Illinois Water Supply Planning



Water Quality

The Governor’s recent initiative stresses the importance of scientifically understanding Illinois’ water supplies in order to help state and local officials manage our finite supplies of fresh water. The primary water uses in Illinois are drinking water, irrigation, and industry, and a certain level of water quality is necessary for all these uses. In addition, the state’s rivers, lakes, and wetlands require certain amounts and quality of water to maintain healthy ecosystems.

Water quality is an intrinsic component of water supply. Much of the groundwater in Illinois is unavailable for economic use because of naturally elevated total dissolved solids (TDS) concentrations that are expensive to remove. Other water supplies have natural and/or human contaminants which may be treated to make them potable, but costs may be prohibitive.

Changing water use demands and patterns can cause unpleasant water quality changes. For example, increased groundwater pumping and resulting cones of depression can induce flow of poor quality water to wells.

Water quality can seriously impair the mechanics of water supplies. The most widespread problem is the precipitation of secondary minerals such as iron hydroxides, calcite, and barite onto well screens, significantly diminishing well pumping capacities.

Water quality information is a useful tool that can help improve understanding of important hydrogeological issues, such as groundwater flow, recharge, age, and groundwater-surface water interactions.

Two high-priority regions have been selected for initial water supply planning: northeastern Illinois, specifically the deep bedrock aquifers underlying the region, and shallow aquifers and surface water within the Fox River valley; and east-central Illinois, specifically the Mahomet aquifer system and surface waters in the Sangamon River watershed. Therefore, studies of water quality with respect to water supply currently focus on these regions.

With respect to groundwater, the aquifers in both regions have natural contaminants whose distributions and solubility controls are not well understood. Withdrawals from aquifers in both regions also are expected to increase, and the effect of that increased pumping on water quality is unknown.

Decreased water availability, whether due to drought or increased withdrawals, can have adverse impacts on surface water quality. Decreased stream base flows and reservoir levels result in more stagnant water conditions which can increase treatment costs and stress indigenous aquatic communities. Point source discharges can become an increasing component of the total flow, which also can degrade water quality. In any case, although it is likely that decreased water availability will result in degraded surface water quality, the precise nature of the impacts requires further study.

Whether drinking water is from groundwater or surface water, treatment generally is required to make the water fit for human consumption. The Illinois Environmental Protection Agency (IEPA) regulates the treatment and quality of public water supplies. More information about this role is found on the Public Water Supply page of the IEPA Web site.

Surface Water Groundwater Interactions

Using Water Quality to Examine Surface Water – Groundwater Interactions and Groundwater Recharge Estimations
Groundwater and surface water are intimately linked. Groundwater in temperate regions such as Illinois typically discharges to surface water bodies such as streams and lakes. This can be observed readily during dry periods when there is no discernible surface runoff but streams continue to flow. This is known as base flow, when 100% of the water in the stream is from groundwater discharge.

Groundwater-surface water interactions are very complex. Groundwater generally discharges from discrete areas rather than evenly throughout a stream or lake bottom. Measurement of certain chemical and physical parameters (i.e., temperature) can help determine where groundwater discharge occurs. Recent investigations relying heavily on chemical data have increased our understanding of these processes, including that most storm runoff spends some time in the subsurface, and most water discharging to streams during storms is “old,” i.e., was present in the subsurface prior to a storm event1.

In certain areas of Illinois, including parts of the Mahomet aquifer system, streams and rivers are directly connected to aquifers. In these areas, surface water may be receiving groundwater discharge under some conditions but discharging into aquifers under other conditions: for example, when there is significant pumping of wells near the surface water body. A recent paper2 provides a method for quantifying surface water – groundwater interactions using thermal records. Water quality sampling can assist in identifying such areas. Water quality measurements also can assist with estimating recharge to aquifers away from surface water connections. Measurements of ions/molecules such as chloride, bromide, or radiogenic and stable isotopes are valuable tools in determining areas of recharge and rates.


1. McDonnell, J. J., M.K. Stewart, and I.F. Owens, 1991. Effect of Catchment-Scale Subsurface Mixing on Stream Isotopic Response. Water Resources Research 27:3065-3073.

2. Hatch, C.E., A.T. Fisher, J.S. Revenaugh, J. Constantz, and C. Ruehl, 2006. Quantifying surface water – groundwater interactions using time series analysis of streambed thermal records: Method development. Water Resources Research 42:W10410, doi: 10.1029/2005WR004787

Other References

U.S. Geological Survey Circular 1139, Ground Water and Surface Water A Single Resource: Many natural processes and human activities affect the interactions of groundwater and surface water. This report presents an understanding of these processes and activities as well as limitations in that knowledge.

Radium and Barium in the Ironton-Galesville Bedrock Aquifer in NE Illinois (pdf ~1.2mb).

Historical Changes in Shallow Groundwater in NE Illinois (pdf ~1.6mb).

Also See: Water Quality: East Central Illinois, Water Quality: Northeastern Illinois

Illinois State Water Survey

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