<- 
 
 Criiot. SuQX>Lu 
 
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 STATE OF ILLINOIS 
 
 WILLIAM G. STRATTON, Governor 
 
 DEPARTMENT OF REGISTRATION AND EDUCATION 
 
 VERA M. BINKS, Director 
 
 • 
 
 GROUNDWATER GEOLOGY 
 
 
 IN 
 
 
 EAST-CENTRAL ILLINOIS 
 
 
 A Preliminary Geologic Report 
 
 
 Lidia F. Selkregg 
 John P. Kempton 
 
 
 Service activities concerning groundwater are performed 
 jointly by the Illinois State Geological Survey 
 and the Illinois State Water Survey. 
 
 
 
 
 DIVISION OF THE 
 
 
 ILLINOIS STATE GEOLOGICAL SURVEY 
 
 
 JOHN C. FRYE, Chief URBANA 
 
 
 CIRCULAR 248 1958 
 
Digitized by the Internet Archive 
 
 in 2012 with funding from 
 
 University of Illinois Urbana-Champaign 
 
 http://archive.org/details/groundwatergeolo248selk 
 
GROUNDWATER GEOLOGY 
 IN EAST-CENTRAL ILLINOIS 
 
 A Preliminary Geologic Report 
 Lidia F. Selkregg and John P. Kempton 
 
 ABSTRACT 
 
 Probabilities for developing domestic, farm, municipal, in- 
 dustrial, and irrigation groundwater supplies in central Illinois range 
 from poor to excellent. This report presents a general summary of 
 groundwater principles , summarizes the geologic factors that con- 
 trol the availability of groundwater, and discusses methods of de- 
 veloping groundwater supplies. 
 
 The maps show 1) probability of occurrence of sand and gravel 
 aguifers; and 2) areal distribution, type, and water-yielding char- 
 acter of upper bedrock formations. 
 
 INTRODUCTION 
 
 This report is the last of a series of Circulars which have been prepared 
 by the Illinois State Geological Survey to describe and outline groundwater con- 
 ditions throughout the State.* The aim of these studies is to provide the public 
 with general information on groundwater principles and with regional information 
 on groundwater availability. This report supplements Circular 19 2, "Water Wells 
 for Farm Supply in Central and Eastern Illinois, " by providing more detailed ex- 
 amination of available data and including evaluation of groundwater probabilities 
 for municipal and industrial supplies as well as for domestic and farm needs. 
 
 The region described is Agriculture Extension District No. 3, which in- 
 cludes the following counties: Champaign, Christian, Coles, DeWitt, Douglas, 
 Edgar, Ford, Iroguois, Livingston, Logan, McLean, Macon, Marshall, Mason, 
 Menard, Moultrie, Piatt, Putnam, Sangamon, Tazewell, Vermilion, and Wood- 
 ford (figs. 1, 3). It comprises 13,852 sguare miles and has a population of a- 
 bout 9 20,500. It includes a large area of highly productive agricultural land on 
 which general farming is the principal enterprise. 
 
 Water is essential to an expanding economy. The location of new indus- 
 tries, the growth of communities, and any forms of land improvement are con- 
 trolled by the water resources of an area. The purpose of this report is to pro- 
 vide information on the availability of groundwater for farm, industrial, and mu- 
 nicipal supplies. It discusses principles of groundwater occurrence and develop- 
 ment and describes basic methods of well construction. 
 
 Although the Illinois, Sangamon, and Embarrass rivers provide large guan- 
 tities of surface water, most of the water supplies utilized directly or indirectly 
 by man in the area come from the much larger subsurface reservoir in the ground. 
 Subsurface water is stored either in the soil zone as soil moisture, where it is 
 available for the growth of plant life, or in the underlying zone of permanent 
 saturation as groundwater, from which it is withdrawn for use by springs and 
 wells. Groundwater is the major source of water supply for farms, cities, and 
 
 * Previous reports in the series (see fig. l) ; listed in "Suggested Reading" on 
 page 35, are available upon reguest from the Survey offices in Urbana, Illinois, 
 
 [1] 
 
ILLINOIS STATE GEOLOGICAL SURVEY 
 
 This series of general 
 outlined and identified by 
 Circular numbers. 
 
 Fig. 1. - Index map to reports on groundwater geology in Illinois 
 published since 1950 or in progress. 
 
 industries in the region covered by this report. The demand for groundwater is 
 great because surface water supplies are not always present where needed, and 
 even where present they commonly require considerable capital outlay for col- 
 lection and treatment. 
 
 The availability, quantity, and quality of groundwater depend upon the 
 nature and arrangement of the earth materials beneath the surface, that is, upon 
 geological conditions. Any groundwater supply, whether for small domestic 
 needs or for the large requirements of a city or industry, can be obtained only 
 where geologic formations that can transmit water are present. Formations that 
 transmit water are said to be permeable and are called aquifers. Because geo- 
 logic conditions differ from place to place, groundwater is readily available in 
 some areas whereas in others it is difficult to obtain. The proper development 
 of the groundwater resources of an area, therefore, is greatly assisted by infor- 
 mation on the distribution and character of the aquifers that may be present. 
 
 The authors wish to acknowledge the helpful assistance given by drilling 
 contractors in providing large numbers of logs of water wells in central Illinois 
 and in supplying information on specific problems of occurrence of water-yield- 
 
GROUNDWATER IN EAST-CENTRAL ILLINOIS 
 
 ing materials and drilling conditions. We also acknowledge the assistance 
 given by Paul Hughes and other members of the Groundwater Section and of 
 other Sections of the State Geological Survey. The data on water quality and 
 well yields contained in this report have been taken from records or published 
 reports of the Illinois State Water Survey. 
 
 Fig. 2. - Source, movement, and occurrence of groundwater. 
 
 OCCURRENCE OF GROUNDWATER 
 
 Because groundwater occurs beneath the earth, hidden from view, it is 
 often regarded as somewhat mysterious, and throughout human history many 
 fanciful explanations have been presented to describe its source, movement, 
 and occurrence. Scientific study has shown, however, that groundwater obeys 
 physical laws or principles that are relatively simple and easily understood, 
 although they may be complex in detail. Figure 2 shows diagrammatically the 
 basic fundamentals of our present understanding of the source, movement, and 
 occurrence of groundwater. 
 
 The source of groundwater is seepage into the earth of some of the moisture 
 that falls as rain, snow, and ice. The tremendous quantity of water that falls 
 on the land surface by precipitation is seldom fully realized; one inch of rain- 
 fall distributed over one square mile amounts to nearly 18 million gallons. 
 However, only a small part of the precipitation actually enters the groundwater 
 reservoir. Most of it falls into lakes and oceans, runs off in streams, or is re- 
 turned to the atmosphere by evaporation and transpiration. The remainder filters 
 slowly into the ground to a level below which all available openings are filled 
 with water. 
 
 The top of this zone of saturation is called the water table. Under water- 
 table conditions, a well drilled or dug remains dry until it penetrates the zone 
 of saturation; the position of the water table is then shown by the level at which 
 water stands in the well. The water table is not level but conforms more or less 
 to the features of the land surface. Where the water table intersects the land 
 surface, groundwater is discharged in the form of springs which feed perennial 
 streams, lakes, and swamps. The water table rises or falls in response to the 
 gain or loss of groundwater in the reservoir, so the water level in a well that 
 
ILLINOIS STATE GEOLOGICAL SURVEY 
 
 KEY 
 •^ ^ Approximate oxis of bedrock volley 
 
 A A' Line of cross section (see fig. 5 ) 
 
 B B' Line of cross section (see fig 5) 
 
 C C' Line of cross section (see fig 6) 
 
 SCALE OF MILES 
 0_ 10 20 
 
 Fig. 3. - East-central Illinois, showing major bedrock 
 
GROUNDWATER IN EAST-CENTRAL ILLINOIS 
 
 valleys, LaSalle anticlinal belt, and lines of cross section 
 
6 ILLINOIS STATE GEOLOGICAL SURVEY 
 
 penetrates the saturated zone (a "water-table" well) rises or falls with the water 
 table. During extended dry periods shallow wells go dry when the water table 
 drops below the bottom of the well. 
 
 Groundwater moves, under the influence of gravity or in response to other 
 pressure differentials, toward points of lower pressure, such as springs or dis- 
 charging wells. The movement is slow because of friction between water and 
 the walls of the small pores or other openings in the rocks. Under such condi- 
 tions groundwater moves slowly by gravity flow in the direction of the slope of 
 the water table. 
 
 Groundwater is said to be confined or under artesian conditions where a 
 saturated aquifer is overlain by a less permeable material that restricts the up- 
 ward movement of the water. Under such conditions the water in the confined 
 layer has a hydrostatic pressure that causes the water in a well to rise above 
 the top of the aquifer. Where pressures sufficient to cause the water to rise 
 above the land surface are encountered in an artesian well, the well will flow. 
 
 To supply a pumped or flowing well, groundwater must move through the 
 aquifer toward the well. Under water-table conditions, pumping lowers the 
 water table in the vicinity of the well and gravity induces groundwater to flow 
 toward the well from adjacent areas. Under artesian conditions, pumping re- 
 duces the hydrostatic pressure in the vicinity of the well, and induces the flow 
 of groundwater toward the well. The depression in the water table, or in the 
 artesian pressure surface resulting from discharge, is in the form of an inverted 
 cone with the well at the center and is called the cone of depression (fig. 2). 
 
 Water is to be found everywhere below the top of the zone of saturation, 
 but successful wells can be constructed only where strata that will transmit 
 water easily are present. The capacity of earth materials to accept, store, 
 and yield water depends on the type, size, number, and degree of intercon- 
 nections that can store and conduct groundwater. Sand and gravel aquifers store 
 considerable water and transmit it readily. Other earth materials, such as clay 
 and shale, may contain as much or more water per cubic foot as sand and gravel, 
 but they hold water in pores so small that it cannot be transmitted in usable 
 quantities to wells. 
 
 Sand and gravel deposits are water-yielding because the openings between 
 the grains are large enough to allow relatively free movement of water. The 
 most permeable water-yielding sand or gravel deposits are composed of grains 
 that are nearly all the same size and coarser than granulated sugar. If large 
 amounts of silt and clay are present in the spaces between the larger particles 
 of sand and gravel they retard the flow of water. Sand and gravel deposits in 
 the area of this report are from a few inches to about 200 feet thick. Deposits 
 a few feet or more thick are generally suitable aquifers for drilled wells. 
 Thinner deposits of sand and gravel may be suitable only for large-diameter 
 dug or augered wells. 
 
 Sandstone formations also transmit groundwater through the openings 
 between sand grains. The water-yielding capacity of sandstone depends upon 
 the degree of cementation, size, and sorting of the sand grains. Any material 
 in the openings between the sand grains reduces the water-transmitting capacity 
 of the sandstone. Some sandstones are so thoroughly cemented that any water 
 present moves through joints and fractures rather than between grains. 
 
 Relatively few wells have been completed in sandstone in the area of this 
 report. Locally, however, the St. Peter sandstone of Ordovician age and thin 
 
GROUNDWATER IN EAST-CENTRAL ILLINOIS 
 
 SYSTEM 
 
 SERIES 
 OR GROUP 
 
 FORMATION 
 THICKNESS (FT.) 
 
 GRAPHIC 
 LOG 
 
 ROCK TYPE 
 
 (DRILLERS TERMS) 
 
 WATER-YIELDING CHARACTERISTICS; 
 
 DRILLING AND WELL CONSTRUCTION 
 
 DETAILS 
 
 Unconsolidated glacial de- 
 posits, alluvium ond 
 wind - blown silt (drift, 
 surface, overburden ) 
 
 Water-yielding choracter variable Large yields 
 from thicker sand ond gravel deposits In bedrock 
 valleys Wells usually require screens ond 
 careful development Chief aquifer in orea 
 
 McLeonsboro 
 
 i i 1 i ~r 
 
 Tradewoter 
 Coseyville 
 
 Mainly shale with thin 
 limestone, sandstone and 
 coal beds 
 
 (Coal Measures) 
 
 Water-yielding choracter variable Locolly shallow 
 sandstone and creviced limestone yield small 
 supplies Water quolity usually becomes poorer 
 with increasing depth Moy require casing 
 
 0-600 
 
 Limestone, sandstone 
 and shale 
 
 To deep to be considered os a source of 
 groundwater in this oreo 
 
 Ste Genevieve 0-120 
 St Louis 
 
 Valmeyer 
 
 Salem 
 Warsaw 
 Keokuk - 
 Burlington 
 
 0-270 
 0-130 
 0-300 
 
 iz ' ' . r~r 
 
 s 
 
 Shale 
 
 sro? 
 
 Cherty limestone 
 
 May be water-yielding in Moson county where 
 these formations ore present ot a shallow 
 depth. In the rest of the area too deep to 
 be considered as a source of groundwater 
 
 DEVO- 
 NIAN 
 
 Not water-yielding 
 
 0-70 
 
 SILU- 
 RIAN 
 
 Niogoran 
 
 Alexandrian 
 
 0-100 
 
 Dolomite ond limestone 
 
 Water-yielding from crevices where encountered 
 ot a shallow depth In most of the orea too 
 deep to be considered as o source of 
 groundwater 
 
 Maquoketo 0-200 
 
 ',',','\ 
 
 z_ t - ;. ^ yizr 
 
 Shale with limestone ond 
 dolomite beds 
 
 Not water-yielding at most places, cosing required 
 
 Galena-Platteville 
 300-430 
 
 Limestone and dolomite 
 
 Not importont os aquifers Creviced dolomite 
 probably yields some water to wells drilled 
 into underlying sandstone 
 
 Chazy 
 
 Glenwood- St Peter 
 150- 300 
 
 Sandstone, cleon, white, 
 thin dolomite and shale 
 at top (St. Peter) 
 
 Dependoble source of groundwater in the north- 
 ern part of the area. Water becomes highly 
 mineralized with increasing depth 
 
 Shokopee 
 200-410 
 
 6 a'A< 
 
 Cherty dolomite 
 thin beds of sandstone 
 
 Prairie 
 
 Du 
 Chien 
 
 Not important as aquifer. Liners in lower St 
 Peter sandstone are commonly seated in 
 upper part of Shakopee 
 
 New Richmond 0-175 
 
 Sandstone and dolomite 
 
 Oneota 
 300-500 
 
 z^ztt-^z: 
 
 ',',' v 
 
 Dolomite with some 
 sandstone beds 
 ( Lower magnesic 
 
 Not important os aquifers in this orea 
 
 y /. 
 
 Trempealeau 
 200-250 
 
 Dolomite with some 
 sondstone beds 
 
 Francoma 
 100-200 
 
 ZZH 
 
 S 
 
 Sandstone, shale and 
 dolomite 
 
 Ironton - Galesvilli 
 125- 215 
 
 f / J s ~r 
 
 /J ±'JZ\ 
 
 Sondstone, cleon, white, 
 thin dolomite bed ot the 
 top ( Dresboch) 
 
 Limestone and sandstone beds ore water- yielding. 
 Water highly mineralized or "brine" in most of 
 the area In the northern port, quality of 
 water unknown 
 
 L_f t 2 Z 
 
 Eau Claire 
 350-500 
 
 / / / 
 
 Shale, dolomite ond sand- 
 stone 
 
 Mt Simon 
 1200 + 
 
 Sondstone, with thin red 
 shale beds 
 
 PRE -CAMBRIAN 
 
 Crystalline rocks extending to great depths 
 
 Fig. 4. - Generalized column of rock formations in east-central Illinois. 
 
ILLINOIS STATE GEOLOGICAL SURVEY 
 
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 00 NOISONIAH 
 
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 O0~l±IW 30 
 
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GROUNDWATER IN EAST-CENTRAL ILLINOIS 9 
 
 fine-grained Pennsylvanian sandstones are groundwater sources (figs. 4, 5, 
 and 7). 
 
 Limestone and dolomite rocks are generally tight and compact, and 
 groundwater moves only through cracks and solution channels. Wells drilled 
 into these rocks are successful if the well penetrates water-bearing crevices. 
 The presence and extent of these openings at any specific location are not 
 readily predictable. The Niagaran-Alexandrian dolomite (Silurian) is a source 
 of groundwater in the northern part of the area where the dolomite is encountered 
 at a shallow depth. 
 
 Silurian and Devonian dolomite and limestone are the source of ground- 
 water along the LaSalle anticlinal belt, where present below the drift (figs. 4, 
 5 , and 7). 
 
 GEOLOGY 
 
 The bedrock formations in east-central Illinois (fig. 4) are layers of sand- 
 stone, limestone, dolomite, shale, and coal, arranged one above the other like 
 the pages of a book. The rocks, now firm and compact, were originally deposit- 
 ed as unconsolidated sediments in coastal marshes or in shallow seas that re- 
 peatedly invaded the continent. The shale was originally deposited as clay; 
 the sandstone was deposited as sand; the limestone was formed by precipitation 
 of carbonates and by accumulation of sea shells on the sea floor; the coal was 
 formed from plants which were buried in the coastal swamps. 
 
 The sedimentary layers, hardened into rock, were later warped and tilted 
 so that today they are no longer horizontal. In east-central Illinois the beds 
 dip eastward and southward to form part of a saucer-like structure known as the 
 Illinois basin. Rocks that are near the surface in Livingston County are en- 
 countered at depths of more than 2000 feet in Macon County, and rocks that 
 are near the surface in Mason County are encountered at depths of more than 
 1000 feet in Macon County (fig. 5). In the eastern part of the basin is a narrow 
 band along which the rocks have been warped upward into arch-like structures 
 called anticlines (figs. 3,5). This feature, the LaSalle anticlinal belt, extends 
 from Ogle County southeast to Wabash County (fig. 1). Along this belt in east- 
 central Illinois some of the formations which in the basin are too deep to yield 
 potable groundwater are near the surface and are locally important groundwater 
 sources (figs. 4,5, and 7). 
 
 After the Pennsylvanian sediments were deposited, the seas retreated 
 and the upper part of the bedrock was deeply eroded. Then, during the Pleis- 
 tocene epoch, great continental glaciers advanced from Canada, overriding the 
 eroded bedrock surface and leaving widespread glacial deposits whose nature 
 and distribution are important factors in the occurrence of groundwater in east- 
 central Illinois (fig. 8). 
 
 A map of the bedrock surface that lies below the glacial drift shows that 
 a well integrated drainage system was developed on the bedrock surface before 
 the continental glaciers advanced (Bedrock Topography of Illinois, Bull. 73). 
 One of the major valleys of the old drainage system extends across the cen- 
 tral part of the State from the Indiana border to the Illinois River valley. This 
 valley takes its name from the village of Mahomet located over the deepest part 
 of the channel in Champaign County (fig. 8). It has been suggested that the ancient 
 river had its source in the Blue Ridge Mountains in North Carolina , flowed north- 
 ward into Ohio, then west across Indiana and central Illinois, and discharged 
 
10 ILLINOIS STATE GEOLOGICAL SURVEY 
 
 into the ancient Mississippi River in Mason County. The valley of the Mahomet 
 River and its tributaries, and much of the ancient Mississippi Valley, were com- 
 pletely buried by the deposits of the great glaciers that covered much of Illinois 
 during the Pleistocene epoch. South of Putnam County, the valley of the ancient 
 Mississippi River is now occupied by the Illinois River. 
 
 Glacial ice sheets that advanced outward from centers of snow accumulation 
 in Canada transported a great volume of rock debris and in melting deposited 
 it as an irregular blanket that covered the eroded layered bedrock. Till - an un- 
 sorted mixture of clay, silt, sand, and pebbles - was laid down under the ad- 
 vancing ice or dumped during its melting. 
 
 Beyond the ice front, sediment-laden meltwaters flowed down valleys, 
 partially filling them with deposits of outwash that consisted of sorted sand, 
 gravel, and finer material. River flats, kept free of vegetation by frequent 
 glacial flooding, were subject to wind erosion, and great volumes of silt were 
 blown into the uplands bordering the valleys to form loess deposits. Till, out- 
 wash, loess, and the sediments of modern streams now cover the bedrock sur- 
 face of east-central Illinois, resulting in a relatively level plain. This plain 
 is broken by broad ridges (moraines) that were deposited along the ice front in 
 a roughly concentric pattern (fig. 8). 
 
 There were four major glaciations - Nebraskan, Kansan, Illinoian, and 
 Wisconsin - but glaciers of only the last three are known to have entered the 
 area of this report. Each glaciation was followed by an interglacial period in 
 which the climate warmed and the ice front melted back. The advance of each 
 glacial lobe greatly modified the drainage of east-central Illinois. The Mahomet 
 River Valley and its tributaries were gradually filled, the course of the Missis- 
 sippi River was diverted westward to its present position, and the Illinois River 
 became the major stream in the area. These changes in the drainage system 
 were very complex. 
 
 The character of the glacial deposits encountered in drilling shows that 
 the valleys were filled at intermittent periods. Sand deposits as much as 200 
 feet thick are found in the deepest part of the valleys. It is believed that they 
 were deposited before or during the advance of the earliest ice sheet when the 
 old valleys were the main drainage system. The deposits in the ancient Missis- 
 sippi River Valley in the Peoria region are called "Sankoty sand, " and those in 
 the Mahomet Valley (Bulletin 73, Bedrock topography of Illinois) have been 
 called "Mahomet sand. " The sand deposits are an important source of ground- 
 water (see county summaries). The later advance of the Kansan, Illinoian, and 
 Wisconsin glaciers completed the filling of the valleys with till interbedded 
 locally with sand and gravel. 
 
 DISTRIBUTION OF AQUIFERS 
 
 Sand and gravel deposits are the most important aquifers along the courses 
 of streams and in the fill of buried valleys. They are also important sources of 
 groundwater in areas where the glacial drift is thick. Figure 6 shows the prob- 
 ability of occurrence of sand and gravel aquifers. The areas shown as "good 
 to excellent" are underlain by thick deposits of unconsolidated material con- 
 taining sand and gravel aquifers. Groundwater for domestic and farm supplies 
 may be obtained readily from this material with small-diameter drilled wells. 
 The probabilities for construction of high-capacity wells for industries and 
 municipalities are good, although test drilling is necessary to locate suitable 
 sand and gravel deposits. 
 
GROUNDWATER IN EAST-CENTRAL ILLINOIS 11 
 
 The areas shown as "fair to good" in figure 6 are underlain by moderate 
 thicknesses of unconsolidated materials that fill shallow valleys or lie on the 
 uplands bordering the main valleys. These materials contain thin and discon- 
 tinuous deposits of sand and gravel. Groundwater for domestic and farm sup- 
 plies is obtained locally in this area from wells drilled in sand and gravel, but 
 in some places good water-yielding deposits are absent and water from the un- 
 consolidated material is obtainable only with large-diameter dug wells. The 
 probabilities of obtaining supplies of water for industrial and municipal purposes 
 are poor to fair. Extensive test drilling generally is necessary to locate water- 
 yielding deposits. 
 
 The areas outlined by dashes in figure 6 correspond to buried bedrock 
 valleys that contain deposits of unconsolidated material as much as 150 to 200 feet 
 thick. Few well records are available in these areas and therefore the presence 
 and extent of sand and gravel deposits is not known. The areas, however, 
 merit special attention in exploration for water-yielding sand and gravel. 
 
 The areas shown as "poor" are primarily bedrock upland. Glacial deposits, 
 if present, are thin or are composed mainly of tight till. Sand or gravel deposits 
 which might supply groundwater are rare and most wells obtain water from the 
 bedrock. 
 
 Figure 7 shows the distribution and water-yielding character of the bed- 
 rock formations that crop out at the surface or lie directly beneath the glacial 
 and alluvial material. 
 
 The Pennsylvanian bedrock, encountered below the drift in most parts of 
 the area, generally is not a reliable source of groundwater. Locally, however, 
 domestic and farm supplies are obtained from creviced limestone, permeable 
 sandstone, or cracked shale and coal in the upper part of the Pennsylvanian 
 bedrock. Throughout east-central Illinois wherever water can not be obtained 
 from the unconsolidated material, and where there are no other usable bedrock 
 aquifers, the upper part of the Pennsylvanian bedrock should be tested. 
 
 Silurian and Devonian limestone and dolomite are suitable sources of 
 groundwater for domestic and farm supplies where they are present just below 
 the unconsolidated material or where they are thinly covered by Pennsylvanian 
 formations. Locally Silurian dolomite may yield large quantities of water. 
 
 In the northern part of the area, municipal and industrial supplies are ob- 
 tained locally from the St. Peter sandstone. This formation is encountered at 
 depths ranging from 450 feet below land surface in the extreme northwestern 
 corner of Livingston County to more than 2000 feet below land surface in the 
 southern part of the area. In Iroquois, Livingston, Woodford, Marshall, and 
 Putnam counties , a few wells have penetrated this formation (see county summa- 
 ries). South of the line shown on figure 7, we do not have records of any well 
 finished in the St. Peter sandstone or deeper formations. The quality of water 
 in the St. Peter sandstone becomes poorer toward the south, and in most of the 
 area the water in the St. Peter sandstone is too salty for use. 
 
 Only one well, the Chenoa City Well No. 1, has penetrated formations 
 deeper than the St. Peter sandstone. This well was finished at a depth of 2035 
 feet in the Oneota dolomite (Ordovician) . 
 
12 
 
 ILLINOIS STATE GEOLOGICAL SURVEY 
 
 Good to excellent; oquifers highly permeable 
 and widely distributed 
 
 Fair to good; aquifers vonable in permeobility, 
 scattered and discontinuous 
 
 / . Area of thicker drift in bedrock volley; little 
 S/ subsurface information available; 
 ' area merits further investigation 
 
 J Poor, sand and gravel deposits generally absent 
 
 \, Limit of Wisconsin glaciation 
 
 Rl I W 10 
 
 -a~m=imap: 
 
 L 4 sii: 4-i-.-:^ Kl '.w, ^ 
 
 A M OVsN;toS-' H 
 
 ;4--+ — 
 
 
 ■ - . ••■ ■I.-- ■ ' ■■ i'?' , 
 
 C H : XR-' 1ST IAN 
 
 ; 
 
 2 : 4 
 
 Fig. 6. - Probability of occurrence 
 
GROUNDWATER IN EAST-CENTRAL ILLINOIS 
 
 13 
 
 R2E 3 4 
 
 N 
 
 and nature of sand and gravel aquifers. 
 
14 
 
 ILLINOIS STATE GEOLOGICAL SURVEY 
 KEY 
 
 Pennsylvonion 
 
 □ Mainly shale with thin sandstone, limestone, ond cool 
 beds. Smoll groundwater supplies obtained from 
 sandstone, limestone, or fractured shales 
 
 Woter ( potoble) wells finished in Pennsylvonion formations N.[ 
 Pre-Pennsylvanian formations 
 Mississippian 
 
 j Kmderhook shale; not woter-yielding 
 
 Y/// \ s,eGen evieve-Worsow limestones; water-yielding where creviced 
 \///A Not u,ili2 ed m this area because of excellent potentiol of 
 shallow drift deposits 
 
 Oevonion 
 
 Dolomite and limestones; water-yielding where creviced 
 Pattern shaded where overlain by Pennsylvonion formotions 
 Silurian 
 
 l yvvv Dolomite; woter-yielding where creviced; upper port 
 KNXSJj mos1 favorable. Pattern shaded where overlain by 
 Pennsylvonion formations 
 
 ^ Generalized southern boundory of water 
 wells finished in St. Peter sondstone 
 or deeper bedrock formotions 
 
 ID 
 
 Fig. 7. - Areal distribution, type, and water-yielding character of upper 
 
GROUNDWATER IN EAST-CENTRAL ILLINOIS 
 
 15 
 
 bedrock formations. (Modified from Geologic Map of Illinois, 1945 ed.) 
 
16 ILLINOIS STATE GEOLOGICAL SURVEY 
 
 DEVELOPMENT OF GROUNDWATER SUPPLIES 
 
 Geologic Conditions That Affect Groundwater Development 
 
 The geologic factors that control the amount and quantity of groundwater 
 available in an area and that must be considered in developing groundwater sup- 
 plies are: 
 
 1) Distribution, depth, thickness, and areal extent of aquifers. 
 
 2) Nature of aquifers , including type of materials , kind of openings 
 (pores or crevices), and presence of substances that affect water 
 quality. 
 
 3) Geologic structure, including regional and local dips of bed, faults, 
 and jointing. 
 
 4) Distribution and nature of relatively impermeable materials. 
 
 The presence of an aquifer in a given area does not guarantee a satis- 
 factory groundwater supply. Care must be taken to select the aquifer most suit- 
 able for the water supply required, to adapt the type and manner of construction 
 of the well to the geologic conditions , and to place the well where it will best 
 maintain the required standards of quality and quantity of water. 
 
 Information on regional geologic conditions pertaining to groundwater sup- 
 plies at prospective well locations is available upon request from the Illinois 
 Geological Survey. The Survey maintains a current file of subsurface informa- 
 tion, including drillers' logs, samples of drill cuttings, and geophysical records 
 from which specific data on formation characteristics for many areas in Illinois 
 are available. Information on well yields, water levels, and water quality is 
 furnished by the State Water Survey. 
 
 Unconsolidated Deposits 
 
 Where extensive water-yielding sand and gravel deposits are present, 
 consideration should be given to developing wells in them rather than in the 
 underlying bedrock. Wells in sand and gravel may have one or more of the 
 following advantages over bedrock wells, particularly deep bedrock wells: 
 shallower water levels, colder water, greater water yields to specific wells, 
 and water of better mineral quality. The disadvantage of wells in sand and 
 gravel is the special construction needed to take full advantage of the water- 
 yielding capacity of the aquifer. 
 
 High-capacity sand and gravel wells require the use of screens which 
 allow sand-free water to flow into the well bore. It is important in developing 
 such wells to choose the size of the screen openings or slots on the basis of 
 the size of the material to be screened. Therefore it is necessary to obtain 
 samples of the aquifer and analyze its particle size in order to select the cor- 
 rect screen size. 
 
 Development necessarily follows construction of a sand and gravel well. 
 In proper development, the finer grained materials in the immediate vicinity of 
 the well bore are drawn through the screen and removed, leaving a natural 
 graded filter that prevents pumping of sand and silt. Good results and yields 
 are usually obtained from sand and gravel deposits by placing an "envelope " or 
 "pack" of selected gravel or coarse sand between the deposit and the screen. 
 The grain size of the particles in the gravel pack must have the proper relation- 
 ship to the grain size and sorting of the formation and to the screen slot size 
 
GROUNDWATER IN EAST-CENTRAL ILLINOIS 17 
 
 to achieve the best results (Smith, H. F.: Gravel Packing Water Wells, Illinois 
 State Water Survey Circular 44). 
 
 The use of slotted pipe or open pipe filled with gravel should be avoided 
 except in very coarse deposits where the well will yield far more water than is 
 pumped. 
 
 The physical characteristics of sand and gravel deposits are generally 
 more variable than those of bedrock formations. For this reason, development 
 of a well in sand and gravel usually requires test drilling before the well is de- 
 signed and constructed. In areas where the presence of suitable aquifers is un- 
 certain, a test drilling program is necessary to determine whether suitable de- 
 posits are present and, if they are, the best location for the well. Electrical 
 earth resistivity surveys frequently are useful in determining the general area 
 where test drilling might be undertaken. 
 
 Test holes are generally small-diameter holes drilled with cable tool 
 (percussion) or with rotary equipment. The test driller's report is an important 
 part of the groundwater development program and should include the following 
 information when obtainable: 1) driller's log of formations penetrated; 2) static 
 water level and changes in water levels during drilling; 3) drilling time of the 
 individual formations; 4) weight and viscosity of drilling mud; and 5) loss of 
 mud or fluid during drilling. Samples of drill cuttings should be saved at five- 
 foot intervals and at changes in the formation. 
 
 Driving a sand point is the quickest and most economical method of well 
 construction, but is practical only where small supplies of groundwater are 
 needed and where such supplies are available from sand and gravel at shallow 
 depth. Conditions suitable for driven wells exist in the Illinois and Sangamon 
 Valley bottoms and locally in the sandy area in Menard, Macon, and Tazewell 
 counties. 
 
 Large-diameter dug wells are most suitable in areas where the uncon- 
 solidated materials are fine-grained and cannot yield water readily to a drilled 
 or driven well. They are used widely throughout much of the area in which 
 glacial material is thin, tight, and underlain by relatively impermeable Penn- 
 sylvanian rocks. Large-diameter wells are excavated by hand only to shallow 
 depths but may be excavated to as deep as 100 feet by power auger, shovel, 
 or bucket. 
 
 In areas in which conditions are favorable for drilled or driven wells, the 
 use of large-diameter dug wells is not recommended because of pollution and 
 maintenance problems. The chief advantage of a large-diameter well is that it 
 can store relatively large quantities of water. Short, intermittent pumping of a 
 large-diameter well does not require immediate release of water from the sur- 
 rounding materials, and the well can refill slowly between times of pumping. 
 Special sanitary precautions should be taken with large-diameter wells (Circu- 
 lar 14A, Illinois State Department of Public Health, Springfield). 
 
 Bedrock Formations 
 
 Wells constructed in bedrock aquifers are less difficult to design than 
 wells in unconsolidated material because the well bore generally is left uncased 
 and little or no development is required. Test drilling is seldom done in bed- 
 rock aquifers, particularly if records of prior drilling in the area are available. 
 
 In east-central Illinois, geologic factors in well construction in bedrock 
 aquifers are:_ 1) type, thickness, depth, and permeability of aquifers; 2) ten- 
 
18 ILLINOIS STATE GEOLOGICAL SURVEY 
 
 dency of formations to sustain open hole without casing or lining; and 3) ten- 
 dency of formations to yield silt or sand during pumping. Creviced dolomite 
 and limestone do not normally require casing or lining. However, where ground- 
 water supplies are obtained from dolomite or limestone with less than about 35 
 feet of overburden, there is danger of bacterial pollution. The open crevices 
 provide little filtering or other purifying action, and polluted water may travel 
 long distances through these openings. 
 
 In developing bedrock aquifers the driller commonly installs surface cas- 
 ing to firm bedrock and continues into the bedrock with an open hole. Where 
 bedrock formations are too weak to sustain an open hole, it may be necessary 
 to continue the surface casing through the weak formation into a more competent 
 underlying formation or to set liners opposite the weak formations. Pennsylvanian 
 shales (fig. 4) generally require casing. Conditions for drilled wells in the bed- 
 rock in east-central Illinois are locally favorable. 
 
 The main aquifers are the Silurian dolomite in the northeastern part of the 
 area and shallow Pennsylvanian sandstones (fig. 7). Because the Pennsylvanian 
 sandstones vary laterally in permeability they are not water-yielding at all sites. 
 Water-pressure fracturing of Pennsylvanian sandstones has, in some instances, 
 increased the initial yields of some wells. In areas where the unconsolidated 
 material does not contain water-yielding sand and gravel deposits (fig. 6) and 
 sandstones are absent,there are slight possibilities of obtaining small ground- 
 water supplies from fractured shales, limestone, or coal. 
 
 Large Groundwater Supplies 
 
 Development of groundwater supplies for municipal, industrial, and irri- 
 gation purposes requires technical advice and careful planning based on all 
 available geologic, geophysical, and hydrologic data. The type, extent, thick- 
 ness, depth, distribution, and water-yielding characteristics of aquifers should 
 be determined in order to estimate the available quantity of water and plan proper 
 well construction. Hydrologic data, such as yields from existing wells, pressure 
 potential of various formations, and water quality should be determined as accu- 
 rately as possible. 
 
 Domestic Groundwater Supplies 
 
 Development of groundwater supplies for domestic and stock use differ 
 from municipal, industrial, and irrigation developments in three important as- 
 pects: 1) the quantity of water needed for domestic and stock purposes is con- 
 siderably smaller and may, therefore, be provided from considerably thinner and 
 less permeable aquifers; 2) the area within which a well can be constructed for 
 domestic or stock purposes is normally small, usually a farmyard or a suburban 
 lot; and 3) the cost of well construction must be low. 
 
 In south-central Illinois geologic conditions are not always favorable for 
 obtaining private water supplies with drilled wells. In areas of thin drift under- 
 lain by non-water-yielding bedrock (figs. 6, 7), dug wells are the most economi- 
 cal way of obtaining groundwater supplies. 
 
 Subsurface geologic conditions generally vary little within the limited 
 area of an individual homesite or farm. However, there may be great changes in 
 geologic conditions with increasing depth. Information on depth of aquifers is 
 valuable for planning the type, depth, and size of the intended well. 
 
GROUNDWATER IN EAST-CENTRAL ILLINOIS 19 
 
 Perhaps the most important considerations in locating private wells are 
 those of sanitation and convenience. Wells should be placed with regard to 
 geologic conditions, surface drainage, topography, and land usage so as to pro- 
 vide maximum protection from harmful bacteria and other objectionable inorganic 
 material. 
 
 The following suggestions may be helpful in planning for individual or 
 farm supplies: 
 
 1) Inventory the water requirements by estimating the amount of water 
 needed for domestic use, stock use, milk cooling, cleaning, and fire protection. 
 
 2) Obtain all available information on the occurrence of water-yielding 
 formations at the location. The maps in this report are designed to give a funda- 
 mental understanding of the occurrence and distribution of the water-yielding for- 
 mations in this area, so that the most suitable type of well can be planned. If 
 additional, more specific information is desired, address the Illinois State Geo- 
 logical Survey, Urbana, Illinois, giving: a) location of property by section, 
 township, and range; b) intended use of the water supply; c) estimate of the 
 quantity of water needed; and d) all information on existing wells on the property 
 or on previous drilling attempts. 
 
 3) Select a well driller with a reputation for constructing wells that have 
 been proved to be trouble-free. Make sure the driller is capable of properly 
 handling the types of aquifers he may encounter at the location. If the well is 
 to be finished in sand and gravel, select a driller experienced in setting well 
 screens. 
 
 4) Check with the State Department of Public Health for regulations and 
 suggestions on proper well construction and location and proper pump housing. 
 The State Department of Public Health discourages the use of well pits on Grade 
 A milk farms unless they are built to very rigid specifications. Properly con- 
 structed well pits are more expensive than other approved methods of pump in- 
 stallation. 
 
 5) Make periodic bacterial analyses of the water supply. Dug wells are 
 more difficult to keep sanitary than are properly constructed drilled wells. Wells 
 drilled into creviced dolomite and limestone formations are, however, also sus- 
 ceptible to bacterial pollution, particularly where the creviced formation is over- 
 lain by thin overburden. 
 
 Role of the Drilling Contractor 
 
 Much of the success of any drilled well depends on the skill and know- 
 ledge of the drilling contractor. A drilling contractor has certain duties and 
 responsibilities to his customers: 
 
 1) The driller should provide an accurate log of the boring when the hole 
 
 is completed. The log should include a description of the formations, information 
 on the static water level, basic construction features of the well (length and size 
 of well casing and screen, etc.), and an indication of the capacity of the well 
 as determined by a pumping test. Copies of the driller's log should be filed 
 with the Illinois State Geological Survey in Urbana. Log books may be obtained 
 by drillers without charge from the Survey. 
 
 2) The well should be constructed in accordance with accepted safe sani- 
 tary practices. The top of the well should be constructed to prevent surface 
 pollution from entering the well or seeping downward around the casing. It is 
 also desirable that well construction allow for measurement of the depth to water 
 without requiring removal of the pumping equipment. 
 
20 
 
 ILLINOIS STATE GEOLOGICAL SURVEY 
 
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 E 
 
 o 
 
 XI 
 
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 2 
 
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 00 
 
GROUNDWATER IN EAST-CENTRAL ILLINOIS 21 
 
 3) The driller should endeavor to take full advantage of any water-yield- 
 ing formations he may encounter. In areas where groundwater conditions are 
 generally unfavorable, it takes a skillful driller to obtain the maximum amount 
 of water from a poor formation. Where sand and gravel aquifers are used as a 
 source of groundwater, the driller should select a well screen on the basis of 
 size and sorting of the formation material. After construction the well should 
 be properly developed. A properly screened and developed well in sand and 
 gravel will not pump an objectionable amount of sand or silt during service. 
 
 4) It is desirable to save samples at 5 -foot intervals for the total depth 
 of drilling, especially for municipal, industrial irrigation, and school wells. 
 The Illinois Geological Survey files samples of drill cuttings received from 
 drillers. The samples may be sent express collect to the Survey where they 
 will be studied and kept on file for reference. Information obtained from samples 
 is vital in effective rehabilitation of old wells. 
 
 COUNTY GROUNDWATER SUMMARIES 
 
 Detailed information on groundwater supplies in the counties of east- 
 central Illinois is given in the following pages. These discussions supplement 
 the geologic information shown in figures 6 and 7. 
 
 Champaign County 
 
 In Champaign County the fill of the Mahomet Valley (figs. 3, 6, 8) contains 
 sand and gravel deposits suitable for development of high-capacity wells. 
 These sand and gravel deposits are generally concentrated in three zones: 
 at depths between 60 and 120 feet, depths between 140 and 170 feet, and below 
 a depth of approximately 200 feet. A typical log (Village of Mahomet test hole 
 no. 1 in sec. 15, T. 20 N. , R. 7 E.) follows: 
 
 Description 
 
 Soil and till 
 
 Sand and gravel clean 
 
 Till 
 
 Sand, some gravel, clean 
 
 Till 
 
 Gravel, sandy, very silty 
 
 Sand and gravel 
 
 At some locations the zones of sand and gravel are nearly continuous vertically 
 and extend from shallow depths to the top of the bedrock. 
 
 Municipal wells for Champaign-Urbana and the Petro Chemical wells lo- 
 cated in Ts. 19 and 20 N. , R. 8 E. , are finished in the lower zone below 200 
 feet. The Northern Illinois Water Corporation well 52 (Champaign-Urbana water 
 supply) is screened from 238| feet to 313| feet. The Illinois State Water Survey 
 reports that in a pumping test conducted September 21, 1956, well 52 yielded 
 1940 gallons per minute with 9.55 feet of drawdown. Shallow deposits above 
 a depth of 120 feet supply groundwater to many domestic and farm wells in the 
 area; however, at many locations the shallow sand is very fine grained so that 
 careful well development is needed. 
 
 Thickness 
 
 Depth 
 
 (ft.) 
 
 (ft.) 
 
 55 
 
 55 
 
 68 
 
 123 
 
 27 
 
 150 
 
 5 
 
 155 
 
 25 
 
 180 
 
 10 
 
 19 
 
 30 
 
 220 
 
22 ILLINOIS STATE GEOLOGICAL SURVEY 
 
 In other parts of the county the glacial drift is generally more than 150 
 feet thick and contains sand and gravel deposits suitable for domestic and farm 
 supplies. Locally municipal supplies have been obtained at Tolono, Pesotum, 
 and Philo. In the southeastern part of the county, sand and gravel deposits are 
 locally absent. Intensive test drilling was needed to locate the water supply for 
 Broadlands and Longview. 
 
 The bedrock may contain water-yielding formations (figs. 4, 7) but,be- 
 cause groundwater supplies are usually available from the unconsolidated ma- 
 terial, water wells rarely penetrate the bedrock. 
 
 Christian County 
 
 Groundwater probabilities in Christian County range from poor to good 
 (fig. 6). Favorable conditions for industrial and municipal wells are found in a 
 strip half a mile to two miles wide which extends from south of Harvel north- 
 east through Morrisonville, Taylorville, and Stonington, on the east side of the 
 Wabash railroad. In this area deposits of water-yielding sand and gravel are 
 common below a depth of about 20 feet. The municipal wells for Taylorville and 
 Stonington were finished at depths of 114 and 131 feet respectively. Some of 
 the knolls and ridges present in Christian County contain water-yielding de- 
 posits of sand and gravel, but their value as sources of municipal or industrial 
 groundwater supplies is restricted by the limited extent of the deposit. 
 
 Sand and gravel deposits are present in the Sangamon River flat and 
 southward in the Mt. Auburn area. At Mt. Auburn a sand and gravel deposit 26 feet 
 thick was encountered at a depth of 44 feet. Outwash deposits in minor stream 
 valleys may yield small industrial and municipal groundwater supplies. Ex- 
 tensive testing, however, is commonly required to locate suitable sources of 
 groundwater in the valley flats. Domestic and farm supplies are generally ob- 
 tainable throughout Christian County except for an area south and west of Pana 
 and in the western part of the county where the drift is thin. In this area, re- 
 ported as "poor" in figure 6, water is obtained locally from large diameter dug 
 wells in the drift or from wells drilled into the bedrock. 
 
 Throughout the county the Pennsylvanian bedrock below the drift is com- 
 posed principally of shale. Locally, sandstone lenses are present and may yield 
 small water supplies. However, drilling into the bedrock should be considered 
 only when a suitable groundwater source cannot be found in the glacial drift; 
 and because of the poor quality of water present in deeper bedrock formations, 
 drilling should not extend below an approximate depth of 200 to 250 feet below 
 land surface. 
 
 Coles County 
 
 Sand and gravel deposits favorable for domestic and farm supplies are 
 present throughout Coles County except for small areas where the drift is thin 
 (fig. 6). These sand and gravel deposits occur at various depths ranging from 
 40 to 100 feet below land surface. In Ts. 11 N. , 12 N. , 13 N. and 14 N. , Rs. 
 7 E. and 8 E. , sand and gravel deposits are more continuous than in other parts 
 of the county and at some places may be the source for small municipal and in- 
 dustrial supplies. 
 
 The Mattoon city wells in sec. 30, T. 12 N. , R. 8 E. , and in sec. 18, 
 T. UN., R. 7 E. , are finished at depths ranging from 40 to 70 feet. In the 
 
GROUNDWATER IN EAST-CENTRAL ILLINOIS 23 
 
 buried valley of the Embarrass River (fig. 6 area outlined by dashes), the drift 
 is thick and locally may contain favorable sand and gravel deposits. 
 
 The Pennsylvanian sandstones, coals, or fractured shale and limestone 
 are local sources of groundwater for small farm supplies throughout the county 
 (figs. 4,7). Because of the poor quality of the water in deeper bedrock formations, 
 drilling generally should not extend below a depth of approximately 300 feet. 
 
 DeWitt County 
 
 In DeWitt County municipal and industrial groundwater supplies are availa- 
 ble from sand and gravel deposits in the buried Mahomet Valley (figs. 3,6,8) 
 where drift deposits as much as 400 feet thick are present. The City of Clinton 
 wells are finished in these deposits at depths of 340 to 360 feet. Local, water- 
 yielding bodies of sand and gravel are found in the upper 200 feet of drift. 
 
 Shallow sand and gravel deposits, which may yield large groundwater 
 supplies, are present in the extreme southwestern corner of the county where 
 they are associated with the outer edge of the Wisconsin drift border (fig. 6). 
 These deposits directly underlie the surface south, west, and northwest of 
 Kenney. 
 
 Groundwater supplies for farm and domestic use can generally be developed 
 in shallow sand and gravel beds in the upper 200 feet of the drift throughout the 
 county. In the northeastern part, conditions are less favorable because sand 
 and gravel layers in the drift are more discontinuous , and locally there is diffi- 
 culty in developing farm or domestic groundwater supplies. 
 
 The Pennsylvanian bedrock underlying the drift is not generally used as a 
 source of groundwater in the county because the glacial drift contains more 
 favorable aquifers. Nevertheless, small supplies of groundwater may be ob- 
 tained from the upper 50 to 100 feet of the Pennsylvanian formations, but they 
 should be tested only after all attempts to develop a well in the drift have 
 failed. 
 
 Douglas County 
 
 Sand and gravel deposits favorable for development of domestic and farm 
 groundwater supplies are generally scattered over Douglas County, but in the 
 northwestern and central part of the County they occur somewhat more consistent- 
 ly. Here the drift is as much as 150 feet or more thick in the buried Pesotum 
 Valley and in two minor tributary valleys. Small municipal groundwater supplies 
 have been developed from sand and gravel at Areola, Arthur, Atwood and Newman. 
 
 Pennsylvanian bedrock, consisting principally of shale, underlies the 
 drift in most of the county and locally may yield small groundwater supplies 
 from thin beds of sandstone or creviced limestone or from fractures in the shale. 
 Along the LaSalle Anticlinal Belt (fig. 5, cross section A-A', and fig. 7), bed- 
 rock of the Mississippian and Devonian systems directly underlies the drift. 
 In this area groundwater is obtained from the Devonian and underlying Silurian 
 dolomites where drift supplies are not available. Where Mississippian shales 
 underlie the drift, drilling must continue through the shale into the dolomite. 
 Tuscola and Villa Grove obtain groundwater supplies from Silurian and Devonian 
 rocks. 
 
24 ILLINOIS STATE GEOLOGICAL SURVEY 
 
 Summary Sample-Study Log of Tuscola Well No. 4, 
 SW| NW{ SE± sec. 34, T. 16 N. , R. 8 E. 
 
 Description Thickness Depth 
 
 (ft.) (ft.) 
 
 Pleistocene series 
 
 Till, sand, and silt 135 135 
 
 Mississippian system 
 
 Shale, some siltstone with thin 
 
 sandstone bed at base 133 268 
 
 Devonian system 
 
 Dolomite, some limestone and 
 
 thin siltstone beds 154 422 
 
 Silurian system 
 
 Dolomite 272 694 
 
 At Villa Grove, the municipal wells are finished at depths of 645 and 627 
 feet into reported Devonian sandstone; the wells penetrate both Pennsylvanian 
 and Mississippian shale. A number of domestic and farm wells also obtain 
 small groundwater supplies from the upper part of the Devonian and Silurian dolo- 
 mites in this area. Deeper drilling is not recommended at sites which are more 
 than a mile away from the area of outcrop of the Mississippian and Devonian 
 formations (fig. 7). 
 
 Edgar County 
 
 Sand and gravel deposits favorable for domestic and farm supplies are 
 present at many places in the western part of Edgar County. In the eastern part 
 the drift is generally thin. Bedrock crops out at many places east of Paris. 
 Small municipal water supplies have been obtained locally after intensive test 
 drilling to locate favorable sand and gravel aquifers. The towns of Hume, Met- 
 calf , Kansas and Vermilion obtain water supplies from wells finished in uncon- 
 solidated material at depths of 55, 9 3, 85 and 103 feet respectively. 
 
 Groundwater is obtained from shallow Pennsylvanian sandstone, creviced 
 limestone, and shale in most of the county (fig. 7). Drilling should be extended 
 into the upper 100 to 150 feet of Pennsylvanian bedrock if water is not obtained 
 from shallow unconsolidated deposits. 
 
 Ford County 
 
 In Ford County municipal and industrial groundwater supplies are avail- 
 able from sand and gravel deposits in the fill of the buried Mahomet Valley (figs. 
 3,6,8). Sand and gravel beds in the fill generally occur at depths ranging from 
 130 to 170 feet below land surface and below a depth of 300 feet. Locally, 
 however, sand and gravel are missing. At Paxton extensive test drilling was 
 necessary to locate a favorable well site. 
 
 Shallow sand and gravel deposits are present in the southern part of the 
 county near Gibson City. These deposits are a dependable source of ground- 
 water for domestic and farm wells and may supply larger quantities locally. 
 Gibson City obtains its water supply from wells finished at depths of 55 to 58 
 feet. The fill of the Chats worth and Kempton Valleys (figs. 3, 6) may contain 
 permeable deposits suitable for small municipal and industrial supplies, but little 
 is known about them. 
 
GROUNDWATER IN EAST-CENTRAL ILLINOIS 25 
 
 Domestic and farm supplies are generally available from sand and gravel 
 deposits throughout the rest of the county. Small municipal supplies have been 
 obtained at Melvin, Piper City, Roberts, and Elliott. In Ts. 27, 28, 29 N. , R. 
 9 E. , drillers report that in places, sand and gravel beds are thin, and careful 
 well construction and development are needed to obtain suitable water supplies. 
 
 Pennsylvanian bedrock underlies the drift in most of the county. Small 
 groundwater supplies may be obtained locally from shallow Pennsylvanian for- 
 mations and from Silurian and Devonian dolomite (figs. 4,7). 
 
 Although the Illinois State Geological Survey does not have records of 
 wells drilled into the St. Peter sandstone in this county, this formation may be 
 a suitable source of groundwater north of Piper City where it is present at depths 
 ranging from 900 to 1100 feet. 
 
 Iroquois County 
 
 The probabilities of obtaining industrial and municipal groundwater sup- 
 plies from the unconsolidated glacial drift are excellent in much of Iroquois 
 County (fig. 6). Sand and gravel deposits in the fill of the preglacial Mahomet 
 and Onarga Valleys (fig. 3) are a source of water for the towns of Watseka, 
 Onarga, Buckley, and Cissna Park. In the eastern part of the county, sand and 
 gravel deposits are a good source of groundwater for domestic and farm supplies 
 and locally for municipal supplies. The town of Sheldon obtains its water sup- 
 ply from sand and gravel at a depth of 116 feet. In the valley of Sugar Creek, 
 sand and gravel deposits are present and locally are suitable for municipal sup- 
 plies. Milford obtains its water supply from such deposits. 
 
 In the northern part of the county, Ts. 28 N. and 29 N. , the drift is thin- 
 ner, and sand and gravel deposits are scattered and discontinuous. Here most 
 wells are finished in the upper 200 feet of Silurian and Devonian dolomites (figs. 
 4, 7). 
 
 Few wells have been drilled below the Kinderhook shale in the eastern 
 part of the county. These are finished in dolomite at depths ranging from 200 to 
 300 feet below land surface. 
 
 The St. Peter sandstone may be a source of groundwater although the only 
 available record of deep drilling in this area is the log of a municipal well drilled 
 at Sheldon in 1898. This well was drilled to a depth of 1850 feet. State Water 
 Survey Bulletin 40 reports that a 1918 water analysis shows a hardness of 1.4 
 gr. per gallon, a residue of 600 ppm, and an iron content of 0. 1 ppm. The yield 
 was reported to be 30 gpm. 
 
 Livingston County 
 
 Groundwater probabilities from sand and gravel aquifers are extremely 
 variable in Livingston County. Although the drift is generally more than 100 
 feet thick throughout the county, it is composed mostly of till with only thin and 
 discontinuous beds of sand and gravel. Locally the sand and gravel deposits 
 are the source of small municipal groundwater supplies, as at Cornell, Cullom, 
 Dwight, Flanagan, and Forrest, where water supplies are obtained from uncon- 
 solidated material within 150 feet of land surface. 
 
 Farm and domestic groundwater supplies are available from the drift in 
 much of the county. Drillers report that in the area of thin drift around Odell 
 (fig. 6), the probability of obtaining groundwater supplies is poor. At some lo- 
 
26 ILLINOIS STATE GEOLOGICAL SURVEY 
 
 cations in this area however, thin deposits of sand are locally present within 
 the drift. Drillers report that with careful well construction and development 
 small supplies of groundwater may be obtained from these sand deposits. 
 
 Throughout the county, domestic and farm supplies are obtained from 
 shallow Pennsylvanian formations (fig. 7). Drilling into the Pennsylvanian is 
 recommended only when a suitable groundwater supply is not obtained from the 
 drift. Water in the Pennsylvanian bedrock is sometimes of poor quality as in 
 the area around Odell where drillers report the water to be slightly salty. 
 
 In the northern and northeastern part of the county, the Silurian dolomite 
 underlying the Pennsylvanian bedrock (fig. 7) is penetrated at depths ranging 
 from 250 to 350 feet and is the chief source of water for many domestic and 
 farm wells in the area. The dolomite thins to the northeast and is absent in 
 most of the northern part of the county. To the south it contains water that is 
 highly mineralized. 
 
 The St. Peter sandstone, which ranges in depth from 450 feet in the ex- 
 treme northwestern corner of the county to 1400 feet at Fairbury, is considered 
 a dependable aquifer by local well drillers. The formation has been the source 
 of water to several municipalities in the county. Odell has a well finished in 
 St. Peter sandstone at a depth of 1,341 feet. The analysis of a sample of water 
 collected on May 4, 1947, shows a hardness of 8.7 gr. per gallon, a residue 
 of 2389 ppm, and an iron content of 0.4 ppm. The town of Chatsworth has a 
 well finished in St. Peter sandstone at a depth of 1285 feet. The analysis of a 
 sample collected in May 1947 showed a hardness of 417 ppm, a residue of 699 
 ppm, and a chloride content of 2.0 ppm (State Water Survey Bull. 40). 
 
 Logan County 
 
 Domestic and farm groundwater supplies can be obtained from sand and 
 gravel deposits in Logan County, in the area east and south of New Holland 
 (fig. 6), and locally in the southern and eastern parts of the county where the 
 drift is thin. 
 
 Municipal and industrial supplies can be obtained from the fill of the buried 
 Mahomet Valley and locally from scattered sand and gravel deposits in the buried 
 Middletown Valley (figs. 3, 6). The villages of Atlanta, Emden, and Hartsburg 
 withdraw water from wells reaching depths between 97 and 191 feet in the Ma- 
 homet Valley fill. Middletown and possibly Elkhart City take water from wells 
 155 and 76 feet deep in the Middletown Valley fill. 
 
 In the remainder of the county, municipal groundwater supplies are local- 
 ly available from sand and gravel and have been developed at Latham and Mt. 
 Pulaski. Lincoln obtains a large groundwater supply from sand and gravel de- 
 posits in Salt Creek, and it is likely that sand and gravel deposits suitable for 
 the development of large groundwater supplies may be present in other parts of 
 Salt Creek as well as in Deer, Kickapoo, and Sugar creeks. 
 
 Domestic and farm supplies are obtained locally in the central and southern 
 part of the county from the Pennsylvanian bedrock (fig. 7). Wells should not be 
 drilled deeper than 150 to 200 feet into the bedrock. 
 
 Macon County 
 
 Conditions are favorable for obtaining municipal and industrial ground- 
 water supplies in the northern part of Macon County (fig. 6). Here the buried 
 
GROUNDWATER IN EAST-CENTRAL ILLINOIS 27 
 
 Mahomet Valley, which is filled with unconsolidated glacial drift material about 
 250 to 280 feet thick, contains thick water-yielding sand and gravel deposits. 
 These deposits are present at various depths but generally occur between depths 
 of 70 to 9 feet and below a depth of 200 feet. The town of Maroa obtains its 
 groundwater supply from sand and gravel deposits at a depth of 81 feet. At Ar- 
 genta a well was finished in sand at a depth of 233 feet. The alluvial deposits 
 along the Sangamon River are also a possible source for municipal and industrial 
 supplies. 
 
 Domestic, farm, and, locally, larger supplies are available throughout the 
 county. Shallow gravel deposits at the front of the Wisconsin glacial border 
 (fig. 3) are a favorable source of groundwater west of Macon, Harristown,and 
 Warrensburg. Drillers report the area between Blue Mound and the Sangamon 
 River to be less favorable. The municipal supply for Niantic is obtained from a 
 well drilled to a depth of 48 feet. Thicker drift behind the border of the Wiscon- 
 sin glaciation contains sand and gravel beds which locally are favorable sources 
 of groundwater. 
 
 The Pennsylvanian bedrock below the glacial drift is composed principally 
 of shale with thin beds of limestone, sandstone, and coal. Because of the wide- 
 spread availability of groundwater from sand and gravel, wells drilled into the 
 bedrock are uncommon in most of the county. In the southern part of the county, 
 in Ts. 15 N. and 14 N. , if water is not obtained from shallow drift deposits, 
 wells should be drilled to the upper 50 to 100 feet of bedrock where small sup- 
 plies may be obtained from water-yielding sandstone or creviced limestone or 
 shale. Deeper drilling is not recommended because the water in deeper bedrock 
 formations is highly mineralized. 
 
 Marshall County 
 
 In Marshall County extensive thick permeable sand and gravel deposits 
 occur along the Illinois River Valley and extend several miles eastward from the 
 river (fig. 6)'. In this area the Sankoty sand forms the base of the glacial deposits, 
 Along the Illinois River Valley the Sankoty sand is overlain by glacial outwash 
 and later alluvial deposits, but outside the valley it is overlain by till. Munici- 
 pal and industrial groundwater supplies can generally be obtained from the San- 
 koty sand. The shallower sand and gravel outwash is also a dependable source 
 of groundwater in the valley. A small, buried bedrock valley in the southwestern 
 corner of the county (fig. 6) is recognized, but not enough information is avail- 
 able to forecast the water-yielding potential of the valley fill. 
 
 In the eastern and western parts of Marshall County, average thickness of 
 the drift is 75 to 100 feet. In these areas domestic and farm supplies can gener- 
 ally be obtained from thin scattered layers of sand and gravel within the drift. 
 
 The Pennsylvanian formations underlying the drift in the eastern and west- 
 ern parts of the county are not considered a dependable source of groundwater. 
 Small groundwater supplies are available from a few wells in the Pennsylvanian 
 rocks, but drilling into these formations should be considered only as a last re- 
 sort. 
 
 The villages of Toluca and Wenona obtain their water supply from the St. 
 Peter sandstone at depths of 2000 and 1855 feet respectively. The State Water 
 Survey reports that the water is highly mineralized at both villages: 1080 ppm 
 chloride, 200 ppm hardness, and a residue of 2322 ppm at Toluca; 490 ppm chlor- 
 ide, 25 6 ppm hardness, and a residue of 1437 ppm at Wenona. 
 
Thickness 
 
 Depth 
 
 (ft.) 
 
 (ft.) 
 
 50 
 
 50 
 
 40 
 
 90 
 
 30 
 
 120 
 
 10 
 
 130 
 
 10 
 
 140 
 
 10 
 
 150 
 
 28 ILLINOIS STATE GEOLOGICAL SURVEY 
 
 Mason County 
 
 Geologic conditions in Mason County and adjoining parts of Tazewell and 
 Logan counties are favorable for developing large groundwater supplies. This 
 area is a wide, bedrock lowland that was formed at the confluence of the ancient 
 Mississippi and Mahomet rivers and is now buried beneath a thick mantle of 
 glacial deposits, mainly sand and gravel. The deposits include ancient stream' 
 fills and later glacial outwash that poured down the Illinois River Valley. Around 
 Havana the deposits have an average thickness of about 150 to 200 feet and are 
 composed of sand and gravel in the upper part, and mainly of sand in the lower 
 part. 
 
 The following sequence, penetrated one and a half miles south of Forest 
 City in sec. 19 , T. 22 N. , R. 6 W. , is representative of the valley fill in the 
 lowland area . 
 
 Description 
 
 Sand fine to medium, some coarse 
 
 sand at base 
 Sand fine to medium 
 Sand fine to medium, silty 
 Sand fine to medium, some 
 
 granular gravel, very silty 
 Sand, fine to medium 
 Shale, dark grey 
 
 Most of the wells in the lowland area are finished in shallow aquifers at depths 
 ranging from 40 to 120 feet below land surface. Wells drilled for the city of 
 Havana are finished at depths ranging from 70 to 90 feet. Domestic and farm 
 supplies are easily obtainable at depths of less than 50 feet with sand points, 
 drilled, and dug wells. 
 
 In the upland areas in the southeastern part of the county, the lower sand 
 and gravel deposits are overlain by glacial till. This sequence of unconsolidated 
 material offers several water-yielding horizons. Sand and gravel lenses that 
 vary laterally in extent, thickness, and permeability are sources of water for 
 many private supplies. As in the lowland area the sand and gravel in the deeper 
 part of the unconsolidated material is a dependable source of groundwater. The 
 following is a sequence of deposits encountered in drilling at Mason City in 
 sec. 8, T. 20 N. , R. 5 W.: 
 
 Description 
 
 Soil 
 
 Sand, fine 
 
 Silt 
 
 Till, silty, brown 
 
 Sand, medium to coarse 
 
 Sand, very coarse, some gravel, dirty 
 
 Till, yellowish, brown 
 
 Sand, fine to medium 
 
 Sample missing 
 
 Sand, very fine to fine 
 
 Sand, very coarse, some gravel 
 
 Sand, medium to coarse 
 
 Thickness 
 
 Deptl 
 
 (ft.) 
 
 (ft.) 
 
 5 
 
 5 
 
 35 
 
 40 
 
 5 
 
 45 
 
 5 
 
 50 
 
 5 
 
 55 
 
 10 
 
 65 
 
 20 
 
 85 
 
 5 
 
 90 
 
 105 
 
 195 
 
 4 
 
 199 
 
 11 
 
 210 
 
 10 
 
 220 
 
GROUNDWATER IN EAST-CENTRAL ILLINOIS 29 
 
 The well was finished at 220 feet, but sand is present below this depth and ex- 
 tends to bedrock. Municipal and industrial supply may be obtained from the low- 
 er sand at most locations in this upland area. 
 
 Mississippian and Pennsylvanian bedrock formations underlie the glacial 
 drift (fig. 7); but because groundwater is available in shallow unconsolidated 
 material, only a few water wells have been attempted in the bedrock. 
 
 McLean County 
 
 Sand and gravel deposits suitable for the development of high-capacity 
 wells are present in the buried Mahomet Valley in the southwestern part of Mc- 
 Lean County (figs. 3, 6). The village of McLean obtains water from these de- 
 posits at a depth of 353 feet. Favorable conditions for the construction of mu- 
 nicipal and industrial wells are also found locally along the ridge-like features 
 (moraines) that cross the county. Thicker drift associated with the moraines 
 contains sand and gravel deposits which are the source of water for the towns 
 of LeRoy, Normal, Saybrook,and Lexington. Small stream valleys, such as por- 
 tions of Sugar Creek Valley, occasionally contain sufficient deposits of sand 
 and gravel for small municipal and industrial supplies. In most instances, how- 
 ever, extensive test drilling is necessary to locate the most suitable sites for 
 well locations. 
 
 Farm and domestic supplies are available throughout most of the county. 
 Because conditions within the drift are so variable, it is advisable to test the 
 entire thickness of drift because water-yielding deposits are commonly present 
 at shallow depths and at the base of the drift just above the bedrock. 
 
 The Pennsylvanian bedrock directly underlies the glacial drift throughout 
 the county. Although little information is available on the distribution of water- 
 yielding beds within the upper part of the Pennsylvanian rocks, a few wells ob- 
 tain small supplies of groundwater from these formations. When all attempts to 
 develop drift wells have failed, the upper 100 feet of the Pennsylvanian bedrock 
 should be tested for the presence of water-yielding sandstone, creviced limestone, 
 or fractured shale beds. Below 100 feet in the bedrock the formations are gener- 
 ally tight and the water is highly mineralized. 
 
 In the northeastern-most portions of the county it is likely that deeper bed- 
 rock formations may yield large groundwater supplies. The village of Chenoa 
 has a well finished in the Oneota formation (fig. 4) at a depth of 2035 feet. How- 
 ever, the State Water Survey (Bull. 40) reports that the mineral content of the 
 water is high, with a residue of 1314 ppm, a chloride content of 540 ppm and a 
 hardness of 229 ppm. 
 
 Menard County 
 
 Unconsolidated deposits, locally as much as 150 feet thick, are associ- 
 ated with the buried Ancient Mississippi Valley, the Sangamon River, and Salt 
 Creek in the extreme northern part of Menard County. These deposits contain 
 beds of sand and gravel favorable for the development of small industrial or mu- 
 nicipal groundwater supplies. The buried Athens Valley (figs. 3, 6) also may 
 contain as much as 150 feet of glacial drift, but little is known about the distri- 
 bution of water-yielding sand and gravel within the fill. 
 
 Sand and gravel outwash deposits along the Sangamon River in the southern 
 part of the county are generally thin and patchy, and bedrock crops out along the 
 
30 ILLINOIS STATE GEOLOGICAL SURVEY 
 
 valley. From Petersburg north, the valley fill thickens somewhat and may con- 
 tain more continuous deposits of sand and gravel. The water supply for Peters- 
 burg is taken from these sand and gravel deposits. 
 
 Sand and gravel deposits are generally thin and discontinuous on the up- 
 lands. Locally these deposits are the source of groundwater for domestic and 
 farm supplies. 
 
 Pennsylvanian bedrock underlying the drift throughout the county is locally 
 a source of groundwater for domestic and farm supplies. 
 
 Moultrie County 
 
 Municipal and industrial supplies may be obtained locally in the flat of 
 the Kaskaskia River, its tributaries, and in the fill of the preglacial Middletown 
 valley which extends from south of Sullivan to the northwest corner of Moultrie 
 County (fig. 3) . 
 
 The glacial fill of the Middletown valley ranges in thickness from 150 to 
 200 feet. At many locations, according to drillers, the fill is composed princi- 
 pally of clay but locally contains sand and gravel deposits which may be a source 
 of groundwater for industrial and municipal supplies. Testing is needed to lo- 
 cate suitable sand and gravel deposits in the Middletown valley area. The "North 
 well" of the town of Sullivan drilled in the northeast corner of sec. 23, T. 13 N. , 
 R. 5 E. , penetrates the following deposits: 
 
 Description 
 
 Pleistocene series 
 Clay 
 
 Sand and gravel 
 Hardpan 
 Gravel 
 Clay and gravel 
 
 A well at Dalton City is finished in gravel at a depth of 108 feet. 
 
 Throughout most of the county, water for farm and domestic supplies is 
 obtained from thin sand and gravel deposits within the drift. Locally in Ts. 13 
 N. , 14 N. and 15 N. , R. 6 E. , sand and gravel are absent, and wells have been 
 drilled into the Pennsylvanian bedrock. 
 
 Pennsylvanian sandstones, coal, or fractured shales and limestone are a 
 local source of water for small farm supplies. Because of the poor guality of 
 water in deeper bedrock formations, drilling should not extend below a depth of 
 approximately 300 feet. 
 
 Piatt County 
 
 In Piatt County excellent water-yielding sand and gravel deposits suitable 
 for municipal and industrial supplies occur in the fill of the buried Mahomet 
 Valley (figs. 3,6, 8). The following sequence of deposits was penetrated at Mon- 
 ticello, sec. 7, T. 18 N. , R. 6 E: 
 
 ickness 
 
 Depth 
 
 (ft.) 
 
 (ft.) 
 
 64 
 
 64 
 
 37 
 
 101 
 
 3 
 
 104 
 
 3 
 
 107 
 
 22 
 
 129 
 
GROUNDWATER IN EAST-CENTRAL ILLINOIS 
 
 31 
 
 Description 
 
 Soil and till 
 
 Sand and gravel 
 
 Till 
 
 Sand and fine gravel 
 
 Sand, some clay and gravel 
 
 Sand 
 
 Sand with organic matter 
 
 Sand 
 
 Gravel 
 
 Thickness 
 (ft.) 
 28 
 12 
 55 
 15 
 40 
 15 
 
 5 
 
 5 
 34 
 
 Depth 
 (ft.) 
 
 28 
 
 40 
 
 95 
 110 
 150 
 165 
 170 
 175 
 209 
 
 The city of Decatur has wells in sec. 30, T. 18 N. , R. 5 E. , finished in 
 sand at a depth of 256 feet. 
 
 The buried Pesotum Valley (fig. 3 and area outlined by dashes in fig. 6) 
 locally may contain sand and gravel deposits favorable for industrial and mu- 
 nicipal supplies and appears worthy of testing. 
 
 Sand and gravel deposits suitable for domestic and farm supplies generally 
 occur throughout the county at depths ranging from 80 to 200 feet. Locally small 
 municipal supplies may be developed in these deposits. For example, the Ham- 
 mond municipal supply is obtained from sand at a depth of 143 feet. Drillers re- 
 port that north of Deland, sand and gravel beds are generally thin and discontin- 
 uous. 
 
 Because of the availability of water from sand and gravel above the bed- 
 rock, wells are rarely drilled into shallow Pennsylvanian bedrock formations 
 (fig. 7). It is possible that locally small farm supplies may be obtained in the 
 upper 250 to 300 feet of bedrock. 
 
 Putnam County 
 
 In Putnam County excellent water-yielding sand and gravel deposits suit- 
 able for high-capacity wells occur in the fill of the buried Ancient Mississippi 
 Valley and locally in the buried Ticona Valley (figs. 3, 6). Domestic and farm 
 supplies are available throughout the county from thin layers of sand and gravel 
 within the drift. 
 
 Pennsylvanian sandstones or fractured limestones are local sources of 
 water for small farm supplies throughout the county (fig. 7). 
 
 In the eastern two-thirds of the county, Silurian and Devonian limestone 
 and dolomite below the Pennsylvanian rocks may be a source of groundwater. 
 These formations occur at depths generally greater than 400 feet below land sur- 
 face. The village of Hennepin obtains its groundwater supply from limestone at 
 depths of 400 to 850 feet. Little information is available on the groundwater 
 potential of these formations in Putnam County. 
 
 The Ordovician-St. Peter sandstone (fig. 4) is the only major deep bed- 
 rock aquifer being utilized in Putnam County. At present the villages of Gran- 
 ville and Standard have wells finished in this formation. At Granville the St. 
 Peter sandstone was penetrated at a depth of 1635 feet below land surface. 
 According to the State Water Survey, water obtained from this well had a hard- 
 ness of 29 6 ppm, a residue of 994 ppm, a chloride content of 305 ppm, and an 
 iron content of 1.7 ppm. At Standard, the St. Peter sandstone was penetrated 
 at a depth of 1600 feet and the water had a hardness of 237 ppm, a residue of 
 
32 ILLINOIS STATE GEOLOGICAL SURVEY 
 
 3279 ppm, a chloride content of 1675 ppm, and an iron content of 2.2 ppm. 
 (Illinois Water Survey Bull. 40, 1948). 
 
 Sangamon County 
 
 Except for a narrow band along the Sangamon River, the probabilities of ob- 
 taining municipal or industrial groundwater supplies in Sangamon County are 
 generally poor. Near Springfield and extending southward in T. 14 N. (fig. 6), 
 the unconsolidated glacial drift is thin and composed of compact clay. In this 
 area groundwater supplies are generally obtained with large diameter dug wells 
 or with drilled wells penetrating the upper part of the underlying Pennsylvanian 
 bedrock. 
 
 In the rest of the county the unconsolidated glacial drift ranges in thick- 
 ness from 50 feet to 130 feet; and sand deposits suitable for the construction of 
 farm and domestic wells have been encountered at depths ranging from 60 to 130 
 feet below land surface in T. 13 N. and T. 14 N. , R. 7 W. and R. 6 W. and in 
 the area of Buffalo and Mechanicsburg. Drillers in the area report that careful 
 well construction and development is necessary to utilize these deposits which 
 are generally thin and fine. Lack of information on water wells in the county 
 make it impossible to evaluate accurately the extent and continuity of water- 
 yielding sand in the drift. The fill of the buried Athens Valley (fig. 3) contains 
 favorable sand and gravel deposits locally. Little is known, however, of the 
 character of the valley fill in Sangamon County. 
 
 The Pennsylvanian bedrock below the glacial drift is composed of shale 
 with beds of limestone, sandstone and coal. Throughout the county small ground- 
 water supplies have been obtained from permeable sandstone, creviced limestone, 
 or fractured shale in the upper 150 feet of bedrock. Drilling should not extend 
 below this depth because mineralized water is usually encountered in the deeper 
 bedrock formations. 
 
 Tazewell County 
 
 The probability of obtaining industrial and municipal supplies from the un- 
 consolidated material is excellent in most of Tazewell County. The area is part 
 of a wide, bedrock lowland formed at the confluence of the ancient Mississippi 
 and Mahomet Rivers and buried beneath thick unconsolidated deposits of mainly 
 sand and gravel. These deposits include stream fills and glacial outwash that 
 poured down the Illinois River Valley- They are generally composed of sand and 
 gravel in the upper part and sand in the lower part. The city of Pekin obtains 
 its water supply from shallow sand and gravel at a depth of 119 feet. Domestic 
 and farm supplies are easily obtainable at depths less than 50 feet with driven 
 sand points or with drilled and dug wells, with the exception of a small area 
 east of Pekin (fig. 6) where the unconsolidated material is thin. 
 
 In the upland area in the east half of the county, the lower sand and gravel 
 deposits are overlain by glacial till. Interbedded with the till are sand and grav- 
 el beds that are sources of water for many private supplies and locally for small 
 municipal supply. The Tremont water supply is obtained from wells finished in 
 sand and gravel at depths of 133 and 154 feet. As in the lowland area, the sand 
 and gravel in the deeper part of the unconsolidated material is a dependable 
 source of groundwater. The following log of the city of Washington Well No. 3 
 shows the seguence of material generally encountered in the upland area: 
 
GROUNDWATER IN 
 Description 
 
 Pleistocene series 
 
 Soil, silt, till 
 
 Gravel 
 
 Till 
 
 Gravel 
 
 Silt and till 
 
 Gravel 
 
 Till 
 
 Sand and gravel 
 Pennsylvanian system 
 
 Siltstone 
 
 CENTRAL 
 
 ILLINOIS 
 
 Thickness 
 
 Depth 
 
 (ft.) 
 
 (ft.) 
 
 83 
 
 83 
 
 11 
 
 94 
 
 69 
 
 163 
 
 10 
 
 173 
 
 27 
 
 200 
 
 18 
 
 218 
 
 32 
 
 250 
 
 120 
 
 370 
 
 33 
 
 375 
 
 The Pennsylvanian bedrock (fig. 7) below the drift is a source of water for 
 farm and domestic supplies east of Pekin where the drift is thin. In the rest of 
 the county, because groundwater is widely available from the unconsolidated 
 material, only a few wells have been attempted in the bedrock. 
 
 Vermilion County 
 
 In Vermilion County dependable water-yielding sand and gravel deposits 
 occur at many places in the fill of the Mahomet bedrock valley and its southern 
 tributaries (figs. 3, 6). The thickness of the drift in the valley ranges from about 
 250 to 350 feet and contains several zones of water-yielding sand and gravel. 
 Although these beds are encountered at various depths within the drift, they are 
 more continuous and usually occur at depths ranging from 80 to 140 feet and be- 
 low a depth of about 200 feet. 
 
 The towns of Hoopeston, Potomac, Rossville, and Rankin obtain water sup- 
 plies from sand and gravel deposits encountered at depths ranging from 110 to 
 230 feet. The water present in the drift is confined at most places, and drillers 
 refer to this area as the "artesian belt". Favorable conditions for developing 
 groundwater supplies from sand and gravel are locally present along the North 
 Fork of the Vermilion River. 
 
 A belt of thick drift in the Danville area is probably associated with the 
 channel of a preglacial bedrock valley (fig. 3). The area is worth testing to lo- 
 cate suitable water-yielding deposits for small industrial supplies. In the area 
 of Allerton, Sidell, and Indianola, sand and gravel beds are absent locally, and 
 sometimes test drilling is required to locate a favorable well site. The thickness 
 of the drift ranges from about 60-120 feet in the south to about 80-200 feet in 
 the north. 
 
 In the other parts of the county, the probability of obtaining groundwater 
 supplies from the drift is fair. Sand and gravel deposits suitable for domestic 
 and farm wells are generally present, and locally, especially east of Rossville, 
 they may yield water for small industrial and municipal supplies. 
 
 South of Danville the probabilities of aquifers above the bedrock are poor. 
 Here, some wells that have penetrated the upper part of the Pennsylvanian bed- 
 rock obtain small supplies of groundwater from shale and thin sandstone or lime- 
 stone beds. With the exception of the areas shown as "poor" in figure 6, every 
 effort should be made to develop a well in the drift before drilling into the bed- 
 rock. The quality of the water obtained from the bedrock formations below depths 
 ranging from 150 to 300 feet may be unsatisfactory. 
 
34 ILLINOIS STATE GEOLOGICAL SURVEY 
 
 Woodford County 
 
 The Sankoty sand occurs at the base of the drift in the western part of 
 Woodford County. This formation and the outwash sand and gravel deposits in 
 the Illinois River Valley are sources of municipal and industrial groundwater sup- 
 ply (fig. 6). In the remainder of the county, large supplies may be obtained lo- 
 cally, although extensive test drilling is usually necessary to locate the best 
 well sites. 
 
 Groundwater supplies for farm and domestic use are generally available 
 from sand or gravel layers within the drift, and therefore relatively few wells 
 are finished in the Pennsylvanian formations (figs. 5,7). In the eastern two- 
 thirds of the county, if water is not obtained from the unconsolidated material, 
 the upper part of the Pennsylvanian bedrock should be tested. 
 
 Two wells for the city of Minonk have been finished in the St. Peter sand- 
 stone (fig. 5). Water from the first well, drilled in 1893 to a depth of 1850 feet, 
 was reported to have a hardness of 239 ppm, a chloride content of 685 ppm, 
 and a residue of 1703 ppm (State Water Survey Bull. 40). Throughout the remain- 
 der of the county, the deeper bedrock formations appear to contain water too 
 highly mineralized for ordinary use. 
 
GROUNDWATER IN EAST-CENTRAL ILLINOIS 35 
 
 SUGGESTED READING 
 
 An Intregrated Geophysical and Geological Investigation of Aquifers in 
 Glacial Drift near Champaign-Urbana, Illinois: John W. Foster and Merlyn B. 
 Buhle, Illinois Geol. Survey Re pt. Inv. 155, 1951. 
 
 Bedrock topography of Illinois: Leland Horberg, Illinois Geol. Survey 
 Bull. 73, 1950. 
 
 Cisterns: Illinois Dept. of Public Health Circ. 129, 1949. 
 
 Disinfection of water: Illinois Dept. of Public Health Circ. 97, 1950. 
 
 Groundwater in the Peoria region: Leland Horberg, T.E. Larson, and Max 
 Suter, Illinois Geol. Survey Bull. 75 (Illinois Water Survey Bull. 39), 1950. 
 
 Geology and mineral resources of the Marseilles, Ottawa,and Streator 
 quadrangles: H.B. Willman and J. Norman Payne, Illinois Geol. Survey Bull. 66, 
 1942. 
 
 Geology and mineral resources of the Beardstown, Glasford, Havana, and 
 Vermont Quadrangles: H.R.Wanless, Illinois Geol. Survey Bull. 82, 1957. 
 
 Individual water supply systems: Recommendation of the Joint Committee 
 on Rural Sanitation: U. S. Public Health Service Pub. 24, 19 50. 
 
 Major aquifers in glacial drift near Mattoon, Illinois: John W. Foster, 
 Illinois Geol. Survey Circ. 179, 1952. 
 
 Pleistocene deposits below the Wisconsin Drift in northeastern Illinois: 
 Leland Horberg, Illinois Geol. Survey Rept. Inv. 165, 1953. 
 
 Public ground-water supplies in Illinois: compiled by G.C. Habermeyer, 
 Illinois Water Survey Bull. 21, 19 25. 
 
 Public ground-water supplies in Illinois: compiled by Ross Hanson, Ill- 
 inois Water Survey Bull. 40, 1950. 
 
 Significance of Pleistocene deposits in the groundwater resources of 
 Illinois: J. W. Foster, Economic Geology, v. 48, no. 7, November 1953. 
 
 Wells, dug, drilled, driven: Illinois Dept. of Public Health Circ. 14, 1951. 
 
 Other general reports on groundwater geology in Illinois, similar in pur- 
 pose and scope to the present study, include the following circulars: C 192, 
 Water wells for farm supply in central and eastern Illinois; C 198, Groundwater 
 possibilities in northeastern Illinois; C 207, Groundwater in northwestern 
 Illinois; and C 212, Groundwater geology in southern Illinois; C 222, Ground- 
 water geology in western Illinois, north part; C 225, Groundwater geology in 
 south-central Illinois; C 232, Groundwater geology in western Illinois, south 
 part. These circulars, published by the Illinois State Geological Survey, are 
 available free on request. 
 
 Topographic maps are available for the area covered in this report. These 
 maps are on a scale of approximately 1 inch to the mile, but in the Champaign- 
 Urbana, Danville, East Peoria, Havana, and Springfield regions they are avail- 
 able also on a scale of approximately 2\ inches to the mile. They are printed 
 by quadrangle and may be obtained from the Illinois State Geological Survey, 
 
36 ILLINOIS STATE GEOLOGICAL SURVEY 
 
 Urbana, Illinois, or from the United States Geological Survey, Washington 25, 
 D. C. , for 30 cents each. Index maps showing the topographic map coverage 
 of the state are free on request. 
 
 Detailed geologic reports have been published for the following quadrangles: 
 Danville, Hennepin, LaSalle, Peoria, Tallula, Springfield. Information on these 
 reports may be obtained from the Illinois State Geological Survey, Urbana, 
 Illinois. 
 
 Illinois State Geological Survey Circular 248 
 36 p. , 7 figs. , 1958 
 
nncnzifr 
 
 CIRCULAR 248 
 
 ILLINOIS STATE GEOLOGICAL SURVEY 
 
 URBANA 
 
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