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. Plain Language Summary: The amount of salt used on roads to melt snow and ice has been increasing for many years in North America. This salt can make its way into nearby surface waters and groundwater, and can cause problems for public water supplies and aquatic ecosystems. In Chicago, where thousands of tons of road salt are used each winter, researchers have found that the amount of salt entering rivers has increased by 60% since 1990. However, they have also found that a lot of the salt is accumulating in the deeper parts of the groundwater system, which means that even if less salt is used in the future, it will take a long time for the system to recover. This study highlights the importance of considering the long‐term effects of salt use on water quality in cities. Key Points: Road salt application in Chicago increased by ∼30% between 1990 and 2020Downstream chloride export from the Chicago region to the Illinois River totals ∼1,000 ktons year−1Groundwater is a major sink for legacy chloride in the Chicago region at depths below 30 m [ABSTRACT FROM AUTHOR]