Risk to Groundwater Supplies in the Lower Illinois Region

The Illinois State Water Survey (ISWS) is commonly asked: “How much water is available to our community?” This question is challenging to answer since it is predicated on identifying a threshold to define available water. In the supply numbers presented below, the ISWS assessed available supply considering how much water can be withdrawn before groundwater contributions to surface water are reduced by  10%, a threshold  above which negative impacts to aquatic species dependent on surface water are expected to occur.

The table below presents the supply (water available to use) and demand (water needed to meet current demands) for each county in the Lower Illinois Region.

Of the seven counties in the Lower Illinois region, demand is estimated to exceed supply only for Pike and Scott counties. The demand in these counties is higher than the other counties. Several counties, such as Calhoun, Jersey, and Morgan, have more than twice as much supply as demand. Macoupin County is not currently projected to be using groundwater to meet water supply.

Another risk to groundwater supply is well failure at communities which have only one well to meet water supply demands. The Lower Illinois Region has four such communities. Three (Gillespie, Staunton, and Palmyra Modesto Water Commission) are in Macoupin County, with the fourth (Waverly) in Morgan County. Care should be taken to ensure the longevity of these wells and to develop a backup supply of water in case of well outages, a not uncommon occurrence.

Most high-capacity wells in the Lower Illinois Water Supply Planning Region are drilled into the alluvial aquifers of either the Illinois or Mississippi Rivers. However, there are lower-capacity domestic wells present throughout the region that are able to withdraw sufficient amounts of water from much lower transmissive geologic materials. Both the high- and low-capacity wells in the region are vulnerable to human derived contamination, as is evident from elevated nitrate levels in both types of wells. As such, it is important to have a regional understanding of aquifer vulnerability to contamination.

The aquifer vulnerability map below shows that the greatest risk to contamination indeed occurs along the major river valleys in the Lower Illinois Region. It is important to note that the most transmissive areas are also the most at risk of contamination; the high transmissivity and high risk of contamination are both driven by the presence of sand and gravel deposits at or near land surface that rapidly transmit water. Areas of highest risk are those with a large proportion of sand and sand layers within 25 feet of land surface.  

Groundwater contamination can be driven by very local pathways that cannot be captured in the regional map shown above. If you have concerns about your well water quality, the best thing that you can do is reach out to the ISWS to  obtain a water quality sample analysis .

This story map is part of a series about Water Supply Planning throughout the state of Illinois. Water Supply Planning is an ongoing research effort conducted by the Illinois State Water Survey and is funded by the  Illinois Department of Natural Resources Office of Water Resources.