Water Quality in the Mahomet Aquifer: Chemical Indicators of Brine Migration and Mixing

Contamination of fresh groundwater by naturally occurring, deep-seated brines can be a water quality problem in parts of Illinois and surrounding states. Sudden changes in water quality, such as an increase in chloride concentrations within private and/or public wells, could be due to other activities in the vicinity of a well or wells. Remnant brines within the Illinois Basin, which underlies much of the state of Illinois, have unique chemical signatures that are useful in identifying the geologic formations within the basin. Chloride–bromide mass ratios are an excellent indicator of the source of salinity in both natural and contaminated waters. With this indicator, errant brines entering wells or breaching the surface as saline springs can be traced to the geologic formations from which the brines originated.

According to previous studies and available data, groundwater from the eastern half of the Mahomet aquifer has extremely low chloride concentrations on the order of 1 mg/L and very low chloride–bromide ratios of about 100. Seepage of even a minute amount of basin brines into the Mahomet aquifer would be noticeable because basin brines can have chloride concentrations of over 100,000 mg/L and chloride–bromide mass ratios ranging from 200 to 800. For example, water quality analyses from wells screened in the Mahomet aquifer within several kilometers of an area of suspected discharge of brine or saline water from bedrock would help define water quality conditions and spatial variability. If chloride concentrations from multiple wells exceeded 10 mg/L, then potential mechanisms for migration of the brine or saline water would need to be evaluated. Chemical analysis of groundwater samples for a range of constituents (including chloride–bromide mass ratios) would be an effective first step in determining whether brine or saline water movement has affected groundwater quality within the Mahomet aquifer. Recent questions about the origin of methane gas entering wells screened within the Mahomet aquifer may be addressed if the gas is also associated with the seepage of even a very small amount of brine into the aquifer. Here we present a methodology for identifying the source of brines seeping into the Mahomet aquifer.

This report, authored by Samuel V. Panno and Walton R. Kelly, is available as a digital download from the University of Illinois' IDEALS repository: Water Quality in the Mahomet Aquifer: Chemical Indicators of Brine Migration and Mixing. To purchase a print copy for $6 (plus shipping costs), visit our online store.