Recent Trends in Chloride and Total Dissolved Solids in Silurian Wells in the Southwest Water Planning Group Region: Indicators of Groundwater Contamination within the Silurian Dolomite Aquifer

The objective of this study was to assess whether chloride (Cl-) and total dissolved solids (TDS) concentrations have continued to increase since 2005 in the Silurian shallow bedrock aquifer in the Southwest Water Planning Group (SWPG) region of Illinois (Will, Kendall, and Grundy Counties). Previous research has indicated that Cl- and TDS concentrations have been increasing in Will County since the 1960s as a result of road salt runoff. Analysis of new data indicates that several community supply wells in the SWPG region that are finished in the Silurian dolomite aquifer show increasing Cl- and TDS concentrations. The median increase in Cl- concentrations was 2.5 mg/L per year, with a maximum of 10.4 mg/L per year. The most affected wells were in the northwestern part of Will County. Deeper wells tended to have lower concentrations. Some of the wells that were sampled over short time frames showed extreme variability, which suggests that contaminated water recharges seasonally and that there is minimal mixing in the aquifers. As was shown in previous studies in the Chicago region, urbanization can seriously degrade the groundwater quality of unconfined aquifers. Increasing Cl- concentrations in the region indicate that road salt runoff is reaching the Silurian dolomite aquifer, even to depths greater than 300 feet. Therefore, recharge from the surface is reaching the dolomite aquifer, passing through any till and sand and gravel layers above it. This connectivity between the land surface and the dolomite aquifer shows the vulnerability of the aquifer to surface contamination and the need to protect it