Illinois Water Supply Planning




Groundwater is an economically important, renewable resource. Each day Illinois uses over one billion gallons of ground water to meet supply needs for drinking water, agriculture, industry, and power generation. Although Illinois aquifers have an estimated combined potential yield of approximately 7 billion gallons per day, those aquifers are neither uniformly distributed throughout the state nor homogeneous in their physical and chemical properties from area to area. It is the responsibility of the State Scientific Surveys to evaluate these resources for the people of Illinois. This is accomplished through a wide variety of field geologic and hydrologic investigations, geologic mapping, and groundwater modeling.

Principal Aquifers
Sand and Gravel Aquifers
Figure 1.
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Groundwater is developed from three principal aquifer types in Illinois. These are generally categorized as sand and gravel aquifers within the unconsolidated geologic materials overlying the bedrock (figure 1); shallow bedrock aquifers lying within approximately 500 feet of land surface (figure 2); and deep bedrock aquifers lying at depths greater than 500 feet of land surface (figure 3). Unlike the boundaries of the other aquifers, the southern boundary of the deep bedrock aquifers as delineated across central Illinois, is not a physical boundary but is an estimate of the 2,500 milligram per liter total dissolved solids (TDS) boundary beyond which the water is unsuitable for most purposes.

Shallow Bedrock Aquifers
Figure 2.
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To illustrate the use of these aquifers for drinking water, figures 4 – 6 (figure 4, figure 5, figure 6) present the locations of community wells overlain on maps of the principal sand and gravel, shallow bedrock, and deep bedrock aquifers, respectively. Two observations are readily apparent from viewing these maps: a) numerous community sand and gravel wells are not completed in the State’s principal sand and gravel aquifers and b) groundwater availability is heavily weighted toward the state’s major river valleys and the northern third of Illinois which is underlain by one or more principal aquifers. Southern Illinois is generally considered groundwater "poor" because of a lack of thick, coarse glacial deposits (except along major river valleys) and relatively low-yielding bedrock. Hence, there is a greater occurrence of surface reservoirs for water supply in central and southern Illinois.

Potential Aquifer Yields
Deep Bedrock Aquifers
Figure 3.
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The term "potential yield" or "safe yield" often is used when applied to aquifers. Potential aquifer yield is the maximum amount of groundwater that can be continuously withdrawn without creating critically low water levels or exceeding recharge. Statewide potential aquifer yield estimates were developed in the late 1960s by the ISWS1. The basis for making such estimates is discussed below. Illinois’ principal aquifers shown in figures 1-3 (figure 1, figure 2, figure 3) are defined as aquifers with potential yields greater than 100,000 gallons per day per square mile (gpd/mi2) and occupying an area greater than 50 square miles2. (Further discussion on potential and sustainable yields is being developed.)

Sand and Gravel community water supply wells
Figure 4.
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Maps of estimated potential aquifer yields for Illinois’ sand and gravel and shallow bedrock aquifers are presented in figure 7 and figure 8, respectively1. The potential yields are expressed as recharge rates, in gallons per day per square mile (gpd/mi2). {A recharge rate of 100,000 gpd/mi2 is equal to 2.1 inches/year.} The total potential yield of sand and gravel and bedrock aquifers in Illinois are estimated to be 4.8 and 2.5 billion gallons per day (bgd), respectively. The total groundwater potential in Illinois, based on full development of either sand and gravel or bedrock aquifers, whichever has the higher recharge rate, is estimated to be slightly less, 7.0 bgd. Pumping from the sand and gravel aquifers reduces recharge to the underlying bedrock aquifers and, therefore, reduces the combined potential yield of the aquifers. This assumption should be given special consideration when groundwater resource development is contemplated in the northern third of the State, where important bedrock aquifers are located.1

Shallow bedrock community water supply wells
Figure 5.
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Principal sand and gravel aquifers underlie only about 25 percent of the total land area in Illinois. About 3.1 bgd, or about 65 percent of the total potential yield of the sand and gravel aquifers in the State, is concentrated in less than 6 percent of the total land area in Illinois and is located in alluvial deposits that lie directly adjacent to major rivers such as the Mississippi, Illinois, Ohio, and Wabash (figure 7). About 0.5 bgd, or about 10 percent of the total potential sand and gravel yield is from the principal sand and gravel aquifers in the buried Mahomet bedrock valley in east-central Illinois and in the river valleys of the Kaskaskia, Little Wabash, and Embarras Rivers in southern Illinois.

Deep bedrock community water supply wells
Figure 6.
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Of the total estimated yield of bedrock aquifers in the State, 1.7 bgd, or 68 percent, is available from the shallow bedrock aquifers (figure 8), mainly dolomites in the northern third of the State. The potential yield of the shallow dolomite varies. In areas where the more permeable shallow dolomites lie directly beneath the glacial drift, the potential yield ranges from 100,000 to 200,000 gpd/mi2. In areas where less permeable dolomites lie directly beneath the drift or are overlain by thin beds of less permeable rocks, the potential yield ranges from 50,000 to 100,000 gpd/mi2. Where the overlying rocks are thick, the potential yield is less than 50,000 gpd/mi2.1

Estimated potential yield of sand and gravel aquifers
Figure 7.
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The "deep bedrock" aquifers of Illinois generally consist of rocks of Cambrian and Ordovician age and are largely restricted in use to the northern third of Illinois. The southward dip of these bedrock aquifers into the Illinois Basin (figure 9) causes water contained in them to become non-potable (containing 2,500 milligrams per liter TDS) south of a line roughly parallel to and 10 to 100 miles south of the Illinois River (figure 3). Recharge to the deep bedrock aquifers is extremely limited, but development of the Cambrian-Ordovician aquifers in northeastern Illinois has been possible because sufficient drawdown is available for pumpage to create large cones of depression to divert water from recharge areas in north-central Illinois, where the upper layers of the Cambrian-Ordovician aquifer lie directly beneath the glacial drift.1 A widely quoted estimate of the potential yield of the deep bedrock aquifer system for northeastern Illinois is about 46 million gallons per day (mgd) based on the distribution of wells in 1961 and about 65 mgd using a more optimal distribution of wells.3 As is discussed in the section on northeastern Illinois, the continued development of the deep bedrock in northeastern Illinois is uncertain and a subject of much study and concern4,5.

Estimated potential yield of shallow bedrock aquifers
Figure 8.
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Potential aquifer yields are based on estimates of groundwater recharge. Geologic and hydrologic data show a tremendous variability in the character, thickness, and hydraulic conductivity within the geologic materials overlying these aquifers. This variability, in turn, causes great variability in recharge to underlying aquifers. Recharge to the sand and gravel aquifers is from direct infiltration of precipitation to shallow aquifers, leakage through confining beds to deeper aquifers, and induced infiltration to aquifers adjacent to major streams and rivers. Recharge to bedrock aquifers is primarily from vertical leakage occurring through the glacial drift or overlying bedrock formations.1

North to south geologic cross section through Illinois
Figure 9.
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Because recharge is not a process that can be observed directly, it is deduced through streamflow measurements, groundwater level measurements coupled with groundwater withdrawal data, or use of idealized aquifer models.6-10. Each method contains inherent measurement uncertainties as indicated by the mapped recharge rate ranges (e.g., 100,000-150,000 gpd/mi2; 1,000,000-3,000,000 gpd/mi2).

A number of studies involving recharge estimation have been conducted since the time these potential yield maps were created11-13, and while each provides some differences in approach and result, they all tend to reasonably agree within the ranges provided on the statewide maps. Given the uncertainties in the measurement of heterogenous aquifer hydraulic properties such as transmissivity; in the preparation of potentiometric surface maps from which hydraulic gradients and diversion areas are determined; and in the determination of time-variant withdrawal rates and streamflow, the potential yield ranges shown on figures 5 and 6 (figure 5 , figure 6) are reasonable and encompass the range of uncertainty that went into the estimate.

The calculations used to prepare the sand and gravel yield map (figure 7) assumed full development of the sand and gravel aquifers, which includes reducing potential recharge to underlying bedrock aquifers as well as potential base flow to streams. Thus, full development of the sand and gravel aquifers may have undesirable effects that are not intuitively obvious. Similarly, the potential yield estimates of the shallow bedrock aquifers (figure 8) assumed full development of the bedrock. Therefore, caution is warranted where additional groundwater resource development is being considered where productive shallow bedrock aquifers are overlain by sand and gravel aquifers, such as in the northern third of the state.

The potential yield of the deep bedrock aquifer recharge area of north-central and northwestern Illinois is estimated to be approximately 20,000 gpd/mi2 whereas, in the rest of the state, the deep aquifers are overlain by shales of the Maquoketa Group, limiting the potential yield to the maximum amount of water that can move vertically downward through the Maquoketa, or only about 2,100 gpd/mi2 14.

Drought and Recharge

Groundwater recharge is arguably one of the least understood and quantified components of the hydrologic cycle. It cannot be measured directly, is highly variable in space and time, and must be inferred from measurements and determinations of related geologic and hydrologic properties.

"The major sources of recharge to aquifers in Illinois are direct precipitation on intake areas and downward percolation of stream runoff (induced infiltration)….Recharge from direct precipitation and by induced infiltration of surface water involves the vertical movement of water under the influence of vertical head differentials. Thus, recharge is vertical leakage of water through deposits. The quantity of vertical leakage varies from place to place and it is controlled by the vertical permeability and thickness of the deposits through which leakage occurs, the head differential between sources of water and the aquifer, and the area through which leakage occurs." 7

The last "major" droughts experienced in Illinois were in 1980-81 and 1988-1989. An analysis of the impact of these droughts on water resources contain analyses of drought impacts on shallow groundwater conditions based on groundwater level data from an ISWS-maintained shallow groundwater level observation well network15,16. This network consists of shallow water-table wells located in areas remote from pumping. The observation wells are shallow (mean depth = 28.5 feet) and, by design, were not completed in the state’s major aquifers. While data from this network are useful for examining impacts of weather and climate on the water table and thus are useful for extrapolating to impacts on shallow wells, the impacts of drought on recharge to the state’s aquifers is less well-documented or understood.

…water stored in thick deposits of glacial drift is available to deeply buried aquifers so that drought periods have little influence on water levels in these aquifers. Ground-water storage in deposits above aquifers and in aquifers permits pumping for short periods of time at rates greater than recharge. However, many aquifers are greatly limited in areal extent and thickness, and pumping at rates much above recharge rates for extended periods results in rapid depletion of aquifers." 7

Shallow sand and gravel and bedrock aquifers do not have water stored in overlying deposits from which they can draw during times of drought. Therefore, water levels in such aquifers are more sensitive to climatic conditions and will decline in response to dry weather. Available drawdown in wells (the difference between the non-pumping water level and the allowable pumping level, such as the top of the well screen) will be correspondingly reduced. The situation can be further exacerbated by the effects of well interference; water demand often increases during drought, causing wells to be operated at higher pumping rates and/or for longer periods.

Alluvial valley aquifers are often in hydraulic communication with the streams occupying the valleys in which the aquifer is situated. In the humid Midwest, groundwater discharge to streams is often a large component of stream flow17 and may be all of the flow in perennial streams during low flow periods, especially during drought. However, as described by Walton7 above, wells completed in these aquifers can induce infiltration of surface water through stream beds. If stream flow is significantly affected by drought, well yields can also be adversely affected. Conversely, pumping wells that are inducing recharge from nearby streams will reduce stream flow - an effect that may be unacceptable if ecological stream flow thresholds are crossed.

Land Use/Land Cover Impacts on Groundwater Recharge

(discussion under development)

Groundwater Flow Modeling (pdf ~2.2mb) -- Presented by Douglas D. Walker, Illinois State Water Survey, at the 2007 Priority Places Workshop: Implementing a Sustainable Water Supply for Kane County’s Future, on September 20, 2007 in Geneva, Illinois.


1. Illinois Technical Advisory Committee on Water Resources. 1967. Water for Illinois, A Plan for Action. Illinois Department of Business and Economic Development, Springfield.

2. O’Hearn, M., and S.C. Schock. 1984. Design of a Statewide Ground-Water Monitoring Network for Illinois. ISWS Contract Report 354. Illinois State Water Survey, Champaign, IL.

3. Walton, W.C. 1964. Future water-level declines in deep sandstone wells in Chicago region. Ground Water, 2 (1): 13-20.

4. Walker, D.D., S.C. Meyer, and D. Winstanley. 2003. Uncertainty of estimates of groundwater yield for the Cambrian-Ordovician aquifer in northeastern Illinois. Proceedings of Probabilistic Approaches and Groundwater Modeling, American Society of Civil Engineers, Environmental and Water Resources Institute Symposium, Philadelphia.

5. Wehrmann, H.A., and V. Knapp. 2006. Prioritizing the State’s Aquifers and Watersheds. ISWS Miscellaneous Report. Illinois State Water Survey, Champaign, IL. In press.

6. Schicht, R.J., and W.C. Walton. 1961. Hydrologic budget for three small watersheds in Illinois. Illinois State Water Survey Report of Investigation 40.

7. Walton, W.C. 1962. Selected analytical methods for well and aquifer evaluation. Illinois State Water Survey Bulletin 49.

8. Walton, W.C. 1965. Ground-water recharge and runoff in Illinois. Illinois State Water Survey Report of Investigation 48.

9. Prickett, T.A., L.R. Hoover, W.H. Baker, and R.T. Sasman. 1964. Ground-water development in several areas of northeastern Illinois. Illinois State Water Survey Report of Investigation 47.

10. Zeizel, A.J., W.C. Walton, R.T. Sasman, and T.A. Prickett. 1962. Ground-water resources of DuPage County, Illinois. Illinois State Water Survey and Illinois State Geological Survey Cooperative Report 2.

11. Schicht, R.J., J.R. Adams, and J.B. Stall. 1976. Water resources availability, quality, and cost in northeastern Illinois. Illinois State Water Survey Report of Investigation 83.

12. O’Hearn, M., and J.P. Gibb. 1980. Ground-water discharge to Illinois streams. Illinois State Water Survey Contract Report 246.

13. Roadcap, G.S., S.J. Cravens, and E.C. Smith. 1993. Meeting the growing demand for water: an evaluation of the shallow ground-water resources in Will and southern Cook Counties, Illinois. Illinois State Water Survey Research Report 123.

14. Visocky, A.P., M.G. Sherrill, and K. Cartwright. 1985. Geology, Hydrology, and Water Quality of the Cambrian and Ordovician Systems in Northern Illinois. Cooperative Groundwater Report 10. Illinois State Water Survey and Illinois State Geological Survey, Champaign, IL.

15. Changnon, S.A., Jr., G.L. Achtemeier, S.D. Hilberg, H.V. Knapp, R.D. Olson, W.J. Roberts, P.G. Vinzani. 1982. The 1980-1981 Drought in Illinois: Causes, Dimensions, and Impacts. ISWS Report of Investigation 102. Illinois State Water Survey, Champaign, IL.

16. Lamb, P.J., Editor. 1992. The 1988-1989 Drought in Illinois: Cause, Dimensions, and Impacts. ISWS Research Report 121. Illinois State Water Survey, Champaign, IL.

17. Grannemann, N.G., R.J. Hunt, J.R. Nicholas, T.E. Reilly, and T.C. Winter. 2000. The Importance of Ground Water in the Great Lakes Region. U.S. Geological Survey Water-Resources Investigations Report 00-4008. Lansing, MI.

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