Isotopes and Halide Investigation Helps Resolve Origin of Elevated Chloride Concentrations in Monitoring Wells of Illinois Landfill

ISGS geochemists recently completed a study examining some geochemical characteristics of groundwater samples from a landfill in southern Illinois where monitoring wells contained elevated concentrations of chloride ions. In addition to the typical chemical analyses, oxygen and hydrogen isotopes as well as halide (chloride, bromide, and iodide) analyses were completed on 25 water samples (both up-gradient and down-gradient of the landfill) to help determine the origin of the elevated chloride concentrations observed in several monitoring wells. The isotopic data showed that the two leachate samples collected from the landfill had a very different isotopic composition compared to the local surface water and groundwater monitoring wells. Based on the isotopic results, no detectable leachate contamination was evident for any of the monitoring wells. However, this did not explain the elevated chloride concentrations in many of the monitoring wells at the landfill. This is important because landfill leachate typically contains thousands of milligrams per liter of chloride, making the chloride ion a more sensitive parameter of contamination compared to the oxygen and hydrogen isotope analyses. Furthermore, in some areas, such as in this case, there are multiple sources of chloride which could result in elevated levels of chloride in shallow groundwater wells.

Using graphing techniques recently developed by the ISGS for distinguishing various sources of chloride in groundwater, the monitoring well samples with elevated chloride concentrations showed characteristics similar to other groundwater samples in Illinois that were known to have been affected by brine and/or road salt contamination. Mixing curves were generated from the halide and chemical data in this study to further evaluate the range of concentrations observed in the data. Data from the monitoring wells followed the mixing curves that were generated between the brine (and road salt) end members and the up-gradient wells rather than along the curves calculated for the landfill leachate and up-gradient groundwater. Thus, the halide results agreed with the isotopic data suggesting the monitoring wells with elevated chloride concentrations were probably not impacted by landfill leachate but instead had been impacted by brine and/or road salt contamination. The physical presence of old oil wells in close proximity to the landfill suggested that brine contamination in shallow monitoring wells in the area was possible, if not likely, corroborating the isotope results.