unit to unit and place to place. The structural relief on the pre-Tertiary hydrogeologic units commonly ranges from 2,000 to 6,000 feet within distances of a few miles and locally is as much as 500 feet within distances of 1,000 feet. Thus, the lower carbonate aquifer, which is generally buried and fully saturated at depths of hun- dreds to thousands of feet below most valley floors, is only partly saturated along flanking ridges. In contrast, in areas where the lower elastic aquitard occurs in struc- turally high positions, the lower carbonate aquifer either has been largely removed by erosion or occurs entirely, or largely, above the zone of saturation. Also, because of complex pre-early Tertiary deformation and deep ero- sion, only a fraction of the 15,000-foot aggregate thickness of the carbonate aquifer is usually present in the zone of saturation. In general, because of its great thickness, several thousand feet of the lower carbonate aquifer lies within the zone of saturation beneath most ridges and valleys of the study area. Vertical displacement, ranging from hundreds to thousands of feet along block faults, affects the subsur- face disposition and saturated thicknesses of the tuff aquitard and the welded-tuff and valley-fill aquifers. Beneath valleys that have deep (700—1,900 ft) water tables, the depth to water also affects the saturated thickness of the tuff aquitard and the welded-tuff and valley-fill aquifers. In Yucca Flat, the valley-fill aquifer is saturated only beneath a 10-square mile area where the aquifer thickness exceeds 1,600 feet. Similarly, the welded-tuff aquifer is only partly to fully saturated beneath the central part of that valley; it is unsaturated beneath margins of the valley, even though it is buried at depths of hundreds of feet. Both intrabasin and interbasin movement of ground water occurs in the region. Intrabasin movement of ground water from welded-tuff and valley-fill aquifers to the lower carbonate aquifer occurs beneath several of the intermontane valleys of the study area. The volume of flow between the Cenozoic hydrogeologic units and the lower carbonate aquifer is usually small, because the aquifers are separated by the thick and widespread tuff aquitard. In Yucca and Frenchman Flats, water leaks downward at a rate less than 100 acre-feet per year in each valley. In east-central Amargosa Desert and on the upgradient side of major hydraulic barriers cutting the lower carbonate aquifer, intrabasin movement is upward from the lower carbonate aquifer into the younger SOUTH-CENTRAL GREAT BASIN, NEVADA-CALIFORNIA; NEVADA TEST SITE hydrogeologic units. Interbasin movement characterizes ' flow through the lower carbonate aquifer underlying most of the valleys and ridges of south-central Nevada. Within the Nevada Test Site, water moves south and southwestward beneath Yucca and Frenchman Flats, Mercury Valley, and the east-central Amargosa Desert toward a major spring discharge area, Ash Meadows, in the Amargosa Desert. The hydraulic gradient ranges from 0.3 to 5.9 feet per mile. Interbasin movement through the carbonate rocks is significantly controlled by geologic structure. In the vicinity of major structures, the lower carbonate aquifer is compartmentalized either through its juxtaposition against the lower or upper elastic aquitard along major normal or thrust faults, by the occurrence of the lower clastic aquitard in struc- turally high position along major anticlines, or by gouge developed along major strike-slip faults. The water levels in the lower carbonate aquifer on opposite sides of such structures differ as much as 500 feet in a single valley and as 'much as 2,000 feet between valleys, although the hydraulic gradient within each aquifer compartment or block is only a few feet per mile. Hydraulic, geologic, isohyetal, hydrochemical, and isotopic data suggest that the area hydraulically in- tegrated by interbasin water movement in the lower car- bonate aquifer is no smaller than 4,500 square miles and includes at least 10 intermontane valleys. This hydrologic system, the Ash Meadows ground—water basin, may, in turn, be hydraulically connected to several intermontane valleys northeast of the study area from which it may receive significant underflow. The principal discharge from the basin, about 17,000 acre- feet annually (about 10,600 gpm) occurs along a promi- nent fault-controlled spring line 10 miles long at Ash Meadows. The discharge of individual springs is as much as 2,800 gpm. Underflow beneath the spring line into the central Amargosa Desert is probable, but its magnitude cannot be estimated. Pahrump and Stewart Valleys, proposed as the major source of the spring discharge at Ash Meadows by earlier workers, contribute at most a few percent of the discharge. The major springs in east- central Death Valley (Furnace Creek Wash—Nevares Springs area) are probably fed by interbasin movement of water from central and south-central Amargosa Desert, but not from Pahrump Valley. Five hydrochemical facies of ground water in and adja- cent to the study area have been distinguished by percentages of major cations and anions. Ground water that has moved only through the lower carbonate aquifer or through valley fill rich in carbonate detritus is a calcium magnesium bicarbonate type. Water that has moved only through rhyolitic tuff or lava-flow terrane, or through valley-fill deposits rich in volcanic detritus, is a sodium potassium bicarbonate type. Water in the lower carbonate aquifer, in areas of downward crossflow from C119 the Cenozoic aquifers and aquitards, is a mixture of these two types and is designated the calcium magnesium sodium bicarbonate type. It is characterized by about equal quantities of the cation pairs calcium plus magnesium and sodium plus potassium. Water in east-central Death Valley, probably a mixture of water of the third type and water from Oasis Valley, is a sodium sulfate bicarbonate type. Shallow ground water, such as that beneath saturated playas, is informally designated as the playa type. The chemistry of this water (dissolved-solids content as high as 50,000 mg/l) varies widely and depends in part onthe depth of the sampling point. Major inferences pertinent to the ground water regimen, made on the basis of hydrochemical data, are as follows: 1. Ground water beneath the Nevada Test Site moves towards the Ash Meadows area. 2. Chemistry of water in the lower carbonate aquifer may not change markedly to depths as great as 10,000 feet. ‘ 3. Leakage of water from the tuff aquitard into the lower carbonate aquifer is probably less than 5 percent of the spring discharge at Ash Meadows. 4. Underflow into the Ash Meadows basin, from Pahranagat Valley, may amount to as much as 35 percent of the spring discharge at Ash Meadows. The estimated velocity of ground water moving ver- tically through the tuff aquitard into the lower carbonate aquifer in Yucca Flat ranges from 0.0005 to 0.2 foot per year; values toward the lower end of the range are more probable. The estimated velocity of water in the lower carbonate aquifer beneath central Yucca Flat ranges from 0.02 to 2.0 feet per day. Velocity in the carbonate aquifer beneath the Specter Range ranges from 2 to 200 feet per day. 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D., 1959, Study and interpretation of the chemical characteristics of natural water: U.S. Geol. Survey Water-Supply Paper 1473, 269 p. 1961, Calculation and use of ion activity: U.S. Geol. Survey Water-Supply Paper 1535—0, 17 p. Hinrichs, E. N., 1968, Geologic structure of Yucca Flat area, Nevada, in Eckel, E. B., ed., Nevada Test Site: Geol. Soc. America Mem. 110, p. 239—246. Hinrichs, E. N., and McKay, E. J., 1965, Geologic map of the Plutonium Valley quadrangle, Nye and Lincoln Counties, Nevada: U.S. Geol. Survey Geol. Quad. Map GQ—384. Hood, J. W., 1961, Water wells in Frenchman and Yucca Valleys, Nevada Test Site, Nye County, Nevada: U.S. Geol. Survey TEI—788, open-file rept., 59 p. Hoover, D. L., 1968, Genesis of zeolites, Nevada Test Site, in Eckel, E. B., ed., Nevada Test Site: Geol. Soc. America Mem. 110, p. 275—284. Hubbs, C. L., and Miller, R. 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Page A Alkali Flat, ground-water chemistry _________ C99 Alluvial-fan deposits. See Valley-fill aquifer. Alluvium, Quaternary ___________________ 9 Amargosa Desert ___________ 4, 13, 14, 19, 20, 37 ground-water chemistry _______________ 99 ground-water movement ______________ 111 potentiometric trough ________________ 71 underflow ______________________ .85 Amargosa River _____________________ 4, 6 Ammonia Tanks Member of Timber Mountain 'I‘uff .31 Aquifers ___________________________ 14 bedded tuff _____________________ .34 distribution ______________________ 47 lower carbonate ______________ 14, 102, 109, ground-water velocity _____________ 113 transmissibility ________________ .20 potentiometric map _________________ 71 saturation _______________________ 47 upper carbonate __________________ .30 valley-fill --------------__._---37, 118 ground-water age _______ ___ - 114 welded-tuff ___________________ 31, 118 Aquitards _______________________ 34, 38 Climax stock _____________________ 46 Gold Meadows stock _________________ 46 ground-water movement control __________ 63 Horse Spring Formation _______________ 78 hydraulic tests ____________________ 4 2 lower clastic _____________________ .39 Pavits Spring Rocks _________________ 78 transmissibility ____________________ 42 tuff - - _ . ____________________ 43, 118 ground-water age _______________ 114 ground-water movement ___________ 109 ground-water velocity _____________ 113 upper clastic __________________ 43, 118 Ash-flow tuff. See Welded tuff. Ash Meadows _______________________ 1 ground-water basin _______________ 75, 85 ground-water chemistry ______________ 104 ground-water movement ____________ 70, 108 potentiometric trough ________________ 71 underflow ______________________ .85 Ash Meadows—Amargosa Desert discharge area _--_ 3 Aztec Sandstone ______________________ 53 B Banded Mountain. _____________________ 15 Basalt. See Lava-flow aquifer. Basin and Range, topography ____________ 9, 13 Beatty, Nev _________________________ 6 Bedded-tuff aquifer ___________________ .34 Belt Range _________________________ 4 Bibliography _______________________ 119 Big Spring ______________________ 75, 81 Bird Spring Formation ________________ 9, 30 Bonanza King Formation ____________ 16, 28, 69 ground-water chemistry ______________ 102 Breccia zones, transmissibility ______________ 34 Brecciation _________________________ 15 C CP Basin __________________________ 60 CP thrust fault, ground-water control _________ 94 Calico Hills ________________________ 106 Calico Hills beds _____________________ 43 INDEX [Page numbers of major references are in italic] Page Cane Spring fault zone _________________ C60 Cane Springs ________________________ 50 Carbonate aquifers, lower __________ 14, 102, 109 ground-water velocity _____________ 113 transmissibility ________________ .20 upper _________________________ .30 Carrara Formation _______________ 13, 28, 39 Caverns _______________________ 115, 118 Caves ____________________________ 16 Chainman Shale, ground-water ____________ 103 Chemistry, facies ____________________ .98 ground-water ____________________ .97 Chinle Formation _____________________ 53 Clark County, Nev _____________________ 4 Clastic aquitard, upper _________________ 118 Clebsch, Alfred, Jr _____________________ 2 Climate ___________________________ 6 Climax stock aquitard __________________ 46 Cold Creek Spring _____________________ 53 Columnar jointing _____________________ 31 Cooling units _______________________ .32 Cordilleran geosyncline __________________ 9 Core samples ________________________ 17 Crater Flat _________________________ 34 Crater Flat Tuff ______________________ 43 ground-water chemistry ______________ 107 Crystal Pool ________________________ 81 D Davis, R. E. ________________________ 37 Death Valley _____________________ 4 , 6, 13 Desert Range ________________________ 13 Desert Valley ________________________ 4 ground-water chemistry ______________ 104 ground-water movement _______________ 62 Devils Hole ______________________ 16, 82 Discharge, springs _____________________ 78 Dissolved solids, lower carbonate aquifer ______ 102 Drill-stem tests _____________________ .20 Dunderberg Shale Member of Nopah Formation 9, 40, 42 ground-water chemistry ______________ 102 E Eleana Formation _________________ 9, 30, 43 Emerick, W. L. ______________________ 37 Emigrant Valley __________________ 4, 37, 97 ground-water chemistry ______________ 104 ground-water movement ____________ 62, 63 Eureka mining district __________________ 29 Eureka Quartzite _____________________ 9 Evapotranspiration estimates _____________ .84 F Fairbanks Spring _____________________ 81 Fanglomerate deposits. See Valley-fill aquifer. Faulting .................. 9, 13, 14, 66, 117 springs control ................... .81 transmissibility ................. 34, 74 Flowmeter surveys ..................... 20 Fluvial deposits. See Valley-fill aquifer. Folding .......................... 117 Foliation .......................... 31 Fortymile Canyon—Oasis Valley ground-water basin .94 Fracturing ...................... 14, 117 filling _________ . ............. 15, 17 porosity ........................ 19 transmissibility .............. .29, 74, 116 Page Frenchman Flat __________ C 2, 4, 9, 11, 12, 37, 97 ground-water chemistry .............. 104 ground-Water movement .............. .57 potentimetric trough ................. 71 springs ......................... 50 tuff aquitard ..................... 44 Funeral Mountains .................... l3 Furnace Creek Wash-Nevares Springs discharge area .95 ground—water chemistry ............... 99 ground-water movement .......... - - _ - 112 spring discharge .................. 112 G Geomorphology ...................... 9 Geosyncline ......................... 9 Gold Meadows stock aquitard .............. 46 Groom Range ..................... 4, 13 Ground water, ‘chemistry ................ .97 contamination ..................... 1 movement .................. 49, 95,118 Ash Meadows ................. 108 Frenchman Flat ................ .57 lower carbonate aquifer ____________ 109 Pahrump Valley ................ 108 regional summary ............... 112 Stewart Valley ................. 109 velocity ..................... 119 Yucca Flat ................... .53 perched ........................ 4 9 Grouse Canyon Member of Indian Trail Formation -34 Gypsum Cave ....................... 17 H Halfpint Range anticline ................. 4 7 Historical geology ................. 9, 12, 13 Horse Spring Formation ............ 12, 43, 105 aquitard ........................ 78 Hydraulic tests, lower clastic aquitard ......... 4 2 tuff aquitard ..................... 4 6‘ upper clastic aquitard ................ 43 welded-tuft" aquifer .................. 34 Hydrochemical facies ................ 98, 119 I Indian Springs ....................... 6 Indian Springs Valley ............ 2, 3, 4, 9, 42 ground-water chemistry _________ 98, 104, 109 ground-water movement ............ 62, 67 Indian Trail Formation .................. 43 Grouse Canyon Member ............... 34 Tub Spring Member ................. 34 Inyo County, Calif ..................... 4 Irrigation .......................... 14 Ilohyetal map ...................... .85 J, K Jackass Flats ............ 2, 4, 11, 12, 14, 31, 97 aquitard ........................ 43 springs ......................... 50 Joana Limestone, ground water ............ 103 Johnnie Formation ................. 39, 68 ground-water chemistry .............. 102 Joints ............................ 14 Kiwi Mesa Basalt .................... .36 0125 C126 Page L Lake Mead, water source ________________ C6 Lakebed deposits. See Valley-fill aquifer. Las Vegas __________________________ 6 Las Vegas ground-water basin _- Las Vegas Valley _________ ground-water chemistry ____________ 98, 109 shear zone ____________________ 13, 67 Lathrop Wells __________________ Lava-flow aquifer ________________ Lava-flow aquitard ____________________ 46 Lawrence Radiation Laboratory _____________ 2 Leakage, computation _____________ _ 110 recharge ___________________ Yucca Flat _ _ - - Lincoln County, Nev. _ Lithology __________________________ 12 Lone Mountain Dolomite, ground water _______ 103 Los Alamos Scientific Laboratory ____________ 2 Lower carbonate aquifer ______ ground-water chemistry _ _ - _ ground-water movement _ - ground-water velocity _______________ 113 potentiometric map _________________ 71 storage coefficient _________________ -28 transmissibility ___________________ _20 Lower clastic aquitard _________________ _37 M Mercury, Nev _______________________ 3,6 Mercury Valley ___________________ 12, 42 ground—water chemistry -_ ground-water movement ______ Miogeosynclinal sedimentation - _ _ _ Monte Cristo Limestone _________________ 9 Montgomery thrust fault ______________ 75, 91 Movement, ground-water ________________ 118 N, 0 National Aeronautics and Space Administration _ __ 6 Nellis Air Force Base ___________________ 6 Nevada Formation, ground water ___________ 103 Nevares Springs-Fumace Creek Wash discharge area 95 ground-water chemistry _______________ 99 ground-water movement ______________ 112 spring discharge - _ - _ _ - _ _ 112 Ninemile Formation ________________ 9 Noonday Dolomite ________________ 42 ground-water chemistry ______________ 102 Nopah Formation _________________ 9, 29, 69 Dunderberg Shale Member _____ 9, 40, 42 ground-water chemistry _ - _ _ _ _ _ 102 Nuclear testing, ban ___________ - - _ - 2 underground ______________________ 1 Nye County, Nev ______________________ 4 Oasis Valley, ground-water chemistry _________ 98 Oasis Valley-Fortymile Canyon ground-water basin 1, 94 Orogeny _________________________ 9, 13 P Pahranagat Range ______________ Pahranagat Valley _______ ground-water chemistry _________ 98, 104, 109 ground-water movement ______________ 110 Pahrump Valley ______________________ 4 ground-water basin _________________ -90 ground-water chemistry -_ groundwater movement -_ Pahute Mesa ___________ ground-water chemistry _______________ 98 Paintbrush 'I‘uff ___________________ 12, 31 Pavits Spring _____________________ 12, 50 INDEX Page Pavits Spring Rocks ________________ C43, 105 aquitard ________________________ 78 ground-water chemistry ______________ 107 Perched ground water ___________________ 4 9 Permeability _______________ 34, 117, 118 ground water movement control _________ 113 Physiography ________________________ 4 Piapi Canyon Group ________________ 31, 36 Pintwater Range anticline ________________ 73 Pluvial periods _________ Point of Rocks Springs -___ Population centers ______ Porosity ______________________ 31, 34, 118 aquitards _______________________ 40 elastic rocks ______________________ 40 lower carbonate aquifer _______________ 17 Potentiometric map ______ lower carbonate aquifer - Precipitation __________ recharge ________________________ 92 Pumping tests ______________________ _20 R Rainier Mesa ____________________ 2, 13, 50 Rainier Mesa Member of Timber Mountain 'I‘uff 31, 34 Ramming effect _____________________ _24 Ranger Mountains _____________________ 15 Recharge __________________________ 86 sources _______________ _ _ _ Resting Springs Range ____________ Reynolds Electrical and Engineering Co _________ 3 Rhyolite. See Lava-flow aquifer. Rock Valley _____________________ 12, 20 ground-water chemistry ______________ 107 lower carbonate aquifer _______________ 20 S Salyer Formation __________________ 12, 43 ground-water chemistry ______________ 107 Sevier Arch _________________________ 12 Sheep Range __________________ -4, 6 springs ___________________ _ -50 Shoshone Mountain Rhyolite ________ __36 Silent Canyon caldera, ground water __________ 94 Skull Mountain Basalt __________________ 36 Snyder, R. P. ______________ __37 Sodium, lower carbonate aquifer _____ .. 103 Specter Range, ground-water velocity ________ 115 potentiometric trough ________________ 72 Specter Range thrust ________________ 73, 75 Spotted Range _______________________ 9 Spring Mountains ___ -4, 6, 9, 11, 13 caves ______________ ground-water chemistry _- springs ______________________ 50, 52 Spring Valley _______________________ 30 Springs ______________________ .50 Big Spring __________________ Crystal Pool _ _ _ - discharge _____ Fairbanks Spring ___________________ 81 Point of Rocks Springs _______________ 79 Spotted Range syncline _______________ 73 Stewart Valley ____________ fault zone ____________ ground-water movement _ _ _ - Stirling Quartzite __________________ 39, 82 Stratigraphy _____________________ 9. 117 Structural geology ______________ 9, 12, 14, 117 ground-water basin boundaries _______ - _ .87 ground-water control ______ _ _ _ -93 Stylolites _____________________ 19 Sulfate, lower carbonate aquifer ____________ 103 Page T Temperature gradients, Ash Meadows discharge area C80 Test drilling, Yucca Flat _________________ 2 Texas Spring ________________ Three Lakes Valley ______ ground-water chemistry - _____ 98, 104, 109 ground-water movement _ ________ 62, 67 Tikaboo Valley _______________________ 4 Timber Mountain caldera, ground-water control ___94 Timber Mountain 'I\1ff __________________ 31 Tippipah Limestone _________________ 30, 66 joints __________________________ 15 Tiva Canyon Member of Paintbrush Thff ____ 31, 34 Topopah Spring Member of Paintbrush Tuff __ 31, 34 Transmissibility ___________________ 34, 117 aquitards _ - _ coefficients _ _ faulting ________________________ 74 fracturing ____________________ 74, 116 lava-flow aquitard _________ _ - _46 lithologic control __________ _ _ _28 lower carbonate aquifer _ - _ - .20 structural control __________________ -28 tuff aquitard _____________________ 46 valley-fill aquifer __________________ _37 Travertine Springs ____________________ 95 Tub Spring Member of Indian Trail Formation ___-34 Tuff aquitard __________________ 43, 118 ground-water age ______________ 114 ground-water movement ______________ 109 ground-water velocity _______________ 113 Underflow - _ recharge ____________________ United States Atomic Energy Commission _____ 1, 2 Upper carbonate aquifer, saturation zone ______ .30 Upper clastic aquitard _______________ 43, 118 Valley<fill aquifer ________________ 14, 37, 118 age of ground-water ______ 114 Vegetation ________ Volcanics _________ Volcanism, Miocene ____________________ 13 W Wahmonie Flat ______________________ 46 Wahmonie Formation __ ___________ 12, 36, 43 ground-water chemistry ______________ 107 Water supply __________________ 6, 14, 31, 37 Welded-tuff aquifer _________________ 31, 118 Well-numbering system ______________ 3 White Pine County, ground water - _____ 103 Whiterock Springs _________________ 50 Williams, W. P _______________________ 37 Willow Spring _______________________ 53 Wood Canyon Formation ______________ 39, 82 Worthington Mountain, caves‘ ______________ 17 Y, Z Yucca Flat ----- 2, 4, 6, 9, 11, 12, 19, 20, 30, 47, 97 aquitard ________________________ 43 Climax aquitard ___________________ 4 6 Gold Meadows aquitard _______________ 46 ground-water chemistry ---------- _ - - 104 ground-water movement ---------- ground-water velocity _- joints _____ leakage ----------- lower carbonate aquifer _______________ 20 potentimetric trough _________________ 71 springs _____---_____ ______ test drilling _______________ tuff aquitard _____________________ 44 Zabriskie Quartzite ____________________ 39 “am $59 . six GEOLOGICAL SURVEY UNITED STATES DEPARTMENT OF THE INTERIOR N. 1,100,000 — N. 1,000,000 A ' ’ :2“ i l N. 900,000 _ ’ »~ 4055 i 30 o N. 800,000 — MDe fl 0' ‘CPCI E. 500,000 «g C154663 i 5 7i10, ‘ 5,; 496 ‘Q‘ q , _ , / E. 600,000 Prepared on behalf of the US. ATOMIC ENERGY COMMISSION _ , , E. 800,000 E. 900,000 ,_ I use : ijflilw :Springs , an» i Ids/71 Springs ' ., ‘1 ,~ \ 1;: - , 47,05 R ’9 ‘ . —— . : ‘9] i1;- 0 <4867imeasurefifi. . 4722 (D < 3650 i 50R (Well ldcation may be in-er‘r'o'rpy a'milel 4065 i 20 / Approximate boundary , ‘3/ ‘ , ofrcalder'a ' ”VALLEY 10/62 ~14a| O 2175 Spring 9 ‘I (11/62 I I I Coyote I \ \ -—24bl W N. 800,000 . 1x. ,,, ,r (m n“ (A 2 «1' : L N. 700,000 — ‘ N. 700,000 . g .; .r f 2765ig10‘ " yo . :17. N. 600,000 — N. 500,000 *4 Base from US. Geological Survey Caliente, Death Valley, Goldfield, and Las Vegas, 1954 100,000—foot grid based on Nevada coordinate system, central zone TeXas Springs” ‘ ‘fo . LL L7, MOTravertine Springs wiring; 32‘ r IQ}? —‘ N. 500,000 E. 900,000 E 600 000 t 000 E. 800,000 E‘ 500'000 51:2“ ' I SCALE 50 000 E 15 20 25 MILES 5 o 5 z s I_I 1__I I——I I———-—— I———-——I 4 E at 2 5 5 o 5 I 15 20 25 KILOMETERS E E9 r: I—I I—-I I—H I :5 CONTOUR INAL200 FEET APPROXIMArE MEAN DATUM IS IVSEA LEVEL DECLINATIDN,1974 AREA OF DOWNWARD CROSSFLOW FROM TUFF AREA OF UPWARD CROSSFLOW FROM LOWER ——. AQUITARD INTO LOWER CARBONATE AQUIFER A CARBONATE AQUIFER INTO TUFF AQUITARD 800°, FRENCHMAR FLAT CP YUCCA Y YUCCA FLAT . , AMARGOSA DESERT SPECTER MERCURY RED Frenc uccg 6000 RANGE VALLEY MOUNTAIN La BASIN PASS Lake SH MEADOWS 67-68 79-69a 81-69 83-68 84<68d 4000' A s HOLE 09 FT PER MILEI v Tt ‘ 7 76-70 DEVIL .__ _ __ __ __ ____ _ _ I —— 7 LI 0.5 FT PER MILE 200° QTaI and Tr Y359 ft HYDRAULIC 2370 ft undifferentiated GRADIENT SEA LEV E L 4—— 2000’ 4000" 6000 Section bends at every test well north of Frenchman Lake Q— Pze \ ch / HYDROGEOLOGIC MAP OF NE ‘IA TEST SITE AND VICINITY, SOUTHE ' EVADA 85—68 Tt m PER MILE YUCCA A _ ELEANA RANGE I A o o 0‘ 2000’ SEA LEVEL 7 2000' ‘ ~ 4000’ 6000' Hydrology by I. J. Winograd, I965; geology by William Tliordarson. 1965. Vertical exaggeration, X 2.6. €p€l«ch contact estimated from areal and subsurface stratigraphic data, ch~Tt contact from gravity surveys and drill hole data. INTERIORi GEOIOGICAL SURVEY, REb'UN. \rIPGINIA’ I975 Hydrogeology by l. J. Winograd, 1965 _ ‘1; —‘ N. 1,000,000 PROFESSIONAL PAPER 712-C PLATE 1 EXPLANATION MAP Upper elastic aquitard Argillite, quartzite, conglomerate, and minor limestone of the Eleana Formation; narrow-line pattern, outcrop; wide-line pattern, area beneath which the upper elastic aquitard is probably the major pre-Tertiary hydrogeologic unit within upper few thousand feet of zone of saturation; pattern includes small areas of the upper carbonate aquifer, T ippipah Limestone Lower elastic aquitard Quartzite, siltstone, shale and minor limestone of the lower part of Carrara Forma- tion, Zabriskie Quartzite, Wood Canyon Formation, Stirling Quartzite, and Johnnie Formation; includes minor Permian to Mesozoic granitic stocks, and in Amargosa Range includes Precambrian metamorphic rocks; narrow-line pattern, outcrop; wide-line pattern, area beneath which the lower clastic aquiv tard is probably the chief pre-Tertiary hydrogeologic unit within the zone of saturation ?——?— Contact Queried where doubtful @___ l—Strike-slip fault Arrows indicate direction of relative movement; dashed where inferred; number is major tectonic feature listed below 6) Thrust fault Sawteeth on upper plate; dashed where inferred; number is major tectonic feature listed below __T______ Normal fault Bar and ball on downthrown side; dashed where inferred; number is major tectonic feature listed below _ __' Antieline Dashed where inferred; number is major tectonic feature listed below G) —.A Inferred syncline Number is major tectonic feature listed below LIST OF MAJOR TLCTONIC FEATURES Y DEVONIAN AND MISSISSIPPIAN V PRECAMBRIAN AND CAMBRIAN Identifying number Name Source on map 1 - - - Timber Mountain caldera - - - Carr (1964) 2 - - - CP thrust fault ----------- Barnes and Poole (1968) 3 — - - Mine Mountain thrust fault - - Hinriehs (1968) 4 - - - Tippinip thrust fault ------- Barnes and Poole (1968) 5 - - - Yucca fault -------------- Hinrichs (1968) 6 - - - Halfpint anticline --------- Do. 7 - - - Spotted Range syncline - - - - Name assigned by authors 8 - - - Pintwater Range anticline - - - Longwell, Pampeyan, Bowyer, and Roberts (1965) 9 - - - None ------------------- Tschanz and Pampeyan (1961) 10 — - - None ------------------- Do. 11 - - - Sheep Range syncline ------ Name assigned by authors 12 - - - Gass Peak thrust fault ------ Longwell, Pampeyan, Bowyer, and Roberts (1965) 13 - - - Las Vegas Valley shear zone - Do. 14 - - - Wheeler Pass thrust fault - - - - Longwell, Pampeyan, Bowyer, and Roberts (1965); Vincelette (1964) 15 - - - Indian Springs syncline ----- Name assigned by authors 16 - - — Montgomery thrust fault - - - (R. L. Christiansen, R. H. Moench, and M. W. Reynolds, unpub. data) and Hamill (1966) 17 — - - Stewart Valley fault zone — - - Name assigned by authors (R. L. Christiansen, R. H. Moench, and M. W. Reynolds, unpub. data) 18 - - - Gravity fault ------------- (D. L. Healey and C. H. Miller, written commun.) 19 - - - Specter Range thrust fault - - (R. L. Christiansen, R. H. Moench, and M. W. Reynolds, unpub. data) 20 - - Roses Well anticline ------- Name assigned by authors (R. L. Christiansen, R. H. Moench, and M. W. Reynolds, unpub. data) HYDRAULIC SYMBOLS All altitudes and contours in feet; datum is mean sea level; well numbers and forma- tions tested given on larger scale maps; see figures 31, 32, 33, 34, and plate 2 . 2381 Test well Well tapping pre- Tertiary hydrogeologic units; number is altitude of static water level; symbol < denotes well was dry or fluid level was declining 2415 t 2 ® 2555 Test well or water well Well tapping both Cenozoic and pre-Tertiary hydrogeologic units; upper number is altitude of static water level in pre—Tertiary aquifer or aquitard; lower number is altitude of static water level in Cenozoic aquifer or aquitard,“ a single number is altitude of composite static water level of both hydrogeologic units; P denotes perched water 2422 (D Test well Well tapping Tertiary hydrogeologic units,“ number is altitude of static water level; R, reported water level; symbol < denotes well was dry or fluid level was de- clining O 2411 Test well or water well Well tapping valley-fill aquifer; number is altitude of static water level; R, reported water level; symbol < denotes well was dry or fluid level was declining ea 2410 t 2 Test well Well tapping valley—fill aquifer and Tertiary hydrogeologic units; number is altitude .....a,... 1M...) A} 1.4;). Lydungnnlngi» unite Q, Spring or cavern Spring or cavern in, or near outcrop of, lower carbonate aquifer; number is altitude of land surface at spring or of water level in cavern 3238 i 5 Q, Spring in Quaternary and Tertiary valley-fill aquifer or lakebeds Number is altitude of land surface at spring Approximate area of extensive evapotranspiration 238 — —— — Potentiometric contour Shows altitude of potentiometric surface for pre-Tertiary aquifers and aquitards; dashed where inferred; contour interval is variable Potentiometric contour Shows altitude of potentiometric surface for Quaternary-Tertiary valley-fill aquifer or for Tertiary h ydrogeologic units; dashed where inferred; hachured, where probable downward crossflow into lower carbonate aquifer occurs; contour interval is variable Inferred ground—water barrier Width of symbol not intended to represent width of hydraulic barrier, which may vary from several tens to a few thousand feet ———— - —— —?~—— Approximate boundary of Ash Meadows ground-water basin Long and short dash, boundary inferred from contact of lower carbonate aquifer and lower clastic aquitard, or from major hydraulic barrier; long dash and dot, boundary inferred from topographic divides and locally from water levels in Cenozoic aquifers; long dash and query, inferred from topographic divides under- lain directly or at depth by lower carbonate aquifer. [n Ash Meadows area basin boundary, hydraulic barrier, and normalfault are coincident, but symbols sepa- rated for clarity SECTION HYDROGEOLOGIC AND GEOLOGIC UNITS SYMBOL GEOLOGIC UNIT HYDROGEOLOGIC UNIT QTal Quaternary and Tertiary valley fill ------ Valley-fill aquifer Tt Tertiary tuff, lakebeds, and lava flows - - - Welded-tuff and bedded-tuff aquifers; tuff and lava-flow aquitards MDe Eleana Formation ------------------ Upper elastic aquitard ch Paleozoic carbonate rocks ------------ Lower Carbonate aquifer CpCl Early Cambrian and Precambrian elastic rocks ----------------------- Lower elastic aquitard 81 -69 II Test well Number is well number; dashed where projected into line of section ‘— ‘— Flow lines Long arrows, flow within the lower carbonate aquifer; short arrows, crossflow Emigrant Valley and Desert Range ----- Kawich Valley --------------------- Yucca Flat ------------------------ Frenchman Flat and Mercury Valley - - - - Specter Range, east—central and southern Amargosa Desert, and western Pahrump Valley ----------------------- Northwestern Amargosa Desert, north eastern Funeral Mountains --------- Black Mountains ------------------- Furnace Creek Wash, Nevares Springs, southwestern Funeral Mountains ----- between tuffaquitards and lower carbonate aquifer or valley fill Potentiometric surface in ore-Tertiary rocks SOURCES OF OTHER GEOLOGIC DATA AREA SOURCE Tschanz and Pampeyan (1961) Ekren, Anderson, Rogers, and Noble (1971) (Harley Barnes, unpub. data) (D. L. Healey and C. H. Miller, unpub. data) (R. L. Christiansen, R. H. Moench, and M. W. Reynolds, unpub. data; D. L, Healey and C. H. Miller, unpub. data) Cornwall and Kleinhampel (1961 and 1964); Jennings (1958) Jennings (1958) Hunt and Mabey (1966) SOURCES OF HYDROLOGIC DATA OUTSIDE STUDY AREA Pahranagat Valley """""""" Pahute Mesa ------------- Sareobatus Flat and Oasis Valley ‘ ‘ Northwest Las Vegas Valley ------ Southwestern Amargosa Desert - - - - Pahrump Valley --------------- AREA SOURCE Eakin (1966) (R. K. Blankennagel and J. E. Weir, Jr., unpub. data 1966) Malmberg and Eakin (I962) Malmberg (1965) Walker and Eakin (1963) Malmberg (1967) 55’ E. 720000 I I | 5515. 720,000 115° 52'30” 37° 15' N. 900,000 I N, 880,000 N. 860,000 57 N. 840,000 r, N. 820,000 37°00’ N. 780,000 Hydrogeology by William Thordarson, 1965 Geology modified from unpublished data of Harley Barnes and files of the U.S. Geological Survey, Las Vegas, Nev. 55’ E. 720000 UNITED STATES DEPARTMENT OF THE INTERIOR prepared on behalf of the 115?:9LOGICAL SURVEY 7 U.S. ATOMIC ENERGY COMMISSION .E. 64clJ,000 llO’ E.66(|),000 5’ E. 680,000 116°OO' E. 700,000 55' E. 720,000 115°52'30" 115°15' E. 640.000 10’ E. 550,000 , E. 680,000 116°00' l . . _ I , . I. _ 8000, . I I I I | 37 15 A I l 7000’ 6000’ BURNT MOUNTAIN 5000' 5000' 4000’ . ' V 37° 15: _ , . , ' TIPPINIP CLIMAX “ 4000' __ 37015; 3000' . , - ' " FAUL'T\ STOCK ‘ . _ N.900,000 —- . ‘ , , RHYOLITE HILLS ' ‘ , . , 3000’ 2000’ . . - . RAINIER 2000’ . 1000’ MESA 1000' " SEA LEVEL 63 SEA LEVEL 0 / \ A-3 / l “ N. 900,000 — - / \ ' 0 / \ N.880,0l% = / \ —— N. 900,000 l 89-68 \\ RAINIER MESA / \ 89-62 5000' N.860,000 — HALF PINT RANGE 4000, Tp . i I 3000 D ELEANA RANGE 2000’ . 1000' l SEA LEVEL 5, B-4 C-5 STOCKADE WASH SYNCLINE RIDGE N. 880,000 _ 10, __ _ 10, N. 840,000 — TIPPINIP FAULTI?) N 880000 \ BANDED \ MOUNTAIN Sites: I. _ E 37°OO' — TIPPINIP FAULTl?) Outline of Yucca Flat N. 860,000 — Y \ N. 800,000 '— — N. 860,000 6000’ 5000’ ELEANA RANGE 4000’ 3000’ 2000 SYNCLINE 55' E . RIDGE 1000 SEA LEVEL D-I ' d N. 780,000 — 5 5! B. GEOLOGIC MAP AT 2,400-FOOT ALTITUDE 36» 52/30., I ‘7 E I I . . 116 o 15, E. 340,000 10 .660,000 5 E. 680,000 116 00 HALF PINT RANGE 20,000—foot grid based on Nevada SCALE 1:96 000 / coordinate system, central1 Zone 1 1/2 0 1 2 3 4 5 MILES / I-—I I—I t; d L— : J:\ I / PAIUTE RIDGE Tp 1 .5 O 1 2 3 4 5 KILOMETERS Farr—W A . l 3:! / _ 115715] E. 640,000 10’ E 050000 5’ E. 680,000 116mg 3/°15' . . . N. 840,000 — / / N. 840,000 / N 900,000 N. 880000 10‘ MINE MOUNTAIN SHOSHONE MOUNTAIN ch 6000’ Outline of Yucca Flat\/ / / / l 5000 4000’ N. 820,000 — 3000' 37°00, _ N. 820,000 2000 3 7°00’ 1 000’ \~§ SEA LEVEL E I 01.860000 TIPPINIP FAULTl?) YUCCA FAULT PLUTONIUM VALLEY N. 840000 N. 800000 — \ //F\ N. 800,000 N. 81’0000 37“OO’ C P HILLS YUCCA PASS Tp YUCCA LAKE THE BENCH 5000’ 4000’ FRENCH PEAK 3000' 55' — I 2000’ _ 55 1000’ TIPPINIP FAULTI?)' ‘ SEA LEVEL N.800,000 » F-1 Ii:— 55‘ k N. 780,000 — — N. 780,000 N. 730,000 . ‘ P 3 E 63:57; e D SAQCTION o I II l l l l I ‘\< 36 52 30 E 540 000 10' E 660 000 I l I l l 36°52'30” 9 q: u . . 116 15 ’ ~ . 5 E4 580.000 1 16°00' E, 700,000 INTERIOR—GEOLOGICAL SURVEY. RESTON, VIRGINIA71975 55' E- 720000 115° 52'30" 36 5c 31016" 15’ E- 540000 IO' E. 550900 5' E 630900 4 HOW 20,009-f00l gl’ld based 01') Nevada SCALE 1348,000 H d l I) William Th (1' - V . 15 o z . coordinate system, central zone y rogeo ogy y or arson, 1965 Base from U.S. Geological Survey l.62,500 it: SCALE 1,96 000 1 V2 0 1 2 3 MILES Tippipah Spring, Papoose Lake, Cane Spring, 1/ l_‘ H "“l H H l—_—'_——‘ l——‘——-—I Frenchman Lake, 1952 s L ,2_. ?__,l 2|______S’ ‘l 5. MILES 55 4r r . 20,000-foot grids based on Nevada 52 g 1 J: H H o 1 2 3 KILOMETERS coordinate Systemmemmlzone 3 § 1 5 o 1 2 3 4 5 KILOMETERS E 5 l—l l—-I I-————I I—I .____I DATUM IS MEAN SEA LE g VEL T . CONTOUR INTERVAL 40 FEET ”PWW‘EME’W DATUM IS MEAN SEA LEVEL DECLI NATION, 1974 HYDROGEOLOGIC MAPS AND FENCE DIAGRAM, YUCCA FLAT, NEVADA TEST SITE, SOUTHERN NEVADA I 9137‘ 50 a; .V, I NY; to N 900,000 IO’ N 880,000 N. 860,000 . IN 840,000 N 870000 3 .7 " I'IO’ 1 N 800,000 FEOIIOO Hydrogeology by l. J. Winograd, 1965 Geology modified from Harley Barnes, (unpub. data) 36° 52’30” 115 ° 52’30" Bar and ball on downthrown side; dashed where inferred; dotted where concealed MAP AND SECTION SYMBOLS FOR PANEL C QTal Tpi Tma Tmr Tpt Tp Mng? PlPt MDe Ccu CpCi EXPLANATION HYDROGEOLOGIC AND GEOLOGIC UNITS Valley-fill aquifer Alluvial fan, fluvial, fanglomerate, lakebed, and mudflow deposits Welded—tuff and bedded-tuff aquifers _ Densely to semiwelded tuff of Piapi Canyon Group; interbedded ash-fall tuff is bedded-tuff aquifer Tuff aquitard Ash-fall and ash-flow tuff, massively altered to zeolite or clay; also includes tuffaceous and clayey lake and fluvial deposits; predominately rocks of the Indian Trail Formation; in- cludes Rocks of Pavits Spring, Salyer Formation, and Wahmonie Formation in southern Yucca Flat Granitic stocks (aquitard) GmmdiOrite and quartz monzonite in stocks, dikes, and sills Upper carbonate aquifer Tippipah Limestone Upper elastic aquitard Argillite, quartzite, conglomerate, and minor limestone of the Eleana Formation; on map C, narrow-line pattern is outcrop; Wide-line pattern, area beneath which the upper elastic aquitard is probably the chief pre-Tertiary hydrogeologic unit within upper few thousand feet of zone of saturation; includes small areas of the upper carbonate aquifer, Tippipah Limestone Lower carbonate aquifer Dolomite and limestone; minor shale and quartzite; on map C, area beneath which the lower carbonate aquifer is prObably the chief pre-Tertiary hydrogeologic unit within the zone of saturation Lower clastic aquitard Quartzite, siltstone, shale, and minor limestone; on map C, narrow-line pattern is outcrop,- wide-line pattern, area beneath which lower clastic aquitard is probably the chief pre- Tertiary hydrogeologic unit within the zone of saturation ._—____l_l‘_l__l Contact Approximate subsurface contact ofhydrogeologic units; on map C, hachured at approximate subsurface contact between lower carbonate aquifer and lower and upper elastic aquitards at the 2, 400-foot altitude; in vicinity of Rainier Mesa, hachured contact at 4,200-foot altitude ____.L____ ........ Normal fault GEOLOGIC UNIT Quaternary and Tertiary valley fill Piapi Canyon Group and Indian Trail Formation, undifferentiated (section only) Ammonia Tanks Member Of Timber Moun— tain Tuff Rainier Mesa Member Of Timber Moun- tain Tuff TOpOpah Spring Member of Paintbrush Tuff Ash-fall(?) tuff Of Piapi Canyon Group Ash-fall and ash-flow tuff massively altered to zeolite or clay; includes some tuffa- ceous and clayey lake and fluvial de- posits; predominantly rocks Of the Indian Trail Formation; includes also Rocks of Pavits Spring and Salyer Formation in southern Yucca Flat Salyer Formation Permian to Mesozoic granitic stocks, dikes, or sills Tippipah Limestone Eleana Formation Paleozoic carbonate rocks; includes minor shale and quartzite Dolomite or limestone Of Ordovician to Devonian age Dolomite or limestone Of Devonian age Devils Gate and Nevada Formations Eureka Quartzite Pogonip Group Bonanza King Formation Carrara Formation, upper half Lower Cambrian and Precambrian elastic rocks HYDRAULIC SYMBOLS NOTE: All altitudes and contours in feet; datum is mean sea level 86-65 . < 2640 (Ch) Test well Well tapping Dre-Tertiary hydrogeologic units; upper number is well number; lower number is altitude of static water level; symbol in parentheses is formation tapped; P, perched water; C. Probably a composite water level of both Tertiary and pre-Tertiary hydrogeologic units; syMbol < denotes well was dry or fluid level was declining 88-66 2415 t 2 (Op) 2555 (Ti) 0 Test well Well tapping luff aquitard and lower carbonate aquifer; upper number is well number; lower numbers are altitude of static water level; symbols in parentheses are formations tapped 87-68 ®< 2628 (Til Test well Well tapping Tertiary hydrogeologic units; upper number is well number; lower number is altitude of static water level; C, probably a composite water level of both Tertiary and pre-Tertiary hydrogeologic units; R, reported water level; symbol < denotes well was dry or fluid level was declining; symbol in parentheses is formation tapped 77-68 632784 i 5 (QTaI, Tma) Test well Well tapping valley-fill aquifer and Tertiary hydrogeologic units; upper number is well num- ber; lower number is altitude of composite static water level of both hydrogeologic units; symbols in parentheses are formations tapped Well tapping valley-fill aquifer; upper number is well number; lower number is altitude of static water level; symbol in parentheses is formation tapped Number above is test hole number; bottom hole formation in parentheses; no reliable hydraulic data available; on map B such wells are shown by solid circle Quadrangle A-1 to A-3 ------ Oak Spring -------- Barnes, Houser, and Poole (1963) B-l to B-2 ------ Rainier Mesa ------- Gibbons, Hinrichs, Hansen, and Lemke (1963) B-3 to B4 ------ Jangle Ridge ------- Barnes, Christiansen, and Byers (1965) C-3 to C-4 ------ .Iangle Ridge ------- D0. D-l to D-2 ------ Tippipah Spring - - - - Orkild (1963) D-4 to D-5 ------ Paiute Ridge ------- Byers and Barnes (1967) El to E-2 ------ Mine Mountain ----- Orkild (1968) F-l to F-2 ------ Yucca Lake ........ Harley Barnes (unpub. cross section) F-3 to F4 ------ Plutonium Valley - — - Hinriehs and McKay (1965) The above listed cross sections were modified only by extending them to sea level. Fence Panels across center Of valley were constructed using an isopach map of alluvium and a StruCture-contour map on pre-Cenozoic rocks by Harley Barnes of the U.S. Geological Survey (unpub. map). Also utilized were lithologic and geophysical logs in files of U.S. 83-68a 02402 (OTaII Test well 83-67 Bl (Op) Test well 2400 ------- Potentiometric contour Shows altitude of potentiometric surface in pre~Tertiary aquifers and aquitards; dashed where inferred; contour interval is variable; contours of potentiometric surface for Cenozoic hydrogeologic units not shown for reasons outlined in text SHOWN IN SECTIONS ONLY 4—— -—r Fault Arrows indicate direction of relative movement 83-68 Test well Number is well number, dashed where projected into line of section Potentiometric surface in pre—Tertiary rocks SOURCES OF GEOLOGIC DATA FOR FENCE PANEL TERTIARY AND QUATERNARY W TERTIARY TO PERMIAN PERMIAN CRETACEOUS PENNSYL- VAN IAN AND V DEVONIAN AND MISSISSIPPIAN CAMBRIAN TO DEVONIAN Y PRECAMBRIAN AND CAMBRIAN HYDROGEOLOGIC UNIT Valley-fill aquifer > Tuff aquifers and aquitards Granitic stocks (aquitards) Upper carbonate aquifer Upper clastic aquitard Lower carbonate aquifer Lower elastic aquitard Geological Survey, Las Vegas, Nevada. Fence panels constructed along base lines that range in altitude from 3,500 to 4,500 feet. 83~666 83-66b BEND IN SECTION BEND IN SECTION 83-66c 940 ft, per mile ‘\?L PROFESSIONAL PAPER 712—C PLATE 2 I 6000 5000’ 4000’ POTENTIOMETFIIC LEVEL 240 ft. per mile < 5 ft. per mile - 3000' I A SYNCLINE VERTICAL EXAGGERATION X 4 HYDRAULIC SECTION SECTION YUCCA FAULT 2000 A' — 6000' 83-666 55 z (projected) a ; 0 fig 83 67 E “I W 83-66c m PIP MDe .__L 4000’ — 2000' _ SEA LEVEL GEOLOGIC SECTION Modified from Winograd and Thordarson (1968, fig. 5) 2000’ Prepared on behalf of the US. ATOMIC ENERGY COMMISSION R 62 E. 115°OO’ 38°OO' UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY 9 MI. TO U. S. 95 E. 900,000 15/ R59E HUNGRY: MI. E- 800,000 R 57 E 30” R 56E. WARM SPRINGS 16 MI. E. 700,000 R 53 E 116°OO' 45' . E. 600,000 15’ R 50 E\ u\ ,/ E. 500,000 30’ R 49 E RABE 45' R 47 E 117°00’ " R E 38°00' ,/ »N. 1,100,000 45' N. 1,100,000 ~____“, \ fill-[010,70 '01!" 30' N. 1,000,000 N. 1,000,030 15’ ' N. 900,000 N. 900,000 / - / .lltu/I 37"00’ 37°00' -'-.-.~‘ .4: K. lcks 1% 17-21:: 7. N. 800,000 N . 300,000 a . Silver K I, g ,1 largets a,” 0 {>03 1 Krilowefi, ) l / 45’ 45’ N. 700,000 \ H N. 700,000 BIG DUN5\\ ' War-02., llowers . .- . O % .-\\ 3/ /' l \ 0 ;/ '/\ 15/ "'9 2/2 1 - ' _. - a *\\\\\\_\ <<‘ _. f \ f .1“ \' \_- I ; \\\\ ‘f [V/ \ / / :_ \ . .- " E / . -' / / 016$ l9—1_ 30' 30' 7 {anion PZWLI PM"! \ N. 600,000 N. 600,000 —‘).V_‘ Lovell Valley 2000 .Ariesmn . _ . . 15' 15' ‘1 ,. _ .- 252 - ‘ ’ " :_ _ _. / \ A F RANGE r. \ \ \ Sky Haven\ 2200 T Lakes N. 500,000 N. 500,000 /Plt‘tm_an ~4pr / ’o ( D "0 ll. 0» k a a a" e \ Foo _/ \. -.\ o \ ‘. . _/ —\- 36.00, I' "R 54 E R 62 E . 36°00' R 3 E. . . R 9 E 1E16°00’ 080E I R, 56 E E 800000 E 900 000 R. 60 E. 15’ R 61 E . - 115 00 E. 000 _ . 700, v I ' I MI. INTERIOR—GEOLOGICAL SURVEY, RESTON, V‘RGlNlA—l975 500. SHOSHONE2 m smov mm 20 m JEAN w Hydrochemistry by I. J. Winograd, 1965 R. 47 E. R. 2 E.45’ 100,000—foot grid based on Nevada coordinate system, central zone 117°oo' JUB/LEE PASS 13 MI. E. 500,000 _ SHOSHONE 13 Ml. BAKER SI Ml. Base from US. Geologlcal Survey SCALE 1.250 000 Caliente, Death Valley, Goldfield, 5 O 5 10 15 20 25 MILES and Las Vegas, 1954 E '_4 1_1 ]_1 l--——-—l |—__.__—] l E 5 0 5 10 15 20 25 KILOMETERS g :r l—l l—l r—-—‘!i l—l 41 CONTOUR INTERVAL 200 FEET DATUM IS MEAN SEA LEVEL APPROXIMATE MEAN DECLlNATION,197d MAP SHOWING GROUND-WATER CHEMISTRY AND HYDROCHEMICAL FACIES, NEVADA TEST SITE AND VICINITY, SOUTHERN NEVADA PROFESSIONAL PAPER 712-C ’ PLATE 5 EXPLANATION HYDRAULIC SYMBOLS .87-62 Test well Well tapping pre- Tertiary h ydrogeologic units; lower carbonate aquifer except well 89-68 which taps the lower elastic aquitard; number is well number @73-66 Test well or water well Well tapping both Cenozoic and pre-Tertiary hydrogeologic units; number is well number o91-74 Water well Well tapping valley-fill aquifer; number is well number (D9075 Test well or water well Well tapping Tertiary hydrogeologic units; number is well number ,5/60-10da Spring or cavern Spring or cavern in, or near outcrop of, lower carbonate aquifer; at Ash Meadows includes springs in lakebeds fed by upward leakage from carbonate aquifer; spring 1 7/53-21 taps the lower elastic aquitard; number is spring number 17/59-34a 9 Spring Spring in Quaternary and Tertiary valley-fill aquifer or lakebeds; number is spring number SD11/47-10ab Spring Spring in Tertiary hydrogeologic units; in Oasis Valley includes springs in valley-fill aquifer; number is spring number CHEMICAL SYMBOLS + Ca + Mg HCO3 CO3 + 504 C1 Na + K 8 O 8 Plotting scheme for pie diagrams, and scale, in milliequivalents per liter Calcium magnesium bicarbonate type water Percentage range of ion pairs given in table 9 Sodium potassium bicarbonate type water Perched water shown in black; percentage range of ion pairs given in table 9 Calcium magnesium sodium bicarbonate and other mixed type waters Percentage range of ion pairs given in table 9 Playa type water ———o 0 Boundary outlining areas where ground water of designated type predominates Solid line, water from lower carbonate and valley-fill aquifers is principally of calcium magnesium bicarbonate facies; short-dashed line, water from tuff, rhyolite, and valley-fill aquifers and aquitards is principally of sodium potas- sium bicarbonate facies; long-dashed line, water from lower carbonate aquifer is of calcium magnesium sodium bicarbonate facies; short-dashed and small- circled line, water from valley-fill aquifer in central and northwestern Amar- gosa Desert is mixture of water from areas IIE, IIG, IIIA, and IIIB. Roman numerals designate enclosed areas; name and statistical summary of water chemistry of each area given in table 8 Wet playa Water table at, or within few feet of, surface Furnace Creek Wash-Nevares Springs area 5757‘ 7DAYS Pa Aim v. ‘7 I a. 3 IENCES DIARY Hydrologic Processes and Radionuclide Distribution in a Cavity and Chimney Produced by the Cannikin Nuclear Explosion, Amchitka Island, Alaska , GEOWL SURVEY l:_I_{_QFESSIONAL PAPEy/l2—D T 0 Prepared on behalf of the US. Energy Research and Development Administration .q. ._ DOM :H ”FR-'70 ”"M ‘3" FEB 15 1978 57 i ”Emmi-"u .. «sub»; Ni;F()i"§-'.~=. U.S. DEPOSE '{ORY 92:5 «3 11978 Hydrologic Processes and Radionuclide Distribution in a Cavity and Chimney Produced by the Cannikin Nuclear Explosion, Amchitka Island, Alaska By HANS c. CLAASSEN HYDROLOGY OF NUCLEAR TEST SITES GEOLOGICAL SURVEY PROFESSIONAL PAPER 712—D Prepared on behalf of the US. Energy Research and Development Administration UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1978 UNITED STATES DEPARTMENT OF THE INTERIOR CECIL D. ANDRUS, Secretary GEOLOGICAL SURVEY V. E. McKelvey, Director Library of Congress Cataloging in Publication Data Claassen, Hans C. Hydrologic processes and radionuclide distribution in a cavity and chimney produced by the Cannikin nuclear explosion, Amchitka Island, Alaska. (Hydrology of nuclear test sites) (Geological Survey Professional Paper 7l2—D) Bibliography: p. 28 l. Hydrology—Alaska—Amchitka Island. 2. Radioactive substances—Alaska—Amchitka Island. 1. United States Energy Research and Development Administration. 11. Title: Hydrologic processes and radionuclide distribution... III. Series. IV. Series: United States Geological Survey Professional Paper 7l2-D. GB705.A4C55 551:4'8 77—9363 For sale by the Superintendent of Documents, US. Government Printing Office Washington, DC. 20402 Stock Number 024-001-03037—3 CONTENTS ‘ Page Page Abstract ................................................ D1 Water quality of the Cannikin-perturbed hydrologic system —- Introduction ............................................ 1 Continued Hydrologic processes in the Cannikin cavity and chimney . . . 3 General discussion 0f quality .Of water samples obtained Events occurring prior to and following condensation of from UA _1 _P1 —Continued . . . , Results from the October 1972 sampling .......... D16 steam in the Cannikin cav1ty ....................... 3 - . . . . . . , Results from the January 1973 sampling .......... 16 Determination of the distribution of chimney poros1ty cre— Re ult f th M 1973 m 1. 19 ated by the Cannikin event ......................... 8 s s rom e ay sa p mg """"""" Heat distribution in the Cannikin cavity .................. 11 _Water'quality 8.8 a descriptor 0f the flow system """" 19 RadioactiVity 1n cav1ty water ............................. 23 Water quality of the Cannikin-perturbed hydrologic system . 12 Beta/gamma activity as an indicator of radionuclide-sorp- General discussion of quality of the water samples ob- tion disequilibrium ................................ 23 tained from UA —1 ——P1 ......................... 12 Tritium activity ..................................... 26 Results from early sampling; February and July 1972 12 Alpha activity ....................................... 26 Localized flow near the UA —1 —P1 drill hole . . . 13 Calculation of cavity radius for the Cannikin event ......... 26 Interpretation of vertical distribution of water Summary and Conclusions ............................... 27 quality in the Cannikin cavity .............. 14 Selected references ...................................... 28 ILLUSTRATIONS Page PLATE 1. Diagrams showing conditions in the Cannikin chimney region from November 6, 1971, to May 3, 1973, Amchitka Island, Alaska .............................................................................................. In pocket FIGURE 1. Map showing location of selected sampling sites, Amchitka Island, Alaska ........................................ D2 2. Map showing intersection A —A’ of projection plane with land surface. White Alice Creek drainage basin and location of selected sites in the vicinity of Cannikin ground zero, Amchitka Island, Alaska ........................... 4 3 —15. Graphs showing: 3. Temperature history of the Cannikin cavity ............................................................ 5 4. Water-level history of the Cannikin chimney and its relation to aquifers and other regions of interest intersected by the cavity and chimney ........................................................................ 7 5. Daily flow contribution to the Cannikin chimney from each aquifer and the White Alice Creek drainage and the combined contribution of all aquifers plus the White Alice Creek drainage . ._ .......................... 9 6. Hypothetical porosity distribution in the Cannikin chimney for three cavity sizes and two days of condensation each ............................................................................................. 10 7. Effect of cavity size and time of condensation on fraction of subsurface void volume filled by day 260' ....... 11 8. Porosity distribution in the Cannikin chimney for Rm = 1.34 and Dc = 60 ................................... 11 9. Temperature profiles in UA —1 —P1 and their relationship to hole construction and natural gradient ........ 15 10. Temperature- and gamma-log profiles in UA —1 -—P1 and their relation to sampling zones B through F ...... 16 11. Results of FLO-PAK survey in UA —1 —P1 .............................................................. 16 12. Relationship between dissolved-solids content and tritium activity in selected samples from UA' —1 —P1 ..... 21 13. Chemical evolution of surface water and shallow ground water and its relation to water from UA —1 —P1 and other subsurface water ............................................................................ 22 14. Comparison of changes in average beta/gamma activity with time determined by two methods -—planchet re- counting and data-regression analysis .............................................................. 24 15. Gross beta/gamma activity versus dissolved-solids content regression lines for the various sampling episodes in UA —1 -P1 and determination of average radioisotope half-life ....................................... 25 III IV CONTENTS TABLES Page TABLE 1. Summary of hydraulic data obtained in hole UAE —1, Amchitka Island, Alaska .................................... D8 2. Chemical and radiochemical analyses of samples from UA —1 —P1 collected from February 1972 to July 1972 ...... 13 3. Chemical and radiochemical analyses of samples collected at selected locations on Amchitka Island, Alaska ...... 14 4. Chemical and radiochemical analyses of samples from UA -—'1 ——P1 collected October 1972 ........................ 17 5. Chemical and radiochemical analyses of samples from UA —1 —P1 collected January 1973 ........................ 18 6. Chemical and radiochemical analyses of samples from UA -1 —P1 collected May 1973 ........................... 20 HYDROLOGY OF NUCLEAR TEST SITES HYDROLOGIC PROCESSES AND RADIONUCLIDE DISTRIBUTION IN A CAVITY AND CHIMNEY PRODUCED BY THE CANNIKIN NUCLEAR EXPLOSION, AMCHITKA ISLAND, ALASKA By HANS C. ABSTRACT An analysis of hydraulic, chemical, and radiochemical data ob- tained in the vicinity of the site of a nuclear explosion (code-named Cannikin, 1971), on Amchitka Island, Alaska, was undertaken to describe the hydrologic processes associated with the saturation of subsurface void space produced by the explosion. Immediately after detonation of the explosive, a subsurface cavity was created sur— rounding the explosion point. This cavity soon was partly filled by collapse of overburden, producing void volume in a rubble chimney extending to land surface and forming a surface-collapse sink. Sur- face and ground water immediately began filling the chimney but was excluded for a time from the cavity by the presence of steam. When the steam condensed, the accumulated water in the chimney flowed into the cavity region, picking up and depositing radioactive materials along its path. Refilling of the chimney voids then resumed and was nearly complete about 260 days after the explosion. The hy- draulic properties of identified aquifers intersecting the chimney were used with estimates of surface-water inflow, chimney dimen- sions, and the measured water-level rise in the chimney to estimate the distribution of explosion-created porosity in the chimney, which ranged from about 10 percent near the bottom to 4 percent near the top. Chemical and radiochemical analyses of water from the cavity resulted in identification of three aqueous phases —ground water, surface water, and condensed steam. Although most water samples represented mixtures of these phases, they contained radioactivity representative of all radioactivity produced by the explosion. Sorp- tion of radioactivity on particulate matter was evident, more than 10,000 times as much activity being found on solids as in solution; however, no selectivity for specific isotopes was observed, and sorp- tion equilibrium had not been reached 400 days after the explosion. Although large amounts of heat were released by the Cannikin ex- plosion, approximately 90 percent of this heat was absorbed by rock and water that fell into the cavity. As heat in the cavity dissipated to the surroundings, additional voids were created by cooling of the rocks that had fallen into the cavity; this new space in part ac- counted for the continued water flow into the cavity observed after chimney filling was virtually complete. The occurrence of zones of low hydraulic conductivity may also have contributed to the delay in filling the cavity. CLAASSEN INTRODUCTION A nuclear device, code-named Cannikin, was deto- nated November 6, 1971, 5875 ft (1790 m) below land surface on Amchitka Island, Alaska, as part of the weapons-testing program of the US. Energy Research and Development Administration (formerly the US. Atomic Energy Commission). For details concerning the Cannikin experiment, see Merritt (1973). After detonation of the device, a reentry hole was drilled into the subsurface cavity formed by the explosion. One of the purposes of the reentry hole was to provide data for a better understanding of the postdetonation hy- drologic processes occurring in a nuclear-explosion-pro- duced cavity and chimney in volcanic rocks. In addition to acquiring hydrologic information, the postdetonation studies were intended to provide data on radionuclide distribution in the cavity and to allow estimation of radionuclide concentrations in cavity water which would be useful for improving predictions of possible contamination hazards associated with un- derground nuclear testing in a saturated zone. Unfor- tunately, conditions under which samples were ob- tained for analysis were less than ideal, and the stated goals were not reached in every respect; nevertheless, valuable data useful for understanding the movement and chemical behavior of radionuclides were obtained. Amchitka Island is the southernmost and largest is- land of the Rat Island group, part of a chain of islands extending from the Alaskan Peninsula and forming the southern boundary of the Bering Sea (fig. 1). The humid climate contributes to a nearly saturated subsur- face environment, composed entirely of volcanic rocks, D1 HYDROLOGY OF NUCLEAR TEST SITES D2 Amy—mm? 453$ wan—EonfiV ,mufim 9595.3 @3023 we r8583 I A 5505 8 2512.0: 25 ate-80.9 it: co:uo_o& .2832 3358; 5203.5 000 2b 35m: 35:3 23 $3.28 Ea: 3:602 000 03 _ _ _ 1 _ _ ..Om.h_o.n OZ<4m_ (VELIUId. amzupuzodxon _ H .Lk. M xiv: I 595... 3.5.3. w o . 0» So . co 5 m m c 1 8° 08 m r00 4'; 85 o 3.2.: .Sw .58 5.2. I Ir MIIPI :03 z n u we E T43 __ 3 l v09 10/09 980 2:6 N. 323:... *0 3 E00. 3.0.. SuN 2580 B 2 «.m0 0 .mmm 50mm 3.0:. 2:4“: oth 9590 25:2240 .I _..I._._._ :35 03‘70 <1 I. .OnoB 2.8a 9220 980 oz<._m_ qmm 3:3 I 2... w. I H “nectar—534‘ m H II 0/ W. I no _ x— a 0/ w p I. .813 m. m I 0/ W. G O u IN II / m I m I: O D m I I: - x . m. m mO_x_ III 0 $0506 :20 wo=< 223 m I ..M M I II IIIIIII IIII I||| a Illlkfificmfinslcfiwwslzl II IIIIIII n N I D m. I 8 U I II x I ¢O_ x_ 3 o 0 Q / x n u 01 3 I / / / I. 3.253 x u/ x a 3 l / D / J 1 w - 0 ~55? x a /x 3 -, / 4, I I 39.65 $20 3.3. 253 man. x x W I .l | IA I 95:36 :5 .3 co::n_::oo cocEEoo OIOI / l ./. .l w x I I O Io. _ I ./ Io/ - I /O O 0.. I /o l /O/ l I o o I 0:: l / / D O/O/O O/O/ _ _ _ _ _ _ b _ _ _ b _ _ _ _ . no. .5 D10 HYDROLOGY OF NUCLEAR TEST SITES then expanded slightly to the regions (indicated by Roman numerals in fig. 4) and the porosity value ob- tained for the subregion then applied to the entire region, to include the entire height of chimney repre- sented by measured water levels. Various values for DC were chosen for each of several ch’s and some of the results plotted in figure 6. Not all the calculated values are plotted, inasmuch as the effects of varying cavity size and day of condensation are well illustrated by using only some of the computed data. The greatest effect on absolute porosity is brought about by varia- tions in cavity size and, consequently, chimney size; for example, an increase in subsurface void volume of 170 percent Rm = 1.00 to ch = 1.40) results in a relative decrease in calculated overall porosity of about 45 per- cent throughout the chimney. The effect of an error in choice of DC is slight; an error of 40 days produces an absolute error of only about 0.01 in the porosity at chimney top and bottom and a negligible error in chimney mid-region porosity estimates. The total flow into the chimney must fill the available void volume — no more, no less. Figure 7 illustrates the effect of varying cavity size and day of condensation on the fraction of void filled by day 260. Day 260 has been arbitrarily chosen as representing completely filled conditions, even though only 95 percent of theoretically fillable chimney has been saturated as indicated by water-level data in figure 4. The value 1.0 on the ordi- nate represents a completely filled (by day 260) chimney. Referring to figure 5, note that shifting the day of condensation to smaller values (earlier times) would result in lower total flow to a chimney of given size; ch = RCH= 1.34 in this example. In figure 7, for Rm = 1.00, no DC, day of steam condensation in the cavity, can be chosen which will generate little enough flow to do anything but overfill the available subsurface void in the allotted filling time. If the subsurface void volume is determined by assuming a spherical cavity (whose radius is determined by the measured radius of . the lower hemisphere) and subtracting the sink volume, and if the aquifer characteristics and infill data that were used in making the calculations are correct, only irrational results are obtained. Uncertainties in both cavity geometry (the cavity may not be spherical, and the void produced may actually be greater than that calculated assuming sphericity) and in collapse- sink volume (surveying errors) produce uncertainty in any determination of the subsurface void, and prevent the condition of total inflow by day 260 (Q1260) equals VSUBSFCfrom being met. This condition must be satisfied for an exact determination of the explosion- produced chimney porosity distribution. Pressure and temperature data prior to day 106, if available, would allow independent determination of day of condensa- 7000 I —2000 Land surface/ 6000— 2 3 5°00- -—|500 3 '— '3 g o uJ 3 2 E u. I n: 5’» n 8 m 4000 m D I) U) U) >. E a; < 4 g —|OOO g E , E q 3000— 4 “J Lu > 5 8 \ m < 0‘ 4 (D " \ a: m ‘ m {f 2000— i t, l 2 I \ . —500 I \ i \ I 0 I000— N 8 o 8 II II II H II II é’s‘.’ sssé’ 3.3. r: ’2. 8 8 iiiiii $0? a? o:K it“ it" 0 l o o 0.|O 0.20 POROSITY CREATED BY EXPLOSION EXPLANATION Dc Day of condensation. in days since zero time R”; Relative coviiy radius FIGURE 6. — Hypothetical porosity distribution in the Cannikin chimney for three cavity sizes and two days of condensation each. TOTAL INFLOW BY DAY 260/ SUBSURFACE VOID VOLUME HYDROLOGIC PROCESSES AND RADIONUCLIDE DISTRIBUTION, AMCHITKA ISLAND, ALASKA 5 I T I I OVERFILLED SUBSURFACE VOID UNDERFILLED SUBSURFACE VOID TIME OF CONDENSATION (DAYS SINCE ZERO TIME) FIGURE 7. — Effect of cavity size and time of condensation on frac- tion of subsurface void volume filled by day 260. tion rather than reliance on inference (D0260) from water-level data as previously discussed. (See p. D5.) On the assumption that the Dc =60 estimate is cor- rect, interpolation between the curves ch = 1.30 and Bro = 1.40 in figure 7 to obtain a curve which would in- tersect Q§6°/ VSUBSFC = 1 at D0 = 60 yields an Rm 2 1.34. Figure 8 shows the resultant chimney porosity distribu- tion, about 10 percent near the bottom to about 4 per— cent near the top. This calculated porosity distribution is very different from that assumed by Fenske (1972) in his prediction of Cannikin chimney infill — 0 percent at the bottom, rising linearly to 14 percent at the top. In the only other published determination of rubble- chimney porosity, Garber (1971) reported a porosity range of about 7 percent near the bottom of the chimney to about 2 percent at a point about 200 ft (61 In) above the chimney bottom. The nuclear device was detonated at a point less than half the depth of Can- nikin, in zeolitized tuff, and Garber’s method of analysis consisted of comparing the volume of water D11 LAND SURFACE I I 6000 — 5°00 _ — I500 3 z I- D < I; D o 8 w < 0 a: E 2 a; 4000p m D m (I) D g (D m E E < q — IOOO E ‘3 E I: 3000_ I: m < I: <1 u: > “J o > m 8 < “ 3 G 2000— t‘.‘ N LIJ “- z —— 500 I000 — o 1 l o 0 one 0.20 POROSITY CREATED BY EXPLOSION FIGURE 8. —Porosity distribution in the Cannikin chimney for ch = 1.34 and DC = 60. removed during a pumping test with the observed inter- ruption in water-level rise resulting from infill. HEAT DISTRIBUTION IN THE CANNIKIN CAVITY The heat generated by the approximately 5 megaton TNT-equivalent Cannikin explosion was calculated by the method of Heckman (1964) to be 5 x 1015 calories (cal). Assuming the heat to be distributed among the D12 various infill materials by day 100, estimates of heat capacity (Hodgman, 1957) were combined with published values for before-detonation rock bulk den- sity and porosity (Lee and Gard, 1971) and the tem- perature from figure 3 to produce the following results for the system Rm = 1.34 and Dc = 60. 1. Heat required to raise temperature of rock melted by explosion from ambient to temperature of cavity region on day 100, AH1 = 7.1 x 1014 cal. 2. Heat required to raise temperature of water con- tained in rock melted from ambient, AH2 = 5.6 X 1013 cal. 3. Heat required to raise temperature of water infill to cavity region (estimated) by day 100, AH3 = 2.0 x 1015 cal. 4. Heat required to raise temperature of saturated rock infill generated by collapse of shock zone above cavity into cavity, AH4 = 2.5 x 1015 cal. Total heat required to raise cavity region contents to estimated conditions on day 100 is 4 2 AH,=5 x 1015. 1:1 The agreement with the amount of heat produced is probably fortuitous, inasmuch as fairly large errors are possible in all of the estimates and a 20 percent error would still be considered a good estimate; nevertheless, the implication that about 40 percent of the energy ap- pears to be deposited in infilling water (AHB) and 50 percent in collapsing saturated rock (AH4) is probably valid. The remaining 10 percent is found in AHI and AH2 above. 'Note that no caloric term has been included to repre- sent conduction of heat away from the cavity region. Preliminary calculations made by the author indicate that such a conduction process is very slow and that an assumption of adiabatic conditions for the cavity is probably a good approximation. The heat balance, of course, tends to confirm this hypothesis. Furthermore, and perhaps most important, the heat balance corroborates the hypotheses made concerning hydraulic events which occurred prior to day 100. The inflow to the cavity calculated in conformance with these hypotheses was used in determining AHa, a major fraction of the total heat in the cavity. Large variations in the hydraulic properties of the system, the day of condensation, or the cavity radius would tend to offset the previously indicated agreement. HYDROLOGY OF NUCLEAR TEST SITES WATER QUALITY OF THE CANNIKIN-PERTURBED HYDROLOGIC SYSTEM GENERAL DISCUSSION OF QUALITY OF THE WATER SAMPLES OBTAINED FROM UA—l —P1 All water samples from UA —1 —P1 were obtained with a thief sampler — that is, the sampling device was lowered to the desired level in the hole, ports were opened to allow water to enter, ports closed, and the device raised to land surface. Use of this method pro- vided water samples from the drill stem, presumed to be representative of water in the cavity/chimney region. Surging with nitrogen or air was done prior to some of the sampling to increase the probability that drill-stem samples would contain a large proportion of formation water. As we shall see, the method of hole completion played a dominant role in the analytical results from samples obtained through thief sampling of UA -—1 —P1. RESULTS FROM EARLY SAMPLING, FEBRUARY AND JULY 1972 The first sample retrieved from the hole was on February 20, 1972 (day 106). The postulated condition of the chimney region at that time is indicated on plate 1H and the analytical results are in table 2. Between February 20, 1972, and April 10, 1972 (day 156), at least 12 700 gal (48 m3) of water of unknown composi- tion, but probably from Constantine Spring, had been poured down the drill stem. From this date to July 15, 1972 (day 251), the hole remained undisturbed, water entering the perforations at zone A and raising the col- umn of water which occupied the drill stem on day 156. The postulated conditions in the chimney region and the drill stem on day 251 are illustrated on plate 1J. Sources of samples taken from various points in the drill stem on days 252 through 255 are shown on plate lJ also. Since the bottom of the introduced-water col- umn represented water residing opposite zone A on day 156, all water in the drill stem below this point repre- sented water entering zone A on a later date, the exact date being determined by the rate of water-level rise as shown in figure 4. By comparing the sampling depths with the water-level data, approximate values were ob- tained for the dates each sample entered the drill stem at zone A, and, thus, these samples became an estimate of water quality in the cavity in the vicinity of zone A on those dates. Because it was suspected that the addition of foreign water prior to day 156 might contaminate subsequent samples, the July sample results were examined for in- dications of dilution by Constantine Spring water (table HYDROLOGIC PROCESSES AND RADIONUCLIDE DISTRIBUTION, AMCHITKA ISLAND, ALASKA D13 TABLE 2. —— Chemical and radiochemical analyses of samples from UA —1 —P1 collected from February to July 1972 [Values in parentheses are corrected for dilution by lithium-tagged water placed into UA -1 —Pl prior to perforating activities at zones B through F in July 1972] . . . ' ed Di d S d d Di 'b ’ Sample identification chemigfzgiilstituenm radiochemiiillzznstituents rafiwhegfc’xflnwisfituenu cggfic‘ilglri): orgross ‘ ' b0 G ll: G be G al h G be / be / wag-g" 35:55:. “5555555.. .. “...... 301:3; 55:55.5 m5: 1...... Sugggnded 555.55 '55.: 5.55555 subsurface datum uranium 37cesium uranium 37cesium ft 1 m meq/L mg/L pCi/L pCi/L pCi/L mg/L pCi/g poi/g ml/g 2—20—72 338 103 0.03 130 8.8 1.7 X 103 1.4 X 107 ... ... . ... 7 —16—72 3207 978 .02 1600 49 1.6 X 102 9.0 X 105 2100 <1.8 3.0 X 102 1.9 X 103 7—17—72 2714 827 .01 1000 8.8 4.8 X 102 1.7 X 109 2300 <2.7 1.2 X 108 2.5 X 103 7—17—72 2230 680 .01 120 1.3 2.9 X 103 2.4 X 109 700 <3.6 2.0 X 10‘ 6.9 X 103 7 —17 —72 1733 528 .03 380 15 8.6 X 101 1.8 X 10" 3600 <1.3 1.1 X 102 1.3 X 103 7 —18—72 1196 365 .04 240 20 2.0 X 102 9.8 X 105 1600 <4.7 2.8 X 102 ‘1.4 X 103 7—18—72 1186 362 .03 220 8.5 1.7 X 102 4.9 X 10" 1300 <4.3 4.4 X 102 2.6 X 103 7—19—72 646 197 .02 230 6.9 2.9 X 102 8.7 X 10" 1200 <3.6 1 2 X 103 4.1 X 103 Zone B . 7—22 —72 654 199 4.61 700 14 3.4 X 102 1.4 X 107 95 <6.3 6 2 X 103 1.8 X 10‘ (700) (23) (5.5 X 102) (2.3 X 107) (1.1 X 10‘) Zone C . 7—23 —72 957 292 3.03 460 5.3 1.4 X 102 2.1 X 105 55 <7.3 1 5 X 10‘ 1.1 X 105 (380) (7.1) (1.8 X 102) (2.8 X 105) (8.3 X 10‘) Zone D . 7—24 —72 1255 383 2.31 2200 67 4.6 X 102 1.3 X 107 350 <4.3 2.9 X 10‘ 6.3 X 10‘ (2600) (83) (5.7 X 102) (1.6 X 107) 5.1 X 10‘) ZoneE . 7—25—72 1544 471 .81 230 18 1.2 X 102 5.4 X 101 76 <5.3 7.1 X 103 5.9 X 10‘ (200) (19) (1.3 X 102) (5.8 X 10‘) (5.5 X 10‘) Zone F . 7—26 —72 1818 554 .53 2600 98 4.1 X 102 8.9 X 10‘5 360 5.8 4.9 X 10‘ 1.2 X 105 (2700) (100) (4.3 X 102) (9.3 x 10“) (1.1 X 105) 3), the most probable source of the foreign water. Un- fortunately, only lithium and dissolved solids values are available for comparison of the July samples with Cons- tantine Spring. This is insufficient for a positive evalua- tion. However, the sample collected about 160 ft (49 m) below the bottom of the introduced—water column (pl. 1J) contains more than eight times the dissolved solids, has more than an order of magnitude greater lithium content than Constantine Spring, and has the highest dissolved-solids content of the suite of samples collected which indicates little contamination. Although the July 1972 samples from UA —1 —P1 identified in table 2 represent the earliest samples col- lected, they were in fact collected rather late in the chimney-filling process. (See pl. 1H, 1, J.) During the period of chimney-filling history represented by these samples the major portion of the chimney is being filled, the cavity region contains water representative of a prior accumulation in the chimney which has drained into the cavity region following steam conden- sation. It is interesting that none of the samples ap- proach formation water expected at the depth of zone A (aquifer 38). (See table 3 for analyses of water from this aquifer.) Two of the samples have dissolved solids and lithium values which are similar to those of aquifers 1 and 2 (table 3), and one sample is lower in dissolved solids than any permanent body of surface water listed in table 3. This latter sample is believed to be composed of a large percentage of condensed steam from the cavity —note its high radioactivity. Why do these samples not have a composition more nearly that of the contributing aquifers? The following discussion is offered as explanation. LOCALIZED FLOW NEAR THE UA—l—Pl DRILL HOLE Figure 9 shows three temperature profiles in UA —1 —P1 and their relation to hole construction. The predetonation natural thermal gradient, as measured in hole UA —1 after a long period of equilibration, is also shown for comparison. Profiles above the top of cement holding the 133/s—in. (340-mm) casing are well below normal; in fact, they are very close to the mean annual surface temperature of Amchitka of 40°F (4°C) (Gonzalez and Wollitz, 1972), indicating the source of flow during the annulus between the drilled hole and casing is of shallow origin. Between the top of the upper and the bottom of the lower cemented intervals flow is restricted sufficiently to allow the temperature to ap- proach normal; below the bottom of the 95/a-in. (240- mm) casing significant flow resumes, and the tem- perature is subnormal until the cavity region is reached. Here, the curves begin to differ significantly from each other. The April 9 log is lower in temperature than one made 101 days later (July 20). This is believed to be caused by the introduced water discussed above. The log of July 20, made prior to perforating intervals B through F, is taken as representative of average cavity temperatures. The slight gradient reversals are proba- bly caused by regions of higher than average fracture D14 HYDROLOGY OF NUCLEAR TEST SITES TABLE 3. — Chemical and radiochemical analyses of samples collected at selected locations on Amchitka Island, Alaska Dissolved Dissolved Sample identification chemical constituents radiochemical constituents f A Gross Gross g oi“ A : 1‘ A alpha beta/ '5. v +5 1‘ + 5 A, '1 as naturalgamma as Depth below 9 5 9 E g E 9 '2 uranium 137cesium Location name land surface (5% E S E E .3 2; g E: of collection point g E, E E a; E E E E E 2‘ 5 5 c8 :3 3 ES (-4 ft m mg/L meq/L meq/L meq/L meq/L meq/L meq/L meq/L mg/L pCi/L pCi/L Constantine Spring1 ......... . . . . . . 15 0.13 0.05 <0.001 2.4 0.12 0.08 1.1 180 <3 5.0 White Alice Creek? .......... . . . . . . 17 .17 .19 <.001 1.4 .05 .06 1.3 150 <0.5 7.7 Cannikin Lake .............. . . . . . . 19 .18 .25 <.001 2.9 .10 .47 2.0 250 1.9 4.7 Precipitation at south hangera ............. . . . . . . . . . .06 .06 . . . .35 ‘ .01 .10 .29 e40 Lake 100 meters west ofmilepost 12 ............. 2.6 .10 .06 . . . .85 .02 .06 .82 70 Lake 980 meters ESE. of Cannikin ground zero ...... . . . . . . . . . .31 .32 <.001 3 0 .03 .46 1.8 e250 Seep 3 ...................... . . . . . . 25 .03 .06 . . . 3.8 .02 .12 2.4 250 1.0 5.6 Well HTH —1 ................ 602 —770 183 —235 1 1 .02 .10 .001 6.1 .02 .87 1.8 310 3.0 4.2 Well HTH —1 ................ 746 —914 227 —279 12 .07 .70 < .001 7.4 .02 2.9 1.7 570 2.2 4.6 Well HTH—3 ................ 169 52 17 .08 .17 <.001 4.6 .07 .33 1.9 330 3.1 1.5 Well UA —1, aquifer 3B ...... 5910 1801 28 <.01 5.5 .01 48 .21 2.9 54 64600 . . . . . Well UAE —1, aquifer 1 ...... 1600 —1850 488 -564 13 .04 3.0 .01 18 .15 1.2 17 1500 Well UAE -1, aquifer 2 ...... 2490 —2580 759 —786 28 .10 2.6 .10 13 .13 3.1 15 1500 . . . . . . Well UAE —1, aquifer 3B ..... 5650 —5850 1722 —1783 22 .01 7.2 <.01 52 .20 3.4 58 4300 <3.3 (5.0 Well UAE -1, aquifer 3B ..... 5000 —7000 1524 —2134 18 <.01 7.7 .01 41 .26 2.9 49 4600 < 15 18 [Average of 17 samples collected from October 1964 to July 1972. Average of 9 samples collected from August 1967 to April 1972. conductivity. The log of July 27 shows the effect of per- forating the five intervals above zone A. The local hy- draulic conditions in the vicinity of the drill stem affected the composition of subsequent samples. We shall see just how in the discussion of samples obtained during perforating operations and during later sam- pling episodes. ‘ INTERPRETATION OF VERTICAL DISTRIBUTION OF WATER QUALITY IN THE CANNIKIN CAVITY Samples from intervals B through F (table 2) were first obtained immediately after sequential perforating and surging. The sequence of perforating was from the lowest interval (B) to the highest (F). Since the hy- draulic potential decreased slightly with depth, it was slightly higher in each zone perforated than in those perforated previously; therefore, water sampled after perforating should have been that which had just en- tered the drill stern. In order to assess the effectiveness of surging in in- ducing water from outside to enter the drill stem, fresh water to which lithium chloride had been added as a tracer was introduced into the drill stem in an amount slightly more than necessary to displace the water already present. This tracer was added just prior to per- forating zones B through F. The data in table 2 associ- ated with the designated zones B through F are the 3Average of 4 samples. e, Estimated from specificAconductance data. results of sampling after perforating each zone and surging. The sample from zone B was the most contami- nated (contained 38 percent tracer water), and the sample from zone F, the least (contained 4 percent tracer water). All the raw data were corrected when possible for tracer-water contamination; these cor- rected results are included in parentheses in the tables. The samples from zones D and F are high in dissolved solids and in radioactivity, but are from zones showing low gamma activities on the gamma log. (See fig. 10 for the results obtained from gamma-logging and their relation to the perforated intervals, zones B through F.) The samples from zones C and E are very low in dis- solved solids and contain considerably less radioactivity but are from zones which logged high in gamma ac- tivity. The sample from zone B is both high in radioac- tivity and from a zone of high gamma activity. Keeping in mind that the drill hole changed the local hydrologic conditions, it is reasonable to postulate that the zones of highest gamma activity are the zones of greatest hy- draulic conductivity (note slight temperature reversals at zones B and C on the temperature log), having car- ried considerable radioactive water during infill and, since drilling of the reentry hole, carried major amounts of near-surface-source water flowing along the route penetrated by the hole. The result is that ra- dionuclides have been deposited on the surfaces of these hydraulically conductive routes during the infill HYDROLOGIC PROCESSES AND RADIONUCLIDE DISTRIBUTION, AMCHITKA ISLAND, ALASKA D15 TEMPERATURE,IN DEGREES FAHRENHEIT 7000 5'0 |_ :1 II U’ a: II II >- < I II CE 1 II 'I < ,_ {I l' - I000 E E I II ._ a: 3000- ii 'i E < July 27, :l :I < LIJ II II LIJ 5 i ' 5 a: 9%inch(240mm)casing I ' to < ' : Cemented < ,_ : l intervals g; m | I LIJ __ | L 2000 l1 E 4',éinch(ll0mm)drill stem F ' lOtoot intervals E I perforated T 500 July 2| to o 26, I972 July 20, l972 (day 256) (letter denotes IOOO- C I zone) July 27, l972 (day 263) B t A I 20 foot interval perforated I *- February 20,l972 0 l l l o 309 325 350 375 ABSOLUTE TEMPERATURE, IN KELVINS FIGURE 9. — Temperature profiles in UA —1 -P1 and their relationship to hole construction and natural gradient. process; the carrier solutions have subsequently been radioactivity but have not accumulated gamma activity flushed by the water flowing near the reentry hole, over and above that of infill water composition,owing to which has resulted in fresh water of low radioactivity in relatively restricted flow conditions. Radioactivity is zones of high gamma activity. Conversely, the zones ex- anticipated to increase with depth in the cavity region hibiting low gamma activity still contain water of ap- because of leaching by downward percolating of infill proximate cavity-infill water dissolved solids and water; therefore, it is not surprising that the lowest D16 HYDROLOGY OF NUCLEAR TEST SITES TEMPERATUREIIN DEGREES FAHRENHEIT l0 I50 2000 I I I ‘ I0 I > >- Gamma log mode July l9,l972 F 3 SI Temperature log mode _ 500 g a: 3 5 July 20,I972 I. g t ’5: E 2 2 S D < o < M u u 2 IOOO c g 2" > IL -—-—-i u. o I: m D: m 3 kW < 3 < ‘0 B (n 0" g . . . (I £3 :3", Somrlmg zone deSIgnatIon E U) LIJ m u“ S O l l l I l l 0 IO 20 30 300 325 350 GAMMA COUNTS PER MINUTE (xI0'3) ABSOLUTE TEMPERATURE, IN KELVINS FIGURE 10. -—— Temperature- and gamma-log profiles in UA —1 —P1 and their relation to sampling zones B through F. zone (B) shows sufficient radioactivity to overshadow the above effects. Three additional visits to the UA —1 —P1 site were made and samples thiefed from the drill stem both before and after surging. These will first be discussed separately. RESULTS FROM THE OCTOBER 1972 SAMPLING The samples obtained from points above zone B prior to surging (October 13—15, 1972) are generally more saline than those obtained after surging (October 18—19, 1972). The data are shown in table 4. The tem- perature log made prior to sampling (identical to the July 27, 1972, log in fig. 9) indicated downward flow in the vicinity of the drill stem. However, the analytical results show little or no flow in the drill stem above zone B, suggesting that the downward flow was outside of the drill stem and that the perforations in zones C through F were plugged. The analytical results of post- surging samples confirm this hypothesis as nearly all samples decrease in dissolved- and radioactive-consti- tuent concentration. Zone D is the exception, and will be discussed later. (See below.) RESULTS FROM THE JANUARY 1973 SAMPLING The analytical results from samples collected during January 1973 are found in table 5. Once again, the sam- ples obtained before surging (January 17 —18, 1973) are different from those obtained after surging (January 21 —22, 1973), but the differences are less pronounced. The post-surging results from zone D display behavior similar to that of the October 1972 sampling — that is, a less pronounced decrease in dissolved solids and radioactivity than observed at other zones. Zone D prob- ably is producing water from a region outside the drill hole in conjunction with fresher water flowing down the drill hole from above. This fresher water enters zones E and F, flows down the drill stem and mixes with the more saline (and more radioactive) water entering at zone D. The same fresh water entering zones E and F is apparently available for entry at zones B and C. A FLO-PAK1 log (essentially a flowmeter survey to deter- mine quantity of water flowing vertically in a hole) pro- duced the data shown in figure 11. These results, ob- tained after surging, show only a small contribution to 1 The use of any brand name in this report is for identification purposes only and does not imply endorsement by the US. Geological Survey. FLOW IN DRILL STEMIIN GALLONS PER MINUTE O 20 40 60 2000 I I I >- >- g Sampling — zone “F g 2 l desi nation —500 a: E < E '5 3:: O —-D < o uJ LIJ UIOOO— _c “$13 > < O <[ Ou- m u. m 01 4 o: <§ —B a m L] 3 E g m m I— m LL Lu 0 I o 2 o 200 400 FLOW IN DRILL STEM,|N CUBIC METERS PER DAY . FIGURE 1 1. — Results of FLO-PAK survey in UA —1 —P1. D17 HYDROLOGIC PROCESSES AND RADIONUCLIDE DISTRIBUTION, AMCHITKA ISLAND, ALASKA .:o:a:m_mou SA A ..oA X MM 1: X MA ASV ovA “MA X MA .3 X MM MA oAv ... AN .. M.A.. No. MM. MMA MVM NhIMAIoA Z... 3 .3 X MM noA X «A MMV MON moA X AA .3 X MM NAV 0AM AA. ON «A. NA. Mo. no. 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B 3955.5 no QED .5533 >33 :30 53—» kucwamzm c525. ma aEEam 3.5.3: mm M. m m. m m m. m. m gone wow—32D 320 \305 $9.0 233 W l W m, J W m m. ‘ 9.3an w. rm. W W W n W. m .332: (7.. (yr un+ (H. (H H m «8:39.53 (I 1 + RENE. (Z 322.:ch 3:35:28 32.5.5212 2:25:ng _u£Ew:vo€u.. 3:25:23 32:55 sciatic vovconmzw 82839 u0>—°mm5 5535.202 aEEam _ NR: .23. E y.— swsott m 3:3 as 353:: 955.8th 8 SE SI HI <2 SE coon—n L353 vague—5:5: .3 533:". .5» 130280 a; ago—2:023 E 325: mug @3255 $3828 Hml HI Vb Sch 8353ch 3335“ Heuwswfifiudx ~35 32525 I .m mania HYDROLOGIC PROCESSES AND RADIONUCLIDE DISTRIBUTION, AMCHITKA ISLAND, ALASKA vertical flow in the drill stem from zone F (less than 1 percent), about 31 percent of the total flow is con- tributed by zone E, 55 percent by zone D, and an addi- tional 14 percent by zone C. Zone B does not seem to contribute, and may actually be an exit point for, some of the water flowing down the drill stem. The perforations in the drill stem apparently became plugged between October 1972 and January 1973, but perhaps not as severely as before the October 1972 sampling. RESULTS FROM THE MAY 1973 SAMPLING The differences between presurging (May 2 —4, 1973) and postsurg’ing (May 8—9, 1973) samples, reported in table 6, are again evident, but to a lesser degree than in any of the previous sampling episodes. The downhole flow pattern is evident, as before, from temperature and FLO-PAK logs (not illustrated but virtually identi- cal to the July 27, 1972, temperature log illustrated in figure 9 and the FLO-PAK log in figure 11) and zone D still contributes the most saline and radioactive water. Figure 12 shows the relationship between dissolved solids and tritium concentration for samples obtained from zones B through F during the July 1972, October 1972, January 1973, and May 1973 samplings. Although there is considerable scatter in the data, two relationships are evident — (1) samples with higher dissolved-solids values generally have higher tritium radioactivities, and (2) the range of dissolved-solids values (and, therefore, radioactivities) decreases from October 1972 to May 1973. This is consistent with the hypothesis that the cavity region was initially filled mainly by formation water from aquifers 3B, 3A, 2, and 1, which have dissolved-solids values in the range 1500 to 4500 mg/L. This water, which would be expected to contain the major amount of radioactivity (ignoring the problems associated with condensed steam estimated to account for about 3 percent of total cavity water), is being diluted by near-surface water which is being recharged to the cavity region by way of the UA —1 —P1 drill hole. As this process continues, some mixing of the native and recharged waters is expected and confirmed by the decreasing range in dissolved-solids and radioac- tivity values from October 1972 to May 1973. A second phenomenon is occurring concurrently with mixing: the reaction of freshly recharged water with the subsurface aquifer matrix. A more detailed discussion of these pro- cesses follows the description of the natural water quality in the vicinity of the Cannikin site (See p. D19, D21.) WATER QUALITY AS A DESCRIPTOR OF THE FLOW SYSTEM To better explain changes observed in the samples from UA —1 -P1, an attempt was made to define the chemical changes accompanying evolution of ground- D19 water composition in the vicinity of the Cannikin site, in the belief that an understanding of the natural system would lead to an understanding of the perturbed system. From published data (Beetem and others, 1971; Schroder and Ballance, 1973) and unpublished data in US. Geological Survey files at Denver, 0010., it was determined that waters from surface, shallow subsur- face, and deep subsurface systems could be dis- tinguished from each other most easily by their respec- tive total dissolved-solids content and sodium ion to chloride ion ratios (Na/Cl). Results of plotting data ob- tained from those systems and from the Cannikin chimney and cavity during the several sampling episodes are shown in figure 13. Analytical data for samples from these sites used in constructing figure 13 are presented in tables 3 through 6. Water recharged to shallow aquifers on Amchitka begins as precipitation; the composition of precipitation on the island is affected by local conditions, mainly sea spray. Thus, the composition varies considerably from time to time, depending on local weather conditions, with high-dissolved-solids precipitation exhibiting Na/Cl ratios resembling that of sea water. The value plotted in figure 13 is an average of the Na/Cl ratio of four samples that were low in dissolved-solids content and are believed to represent average precipitation. When precipitation becomes part of the surface-water system, it reacts with the various organic and inorganic components at and near ground surface. The major result is the increase in sodium ion concentration rela- tive to chloride ion as the concentration of dissolved solids increases. The large scatter in data among the surface-water locations plotted reflects the wide variety of surface-water environments present on Amchitka. A very small fraction of the surface water probably enters the shallow ground-water system (arbitrarily defined as less than about 1000 ft or 305 m in depth), but that which does continues to react with the volcanic-rock matrix of the hydrologic system, increasing the Na/Cl ratio further. The natural vertical hydraulic conductivity between the shallow aquifers (represented by wells HTH —1 and HTH —3) and the deeper aquifers (represented by aquifers 1, 2, and 3B in wells UAE —1 and UA —1) is ap- parently very low, resulting in the large diffusion zone in the Ghyben-Herzberg lens proposed for Amchitka by Fenske (1972). The location in figure 13 of plots of sam- ples from these deeper aquifers suggest that these aquifers are within a very large zone of diffusion (or mixing) between a hypothetical sea water of present- day composition and shallow ground water of undeter- mined composition but probably similar to waters of wells HTH —1 and HTH —3. Examination of plotted data for samples obtained from UA —1 -P1 during the three major sampling HYDROLOGY OF NUCLEAR TEST SITES D20 .cczm :uEy—u :Z _ z: X m6 A: X wd vmv ovm we X CA 5w m.vV ... Hm. Wm H6 8V N". am; vmm mwlmlw ..... m «EON A: X VA A: X n; 3V o2 v.2 X «A A: X MA finv 03 mm. Wm ah no. 2. «mm hnm mwimlm ..... 0 BEN A: X Nd A: X Nam wm cm; A: X v.w A: X HA Z 03 mm. Nd 9m 5. S. mwm mnmfi mm: min ..... D BEN A: X we NON X m6 amV mm A: X 5v 3: X GA mud omv mm. fiw fie No. S. :v 31.: mulwlm ..... m 98N A: x alm A: X Qm omv mm A: X Em A: X 0A QwV 02. mm. fim fio mo. 2. war. 23 mplwlm ..... m 0:0N A: X ”m NS X m; mm 02 A: X mA vw Dov oov mm. 0N mi 8. pm. :w moon mp: min ......... 3 A: X m4. 1: X w.m mmv o2 wofi X NA A: X MA wSV own on. mam vs S. 3. mg vmm mnlvlm ..... m mGON 2: x E Ni x 3 a own .2 x g .3 x m: as owv S. v.“ 3 mo. mo. 3w 2w Elvin ......... S A: X Nd “cm X Wm mm mu 2: X MA .3 X NA m.wV omw mv. 9N m,» S. vm. mmm 5mm $.le ..... O 95M A: X «mm “3 X md mv mm 2: X “A A: X v4 2 03 ow. wfi we S. E. mmm NH: 2.le ......... 3 EH X H.» N3 X A: mHV com A: X hm a: X we m.mV omw mm. wd fim S. E. mwm mmmfi mu! mlm ..... Q “EON mod X mm A: X Qw wHV can A: X mg.“ A: X FA mm 25 om. mN fim no. om. wmv no: MEI min ......... 3 «S X a: N2 X m.m on CNN We X HN A: X wé EV 0mm Nb. we 3 No. hm. :v “.va mKI mlm ..... m chN we X w.@ .2 x 2 NS x 3 iv 8m é x 2 _S x 3 Ev 82 xx 2 E S. 3. :m 33 2&4. ......... S A: X Wm we X NE NV OS A: X Nam a: X mg m: omw :a 0N OH vo. mm. vmm mm: mul mlm ..... m BEN i: x 0.8 S: x as :2 x we .3 x 3 NS x we 5 8v 3: x 3 a: x ma, 2V o5 :. Z” 3 am. 3. :w 88 3| mlm ......... S :2 X 09 E: X ”E A: X m4 1: X wd :V ovm voH X vd A: X flu m.mV 05w 2. fin Nam om. no. mvw Sum mblmlm ......... 3 A: x Na 1: X m.m av o3 voH X wg A: X n4 3 omw omd EN ms vo. 2.0 mag swam mnl mlm. ......... 3 3:: whom 359 ARE ‘SOn 66m 65a éwE $3.: 1:62: £52.: 439: 1:38 E a 5:532: EEEE: . E333: E255 L M. O W», n 0 E35 8353: 558:8 23: 3 aEEam 3.5.3: 3 mE_Mm E5575. 3 2.:an .953: an w W W W W m @553: «o 3.5 5:83 :33 320 iii $9.0 vac—5 mam :32. macho man—a nacho m. M W m m. m mafia 3:3me 53:: W W m w m m 5555933 Up.“ [F 9H 1W. 3+ 3953‘. ( 1 (.6 32058... 3:05:28 3252.92an mezaflm—So _uo_Ew.—uo€§ mucwgcmcou 32593 5.5.5.539 vwvzwnmsm “Sigma “52235 523:2:03 vEEum _ «be 33. E a smash: m 8:3 3 3.33:: 9.55.8.“th 3 Sin :7 T <3 SE Eon—n .533 6033.825: .3 cor—2% 5.. 138:3 v.5 momwfinwusn E. 3:32 mum: ‘33 B3838 ET ~I v5 Sat 33:3ch 3.9355. Ncowswfiowfis N33 ESSEKO I .w amid. HYDROLOGIC PROCESSES AND RADIONUCLIDE DISTRIBUTION, AMCHITKA ISLAND, ALASKA D21 leoa _ T I l I I I I I I l I I I l I r- |X|07 L— a: - “J . t I _J u— m — LU Q. .— U) ‘5 _ a a D U . . O Data-reqvossuon curve . 4 6 g “'06 _ Tritium activity = l.28x|0 (dissolved—solids content) '5.72 x IO _ z 3 : >: r- - p... _ _ > I: ' ’1 ‘ U d 2 " - 2 — o - t‘ 0 EXPLANATION 0‘ xx 0 oo o ’- x’“ ago 0 July l972 samples “('05 .— x I5, 0 x October I972 samples __ Z X o 0 January l973 samples : : ‘3 U Mayl973 sompIos : x D o . _ h D — .1 ”'04 l I | l I l l I 1 l I l 1 I l 0 500 IOOO |500 2000 2500 3000 3500 4000 DlSSOLVED-SOLIDS CONTENT, IN MILLIGRAMS PER LITER FIGURE 12. — Relationship between dissolved-solids content and tritium activity in selected samples from UA —1 —P1. episodes of October 1972 and January and May 1973 reveals the effect on water quality of fresh water being rapidly introduced into the subsurface system (“shorts circuiting” the natural, unperturbed system). It was an- ticipated that this introduced water would undergo two phenomena: mixing with water from aquifers 1, 2, and 3 already present in the cavity, and reaction with the aquifer matrix on its path from near the surface down- ward to the cavity region. The data generally confirm the mixing and reaction hypothesis. The bulk of the data lying close to the postulated shallow-system reac- tion coordinate represents a combined subsurface reac- tion coordinate, paralleling the shallow-system coordi- nate, combined with mixing of water in various stages of reaction with native waters (for example, from aquifers 1, 2, and 3) residing in the cavity region. The reason for the five samples with Na/Cl ratios below those of aquifers 1 through 3 is not certain; possibly, reactive components in the cavity region produced by the nuclear explosion have introduced additional chloride or removed sodium, altering the predetonation water quality somewhat. ‘ D22 HYDROLOGY OF NUCLEAR TEST SITES l IIIIIIII l 1 IIIIIII I I | oSea water EXPLANATION — a Data from selected sampling sites — DATA FROM UA-I-PI: OOctober l972 samples a January I973 samples I May I973 samples I x IO‘ — -— — Aquifer 38(UAE-l)c Aquifer 38(UA-I) _ gAqulfer 38(UAE-I) o o .. h _ Result of mixing water from HTH-l 746-9I4 feet (227-279 meters) with _ 1A sea water in various proportions — o Aquiler 2(UAE-I) O le03— ,— _Result of mixing While Alice Creek water with sea water In various proporlions\ Well HTH-I 746-9I4 feet(227-279meters) _ > Well HTH-3 |69feet(52 meters) —' dWeII HTH-I 602—770 feel(I83-235 meters) Lake 3200feet (980 meters) E s E. of Cannikln Ground Zero Constantine Spring . ‘ White Alice Creek 0 DISSOLVED-SOLIDS CONTENT, IN MILLIGRAMS PER LITER IxIOz— — _ Lake 330feet(|00meters) :. west of milepost l2 Precipitation at south hangar ”'0! 1 1 llllllI I 1 I llillI l I 1 mo" leo° IxI0' SODIUM ION TO CHLORIDE ION RATIO FIGURE 13.. ——Chemical evolution of surface water and shallow ground water and its relation to water from UA —1 —P1 and other subsurface water. HYDROLOGIC PROCESSES AND RADIONUCLIDE DISTRIBUTION, AMCHITKA ISLAND, ALASKA RADIOACTIVITY IN CAVITY WATER As previously indicated, study of the tables contain- ing radiochemical data from UA —1 —-P1 reveals the general increase in radioactivity with increased dis- solved-solids content of the samples. Because of its 12- year half-life, the tritium content seems to correlate better than gross beta/gamma activity with dissolved- solids content (the average beta/gamma activity half- life was determined and will be discussed later). This correlation supports the hypothesis that the major infill to the cavity and chimney was from aquifers 1, 2, and 3 and that, by the time the perturbation in the flow system caused by the drilling of UA —1 —P1 occurred, at least some of the radioactivity produced by the explo- sion had been distributed throughout the more saline subsurface water from aquifers 1, 2, and 3 in the cavity region. A few exceptions to this generalization exist in data obtained during July 16 —19, 1972, as reported in table 2. The samples representing water that entered zone A during the period of days 160 —236 are generally higher in radioactivity for a given dissolved-solids con— tent than those samples obtained later, and represent- ing later times in the cavity history. Some of the sam- ples undoubtedly contain condensed steam from the cavity, which is expected to harbor large amounts of tri- tium and radionuclides with gaseous precursors —for example, note that the sample collected at 2230 ft (680 m) on July 17, 197 2, has the lowest dissolved solids but the highest tritium and gross beta/gamma activities. Once the effect of condensed steam has been minimized by convective mixing stimulated by the inception of sig- nificant downward flow at the sampling points (that is, after July 20, 1972), the relationship between dissolved solids and radioactivity is simplified, representing com- binations of nonradioactive fresh water with radioac- tive water of a salinity roughly comparable to bulk infill to the cavity and chimney. It was anticipated that a plot of salinity (as dissolved solids) versus radioactivity, ex— trapolated to a salinity value similar to that of aquifer 3B, would result in a reasonable estimate of bulk cavity- water radioactivity (that is, hydrologic-contaminant source term), but the scatter of the data was too great for a reliable extrapolation. It is indeed unfortunate that samples could not be obtained by pumping, which would have thereby minimized the effect of the fresh water introduced through the UA —1 —P1 drill hole. BETA/GAMMA ACTIVITY AS AN INDICATOR OF RADIONUCLIDE-SORPTION DISEQUILIBRIUM Although the samples collected from the reentry hole probably were not representative of bulk cavity water, the radioactivity present in the samples was reasoned D23 to be a representative sample of isotopes present in the aqueous phase but at lower concentration. Thus, measurements of radioactivity in any given sample would be an indication of the characteristics possessed by the true radioactive source water. Because very little alpha activity was observed in any of the samples and specific radionuclide analysis was prohibited by cost and manpower limitations, the changes with time in the gross beta/gamma activity (except for tritium) were chosen to describe changes in aqueous-phase radioac- tivity. Recounting of planchets prepared for gross beta/gamma analysis and calibrated using cesium-137 yielded changes in radioactive content with time for a given sample; all Such recounts made on samples col- lected during a given sampling episode (for example, October 1972) from locations at or below. zone F together with the initial counting data and the time in days which elapsed between the first counting and the recount were substituted into equation 1 to determine the average half-life for beta/gamma activity for each sample during that period. ,1 = —0.693t ” l_—n(A/A0) , where this the average half-life, in days, of the beta/- gamma activity for the time period of interest; t is the length, in days, of the time period of in- terest; A0 is the concentration of radioactivity at the beginning of the time period; A is the concentration of radioactivity at the end of the time period. Some samples were recounted more than once and others (May 1973) unfortunately were not recounted at all. Arithmetic averages were computed of all half -lives determined on samples from a given sampling episode and plotted versus the day since zero time representing the midpoint of the first count to second count (or sec- ond count to third count) time period. Curve A of figure 14 resulted from these computations. A different method of analysis was also undertaken. Figure 15 shows the gross beta/gamma data resulting from analysis of all samples obtained from below zone F in UA —1 —P1 during the four major sampling episodes plotted against the dissolved-solids content. Regression lines were fitted to the data, and the intersection of those lines with the arbitrarily chosen dissolved-solids value of 1450 mg/L was determined. Inasmuch as it was assumed that the samples represented mixtures of radioactive water of specified dissolved-solids content with nonradioactive water of also specified, but lower, dissolved-solids‘content, differences in radioactivity be- tween samples of given dissolved-solids content col- lected on two different dates should have represented D24 lOOO l l on O O [1111 1111 AVERAGE BETA/GAMMA HALF-LIFE,|N DAYS IOO -_ ., .1 —‘ 50 -l 0 Data from planchu ' recounting, CurvoA - A Data from Figure 15. _ Curve 8 I I l 800 400 500 600 DAYS SINCE ZERO TIME FIGURE 14.—Comparison of changes with time in the average beta/ gamma activity determined by two methods— planchet re- counting and data-regression analysis. only radioactive decay. The average half -life of beta/gamma activity for the three intervals indicated in figure 15 was computed by using equation 1. As shorter half-life activities decay to lower con- centrations, the average half-life of remaining activity increases, as indicated by the values of tv2 shown in figure 15. These values were plotted versus the average counting day and resulted in curve B in figure 14. The plots of half-life versus time for the two methods used differ significantly through about day 480, the half-lives calculated using differences between two linear regres- sions being shorter. At first glance (refer again to fig. 14), the disagreement might be the result of the large scatter of data for July and October 1972 around their respec- tive regression lines. This cannot be the cause, however, because similar scatter is present in computation of arithmetic means from planchet recounting data. The cause seems to lie in the assumption that only radioac- tive decay accounts for the decrease in activity in sam- ples of given dissolved-solids content between two sam- pling episodes. Recall that the determination of half-life by the method of linear regression involves using the HYDROLOGY OF NUCLEAR TEST SITES ratio of gross beta/ gamma activities of two successive sampling episodes. If the value of A (eq 1) is lower than that which would result from radioactive decay alone, a smaller value of this computed. Therefore, the most probable reason for curve B of figure 14 lying below curve A is that a certain fraction of activity is being progressively removed from solution by sorption on par- ticulate matter in the cavity region. Consequently, one would expect the distribution coefficient (K d) to increase with time; this is not observed because the K d values are very large (greater than 10 000 as found in tables 2, 4, 5, and 6) and small changes in solution concentration, which greatly affect the t,/2 determinations, have little effect on K d. Furthermore, as the bulk carrier solution becomes more dilute, radioactivity is expected to be transferred from the solution to the solid phase (that is, the sorption selectivity increases) and the calculated dis- tribution coefficients should be negatively correlated to solution dissolved-solids content. This behavior was also not observed, the average distribution coefficient, ~2.5 X 104 ml/ g, remaining independent of total solu- tion ionic strength as measured by dissolved-solids con- tent. At about 500 days since zero time, curves A and B of figure 14 approach each other, indicating approach to sorption equilibrium. The approximate time of 500 days for sorption equilibrium to be reached probably is longer than the time to equilibrium which would be ob- served in a system not disturbed by flow down the reen- try hole. These data clearly show that instantaneous sorption equilibrium does not exist in explosion-cavity situations. A quantitative estimate of rate constants from the data is not possible, however. Comparison of sample radioactivity values and their corresponding collection dates (days since zero time) with information contained in classified documents concerning the amounts and kinds of radioactivity ex- pected to be produced by the Cannikin explosive con- firm the presence of isotopes whose half -lives are simi- lar to those calculated and used to produce curve A in figure 14. Thus, one may infer that the distribution of radioactivity in the aqueous phase is similar to the dis- tribution of all radioactivity produced by the explosion. Although fractionation of certain isotopes and chemical forms must occur, it is apparently insufficient to cause large changes in the distribution of gross beta/gamma radioactivity that existed during the time interval after detonation considered in this study. At longer times after detonation, amounts and kinds of residual activity probably are controlled by selective sorption equilibria, and only loosely bound activities will remain in solution, but for intermediate times (250 to 550 days) no such selectivity is apparent. Studies now underway at older explosion sites should illuminate the question of ra- HYDROLOGIC PROCESSES AND RADIONUCLIDE DISTRIBUTION, AMCHITKA ISLAND, ALASKA D25 4500 , I I I EXPLANATION O Julyl972 samples 4000 x October I972 samples 0 January I973 samples I May I973 samples 3500 indmated. 3000 N 0| 0 O N O O 0 I500 DISSOLVED-SOLIDS CONTENT, IN MILLIGRAMS PER LITER h/g = 50 days r,,2 Average beta/gamma activity half-life calculated for period I000 ‘ _. \\\\ ’I/z =66 days \\\ . \\ \\\\ fI/Z = 330d0ys 500 ‘\ d o l I I I l I 0 I00 200 300 400 500 600 GROSS BETA/GAMMA ACTIVITY,|N PICOCURIES PER LITER 700 FIGURE 15. —Gross beta/gamma activity versus dissolved—solids content regression lines for the various sampling episodes in UA —1 —P1 and determination of average radioisotope half-life. D26 dionuclide distribution in the aqueous phase at longer times. TRITIUM ACTIVITY As previously postulated, samples collected from UA —1 —P1 represent mixtures of radioactive water of dissolved-solids content similar to that which filled the cavity (mainly from aquifer 3B) and nonradioactive low-dissolved-solids water which entered the cavity from near ground surface by downward flow in the vicinity of the drill hole. Because tritium values were not significantly affected by decay during the entire sampling period and because the above postulate leads to a linear relationship between dissolved solids and tri- tium concentration, a linear-regression analysis was performed on the tritium and dissolved-solids data. Not all analyses were used: samples collected from above the highest perforations (zone F) and samples collected prior to July 22, 1972, were omitted, the former because they likely did not represent water from outside the drill stem and the latter because some of the samples clearly indicate the presence of highly tritiated con- densed steam from the cavity prior to the mixing in- duced by downward movement of water in the UA —1 ~P1 drill hole. Figure 12, introduced earlier, is a semilogarithmic plot of tritium versus dissolved solids. Inasmuch as a linear relationship is postulated, the reader might question the use of a semilogarithmic plot; it was necessitated by the four-order-of—magnitude range in tritium values. The best linear-fit regression line is also shown. A correlation coefficient of 0.73 for these data strengthens the simple-mixing postulate as reasonable. Some of the data scatter is a result of the reaction of the introduced fresh water with the rock matrix with which it comes in contact, as discussed earlier. This reaction causes higher dissolved-solids values for samples that have mixed in given proportion with the radioactive cavity water than would be pre- dicted by simple mixing of two waters. This is par- ticularly evident in the portion of figure 12 between 300 and 1000 mg/L dissolved solids and in the vicinity of 1 x 10-5 pCi/L tritium activity: the October 1972, January 1973, and May 1973 samples fall in the same dissolved-solids sequence as in figure 13 where their placement describes a reaction coordinate of surface water with subsurface rock. The extrapolation of the tritium versus dissolved- solids content data to a dissolved—solids value of aquifer 3B (table 3) to determine radioactive-source water con- centration in the cavity is tempting, but the assumption must be made that the cavity-fill water is completely homogeneous throughout. This assumption is probably not valid, as indicated by the radioactivity in the sam- ples collected prior to July 22, 1972. Some of these sam- HYDROLOGY OF NUCLEAR TEST SITES ples presumably contain the condensed steam which apparently had not mixed with water from aquifer 3B in the 4 months that had elapsed from day of condensa- tion to the day that water entered the drill stem. What did occur in the region of the cavity near the UA —1 —P1 hole is mixing of water from aquifer 3B with water from near surface by strong local convection induced by the cool downward-moving current of water from near sur- face. Generalized cavitywide convection of any mag- nitude is not apparent from examination of the availa- ble data. ALPHA ACTIVITY Alpha activities, measured using natural uranium as a calibration standard, were either undetected or generally as low as those measured in the natural en- vironment prior to Cannikin. There are a few notable exceptions which may be found in table 2. No identifica- tion of specific radionuclides was made on these sam- ples and it is not known whether they represent alpha activity generated by the nuclear device or if present as a consequence of the large amounts of drilling fluid used in drilling UA —1 —P1. These drilling fluids con- tained significant natural alpha activity in the form of uranium and thorium. Very small amounts of these fluids, if present in a sample, could account for the ob- served alpha activity. Three of the samples in table 2, those collected on July 19, 22, and 23, 1972, were specifically analyzed for plutonium isotopes 239 and 240 and also for uranium 238 and 235 (Eric T. deJonckheere, Jr., written com- mun., 1975). No plutonium was detected, and the uranium-isotope ratio indicated natural uranium was present in the three samples. CALCULATION OF CAVITY RADIUS FOR THE CANNIKIN EVENT Although the exact energy yield of the Cannikin nuclear explosion, the measured radius of the lower hemisphere of the cavity, and the cavity void volume are not available because this is classified information, an approximate radius and void volume of the cavity can be established using published data on the effects of . underground nuclear explosions (Butkovich and Lewis, 1973), and generalized information on the Can- nikin event (Merritt, 1973). Thus, the radius and volume of the cavity formed by a nuclear explosion can be determined by the relationship R = CWI/a L‘ — (p-h)a where RC is the cavity radius of the lower hemisphere, in meters; Wis the energy yield, in kilotons; p— is the HYDROLOGIC PROCESSES AND RADIONUCLIDE DISTRIBUTION, AMCHITKA ISLAND, ALASKA average overburden density, in g/cma; and h is the depth of burst, in meters. The exponent (1 depends on the water content of the medium at the point of burst (Higgins and Butkovich, 1967), and C is a constant equal to about 100 using this set of units. For the Cannikin event, stated to be a nuclear test of somewhat less than five megatons (Mt) yield, where: 5000 kt (Merritt, 1973) - 100 (Butkovich and Lewis, 1973) 2.3 g/cm3 (Lee, 1969) 1790 m (Merritt, 1973) 0.307 (6.5 weight percent water) (Higgins and Butkovich, 1976; Lee and Gard, 1971) Q S‘DIQQ I Then, for W: 5000 kt, RC = 133 m. SUMMARY AND CONCLUSIONS The Cannikin nuclear explosive was detonated November 6, 1971, on Amchitka Island, Alaska, and an underground cavity was immediately created around the explosion. The stress placed on the overlying rock by the cavity was relieved by collapse of the over- burden, creating a rubble chimney extending from the cavity to land surface. Increased vertical hydraulic con- ductivity over that which existed in the undisturbed en- vironment resulted, and water from surface and sub- surface sources flowed into and down the chimney to fill the new void created by the explosion. Water percolat- ing toward the cavity region encountered an upward- moving front of high-temperature steam and water which severely retarded downward movement of water by two mechanisms—(1) vaporization of the down- ward percolating water, and (2) decreased hydraulic conductivity owing to two-phase flow conditions exist- ing at the steam/water interface. Initially the interface moved upward; then, when sufficient cooling had occur- red, it retreated downward. During the downward flow retardation of water from surface and subsurface sources continued to accumu- late in the chimney above the cavity. When the pressure and temperature conditions had allowed steam condensation to occur throughout much of the cavity, water which had accumulated in the chimney flowed downward, filling the cavity. Flow into the upper part of the chimney recommenced, and the progress of refilling the chimney was recorded by periodic water- level measurements in a reentry hole drilled into the Cannikin chimney and cavity. The reentry-hole water levels represented combined measurements of water-vapor pressure in the cavity and hydraulic head in the chimney; consequently, to ob— D27 tain true chimney water levels, the reentry-hole water levels were corrected for the effect of water vapor by using the estimated temperature history of the cavity. The water-level rise in the chimney thus obtained was combined with aquifer-property data, surface«water in- flow, and an estimate of the magnitude of total new sub- surface pore space created by the explosion to estimate the vertical distribution of new porosity within most of the chimney. Aquifer-property data had been obtained from a test hole near the Cannikin emplacement hole and surface-water inflow data were estimated from stream records obtained prior to detonation. Choice of day of steam condensation (day 60) and relative cavity radius of 1.34 (to obtain a corresponding subsurface void volume) were made to best fit the observed phenomena. The resulting calculated porosity distribu- tion, which was in disagreement with published predic- tions, was 10 percent near the bottom of the chimney, ~ decreasing to 4 percent near the top. As the magnitude of subsurface void is dependent on cavity size and shape, and therefore not known with certainty, calcula- tions were made using several values of cavity radius. Furthermore, the water inflow was controlled by aquifer properties and the time that the chimney filling was renewed, immediately after steam condensation in the cavity. Therefore, different values for day of steam condensation in the cavity were paired with different values of cavity radius (directly related to magnitude of subsurface void) to determine the sensitivity of calcul- ated porosity values to errors in estimating these parameters. It was found that a large error in estimate of magnitude of subsurface void (for example, 170 per- cent) results in a small relative error (45 percent) in porosity. The error introduced by an improper choice of day of steam condenstaion in the cavity is also small. The choice of D C: 60 and ch = 1.34 as most probable was corroborated by calculations of distribution within the cavity of heat produced by the explosion. In contrast to the hydraulic data, interpretation of the chemical information obtained from samples col- lected in the vicinity of Cannikin presented greater difficulty. The primary reason for this lay in the method of completion of the reentry hole and the tech- niques used in obtaining samples therefrom. Downward flow of water from near land surface with- in and in the vicinity of the reentry hole was identified by temperature and FLO-PAK logs. Consideration of the water-quality data in light of this phenomenon made it possible to identify the effect of this near-sur- face water on the samples collected. Generally, the most radioactive samples approached a chemical com- position similar to the native saline water near the cavity. The saline water was diluted by fresher water from near land surface. In addition to acting as a dilu- tant, the fresh water underwent changes in chemical D28 composition as it flowed downward, not unlike that which occurred in the undisturbed system prior to the detonation but much compressed in time. It is presumed that contributions of near-surface water to the overall chimney underwent similar changes. The concept of simple-mixing of two waters would theoretically allow calculation of radioactivity in un- diluted cavity water; however, reaction of dilutant with aquifer matrix and the indication that water in the cavity region was not quantitatively (completely) mixed, contributed to data scatter that obviated precise extrapolation. Indications from the general chemical data that quantitative mixing had not occurred in the cavity were corroborated by an analysis of the radiochemical data. Changes with time in radiochemical composition of the water samples indicated that even radiochemical equilibrium had not been achieved in the 18 months which had elapsed since detonation. This water-quality heterogeneity and consequent radioactive dise- quilibrium is perhaps not surprising, as it is difficult to postulate a mechanism for complete mixing to be rapidly attained. Large thermal gradients do not ap— pear to have persisted in the Cannikin cavity. It is cer- tainly possible that “hot spots” existed at time of aban- donment of the site, but, if randomly distributed, they would contribute little to mixing of the cavity water. Thus, it is concluded that diffusion is the only process by which mixing would continue. At the time of site abandonment, hydraulic equilibrium, as indicated by comparison of chimney water level with predetonation conditions, also had not been reached. This could be explained either by further cooling of the “hot spots” or by continued saturation of pore space of low hydraulic conductivity. With in- creased vertical hydraulic conductivity in the vicinity of Cannikin caused by chimney formation, a new hy- draulic system could be formed which would result in deeper circulation of near-surface water than was pre- viously possible. Because the more transmissive aquifers lie well above the cavity region, it is believed that this deepened circulation will not be significant below aquifers 1 or 2. SELECTED REFERENCES Ballance, W. C., 1970, Hydraulic tests in hole UA —1 and water inflow into an underground chamber, Amchitka Island, Alaska; US. Geol. Survey rept. USGS —474 —72, 54 p.; available only from US. Dept. Commerce, Natl. Tech. Inf. Service, Springfield, Va. 22161. 1972, Hydraulic tests in hole UAE —1, Amchitka Island, Alaska: US. Geol. Survey rept. USGS «474 —102, 32 p.; available only from US. Dept. Commerce, Natl. Tech. Inf. Service, Springfield, Va. 22161. nus. Government Printing Office: 1977-777-102/17 HYDROLOGY OF NUCLEAR TEST SITI‘E Beetem, W. A., Young, R. A., Washington, C. L., and Schroder, L. J., 1971, Chemical analyses of water samples collected on Amchitka Island, Alaska: US. Geol. Survey rept. USGS-474 -135, 18 p.; available only from US. Dept. Com- merce, Natl. Tech. Inf. Service, Springfield, Va. 22161. Butkovich, T. R., and Lewis, A. E., 1973, Aids for estimating effects of underground nuclear explosions: Univ. California rept. UCRL —50929, Rev. 1, p. 7. Carr, W. J., and Quinlivan, W. D., 1969, Progress report on the geology of Amchitka Island, Alaska: US. Geol. Survey rept. USGS —474 —44, 15 p.; available only from US. Dept. Commerce, Natl. Tech. Inf. Service, Springfield, Va. 22161. Dudley, W. W., Jr., 1970, Nonsteady inflow to a chamber within a thick aquitard: U.S. Geol. Survey Prof. Paper 700 —C, p. 0206 —C21 1. Fenske, P. R., 1972, Event»related hydrology and radionuclide transport at the Cannikin site, Amchitka Island, Alaska: US. Atomic Energy Comm., Nevada Operations Office rept. NVO —1253 —1, 41 p.; available only from US. Dept. Commerce, Natl. Tech. Inf. Service, Springfield, Va. 22161. Garber, M. S., 1971, A method for estimating effective porosity in a rubble chimney formed by an underground nuclear explosion: U.S. Geol. Survey Prof. Paper 750 —C, p. C207 —CZO9. Gonzalez, D. D., and Wollitz, L. E., 1972, Hydrological effects of the Cannikin event: Seismol. Soc. America Bu11., v. 62, no. 6, p. 1527 —1542. Gonzalez, D. D., Wollitz, L. E., and Brethauer, G. E., 1974, Bathyme- try of Cannikin Lake, Amchitka Island, Alaska, with an evalua- tion of computer mapping techniques: U.S. Geol. Survey rept. USGS—474 —203, 20 p.; available only from US. Dept. Com- merce, Natl. Tech. Inf. Service, Springfield, Va. 22161. Hantush, M. S., 1959, Nonsteady flow to flowing wells in leaky aquifers: Jour. Geophys. Research, v. 64, no. 8, p. 1043 —1052. Heckman, R. A., 1964, Deposition of thermal energy by nuclear ex- plosives, in Engineering with nuclear explosives: Third Plowshare Symposium Proc., US. Atomic Energy Comm. Rept. TID —7695, p. 259 —304. Higgins, G. H., and Butkovich, T. R., 1967, Effect of water content, yield, medium, and depth of burst on cavity radii: Univ. Califor- nia rept. UCRL —50203, 24 p. Hodgman, C. D., ed., 1957, Handbook of chemistry and physics, 39th ed.: Cleveland, Ohio, Chem. Rubber Publishing Co., 3213 p. Lee, W. H., 1969, Some physical properties of rocks in drill hole UAE -1, Amchitka Island, Alaska: US. Geol. Survey rept. USGS —474 —48, 13 p.; available only from US. Dept. Commerce, Natl. Tech. Inf. Service, Springfield, Va. 22161. Lee, W. H., and Gard, L. M., Jr., 1971, Summary of the subsurface geology of the Cannikin site, Amchitka Island, Alaska: US. Geol. Survey rept. USGS —474 —132, 24 p.; available only from US. Dept. Commerce, Natl. Tech. Inf. Service, Springfield, Va. 22161. Merritt, M. L., 1973, Physical and biological effects of Cannikin, US. Atomic Energy Commission rept. NVO ~123, 106 p. Morris, R. H., 1973, Topographic and isobase maps of Cannikin sink: U.S. Geol. Survey rept. USGS —474 —125, 8 p.; available only from US. Dept. Commerce, Natl. Tech. Inf. Service, Springfield, Va. 22161. Pirson, S. J., 1958, Oil Reservoir Engineering, 2d ed.: New York, N.Y., McGraw-Hill Book Co., 733 p. Schroder, L. J., and Ballance, W. C., 1973, Summary of chemical and radiochemical monitoring of water for the Cannikin event, Amchitka Island, Alaska, fiscal year 1972: US. Geol. Survey rept. USGS—474—167, 39 p.; available only from US. Dept. Commerce, Natl. Tech. Inf. Service, Springfield, Va. 22161. PLATE 1 22Hmom< mmmhmz 7500 PROFESSIONAL PAPER 712—D Du nu I Du E T um I nu "n m1 F nu T mu E mm m1 mm mm mm DW WU mm mm mm. mm nu m1 mw I mm mm nu nu ed ground surface ' Aquifer 1 6000- 5000- 60 DAYS(JANUARY 5,1972) C. nu 7 9 om nn E on um E 2DAYS £3. A.1hMNUTE(NOVEMBER 6.197“ F.90 DAYS(FEBRUA 6000~ '5000~ RU RY 20.1972) S Y An nu n0 0 um UARY 14,1972) R S Y A D O O 1 nu Y 4,1972) R ) 2 \l 7 2 9 7 1 9 1 1| 5 1 1 5 Y 1 nn u. An I; U U mm nu A ( nu cu Y Y E E D E T A F F. D PM 0 M 010 a m 2 wlm m m . 1% 1i m 1 E S t m n” c mw‘ .1 d 0 w W mw 4 m an f .M W»... 02 W A N SM ee 10 O 0t H? 0 _|. n pm nu m1 e . t O N .1In S. A d0 r n1 8 L It :1 0 D: C e. m1 X me If n/_ o.w AH Mw An :2 W.d _ A A 9 nu 0 ) 2 7: \1 fi 9 n 010 1 y 9 2 1 1 ’ v. ou nn v. A“ I. U U N J A ( um cu s m Y A D D 5 m _/ Mm Sn 6 . O1 . / nu mum n CQt .._l .015 8 mr] / U8] 3. S.M.H Qu aWd .I AH 14 a > m>om O 0 0 0 5 0 0 1 4; 9 a.” z >m3? H \ +\\\+ + + + + /+ V // \+\ + + + / I/ \ Geology by H.Cornwall, u F. Kleinhampl, l96l, \\ @ l964,F.’ Orkild,written \‘ communication “ Base by US. Geological J Survey. I954 O 5 MILES I I 1 l l I | l l I I l 0 5 KILOMETERS FIGURE 2.—Geology and sampling sites. E4 HYDROLOGY OF NUCLEAR TEST SITES TABLE 1.—Description of sampling sites [QaL alluvium of Quaternary age; TV, volcanic rock of Tertiary age; samples collected July 3—6, 1967] Sa'rinbgle Site Site Sample Probable Depth Discharge number coordinates name source aquifer 881/ type m it Ms min 1 108/47—14bab _ _ Spring Qal _ _ - _ 6 100 2 108/47— 27cba _ _ Well Qal 2 6 _ _ _ - 3 108/47—3laab _ _ Spring Qal _ _ _ _ .7 1 1 4 103/47— 32dda _ _ do Qal _ _ _ _ 14 225 5 103/47—33aab _ _ do Qal - _ _ _ 16 255 6 10S/47— 30dcc Well Qal 37 121 3 50 7 118/46— 26bbb Upper Indian Spring Spring Tv _ _ _ _ - - _ _ 8 118/46—26bcc Lower Indian Spring do TV _ _ _ _ .5 8 9 118/47-3cdb _- do TV __ __ 2.5 40 10 1 18/47— 4cad _ _ do Qal _ _ _ _ .6 10 11 115/47—10caa __ do TV __ -- 3 1 49 12 113/47—10bcc _- do TV __ __ _ _ _ _ 13 118/47—16dcd Burro Hot Springs do TV _ _ _ - _ _ _ _ 14 118/47—16bdc do do TV _ _ - _ _ _ _ _ 15 118/47—18acd Crystal Springs do TV _ _ _ _ _ _ _ - 16 115/47—2lacc _ _ do Qal - _ - _ _ _ _ _ 17 118/47—21dbb _ _ Well Qal _ _ _ _ 1 6 26 18 118/47—21aba _ _ Spring Qal _ _ _ _ _ _ _ - 19 115/47—21aba __ do TV __ _ _ _- __ 20 11S/47— 27cba _ - Well TV 17 55 1 20 21 115/47— 28aac _ _ do Qal 8 25 .06 1 22 118/47— 28dac Ute Spring Spring 31 - - _ _ 1.6 25 23 118/47— 33bac _ _ do _ _ _ _ 1.6 25 24 118/47—10ccb __ do TV __ _ _ _- __ 25 128/47— 5cda Beatty Spring do TV _ _ _ _ _ _ _ _ 26 12S/47-6cdd _ _ Well Qal 55 180 5 80 27 12S/47— 7dbd - _ do Qal 91 300 14 230 28 12S/47— 20bbb _ _ do Qal _ _ _ _ 6 100 29 128/47— l9adc - - do Qal _ _ _ _ _ _ _ _ TABLE 2.—Analyses of selected major chemical constituents [Sampling-site numbers keyed to figures; samples collected July 3—6, 1967. Qal=Alluviurn (:if' ngdtemary age; TV: Volcanics of Tertiary age. Concentrations in millimoles per liter, except as m ica Dissolved constituents Total dis- Sampling— Probable TemperL cal- magne- potas- bicar- chlo- fluo— solved lite aqui er ature pH cium sium sodium sium bonste ride ride sulfate silica solids number source (°C) (Ca) (Mg) (Na) (K) (HCOa) (Cl) (F) (804) (Si&) (mg/L) 1 :11 29.0 8.1 0.18 0.01 6.22 0.21 3.39 1.44 0.22 0.86 0.95 458 2 a] 19.0 7.7 .55 .06 7.44 .22 4.57 1.83 .99 1.06 1.03 573 3 51 19.5 7.6 .58 18 4.35 .20 3.75 1.18 13 .55 1.18 421 4 Qal 22.0 7.6 .75 22 5.96 .00 5.08 1.04 12 1.00 1.03 523 5 Qal 22.0 7.8 .75 19 7.35 .23 4.85 1 92 23 1.07 .90 584 6 $31 22.5 7.8 .60 .19 4.35 .20 3.80 1.13 09 .61 1 20 424 7 26.5 8.7 .00 .01 2.57 .04 1.90 .39 02 .15 73 192 8 '1V 21.0 7.9 .15 .04 2.48 .04 2.07 .42 02 .18 80 213 9 1V 23.0 8.2 .40 .04 5.31 .12 3.00 1.27 15 .98 78 420 10 Qal 21.0 7.7 .65 18 9.70 .22 6.23 2.26 27 1.35 1 03 726 11 1V 24.0 8.1 35 .03 8.53 .06 5.41 1.52 32 1.13 63 583 12 1V 18.5 7.6 35 .02 6.79 .18 4.79 1.18 24 .95 85 511 13 1V 36.5 7.8 45 .02 7.53 .20 4.39 1.33 32 1.32 100 572 14 W 36.5 7.8 42 .02 7.13 .19 4.15 1.21 30 1.25 1 07 546 15 W 24.0 7.7 55 .15 2.18 .09 2.33 .59 03 .23 75 251 16 Qal 31.5 7.7 58 12 10.09 22 6.10 1.95 32 1.65 1.00 750 17 Qal 29.0 7.7 62 13 10.57 21 6.44 2.03 32 1.74 .93 783 18 $31 26.0 7.9 65 13 10.70 21 6.46 2.03 32 1.74 .90 777 19 41.0 7.6 32 02 6.53 20 3.65 .99 32 1.21 .90 509 20 'IV 21.5 8.0 90 21 5.00 28 3.03 2.62 37 2.27 .98 396 21 Qal 18.0 9.1 25 19 13.70 23 8.39 1.92 34 1.76 75 997 22 gt] 21.0 8.2 21 00 10.83 06 5.08 .76 20 .73 77 737 23 34.0 8.3 30 03 4.87 12 2.84 1.27 14 .85 83 355 2‘ '1‘! 21.0 8.2 32 11 5.39 15 3.03 .73 2O .73 1 00 414 25 '1‘! 24.0 7.9 80 18 4.61 19 3.21 1.07 02 .97 90 384 26 Qal 21.5 7.9 68 13 4.57 26 3.47 2.06 .32 1.86 __ 440 27 20.0 7.7 63 15 11.14 26 6.49 2.06 .32 1.36 1.10 814 28 18.5 7.8 68 15 11.05 26 6.39 2.17 .31 1 91 1.12 820 29 ll 20.0 7.7 95 23 12.62 26 7.20 2.82 .33 2 60 1.12 1 040 GEOCHEMISTRY OF GROUND WATER, TUFFACEOUS ROCKS, OASIS VALLEY, NEVADA E5 TABLE 3.—Analyses of selected minor chemical constituents [Sampling-site numbers keyed to figures; samples collected July 3—6, 1967. Concentrations in micromoles per liter] Dissolved constituents Sampling- Boron Co per Iron Alum- Stron- £18; n‘fn'n‘fier (B) (8m (Fe) Tm lg? (Ba, 1 __ 0.03 0.36 0.22 0.17 0.03 2 21.3 .01 .18 .26 1.71 .09 3 45.3 .13 .20 .15 2.05 .09 4 53.6 .03 .18 .074 4.45 17 5 31.4 .05 32 .45 2.17 18 6 47.1 .02 .32 .37 2.17 09 7 15.7 .02 .07 .41 .10 01 8 16.6 .02 16 .55 .34 02 9 17.6 .02 36 .45 1.08 06 10 41.6 .02 14 .26 3.19 14 11 19.4 .05 .27 .11 1.48 04 12 __ .03 .81 1.48 .46 04 13 18.5 .09 .43 .26 1.37 17 14 17.6 .05 1.52 .03 1.37 19 15 10.2 .03 .25 15 .51 02 16 36.0 .03 .89 3.34 2.51 03 17 35.1 .08 1.07 .19 2.86 03 18 30.5 .03 .53 .26 2.97 03 19 15.7 .05 .18 .04 1.37 20 20 14.8 .05 1.07 1.11 .23 02 21 38.8 .03 1.25 .26 4.56 11 22 34.2 .05 8.41 .93 1.03 02 23 _ _ .06 20 .30 .23 01 24 16.6 .02 16 .52 .68 04 25 20.3 .05 18 26 .86 02 26 43.4 .03 1.07 .11 1.37 04 27 46.2 .03 .89 1.12 1.37 06 28 53.6 .06 16 1.67 11 07 1 29 , 1 1 A _ _ _ 1 -- __ 'No analyses for sample site 29. of Oasis Valley (fig. 2) and to a lesser extent in the Bullfrog Hills southwest of Springdale. These rocks have been subjected to intense thrust faulting and fold- ing. The formations range in age from Early Cambrian to Late Mississippian and have a composite thickness of about 7,600 m (25,000 ft). The sequence in Oasis Valley consists of stratified layers of shale, siltstone, quartzite, limestone, and dolomite. Carbonate rocks predominate in the upper half of the sequence, whereas shales and related rocks make up most of the lower half. ROCKS OF TERTIARY AGE Most of the bedrock exposed in Oasis Valley consists of ash—flow tuff, ash-fall tuff, rhyolite flows, and minor basalt flows of Tertiary age that emanated from local volcanic centers. The formations of most interest in the Oasis Valley drainage basin are the Paintbrush 'Iliff, the Timber Mountain Tuff, and the Thirsty Canyon TM)". The formations are multiple—flow units ranging in rock type from rhyolite to quartz latite. The rocks range gradationally from nonwelded to densely welded. Many exposures display extensive jointing and fracturing. ALLUVIUM OF PLEIS’IOCENE AND HOIDCENE AGE Much of the surface area of Oasis Valley is covered with alluvium. Cornwall and Kleinhampl (1961) and other workers have mapped this alluvial cover as two separate units. The older unit, classified as gravels of Pleistocene age, represents old dissected alluvial fans. The younger unit, presently forming alluvial fans, is mapped as alluvium of Holocene age. The gravel in these newer fans grades into sand and silt toward the topographic bottom of Oasis Valley. With the exception of alluvium associated with rocks of Paleozoic age found in the extreme southern part of Oasis Valley, both units are composed of detritus originating from the tuff formations. The exact thickness of the total alluvial cover is not known, but it is estimated by Malmberg and Eakin (1962) to be as much as several thousand feet in the central part of Oasis Valley. The alluvium is much thinner in the southern part of Oasis E6 Valley. Based on unpublished geophysical data of the US. Geological Survey, the estimated thickness of al- luvium resting on the Paleozoic bedrock within the Amargosa Narrows (fig. 2) is 4—5 in (12—16 ft). HYDROIDGY SURFACE WATER The Amargosa River flows in a few reaches in the central part of Oasis Valley during part of the winter and early spring until evapotranspiration becomes sufficient to draw down the potentiometric surface below the stream bottom. Flow at other times is inci- dental with thunderstorms in the headwaters of Thirsty Canyon or Beatty Wash. Surface flow from springs also occurs along short reaches of the valley floor. GROUND WATER The two principal aquifers in the Oasis Valley basin are tuffaceous rocks in the highlands and alluvium be- neath the valley floor. Blankennagel and Weir (1973) concluded that the ground water in the tuffaceous rocks related to those surrounding Oasis Valley was transmitted principally through extensive fracture systems produced by contractional cooling during so- lidification and by post-depositional faulting. Where such rocks are saturated, are broken by recent fault- ing, or intersect the land surface, ground water issues as springs and seeps. Such is the case for several sample locations shown in figure 2 along the east side of Oasis Valley between Beatty Wash and Thirsty Canyon. Ground water infiltrates from springs and subsur- face flow into the valley-fill alluvium which occurs principally along the Amargosa River and its tri- butaries. At most places, this valley fill is saturated to within several feet of the surface. Malmberg and Eakin (1962) suggested that where the cross-sectional area of the alluvial fill is decreased by underlying, less permee able material, water is forced to the surface and issues as springs, seeps, and swampy areas. Some artesian wells have been developed in alluvium where ground water is confined beneath layers of clay. Figure 3 shows the approximate locations of the ground-water flow paths in the alluvium; the locations are based on topographic and geographic distributions of the valley fill. RFCHARGE AND DISCHARGE Malmberg and Eakin (1962) estimated an average annual recharge from precipitation within the Oasis Valley drainage basin of 0.31 hm3 (250 acre-ft), and an annual discharge from Oasis Valley by evapotranspira- HYDROLOGY OF NUCLEAR TEST SITES tion and ground-water underflow of 3.0 hm3 (2,400 acre-ft). Based on the above estimates, they postulated a significant inflow of ground water from outside the Oasis Valley drainage basin, probably as recharge from Pahute Mesa, Gold Flat, and other areas to the north and northeast (fig. 1). Blankennagel and Weir (1973) substantiated this conclusion by defining a southwest- ward-trending potentiometric surface under Pahute Mesa in the direction of Oasis Valley. The total under- flow passing southwestward through tuffaceous rocks beneath Pahute Mesa was estimated at 10 hm3 (8,100 acre-ft) annually, only part of which probably re- charges Oasis Valley. Of this total volume, perhaps 3.9 hm3 (3,200 acre-ft) is recharge from precipitation on Pahute Mesa, and the remainder may be underflow from Gold Flat and other areas to the north. Altitudes of the water table indicate a gradient of approximately 4 m/km (2O ft/mi) between Gold Flat and the upper spring in Oasis Valley, (sample location 1 in fig. 2). Winograd and Thordarson (1975), in defining the gen- eral hydrologic systems in the south-central Great Basin, also suggested that the Oasis Valley ground- water system was recharged by underflow through tuf- faceous rocks from Pahute Mesa and Gold Flat. Evapotranspiration accounts for much of the ground water discharged from Oasis Valley. The principal species of phreatophytes, including salt grass, Ber- muda grass, greasewood, and salt bush, occur along the flood plain of the Amargosa River. The average depth to water below land surface in most phreatophyte zones is 2—3 in (6—10 ft). Evaporation also occurs locally from spring pools and swampy or seep areas. Based on phreatophyte cover, open-water acreage, and assumed rates of evapotranspiration, Malmberg and Eakin (1962) estimated an evapotranspiration discharge of 2.5 hm3 (2,000 acre-ft). They estimated the average ground-water discharge southward toward the Amar- gosa Desert (fig. 1) through the thin alluvium overlying the Paleozoic bedrock in the Amargosa Narrows to be 0.5 hm3 (400 acre-ft) annually. GEOCHEMICAL ENVIRONMENTS The geochemical history of the Oasis Valley ground water can be broadly classified into two stages based on differing aquifer characteristics. Most ground water in Oasis Valley has presumably moved from areas of re- charge through the fracture system of the tuffaceous aquifer. In this first stage, dissolved carbon dioxide originating in the soil zone of the recharge area reacts with mineral phases in the tuffaceous rocks. The specific chemical composition of the ground water at any given point along the flow path is a function of the kinetic rate of reaction, and hence the total residence time spent within the tuffaceous aquifer. Ground wa- GEOCHEMISTRY OF GROUND WATER, 'l‘UFFACl‘IOUS ROCKS, OASIS VALLEY, NEVADA E7 ”6° 50' 45' “6°40 o ' I . _' 37 05 { Thirsty. Canyon EXPLANATION .' / O$ /}J4:;8}\ O \” V$‘{/ Sample site with (J V/// probable ground-water / source in alluvral aqunfer /. 2 Sample sile with probable ground-water source in tuffaceous aquifer .../ \"a‘ (509) \ Dissolved solidsin milligrams per liter K" SPRINGDALE J Approximate location of named flow path in valley alluvium 37°oo' — — 600 11 .. __._ 2 ©(583/ m/ lsograds showing increase Crys/a/ ‘/ in dissolved solids along ,. _ Springs 15 \ 'M (4,?" main Thirsly Canyon- Oasls Volley \ c 4 .. (546),/572) i? ’1 \- > 783),(777} 9 "\ \ V 20 Q >92 78¢ (3.96) / o Ind/an / "'-- $ (2/.{/©Spr/wgs o, . \ a.gh//// 3 \\ \ - W ...// ._ \\\ / 0 £514 ‘\ K, ,‘ / Burton \ . , 5/": 800 * . Mountain \ ; 5 36°55 —~ 2. /© — (440 .0 / (384) Beany . ' oRhyolile . H (8%) \ "\ FLUOR‘S—FAR // ‘ ”pf/591:7 4/100 (l l ‘l ,,~— °°<° 3” . m 9050 \\ o / m 40 '0 ’0 \l (/0 ) ‘1 (820) Bare. \\ l\/\ Base by US. Geological J ‘ I000 M°“"’°'" \ \ Survey. l954 O 5MILES l I l I l I | I I I I [ 0 5 KlLOMETERS FIGURE 3.—The approximate locations of the ground-water flow paths in the alluvium and the regional distribution of dissolved solidsTn—fliE alluvial and tfii‘l‘aceous aquifers. E8 ters from Indian Springs and Crystal Springs (fig. 3) that are recharged locally from the Bullfrog Hills probably have relatively short residence times within the tuffaceous aquifer. Dissolved solids in these ground waters are considerably lower than they are in ground water issuing from springs along the east side of Oasis Valley. Ground water from the springs on the east probably has much more distant recharge areas within the tuffaceous aquifer to the north and east of Oasis Valley. The second stage in the geochemical history of Oasis Valley is the upward percolation of ground water by subsurface flow into the shallow alluvial aquifer: Fig- ure 3 shows that the concentration of dissolved solids increases progressively along the main Thirsty Canyon-Oasis Valley flow path. Ground water in the alluvium at site 1, the uppermost sampling point on the flow path, contains 458 mg/L dissolved solids. Ground water in the alluvium at the lower end of the flow path contains 1,040 mg/L dissolved solids or more than twice the amount contained at site 1. Although some addi- tional reaction may occur between the ground water and tuffaceous detritus in the alluvium, the principal mechanism appears to be an increase in chemical con- centration caused by a decrease in the volume of ground water owing to the immediate proximity of the water table to the atmosphere and soil zone. The de- crease is probably the result of both direct evaporation and transpiration through the vegetation cover. CHEMICAL CHARACTERISTICS CONTROLLED BY THE TUFFACEOUS AQUIF ER Although much of the ground water in the alluvium may have undergone concentration increases and reac- tions due to evapotranspiration, the chemical composi- tion of all ground water in Oasis Valley is assumed to generally reflect a solute composition determined by interaction with solid phases of the tuffaceous rock. This interaction consists of dissolution and hydrolysis of primary mineral phases and precipitation of secon- dary minerals. EQUILIBRIUM RELATIONSHIPS The percentages of primary mineral phases vary in the tuffs with changes in bulk chemistry and in de— gree of devitrification. The most common minerals are glass, alkali feldspar, plagioclase, quartz; lesser amounts of biotite, clinopyroxene, and opaque oxide occur (Cornwall and Kieinhampl, 1964; Lipman, and others 1966). The principal alteration minerals, which formed as a result of ground-water action, include clay minerals, zeolites, cristobalite, and possibly potassium feldspar. The clay minerals are principally montmoril- lonite and montmorillonite-illite mixed-layer clays. HYDROLOGY OF NUCLEAR TEST SITES The zeolites are primarily of the sodium-potassium types, dominantly clinoptilolite and analcime and, to a lesser extent, mordenite and chabazite (Hoover; 1968). Table 4 shows the mean average thermodynamic saturation of both alluvial and tuffaceous ground wa- ters with respect to a number of silicate minerals ob- served in the tuffaceous rocks of Oasis Valley. The term, IAP/Ks, defines the ratio of the actual aqueous ionic activities products (IAP) to the expected sol- ubility products in equilibrium with a given mineral phase (Ks). If IAP/KS is less than unity, the ground water is undersaturated and will tend to dissolve a particular phase. If IAP/KS is greater than unity, the water is supersaturated and may tend to precipitate a given phase out of solution. Whether a mineral does indeed form depends both on its stability with respect to other minerals and kinetic factors influencing nucleation and precipitation. The ground water is supersaturated with respect to the potassium feldspar, adularia, which has been observed as a ground-water alteration product (Hoover, 1968). The water is under- saturated in relation to albite and anorthite, and two end members of the plagioclase solid solution. Plagio— clase is found only as a primary mineral. The ground water is also supersaturated in terms of three clay minerals; kaolinite, montmorillonite, and illite. The aqueous silica concentration displays near-saturation with respect to silica gel and supersaturation with re- spect to both cristobalite and quartz. Although data is not given in table 4, several of the springs emanating from tuffaceous rocks in the east central part of Oasis Valley are also saturated with calcite. Table 4 is incomplete owing to the lack of ther- modynamic data for a number of zeolite phases. Anal- cime, one zeolite for which solubility data exist, is un- dersaturated by more than two orders of magnitude. Such a high degree of undersaturation is of interest because analcime occurs extensively in some altered tuffs adjacent to Oasis Valley to the north and east (Hoover, 1968). Lack of saturation suggests that on- going zeolitization, as documented by Benson (1978), TABLE 4.—Comparison between mean ionic-activity products of Oasis Valley ground water and solubility products of selected silicate phases Ionic Activity Solubility Mineral Product (IAP) Product (KS) IAP/KS at 25°C Adularia ______ 1.2X10‘20 2.7)(10’21 4.4 Albite __________ 5.2X10’19 1.0><10‘18 .52 Anorthite ______ 9.8><10‘22 3.8><10‘20 .026 Kaolinite ______ 2.0X10‘37 1.2X10’37 1.7 Montmorillonite 5.0><10‘28 2.0x10-30 250 Illite __________ 3.8><10’40 5.4><10‘41 7.0 Potassium—Mica 5.5 X 10‘51 7.9 X 10—50 .070 Analcime ______ 1.6X10’15 2.O><10*13 .0080 Silica gel ______ 9.1><10—4 9.6X10‘4 .95 Cristobalite _,,19.1><10‘4 2.6><10’4 3.5 Quartz ________ 9.1 ><10’4 9.9><10_5 9.2 GEOCHEMISTRY OF GROUND WATER, TUFFACEOUS ROCKS, OASIS VALLEY, NEVADA probably occurs in localized geochemical environments that are not reflected in the average composition of ground water entering Oasis Valley. The existence of such localized environments is supported by the work of Gibbons, Hinrichs, and Botinelly (1960), who found that many of the zeolites in southern Nevada occur above clay zones and some densely welded tuffs. These workers suggest that the confining effect of imperme— able layers slow the rate of flushing of solute species from the aquifer. The increase in aqueous concen- trations as a result of continuing reaction with the tuf- faceous rocks causes zeolitization. At least in the case of analcime, most of the ground water contained in Oasis Valley never reaches a composition that permits zeolitization. Although individual minerals listed in table 4 may be saturated or supersaturated with respect to the ground water, their existence in the natural system is not guaranteed because stability with respect to other solid phases is not considered. Activity diagrams de- veloped by Garrels and Christ (1965), Helgeson, Brown, and Leeper (1969), and other workers depict the stability fields of mineral species coexisting with vary- ing aqueous solutions. Compositions for the Oasis Val- ley ground water are plotted for the systems K20 — MgO — H2O — A1203, NaZO — MgO — H20 — A1203, and CaO — MgO — H2O — A1203 in figure 4. Such stability diagrams are based on the free energy of for- mation for each solid phase considered. Previous literature has cited the free-energy data for montmorillonite as estimated through indirect means by Helgeson (1969), Hemley, Meyer, and Richter (1961) and Marshall (1949). Those estimates were based on the assumption that the stability of montmorillonite is dependent on the nature of the absorbed ion (i.e., sodium montmorillonite, calcium montmorillonite, etc.). Recently, Kittrick (1971a, b) has shown that the solubility of montmorillonite depends upon ions com- mon to the crystal structure and upon the magnitude of the exchange capacity, but it is independent of the type of exchangeable ion and its activity in solution. Kit- trick measured experimentally the free energy of for- mation and the solubility products for montmoril- lonites from Aberdeen, Mississippi, and Belle Fourche, South Dakota. Table 5 shows selected chemical compo- nents normalized to silica for Kittrick’s montmoril- lonites and two montmorillonites that occur in or adja- cent to Oasis Valley; the latter are from the Grouse Canyon Member of the Belted Range Tuff (Indian Trial Formation of former usage) and the Ammonia Tanks Member of the Timber Mountain Tuff (Carr, 1974). The principal variations present in table 5 are due to sub- stitutions of magnesium and iron for aluminum in the octahedral coordination sites and the replacement of E9 silica by aluminum in the tetrahedral sites. These sub- stitutions are responsible for the charge imbalance which is neutralized by absorbed cations. A comparison between the montmorillonite from Aberdeen, Mississippi (Kittrick, 1971a, b) and the montmoril- lonites of the Grouse Canyon and Ammonia Tanks Members show that aluminum and magnesium are only slightly deficient in the latter samples. This defi- ciency may be partly due to the presence of unaltered silica glass in the samples. Montmorillonites of the Grouse Canyon and Ammonia Tanks Members show less iron by a factor of two in comparison to the Aber- deen sample. If montmorillonites of the Grouse Canyon and Ammonia Tanks Members represent the average composition of montmorillonite formed by Oasis Valley ground water, an assumption must be made that the substitution between aluminum and iron in the quan- tities shown in table 5 does not affect the free energy of formation of the Aberdeen montmorillonite. The de- gree of magnesium substitution in the montmoril- lonites of the Grouse Canyon and Ammonia Tanks Members is assumed to be equal to that found in the Aberdeen montmorillonite. A decrease in the iron con- centration is balanced by an increase in aluminum. The charged montmorillonite structure is assumed to be balanced by hydrogen-ion absorption which permits expression of the chemical activities as ratios in terms of hydrogen-ion activities. Because the type of ad- sorbed cation does not greatly affect the free energy of formation of the montmorillonite structure (Kittrick, 1971a, b), the exchange of hydrogen ion for sodium, potassium, or calcium should not significantly change the stability relations. The equation below, which de- fines the stability boundary between a montmorillonite of the average composition of samples from the Grouse Canyon and Ammonia Tanks Members and potassium feldspar, is used as an example. An equilibrium expres- sion between the two minerals can be written in terms of the logarithms of the aqueous and solid activities and an equilibrium constant. 6-31 108 aKAlSiaOs (feldspar) + log aH+ +0.61103 an+++ + 1.73 log aMg++ + 12.61 log amo — 6.31 log ax+ — 4.23 log amsw‘ — 381 10g aHAzMgAsFemAlissSlamoiMOH): (Montmorillonite) = 11588 The expression on the right hand side of the equation is the logarithm of the equilibrium constant. Assuming that the activities of the solid phases and water are unity, the above expression can be rearranged to give: ape+++ 3311+ MEfi++0.61 log a 1.73 log aim a — 6.3110ga—:—:- — 4.23 log 811.510. = 1.688 E10 In the above equation as well as in the other stability expressions, the iron-to-hydrogen activity ratio and the silica activity are assumed to equal constants defined by the average solute concentrations of the Oasis Val- HYDROLOGY OF NUCLEAR TEST SITES TABLE 5.—Comparisons of selected chemical components normalized to silica for montmorillonites from tuffaceous rocks that are used in stability calculations Montmorillonites atoms Er unit cell Mg Fe Al Si ley ground water. Substltuting 17.66 for log aFe+++ Aberdeen, Missl ______________________ 0.45 0.34 1.49 3.82 a3H+ Belle Fourche, s. D? __________________ .28 .23 1.53 3.82 Oacsris Vallgy, Nev.3: b _ _ 4 . ' ‘ ‘ rouse anyon Mem er of the and 3 Q for .log amslo. Into the above equatlon glves Belted Range Mf _________________ '42 .17 1‘39 382 the relationshlp: Ammonia Tanks Member of the Timber Mountain ’Iliff ______________ .38 .14 1.29 3.82 3M3” aK+ _ 'K'tt ' k 1971b 1.73 log 32H+ - 6.31 log a—I-I+_ 21.943. iglfigiléilwl“: Variables in this equation are defined as coordinates in the first diagram of figure 4A. The resulting linear plot represents the stability boundary between mont- morillonite and adularia as a function of varying mag- nesium, sodium, and hydrogen activities. Other stabil- ity boundaries shown in figure 4 are derived in an analogous manner. Solid lines in figure 4 define the stability field for a montmorillonite with an iron concentration equal to the average concentration found in the montmoril- lonites of the Grouse Canyon and Ammonia Tanks Members. The more extensive montmorillonite- stability field represented by dashed lines is for an iron concentration equivalent to the Aberdeen montmoril- lonite while the smaller field defined by dashed and dotted lines is for a montmorillonite that contains no iron. At a specific aqueous-iron concentration, the sta— bility fields of montmorillonites containing succes- |8 I I I I I I .- I15 . I: ‘6 :5 5, Is; I6 MAGNESIUM CHLORITE MAGNESIUM CHLORITE .'3_ _ on"? MAGNESIUM CHLORITE .g g Illa“ - U 0 _________ ' I— _Tu' l4 — Lu _' / ,4 _________ —O-u_sls—VGlle-y._H—-, I: .' I _.....W___l.___.______7 ground water / g. / Oasis Volley / . .1 - groundwater - I2 - ~ / =" ’3' — — / — V" . 3’ g.- In" é‘v . N I w ’4", ’. i' ['3 l” '. ll? cv l t I - 4" I~ ° ’3: r: igv/ N v I + z 9 I '7: 2 .' ,9 z 0 i: I, + N o “J . O o I o \_ / o 4" 0+ |o_._l _|: .1)! \_E:Ig _ .__1 ~- \_ r]— 2 :I: :1 3:] Z In)", 3' . J; =1 o, «v 0 l 00 I: 0/ o 65'..,' .10) I: 3'. G.) El 0 \— J I ‘5 . I A o (0/ \n; o/ o 2 3'] =1 s I v 2 a. to ,,,~/ 0 '- .-:'I L: J 8 l\‘ I- 3/ g -\ I _1 3-5 03/ - -2 ;/\< :lq' _ -5 $.\\. ‘3/ _ 2 ~/ F =- 1: :, a? 2 ->/ v i/ 'I g u‘/ T '11:? olf/ 3 C/ . q, . v ’ 2 5 / k - / e s / :8 ,1 6 - — 3 lg. — — l . v? '9], _- ..... cu‘ .° l 12’ F _--—J ., _ ..... +.____./§ a, - I °:r/ .II I 8/ 4 — — _ 5/ _ _ | / _ .'/ I "I I 2 / ADULARIA J .‘I ALBITE _ i ll ANORTHITE _ 6% I :II I I OQ") ll KAOLINITE III KAOLINITE / I .' I o *3? .’ I L I -.’ I I ll I 1 I 2 4 6 8 |04 6 8 l0 I2 8 IZ IS 20 24 LOG 0K+/OH+ LOG °Na+/°H+ LOG 0Co++/QH+2 A B C FIGURE 4.—Aqueous activity diagrams depicting montmorillonite stability. GEO(IHI{MISTRY ()F GROUND WATER, TUFFACEOUS ROCKS, OASIS VALLEY, NEVADA sively more iron expand at the expense of other min— erals that do not contain iron. Regardless of the lattice-iron concentrations, how- ever, figure 4 depicts montmorillonite as the stable phase in equilibrium with Oasis Valley ground water. The stability of montmorillonite as a weathering product is documented by its occurrence in the tuffs surrounding Oasis Valley as well as in other tuffaceous rocks (Sheppard and Gude, 1968, 1973). Kaolinite oc— curs at lower ionic-activity ratios (fig. 4), and illite is unstable under all the conditions shown in the dia- grams. Some of the clays in the tuff formations of Oasis Valley are described as mixed-layer minerals which represent a presumably continuous gradation between montmorillonite and illite. Reliable thermodynamic data exist only for the two end-member clay minerals; therefore, stability fields for mixed-layer clays, though not shown in figure 4, may represent stable phases. Because of the lack of thermodynamic data, the sta- bility fields for a number of zeolites commonly found in the tuffaceous rocks surrounding Oasis Valley (Hoover, 1968) are missing from the diagrams. The metastable boundary between analcime, the only zeolite for which data exist, and an iron-free montmorillonite is shown for the Na20 — MgO — H20 — A1203 system in figure 4 as a dotted line. Other zeolites such as clinoptilolite and mordenite have higher ratios of silica to aluminum in their struc- tures than does analcime. These zeolites would have stability boundaries with slopes similar to that of anal- cime, but because of the relatively high silica concen- trations found in Oasis Valley ground water, the boundaries would be shifted toward the upper left- hand corner of the diagrams in figure 4. These bounda- ries may encroach upon the montmorillonite stability field. The metastable analcime—montmorillonite boundary qualitatively demonstrates that water con- taining moderate amounts of magnesium leads to montmorillonite formation, whereas water containing sodium and potassium but deficient in magnesium fa— vors analcime, clinoptilolite, and mordenite formation. Some water samples taken from tunnels driven into zeolitic tuffs at the adjacent Nevada Test Site have very low concentrations (<5 X 10‘6 moles/liter) of mag- nesium and may be in equilibrium with respect to zeo- lite minerals. The variability in hydrogen activities appears to be a controlling factor in the linear data distributions for Oasis Valley ground water shown in figure 4. This de- pendency is only apparent because the maximum and minimum activities of magnesium, sodium, and potas- sium, when divided by hydrogen activities, do not necessarily correspond to maximum and minimum ratios shown in the stability diagrams. Also, the distri- Ell bution of data in the magnesium-sodium system, and to a lesser extent in the magnesium-potassium system, have slopes of approximately 2 to 1. These slopes are the same as the logarithmic ratio of hydrogen activities that occur in the denominators of the two coordinates, log aMg++/a2H+ versus log aNa+/aH+ and of the coordi- nates, log aMg++/a2H+ versus log aK+/aH+. This variabil- ity in hydrogen-ion activities is expected because ground water in the alluvium is subjected to varying degrees of atmospheric and soil-zone interactions that affect the carbon-dioxide partial pressures and the pH. The distribution of data in the magnesium-calcium system, as shown in figure 4C, is controlled principally by equilibrium with calcite. SOURCES OF DISSOLVED SPECIES SILICA As shown by the equilibrium data in table 3, the aqueous concentration of silica is apparently controlled by equilibrium with silica gel. This relationship is sup- ported by silica gel observed in fractures in tuffaceous rocks. From thermodynamic considerations, silica gel represents a metastable phase in relation to both quartz and cristobalite. Secondary low cristobalite and quartz (Ransome, and others, 1910; Hoover, 1968) within the groundmass and fractures of the tufi's pro- bably represent subsequent crystallization of silica gel. A median value for silica concentrations in ground water is 0.6 millimole per liter (Davis, 1964) compared with the average concentration of 2.5 millimoles per liter silica in Oasis Valley ground water. Similarly high concentrations are found in ground water derived from other tuffaceous rocks (White and others, 1963). The principal silicate phases that are present in the tuffs and that could be adequate sources for silica are feldspars, quartz, and volcanic glass. Feth, Roberson, and Polzer (1964) showed that the principal source of silica in ground water from granitic rocks in the Sierra Nevada, the bulk chemical composition of which is similar to the tuffs, was the dissolution of plagioclase and, to a lesser extent, of potassium feldspar: The silica concentration of these waters averaged one-third of the concentrations found in the Oasis Valley ground water. This difference suggests that volcanic glass and not feldspars is the major contributor of high silica concen- trations in the Oasis Valley groundwater. Jones (1966) and other workers have recognized that glasses, which are much more soluble than crystalline silicate mine!» als, can produce high silica concentrations in ground water. Water can alter volcanic glass by hydration and leaching. Both of these processes can contribute silica to the aqueous system. Assuming constant aluminum E12 concentrations, Lipman (1965) estimated that volcanic glass in the tuffs of southern Nevada had been leached of several percent more silica than had the crystalline rocks of the same composition and origin. SODIUM, POTASSIUM, CALCIUM, AND MAGNESIUM The source of the major cations in the ground water in the tuffaceous aquifer must be potassium-feldspar, plagioclase, and volcanic glass. Comparison between Oasis Valley ground water and water derived from the springs in Sierra Nevada rocks with about the same bulk composition (Feth, and others, 1964) shows a major difference in solute compositions. In the case of water from the granitic terrane, sodium and calcium are roughly equal as dominant ions whereas water from the Nevada tuffs exhibits a sole dominance of sodium, which comprises roughly 90 percent of the ca- tions present. The dissolution of plagioclase in the granitic rocks of the Sierra Nevada is well documented as the prime source of aqueous sodium and calcium by Garrels and MacKenzie (1967). Although the plagioclase in the tuffaceous rocks is slightly deficient in calcium (AN25 ——ANso) as compared with the plagioclase in the granitic rocks (Ast—AN40), breakdown of plagio- clase is unlikely to produce the much higher sodium- to-calcium ratios found in Oasis Valley ground water. A major source of high aqueous sodium concen- trations appears to be volcanic glass. Table 6 shows a comparison between the relative molar percentages of sodium, potassium, calcium, and magnesium found in the glass and aqueous phases. The composition of the glass represents an average of three analyses reported by Lipman (1965) for tuff formations found in or around Oasis Valley. The aqueous composition is the average analyses for ground water in the tuffaceous aquifer of Oasis Valley not saturated with respect to calcite. As is apparent from table 6, the Oasis Valley ground water cannot be described by a simple congruent disso- lution of the glass phase. The percent of sodium is in- creased by a factor of two in the aqueous state, mag- nesium is decreased by a factor of two, and potassium is decreased by a factor of fourteen. Relationships between the glass and aqueous phases shown in table 6 have been predicted by other work- ers. Lipman (1965), in a chemical comparison of vol- canic rocks of southern Nevada, showed that hydrated glasses are characterized by higher K20/Na20 ratios than crystallized rocks. As an approximation, Lipman suggested that about 0.5 percent Na20 has been re- moved by hydration and leaching of the glass phase while K20 remains relatively immobile. Truesdell (1966) determined the ionic-exchange constants of sev- HYDROLOGY OF NUCLEAR TEST SITES TABLE 6.—Comparisons of relative molar percentages of selected cations found in the volcanic glass and Oasis Valley ground water Volcanic Ground glass' water2 Ca ________________________ 8 8 Na ________________________ 46 87 Mg ________________________ 4 2 K ________________________ 42 3 ‘Average of three analyses (Lipman, 1965) for tuffs in or around Oasis Valley. 2Average of all water analyses (this paper). eral natural and synthetic glasses by use of an elec- trode method, and he found that glasses preferentially absorbed hydrogen, potassium, and, to a lesser extent, calcium relative to sodium. He predicted that waters in the vitric tuffs would be sodium-rich. BICARBONATE The dominant anion present in the tuffaceous aquifer is bicarbonate. The occurrence of carbonate minerals is minimal in the tuffaceous rocks (Lipman and others, 1966). The principal source and control of bicarbonate is the reaction of dissolved, soil-zone car- bon dioxide with various mineral phases. In the case of glass, Budd (1961) has demonstrated that in neutral solutions similar to Oasis Valley ground water, elec- trophilic reactions occur in which hydrogen enters the glass structure as hydronium ion and exchanges with sodium or other cations bonded to lattice oxygen atoms. As hydrogen atoms are consumed in this exchange reaction, dissolved carbon dioxide will disassociate to produce bicarbonate ions. FLUORIDE AND CHLORIDE Ratios of fluoride to chloride in tuffaceous rocks northeast of Oasis Valley vary between 0.5 and 2.0 (Noble and others, 1967). Ground water entering Oasis Valley through such rocks has ratios generally an order of magnitude less. This lack of fluoride mobility in comparison to chloride agrees with observations that, during alteration of volcanic glass, fluoride remains in the solid phase. Noble and others (1967) compared fluoride and chloride contents of a number of non- hydrated and hydrated glasses of the same origin and found that four-fifths of the chloride and less than half of the fluoride were lost during hydration. CHEMICAL CHARACTERISTICS CONTROLLED BY THE ALLUVIAL AQUIFER CONCENTRATION INCREASES Figure 5 illustrates trends in concentration as a function of a distance of approximately 24 kilometers GEOCHl-LMISTRY OF GROUND WATER, TUFFACEOUS ROCKS, OASIS VALLEY, NEVADA E13 KILOMETERS KILOMETERS 0 5 l0 IS 20 O 5 10 IS 20 '-3 I l I l '3-0 I I I I 29g I.2 — - I2.o— 27 28 29 2r 27o 28 H — O—OG II.o- 16 17 $0 —0- 5 10 ——O 16 13/ / LO —1/0 \OO 8'02 I0.0- 100/ O18 - > \/ 0.9 -' 17 - 9.0 - - 0.8 - 4'“ 8.0- — "Z: 9 5 : O 7 ' O 6 : 7.0 '/O ’ I: >- ' k — I: O\ R CO" 5 )1 O/G) flo‘o 3' 0‘5 ' / OO - < 6.0- O _ —' O 5' o E 0.5 - - 5 5.0- — O 0.4 - A 4.0-/ _ F' > 0.3 - /CDO + O‘gg 3.0- E K o— _ .5 48‘ ./ c 0 CI / ozi O o\®O H O 2-°-/o/ \%%3—::=%=9’- / 0‘0 Mg 0’ D —O/ 504 O.l ' O - l-OI/O _ >/ 0 I I I I I I I l l l l I I I o I I I l L 1 l I l I I I I o 5 IO I5 0 5 10 I5 MILES M|LES Sample numbers are keyed to tables 1,2,and 3 FIGURE 5.—Changes in concentration of dissolved chemical constituents as a function of distance along the alluvial flow path. (15 miles) along the Thirsty Canyon-Oasis Valley flow path. The data on the far left-hand side of the figure are from the northernmost sampling point (site 1) located near the mouth of Thirsty Canyon. The data on the far right-hand side are from the southernmost point (site 29) in the flow path located at the outlet of the Amar- gosa Narrows. Sodium, bicarbonate, fluoride, and chloride show rapid increases in concentrations along the initial 8 kilometers (5 miles) of the flow path in the alluvium. Throughout the remaining distance, except for the final sampling point, these species Show a more moderate increase. Calcium and magnesium display even greater concentration increases along the initial part of the flow path; silica and potassium show only minor enrichment. RATIOS OF SOLUTE COMPONENTS SODIUM, BICARBONATE, AND CHLORIDE Figure 6 shows the aqueous concentrations of bicar- bonate and chloride plotted against sodium. Included in the diagrams are data for ground water in the tuf- faceous and alluvial aquifers of Oasis Valley. Num- bered data points correspond to samples listed in tables 1, 2 and 3. Also shown in figure 6 are additional geochemical data for ground water from the tuffaceous aquifer under Pahute Mesa (Blankennagel and Weir, 1973) and Gold Flat (unpublished data from files of US. Geological Survey). The latter two areas represent probable recharge areas for Oasis Valley ground water. The most significant observation is that nearly all the data presented in figure 6 plot on the same linear E14 MOLALITY BICARBONATE x I03 MOLALITY CHLORIDE x I03 9.0 HYDROLOGY OF NUCLEAR TEST SITES 8.0- 6.0 I Thirsty Canyon - Oasis Volley 00 I I I I I I I I I I I l I 0.0 |.O 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 l0.0 |l.0 l2.0 |3.0 |4.0 MOLALITY SODIUM x Io3 0.0 |.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 |0.0 ||.O l2.0 |3.0 14.0 3.5 I I I I I I I I I I I I I 3.0 - - 29 z / ’31. 2.5 - ‘ Thirsty Canyon — 2.0 - - A I. 5 - 1 - 9 / 25 3.6 CD // @913 12 H)— ’/ s 4‘ Gd)? _ / rm 0 e Indian Springs ’ ‘920 p g 0.5 - 26 - m, M 00 [1/ l I l l I l J 1 l EXPLANATION ®—. Variation in concentration along Specified alluvial flow path 26. Oasis Valley alluvial aquifer 20© Oasis Valley tuffaceous aquifer A Pahute Mesa E] Gold Flat ———— Average concentration trend for all data Sample numbers are keyed to tables 1,2,and 3 FIGURE 6.—Concentration trends of sodium, bicarbonate, and chloride. GEOCHEMISTRY ()l’ GROUND WATER, TUFFACEOUS ROCKS, OASIS VALLEY, NEVADA trends. The data for the tuffaceous aquifer, both in the immediate vicinity of Oasis Valley as well as for Pa- hute Mesa and Gold Flat, plot in the more dilute region of the diagrams. Alluvial water compositions repre- sented for specific flow paths in figure 6 range from overlapping the dilute region at the initial stages along the flow paths to considerably higher concentrations farther downgradient. The similarity in compositions supports the suggestion of Blankennagel and Weir (1973) that water beneath Pahute Mesa is chemically related to Oasis Valley ground water. The linearity of the data suggests that the water in the alluvium can be produced from tuffaceous compositions by concentra- tion increases due to evapotranspiration. Sodium and chloride appear to be neither selectively added nor re- moved from the water during this process. Some varia— tions in bicarbonate composition may be expected due to local dissolution or precipitation of carbonate min- erals. The Springdale flow path appears anomalously high in bicarbonate, probably due to the dissolution of locally occurring Paleozoic carbonates (fig. 2). The slightly larger increase in bicarbonate along the initial part of the Thirsty Canyon-Oasis Valley system (fig. 6) may also represent dissolution of carbonate minerals in the alluvium, and the slight decrease along the final portion may represent precipitation after calcite satu- ration has been achieved. However, bicarbonate, like sodium and chloride, seems to be principally controlled initially by interaction with tuffaceous rocks and by later modification due to evapotranspiration. Figure 7 shows the relationship between bicarbonate concentrations and the partial pressure of carbon dioxide in equilibrium with Oasis Valley ground water calculated from the expression 10g PC02 : 10g amua— PH _ 10g Ks where log P002 and log amvoa— are thelogarithms of the partial pressure of carbon dioxide (P002) and the ac- tivity of bicarbonate respectively. Log KS is the logarithm of the solubility constant. Pco2 increases rapidly along the Thirsty Canyon-Oasis Valley and the Springdale flow paths relative to that associated with ground water in the tuffaceous aquifer. Increases in P002 with increasing bicarbonate are reverse to the re- lationship previously discussed for the tuffaceous aquifer, in which bicarbonate is produced and C02 is consumed during the reaction with silicate minerals in a closed system. Rapid increases in dissolved CO2 in the alluvial aquifer suggest that the hydrologic system interacts with the atmosphere through the root-zone transpiration of C02 by the relatively dense vegetation along the valley bottom. E15 'l.8 I I I l I I l -2.0 — gafl' '1 4 fi"18 v . 27 ¢ 10...,i .29 -2.2 — 19 6/}J: 17 28 m N SPRINGDALE ’IE 0: ’ 5 .16 L” -24 — 14 - E - ’03 @‘2 THIRSTY CANYON OASIS VALLEY 8 @15 " 119 E ‘2 6 — ’ _ <1 I ' I N o -2.8 ._ l .1 _ o I 0- I 23 24 \ <9 I 9 920 \ 9 -30 _ I 26.025 _. l I -3.2 — 'l _ , INDIAN SPRINGS -3.4 — : _ l I 071 I 1 I I I o l 2 3 4 5 6 7 8 HCO; MOLES x io+35 EXPLANATION 26. Oasis Valley alluvial aquifer 79 Oasis Valley tuffaceous aquifer 9—». Data points along specified flow path General trend of increases in Oasis Valley Sample numbers are keyed to tables 1,2,and 3 FIGURE 7.—Progressive increases in ng with increasing bicarbonate. POTASSIUM AND FLUORIDE Figure 8 shows the aqueous concentrations of potas- sium and fluoride plotted against sodium. No data exist for fluoride concentrations in ground waters of Pahute Mesa and Gold Flat. Both potassium and fluoride show concentration increases in the alluvial aquifer but at a progressively decreasing rate relative to sodium. As- suming that sodium concentration increases reflect relative volume decreases of ground water by evapo- transpiration, the decrease of potassium and fluoride concentrations relative to sodium suggests that their absolute amounts in the aqueous system are being di- minished. The mechanism for the removal of potassium is not certain. Table 4 shows that the average aqueous sys— tem is saturated by a factor of more than two with respect to potassium feldspar. Proven difficulties in nucleation (Berner, 1971) plus the instability of potas- sium feldspar with respect to montmorillonite (fig. 4) tend to discount potassium feldspar as a likely sink for potassium. A possible mechanism for removing potas- sium is by absorption and incorporation into the lattice E16 HYDROLOGY OF NUCLEAR TEST SITES 4.0 I I T I I I I I I I I T I .0‘ O I I" U! I MOLALITY FLUORIDE mo3 5 '2; I I LO- 0.5- 0.0 l l l I I l I L 0.0 6.0 7.0 8.0 9.0 l0.0 “.0 12.0 [3.0 |4.0 MOLALITY SODIUM x Io3 0.0 [.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 I0.0 ||.O l2.0 I3.0 I4.0 3.0 I I I I I I I I I I I I I 2.5 - MOLALITY POTASSIUM on3 3. | 0.5 - 0.0 EXPLANATION 26. Oasis Valley alluvial aquifer (9—. Variation in concentration along Specified 7 alluvial flow path @ Oasis Valley tuffaceous aquifer A Pahute Mesa E] Gold Flat —————- Average concentration trend for all data Sample numbers are keyed to tables 1,2,and 3 FIGURE 8,—Non1inear average concentration trends of fluoride and potassium relative to sodium. GEOCHEMISTRY OF GROUND WATER, 'I‘UFFACEOUS ROCKS, OASIS VALLEY, NEVADA of clay minerals. Although large quantities of clay were deposited in the alluvium of Oasis Valley under subaerial and lacustrine conditions by erosion and weathering of the surrounding tuffs (Malmberg and Eakin, 1962), little is known concerning their specific mineralogy. However, degraded illites and montmoril- lonites, which are common clay minerals in most arid soil zones, adsorb and fix potassium. These minerals are known weathering products of the tuffs in and around Oasis Valley. Also the vegetation cover in Oasis Valley may remove significant amounts of potassium. As will be shown in the section entitled “Equilibrium Relationships,” the mineral fluorite appears to be the major sink for the removal of fluoride from the ground water in the alluvium. CALCIUM AND SULFATE Both calcium and sulfate show anomalous increases in relation to sodium, chloride, and bicarbonate that cannot be explained by increases due to evapotranspi- ration. Figure 9 shows the concentrations of calcium and sulfate plotted against sodium. Along the initial part of the main flow path labeled Thirsty Canyon- Oasis Valley, the calcium concentration increases by a factor of four, which is double the increase observed for sodium along the entire system. Figure 10 shows a comparison between calcium, sodium, and magnesium distributions for the Thirsty Canyon-Oasis Valley and Springdale flow paths. Even higher calcium per- centages in the Springdale system are apparent. This high calcium content, coupled with the anomalously high bicarbonate-to-sodium ratios shown in figure 6, suggest that calcium carbonate is being dissolved in the Springdale system. The source of calcium carbo- nate is probably in the Bullfrog Hills. Carbonate rock crops out locally in the northern Bullfrog Hills west of Springdale. Ransome, Emmons, and Garrey (1910) also reported calcite in fractures and mineralized veins within the hydrothermally altered zones of the Bull- frog Hills. The calcite displays cavernous structure indicating dissolution by ground water. Elsewhere, such as in the Thirsty Canyon-Oasis Valley system, both calcium carbonate and calcium sulfate may occur in lacustrine beds within the alluvium. The distribution of sulfate plotted against sodium is also shown in figure 9. The slope of the sulfate-to- sodium ratio is greatest in both the tuffaceous and a1- luVial aquifers of Oasis Valley relative to the Pahute Mesa and Gold Flats data. The slope is also greater for sulfate to sodium than for bicarbonate or chloride to sodium in the flow system in the alluvium. Like cal— cium, an additional source of sulfate must be contrib- E17 uted to the Oasis Valley ground water. Unlike calcium, however, figure 10 shows that the highest percentage of sulfate relative to bicarbonate and chloride in the allu- vial aquifer is concentrated in the main Thirsty Canyon-Oasis Valley flow path. The primary source of most of the additional sulfate seems to be hydrothermal. Hot Springs (sample site 13), a major thermal (41°C) spring in the area, shows an abnormally high sulfate-to-chloride ratio. Also, the mineralogy of the hydrothermally altered tuffaceous rocks in the Bullfrog Hills suggests that sulfate may have been contributed locally from sulfide minerals. Although pyrite is not now abundant near the surface, the large amounts of limonite (Ransome and others, 1910) suggest that pyrite has been oxidized with the concurrent production of sulfate. EQUILIBRIUM RELATIONSHIPS As discussed in the section dealing with equilibrium relationships, the ground water in the tuffaceous aqui- fer is stable with respect to silica gel, montmorillonite, and possibly mixed-layer clay minerals. When the ground water moves from this aquifer into the alluvial fill of Oasis Valley, stability is maintained between this mineral assemblage and the aqueous phase. Silica con- centrations remain constant throughout the flow sys- tem in the alluvium and appear to be continually con- trolled by silica-gel equilibrium. Montmorillonite sta- bility appears to be maintained with increasing solute concentrations as the reaction paths advance across the montmorillonite stability field toward the chlorite boundary (fig. 4). However, increasing chemical concentrations in the ground water of the Oasis Valley alluvium produce saturation with respect to several mineral phases in addition to those generally associated with the tufface- ous aquifer. Figure 11 shows the ionic-activity product of calcium fluoride plotted against the ionic-activity product of calcium carbonate. The figure also includes the stability fields for calcite and fluorite. Fluorite solubility is that given by Smyshlyaev and Edeleva (1962). Note that all ground-water samples associated with the tuffaceous aquifer are undersaturated with respect to fluorite and that the majority of samples are undersaturated with respect to calcite. Those waters that are calcite—saturated are confined to springs emanating along the east-central side of Oasis Valley and may be associated with carbonate deposition dur- ing local hydrothermal alteration of the tuffaceous rocks (Ransome and others, 1910). Ionic-activity products for both calcium carbonate and calcium fluoride become progressively greater E18 MOLALITY SULFATE x Io3 MOLALITY CALCIUM x I03 2.50 2.25 2.00 L75 |.50 |.25 |.00 0.75 0.50 0.25 0.25 0.20 0.|5 0.|0 0.05 HYDROLOGY OF NUCLEAR TEST SITES 8. l l l l l 1* l l l l I I F 29 _ 21.- ' 16 - _ Thirsty Canyon- _ Oasis Valley 10 — —I _ .22 _ . 15 fig 7 / I l I I l I I I I I I I I 0 I0 2.0 3.0 40 5.0 6.0 7.0 8.0 9.0 |00 “0 I20 I30 I40 MOLALITY SODIUM x Io3 0.0 I0 20 3.0 4.0 5.0 6.0 7.0 80 9.0 |00 l|.0 |20 I30 I40 l | l l I I l | I l l l l Thirsty Canyon— 29 ‘ 4 5 Oasis Volley ‘ .21 I I I I I l I I EXPLANATION 1. Oasis Valley alluvial aquifer @—. Variation in concentration along specified 7® Oasis Valley tuffaceous aquifer alluvial flow path in Oasis Valley ----- — Average concentration trend for Pahute Mesa A Pahute Mesa and Gold Flat data C] Gold Flat Sample numbers are keyed to tables 1,2,and 3 FIGURE 9,—Average concentration trends of sulfate and calcium relative to sodium. GEOCHEMIS’I‘RY ()F GROUND WA'I‘ER, TUFFACEOUS ROCKS, OASIS VALLEY, NEVADA E19 downgradient along the Thirsty Canyon-Oasis Valley Indian Springs in the Bullfrog Hills shows a rapid in- and Indian Springs alluvial flow paths. Ionic-activity crease in the ionic-activity product of calcium carbo— products increase until they intersect the stability mate but only a minor increase in calcium fluoride. This fields of both calcite and fluorite, Beyond this point in the flow path, the ground water shows only minor and erratic variations in the ionic-activity products. This "0'2 i would indicate that calcium carbonate and calcium fluoride are being precipitated from the alluvial system 404 _ as the volume of ground water is being reduced by evapotranspiration. Ground water originating from -|0.6 Nu+ —|0.8 —- |OO°/o —II.o — -l|.2 — -||.4 — 21Q©9 — 924 u‘.“ 6 z 0 -ue — . 9 - 0 l- (L < <_t g ,_ ‘9 "L8 ‘ UNSATURATED AQUEOUS SOLUTION 3‘, ‘ o .J uJ ’— 9 —l2.0 — < _ U 50% _ 50% 422 _ _ Ca++ HCO 3 Mg++ IOO°/o _ [2.4 — - 15 © —|2.6 l— ._ Indian Springs\__ —l2.8 _ _+ 43.0 — -— _|3.2 I l l I I l —9.4 —9.2 —9.0 -8.8 —8.6 -8.4 —8.2 —s.o LOG 'APCOC03 EXPLANATION Oasis Volley 26. Oasis Valley alluvial aquifer 509/0 500/0 l5© Oasis Valley tuffaceous aquifer CL” so 2 Sample numbers are keyed to tables 1,2,and 3 FIGURE 10.—Triangular diagrams depicting relative concentrations FIGURE 11.——lonic-activity products of calcium fluoride compared to of selected cations and anions. calcium carbonate. Saturation values are at 25°C. E20 rapid saturation with respect to calcite but not fluorite suggests again that calcium carbonate is being dis- solved along the Indian Springs flow path. Figure 12 shows the approximate geographic distri- bution of the saturation boundaries for calcite and fluorite. Equilibrium with respect to calcite occurs in the lower three-fourths of the main alluvial flow path that originates at the mouth of Thirsty Canyon. Equilibrium is also achieved throughout the Spring- dale flow path and the lower part of the Indian Springs flow path. Both of these systems have recharge areas. in the Bullfrog Hills and show abnormally high cal- cium carbonate concentrations as compared with the dissolved-solids concentrations. Fluorite saturation is limited to a narrow strip in the lower center of the valley; it coincides with the area in which the ground water would be subjected to maximum evapotranspira- tion. Most of the springs issuing from the tuffaceous aquifer, with the exception of those in the central part of Oasis Valley, are not saturated with calcite or fluorite. The concentration of aqueous magnesium also ap- pears to be controlled by an equilibrium relationship although the exact mechanism is less well defined than for calcium and fluoride. Initially, the percentages of magnesium increase downgradient in the water in the alluvium at a much faster rate than do the percentages of sodium, chloride, and bicarbonate (fig. 5). This rapid increase suggests that, like calcium, magnesium is being added to the system from an additional source. Figure 13 shows the ionic—activity product of mag- nesium carbonate plotted against calcium carbonate. Included in the figure are the stability fields for calcite, magnesite, and dolomite. Both Thirsty Canyon and In- dian Springs flow paths display initial rapid increases in the ionic-activity products of magnesium carbonate and calcium carbonate. However, both systems reach steady states downgradient, after which the ionic- activity products show only minor erratic increases and decreases. As in figure 11 for the calcium carbonate versus calcium fluoride system, such a steady—state situation suggests equilibrium conditions between aqueous and solid phases. The limiting value for cal- cium carbonate activities is controlled by calcite equi- librium (fig. 11). The distribution of the data suggests that calcium carbonate and magnesium carbonate ac- tivities also approach the solubility of dolomite from undersaturation. The inability of many, even highly supersaturated, waters to precipitate dolomite is well documented (Berner, 1971). Therefore, dolomite precip- itation as a means of controlling magnesium concen- trations in saturated or near saturated systems such as Oasis Valley is very improbable. However, waters that are dissolving dolomites exhibit solute concentrations which approach saturation with respect to dolomite HYDROLOGY OF NUCLEAR TEST SITES (Hem, 1970). Figure 13 suggests that the limiting value for magnesium concentrations in the Oasis Valley sys- tem is based on the ability of the ground water to dis- solve dolomite. Once the system is almost saturated with dolomite, little more magnesium is introduced into the system. This theory implies that dolomite dis- solution and not evapotranspiration is the dominant control on the maximum magnesium concentrations found along the flow paths in the alluvium. Dolomite crops out extensively in the hills southwest and south- east of Beatty and is present at depth under the Bullfrog Hills (Ransome and others, 1910). Oasis Valley ground water could come in contact with these rocks or with alluvium derived from them. The Indian Springs system, which is initially greatly undersaturated with respect to both calcite and dolo- mite, shows a calcium to magnesium slope of 1 to 1 (fig. 13). This slope would be expected from a congruent dolomite dissolution. The Thirsty Canyon system is initially much closer to calcite saturation. If dolomite reacts similarly to magnesian calcites, as proposed by Plummer and Mackenzie (1974), dissolution becomes incongruent near calcite saturation. Incongruent disso- lution produces dominantly aqueous magnesium car- bonate. This mechanism would explain why the magnesium-to-calcium ratios in the Thirsty CanyonL Oasis Valley system are higher than those in the In- dian Springs system. MASS BALANCE OF CHEMICAL SPECIES As a means of summarizing the behavior of aqueous chemical species in the alluvial aquifer, a mass balance comparison is made in table 7 between chemical com- positions of ground water recharging and discharging the alluvium. Line 1 in table 7 lists the chemical com- position of ground water at site 29 below the mouth of the Amargosa Narrows (fig. 2). Assuming that the Paleozoic carbonate basement within the Amargosa Narrows is relatively impermeable, all ground water discharging from the alluvium in Oasis Valley must pass through the 3—4 meter (9— 12 ft) thickness of allu- vium at this point. Sample 29, immediately down- gradient from the Amargosa Narrows should, there- fore, be representative of the composition of ground water discharging the alluvium. An estimate of the chemical composition of water recharging the alluvium from the tuffaceous aquifer is more difficult owing to the number of springs and seeps as well as probable subsurface flow. As discussed by Malmberg and Eakin (1964) and Blankennagel and Weir (1973), most ground water that recharges the al- luvium probably moves considerable distances through the tuffaceous aquifer from areas north and east of GEOCHEMISTRY OF GROUND WATER, TUFFACEOUS ROCKS, OASIS VALLEY, NEVADA E21 lll6° 50' 45' ||6°40' 37°“ i ' Thi's'y~’°°"y°" EXPLANATION 15/--.\ , .- V / Calcite saturation Oasis V Fluorite saturation Mountain 2 '. . - ’\_ O ‘ 2 "\ .../ Water samplelocatlon /o -4._ g» \ «g: s b k d 5,? 50 azalzgzmlzfm ”}“s//”’ "i~2'q,4:// 03‘1/// We; 37°00“ — ¢t¥ — I a 1 .- I O /r' Crysla/ ' /II¢ O / / Spring: 15 \ ~ ’W’ ‘ 2 \.. :‘E '. " 4'0 . 7 a a? \Qoglnd/‘an / ...._ 55 Springs 910” wash / S \ ._/ '\_.. 36°55'— _ fiRhyoiite // r // Fl99'399— \Qoo’o / O. 0 I.’ margosa ‘ \l‘ @ arrows THJ \\ “ Bare. 1 Base by US. Geological J M0untaIn Survey. I954 O SMILES i I I l I I | I l I I I 0 5 KILOMETERS FIGURE 12.—Geog'raphic distribution of calcite— and fluorite-saturated ground water. E22 LOG IAP Cocos HYDROLOGY OF NUCLEAR TEST SITES ~90— UNSATURATED AQUEOUS SOLUTION \\ — s \ \ \ _9.2 I I I I I I l I I I \\ —I0.0 —9.8 —9.6 —9.4 —9.2 —9.0 —e.8 —8.6 —8.4 —-8.2 —a.o ~7.8 LOG IAP Mg C03 EXPLANATION 1. Alluvial aquifer 8@ Tuffaceous aquifer Sample numbers are keyed to tables 1, 2,and 3 FIGURE 13,—Ionic-activity products of magnesium carbonate compared to calcium carbonate. Oasis Valley. Chemical composition would be expected to be similar at individual points where this regional tuffaceous aquifer recharges the alluvium in Oasis Val- ley. The general similarity in chemical compositions of ground water in the tuffaceous aquifer is shown in table 2. The principal exceptions are samples 7, 8, and 9 from springs in tuffaceous rocks to the west of Oasis Valley (fig. 3) which represent local recharge from the Bullfrog Hills. The average composition of the remain- ing 11 water samples from the tuffaceous aquifer (table 3) is, therefore, assumed to represent the chemistry of water recharging the alluvium and is shown on line 2 in table 7. In the preceding discussion of chemical behavior, sodium and chloride appeared to be the two species that were neither added to nor removed from the ground-water system in the alluvium. The increases in sodium and chloride concentrations between lines 1 and 2 in table 7 must, therefore, reflect the correspond— ing decrease in the volume of water as the result of evapotranspiration. Assuming a volume reduction of 2.13 overestimates the sodium-discharge concentration by 0.37 millimole (line 4), or 3 percent (line 5), and underestimates the chloride concentration by 0.11 millimole, or 4 percent. Evapotranspiration probably affects only the shal- low ground water directly adjacent to the root zone and atmospheric interface. However, chemical trends along the flow paths are preserved irrespective of the sam- pling depth (0—100 m, 0—300 ft, table 1). Dispersivity along the flow path apparently produces chemical homogeneity within the zone sampled. If the alluvium thickness is more than 660 m (2,000 ft) in the central part of Oasis Valley, as suggested by Malmberg and Eakin (1964), concentrations may be more dilute at greater depth. However, no mixing of more dilute ground water with the shallow alluvial water occurs as the flow system approaches the hydrologic constriction at the Amargosa Narrows. The mass-balance calcula- tion involving only discharge of shallow ground water appears to describe the hydrologically—active system in the alluvium. The estimate in this report that slightly more than half (53 percent) of the ground water contained in GEOCHEMISTRY OF GROUND WAFER. 'l‘UFFACEOUS ROCKS, OASIS VALLEY, NEVADA E23 TABLE 7.—Comparisons of selected chemical species in ground water from the mouth of Thirsty Canyon and the exit of Amargosa Narrows [Constituents in millimoles per liter] Na+ K+ Ca++ Mg++ H00; so; or F— iLSio. 1. Water composition at Amargosa Narrows 12.62 0.26 0.95 0.23 7.20 2.60 2.82 0.33 1.12 2. Average water compo- sition of tuffaceous aquiferl 6.10 .17 .45 .07 3.68 1.12 1.27 .24 .90 3. Average water compo- sition of tuffaceous aqulfer X 2.13 12.99 .36 .96 .15 7.84 2.39 2.71 .51 1.92 4. Molality difference2 — .37 — .10 — .01 + .08 — .64 + .21 + .11 — .18 — .80 5. Percentage gain or loss3 —3 —28 —1 +53 —8 +9 +4 —35 —42 lMean average of water compositions from tuffaceous aquifer (table 3) excluding locations 7, 8, and 15. 2Line 1 minus line 3. ”Line 4, divided by line 3, times 100. Oasis Valley is discharged by evapotranspiration is in variance with the findings of Malmberg and Eakin (1962), who estimated that more than 80 percent of the ground water was lost by this process. Malmberg and Eakin based their value on estimates of total areal cov- erage by phreatophytes, average depth of ground wa- ter, and rates of evaporation. Major variations exist between lines 1 and 3 (table 7) for the other chemical species that cannot be explained by concentration due to evapotranspiration. Silica, fluoride, and potassium show decreases and magne- sium and sulfate show large increases. An estimate of the sources and sinks for the concentration excesses and depletions shown in lines 4 and 5 can be made by considering mass-balance relationships. Based on an initial volume of 1,000 L and assuming a volume-reduction factor of 2.13, 0.80 mole (line 4) or 42 percent (line 5) of the total silica is lost from the ground water in the alluvium. Ground water recharg- ing the alluvium from the tuffaceous aquifer is nearly saturated with respect to silica gel (table 4). The loss of 42 percent of the total silica can thus be explained by silica gel precipitation as silica concentrations are in- creased in excess of saturation by evapotranspiration. Similarly, 0.18 mole or 35 percent of the dissolved fluoride is lost by fluorite precipitation (fig. 11); Ca++ + 2F; —> caze(flunritt~)- A small amount, 0.09 mole, of dissolved calcium is also lost during the formation of fluorite. This amount, when subtracted from a net calcium deficit of 0.01 mole, produces a deficit of 0.10 mole. The most likely source of excess magnesium (0.08 mole or 64 percent of the total) is from the incongruent dissolution of dolomite. MgCOa (dolomite) + H+ —> Mg++ + HCOE. The excess of magnesium must be balanced by 0.16 mole of excess bicarbonate. This reaction would further increase the bicarbonate deficit to 0.80 mole. Concentration increases in calcite-saturated ground water (fig. 12) results in calcite precipitation and the loss of bicarbonate; Ca++ + HCO§—> CaCOs (calcite) + H+. However, the loss of 0.80 mole of bicarbonate in the above reaction is not balanced by the loss of only 0.10 mole of calcium. The excess sulfate (0.24 mole or 9 percent) suggests that the dissolution of calcium sulfate, CaSO4 (gypsum) —9 Ca++ + 804: may be a source of both calcium and sulfate. The net reaction for calcium sulfate dissolution and calcium carbonate precipitation becomes C3804 (gypsum) + HCOs_—> 08003 (calcite) + I‘I+ + 804: in which calcium is conserved. Net calcium conservation is demonstrated in table 7. The deficit of 0.60 mole of bicarbonate is approximately balanced in the above reaction by an increase of 0.48 mole of sulfate. SUMMARY AND CONCLUSIONS The abundance of springs and shallow wells in Oasis Valley provides an excellent opportunity for a detailed geochemical investigation of ground water associated with tuffaceous rocks. These geochemical data, in turn, permit additional insight into the hydrologic features that control the volume and movement of ground water in Oasis Valley. Most of the dissolved solids, represented principally by sodium, bicarbonate, and silica, are derived from reaction with the tuffaceous rocks. Comparison of these high solute concentrations with reported analyses for E24 intruded crystalline rocks of the same chemical compo- sition suggests that hydrolysis and incongruent disso- lution of the volcanic glass phase are the principal reactions between the water and the tuffaceous rock. The relative deficiencies in silica and sodium in hy- drated glasses reported by other workers support this conclusion. Bicarbonate is the product of the disassoci- ation of soil-derived carbonic acid by hydrolysis and of ion exchange involving the glass phase. Chloride is also leached preferentially relative to fluoride in the glass phase. The kinetic-reaction rates of tuffaceous rocks appear to be significant in determining chemical concentrations because relative concentrations are at least partially a function of distance and presumably of time spent in movement between recharge and dis— charge areas. The ground water infiltrates from the tuffaceous rocks into the valley alluvium through springs and subsurface flow. Direct exchange with the atmosphere as well as transpiration through the vegetation cover occurs as the result of the immediate proximity of the water table to the land surface and soil zone. The re- sulting decrease in the volume of ground water leads to an increase in chemical concentration. Increases of sodium, chloride, and bicarbonate concentrations plot on the same linear reaction paths as do initial concen- tration increases attributed to chemical reaction with the tuffaceous rocks. Mass-balance calculations indi— cate, however, that more than 9 percent of the total sulfate and 53 percent of the magnesium can be attrib- uted to sources outside those in the tuffaceous rocks. Calcium and magnesium concentrations in the ground water are higher in areas to the south and west of Oasis Valley where dolomites and limestones locally crop out. Mass-balance equations indicate that sub- stantial sulfate must be associated with increases in calcium; this relation suggests local dissolution of cal- cium sulfate minerals either in lacustrine deposits in the valley floor or as hydrothermal alteration products of sulfide minerals. One of the more interesting geochemical features of Oasis Valley is the aqueous concentration controls ex- hibited by various mineral phases. Dissolved silica re— mains constant, corresponding to saturation with re- spect to amorphous silica. Excess silica produced by chemical reaction and evapotranspiration is removed from the aqueous system as silica gel. Silica gel is ob- served in fracture fillings at numerous locations in the tuffaceous rocks. Montmorillonite represents the sta- ble clay weathering product formed by Oasis Valley ground water. Although the mineral occurs in the tuffs and alluvium, an aqueous supersaturation of more than two orders of magnitude suggests that formation is slow and montmorillonite does not significantly in- HYDROLOGY OF NUCLEAR TEST SITES fluence the bulk ground-water composition. Analcime is another common mineral occurring extensively in zeolitized zones within the tuffs. However, undersat- uration of an order of magnitude with respect to the aqueous system indicates the zeolitization may occur in localized geochemical environments that are not re- flected in the bulk ground-water composition found in Oasis Valley. With increases in concentration in the alluvium, owing to evapotranspiration, the ground water be- comes saturated with calcite, barite, and fluorite. For all three minerals, the aqueous concentration does not exceed saturation even with additional decreases in the volume of ground-water. This situation implies active precipitation of these mineral phases. Approxi- mately 35 percent of the initial fluoride is precipitated in the valley alluvium. Magnesium initially exhibits large increases in concentration in the alluvial aquifer, but downgradient magnesium reaches a constant concentration that is slightly undersaturated with respect to dolomite. The maximum magnesium concentration seems directly re- lated to the inability of the ground water to dissolve dolomite in excess of the saturation value. The initial amount of potassium is decreased by approximately 28 percent in the valley alluvium; this decrease is prob- ably a result of adsorption and fixation by clay min- erals present in the alluvium and soil zones. Two geochemical contributions can be added to the understanding of the hydrologic processes in Oasis Val— ley. The geochemical history of the ground water in Oasis Valley seems to be generically related to ground water contained under Pahute Mesa and Gold Flat as suggested by previous workers. Concentrations of three major conservative components, sodium, bicar- bonate, and chloride, plot on a well-defined linear trend. Water in the recharge areas of Pahute Mesa and Gold Flat plots in the dilute portion of the trend, and the water of Oasis Valley, representing a more ad- vanced stage in the reaction sequence farther down- gradient, plots in the more concentrated part. Secondly, mass balance calculations show that slightly more than half the ground water entering the alluvium in the floor of Oasis Valley is lost as a result of evapotran- spiration. This value is considerably lower than had been previously estimated by more indirect methods; consequently, considerably more ground water is dis— charged by underflow to the Amargosa Desert than had been previously anticipated. REFERENCES Benson, L., 1978, Mass transport processes in vitric tuffs of Rainier Mesa, Nevada: Geol. Soc. America Bull. (in press). GEOCHEMISTRY OF GROUND WA'I‘ER, 'I‘UFFACEOUS ROCKS, OASIS VALLEY, NEVADA Berner, R. A., 1971, Principles of chemical sedimentology: New York, McGraw Hill Book Co., 328 p. Blankennagel, R. K., and Weir, J. E., Jr., 1973, Geohydrology of the eastern part of Pahute Mesa, Nevada Test Site, Nye County, Nevada: US. Geol. Survey Prof. Paper 712—B, p. B1—35. Brown, Eugene, Skougstad, M. W., and Fishman, M. J., 1970, Methods for collection and analysis of water samples for dis- solved minerals and gases: U.S. Geol. Survey Tech. Water- Resources Inv., Book 5 Chap. A—1, 160 p. Budd, S. M., 1961, The mechanisms of chemical reaction between silicate glass and attacking agents—Part 1. Electrophilic and nucleophilic mechanisms of attack: Phys. Chem. Glasses, v. 2, p. 111—114. Carr, W J ., 1974, Structure and clay alteration, in Results of explora- tion of the Baneberry site, early 1971: US. Geol. Survey rept. USGS—474—145, p. 7-18; available only from US. Dept. Com- merce, Natl. Tech. Inf. Service, Springfield, VA 22161. Carr, W. J ., and Quinlivan, W. D., 1966, Geologic map of the Timber Mountain quadrangle, Nevada: US. Geol. Survey Quad. Map GQ—503, 1262,500. Cornwall, H. 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