Road Salt Legacies: Quantifying Fluxes of Chloride to Groundwater and Surface Water Across the Chicago Metropolitan Statistical Area

Freshwater chloride concentrations have been increasing in North American surface waters for decades, largely driven by increases in the use of road salt, which is commonly applied as a deicer. In Chicago, thousands of tons of road salt are applied to roadways each winter, and increases in surface water chloride concentrations have been noted across the region since the mid‐1960s. While much of the applied salt runs directly off to nearby waterways during snowmelt events, some percolates to groundwater, affecting public supply wells and increasing the amount of chloride released to streams as baseflow during the non‐salting season. In the present study we have developed a spatially distributed chloride mass balance across the Chicago Metropolitan Statistical Area (CMSA) for a 30‐year period (1990–2020) to better our understanding of long‐term chloride fluxes and storage. Our results show that inputs of road salt to the region increased by 33% between 1990 and 2020. During this same period, riverine chloride loads across the region increased by 60%. Despite these increases in riverine chloride export, we find that chloride is accumulating in CMSA groundwater at a rate of ∼480 ktons year−1. We show that shallow aquifers, <30 m, exhibit only seasonal chloride storage, without long‐term accumulation. In contrast, at depths below 30 m, we find chloride concentrations to be increasing over time, indicating that legacy chloride is accumulating at deeper depths in CMSA groundwater. The present results highlight the importance of legacy chloride to long‐term water quality dynamics in North American cities.