Road Salt Legacies: Quantifying Chloride Fluxes to Groundwater and Surface Water across the Chicago MSA

Freshwater chloride concentrations have been increasing in lakes and streams across North America for decades (Dugan, Bartlett, et al. 2017; Sujay S. Kaushal et al. 2018). In northern cities, these increases are primarily driven by increases in the use of road salt, a common road deicer. In the city of Chicago, many thousands of tons of road salt are applied to roadways every winter. Accordingly, increases in surface water chloride concentrations have been noted across the region since the mid-1960s (W. R. Kelly and Panno 2012). While much of the applied salt runs directly off to nearby waterways during snowmelt events, some percolates to groundwater, affecting public supply wells and leading to elevated baseflow concentrations of chloride in nearby streams outside of the winter-spring salting season. In the present study we have created a spatially distributed chloride mass balance across the Chicago Metropolitan Statistical Area (CMSA) for a 30-year period (1990-2020). With this mass balance, we can estimate the magnitudes of legacy chloride accumulating within the CMSA and also determine where these legacies may be having the strongest effects on both aquatic ecosystems and the public drinking water supply. Our results show that inputs of the two largest sources of chloride to the region, road salt and wastewater, increased by 33% and 28%, respectively, between 1990 and 2020. During the same period, riverine chloride loads leaving the CMSA increased by 60%. This has maintained a steady rate of chloride accumulation in the CMSA (480 ktons y-1), totaling approximately 14,000 ktons by the end of the study period. The dynamics of subsurface chloride storage associated with this accumulation were also identified in this study. Shallow aquifers (0-30 meters) consistently displayed short term chloride storage dynamics, increasing their chloride concentrations in the winter and spring in response to road salting but remaining unchanged during summer baseflow conditions. However, at depths below 60 meters, late summer chloride concentrations steadily increased over time, indicating legacy chloride is likely accumulating at deeper depths in the CMSA subsurface. The results highlight the importance of legacy chloride to long-term water quality dynamics in northern North American cities.