Illinois Basin brines have unique chemical signatures (e.g., chloride/bromide [Cl/Br] mass ratios) that are useful in identifying the geologic formations from which they originated. Here we present a methodology for identifying the source of brines that have mixed with freshwater aquifers, and in this specific case, the Mahomet aquifer, a major source of water for more than 500,000 people in 15 counties throughout east-central Illinois. The use of Cl/Br mass ratios is generally applicable to water chemistry results from wells throughout the Mahomet aquifer and can be used in conjunction with other water chemistry analyses to evaluate water quality. In addition, this technique potentially could be used to evaluate water quality results from domestic water wells in the vicinity of Mahomet, Illinois, where a subsurface natural gas leak occurred in 2016 from a well in a gas storage field.
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