American Bottoms: Risk to Groundwater Supplies

Groundwater is an essential water resource for the American Bottoms Region. Groundwater is accessed from underground aquifers using pumping wells that withdraw and distribute the water to locations where it is needed via pipes and canals. Groundwater is used in the American Bottoms Region primarily for drinking water and to support businesses.

The geological framework of the American Bottoms region controls the regional groundwater dynamics - defining where aquifers occur and how groundwater behaves/flows. The region's primary aquifer is near land surface and water moves through the sandy, gravely subsurface quickly. As such, local flow paths are expected to be relatively short, and the amount of time groundwater spends in the subsurface is likely measured in days or weeks. Other regions' groundwater (e.g.,  Northeastern Illinois ) follow flow paths that are much longer and deeper, and the length of time groundwater resides underground is measured in centuries or millennia. At the end of the flow paths, groundwater emerges at land surface and discharges to rivers, streams, lakes, springs and wetlands (Kolata and Nimz, 2010).

Many factors impact groundwater levels, including climate change, population growth, rainfall rates, and exchange with surface water. In general, risk to groundwater resources can be discussed from two perspectives: the volume of available water, and the quality of the available water. Concerns associated with risk to the available water volume might include the desaturation of regional aquifers (i.e., aquifers drying up and no longer storing water), or whether water enters the aquifer fast enough to replenish what is removed by human pumping. The primary issue associated with water quality risk is whether the groundwater is potable or safe to drink.

The primary source of water for the American Bottoms Water Supply Planning Region is the eponymous American Bottoms aquifer that underlies the Mississippi River floodplain stretching from the north-central portion of Madison County, south through St. Clair County, to the southwestern portion of Munroe County. The main portion of the aquifer occupies the western halves of Madison and St. Clair counties where the aquifer reaches an approximate maximum width of ten miles from west to east. The ISWS has historically operated a network of 11 observations wells across the aquifer with most sites starting in the 1950s. Unfortunately, data stopped being collected from many of these sites in mid-to-late 2000s. Despite this, the period over which these data were collected still represents a broad range of conditions and can be used to put the current state of the aquifer in context. Two of these historic sites have seen data collection resume in recent years: SWS No. 2 (operated under the WARM program) and the Blast Furnace site.

Below is a graph of water levels at the SWS No. 2 site (near Edgemont, Illnois). Water levels were lowest in the mid-1950s and late-1960s at approximately 20 feet below ground surface and have since recovered to an approximate average depth to water of 12.5 ft from the 1970s to present.

In addition to the SWS No. 2 site, the other long-term site collecting data is at the Blast Furnace near Granite City, Illinois to the northwest of Horseshoe Lake. During the mid-1950s, this site was close to extensive industrial local groundwater pumpage and as a result had low groundwater levels, approximately 25 feet below ground surface (bgs) (see figure below). Groundwater levels recovered from this historic low and from 1960 to present groundwater levels have averaged approximately 7 feet below ground surface.

Of the 11 historic groundwater level observation sites, only the Collinsville site was near a major public water supply system. Groundwater levels at Collinsville were at historic lows during the mid-1950s (approximately 32 feet bgs), but recovered to pre-1950 levels before the end of the decade (approximately 23 feet bgs; see figure below). Since the 1960s, groundwater levels have steadily declined from approximately 23 feet bgs to approximately 32 feet bgs with the lowest measurements dropping below the 1950s historic low to 35 feet bgs in the mid-2000s.

The historic low water levels observed at these three sites during the 1950s were a result of heavy industrial groundwater demands occurring contemporaneously with a historic, multi-year drought. These industrial demands on the aquifer greatly decreased during the period from 1960 to 1980. As seen from the SWS No. 2 and Blast Furnace Observation sites, groundwater levels in the ambient aquifer (away from major pumping sources) have recovered and are well above historic lows. This indicates that the quantity of water the American Bottoms Aquifer is more than sufficient to satisfy modern demands. Groundwater levels near major water supply systems where ground water levels have declined, such as Collinsville, IL, may not follow this trend and merit further investigation.

Contamination of groundwater resources from industrial waste disposal has led to multiple US EPA Comprehensive Environmental Response, Compensation and Liability Act (CERCLA; commonly known as Superfund) sites being identified in the region (embedded map below). The region also contains contaminated sites that are proposed but not formally included on the National Priorities List (NPL). In addition to these CERCLA sites, numerous other potential sources of contamination exist in the area, including Resource Conservation and Recovery Act (RCRA) sites and other Illinois and U.S. Environmental Protection Agency monitored sites (large underground storage tanks, landfills, etc.).

For the first half of the 20 ^th  century, the groundwater resources of the American Bottoms region were heavily utilized for industrial purposes. Good water quality and lots of it made the region attractive to industries and municipalities. At present, the quantity of available groundwater is regularly renewed by infiltration from the Mississippi River and recharge from precipitation. However, long-term unmitigated and unregulated industrial waste disposal has led to contamination in numerous locales throughout the region.

• Killey, M. M., and D.R. Larson, 2004, Illinois Groundwater: A vital geologic resource: Illinois State Geological Survey, Geoscience Education Series 17, 61 p.
• Kolata, D. R., & Nimz, C. (2010). Geology of Illinois. Champaign, Ill: University of Illinois at Urbana-Champaign, Institute of Natural Resource Sustainability, Illinois State Geological Survey.
• United States Environmental Protection Agency. (2023). Superfund National Priorities List (NPL) Where You Live Map. Retrieved from  https://epa.maps.arcgis.com/apps/webappviewer/index.html?id=33cebcdfdd1b4c3a8b51d416956c41f1 

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.