INSIGHTS ON THE DRIVERS OF CHLORIDE ACCUMULATION AND VARIABILITY IN GROUNDWATER FROM A WEEKLY DATASET COLLECTED FROM COMMUNITY WELLS IN NORTHEAST ILLINOIS

This study examines spatial and temporal trends of a three-year dataset of weekly chloride measurements at six wells collected by water operators at a community in northeast Illinois. In the shallow aquifers of northeast Illinois, chloride contamination from winter deicers has been well studied, with county-wide groundwater sampling campaigns and the development of a groundwater flow and chloride transport model that simulates chloride accumulation in the area from 1950 to 2020. Still, while time series of long-term (decades) chloride concentration at wells are available and critical for calibrating the model, the need for higher-frequency data to document shorter term (months) trends in groundwater chloride concentrations has been emphasized in previous studies. This research addresses this gap by analyzing the weekly dataset to better understand trends of groundwater chloride concentrations. Concentrations at these wells are elevated and several wells’ concentrations exceed the USEPA secondary standard of 250 mg/L. Three of the wells are in the commercial sector of the community and three are in the residential sector and the difference in land use is impactful, with wells in commercial areas having higher chloride concentrations than those in residential areas. At two of the three wells in the commercial sector, chloride in groundwater exhibits seasonality. Field sampling and modeling results suggest that stormwater infrastructure near wells could be the source of seasonal signals in the chloride data, with the chloride from winter deicers in the winter meltwater being the source of the spring peaks. Chloride contamination is detrimental to aquatic species, increases corrosivity of water, and raises water treatment costs. These results corroborate an insight from the groundwater flow and chloride transport model that within commercial sectors, small areas of point source recharge of chloride are impactful to chloride concentrations at localized areas of the aquifer.