356-2 Persistence of Chloride in Chicagoland’s Shallow Aquifers Under Sensible Salting Scenarios

Abstract:

Decades of road salt use in the Chicagoland area has led to an accumulation of chloride in the region’s shallow aquifers, affecting both municipal production wells and domestic wells. The increase in chloride has prompted several communities to implement expensive treatment options and adopt sensible salting practices to mitigate against increased corrosivity and associated total dissolved solids (TDS), yet chloride remains elevated in several collar counties with regions exceeding USEPA’s secondary MCL of 250mg/L. We present a chloride modeling study in Kane County, IL using a MODFLOW-NWT/MT3D-USGS model that incorporates transient recharge and variable chloride application rates based on land use. The model is calibrated to decades of chloride data gathered from municipal and private wells open to glacial sands and gravels or the underlying weathered bedrock, and simulates chloride out to 2050 and beyond under various sensible salting scenarios. Scenarios included chloride reductions on major roads only, the entirety of the county road network, and commercial areas that are point sources of chloride. We discuss: 1) the importance of land use and model geology refinement to achieve calibration, 2) the persistence of chloride that remains in the shallow aquifers even under aggressive salt-reduction strategies, and 3) the implications of slow residence time in the shallow aquifers for PFAS contamination and transport. Understanding the movement and transit times of chloride has important implications for other shallow aquifers in northern latitudes as they evaluate risk to their shallow water supplies.

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