Road salt applications mobilize trace elements from roadside soil to shallow groundwater.

In regions where deicers are applied to roadways, micronutrients and toxic trace elements may be mobilized from soil material into soil porewater. These elements may subsequently migrate with soil porewater to surface waters and groundwaters, potentially leaching the soil of micronutrients or introducing toxins to water resources. Our study thus aims to quantify the timing and extent of trace element releases from soil material to soil porewater and groundwater in response to deicing events. We sampled soil porewater near a road at a rural site for trace elements and compared the results to salt applications and soil porewater Na and Cl levels. We also assessed trace element, Na, and Cl concentrations in a karst spring at the rural site and a karst spring at an urban site to evaluate the role of land use in conveying these contaminants to groundwater. We found that certain trace elements (e.g., As, Ba, Fe, Sr) peaked concomitantly with Na and Cl in soil porewater at the rural site after road deicing events, suggesting their release due to excess salt inputs to the soil. We did not observe increases in trace element concentrations at the rural karst spring following individual road salt applications, likely due to low deicer inputs and trace element levels across its recharge basin. However, at the urban site, we observed that other assemblages of trace elements (e.g., As, Cu, Li) in the karst spring peaked with deicing-related Na and Cl pulses. We also found positive and significant correlations between salt applications to the recharge basin and exports of some trace elements (e.g., As, Cu, Li, Se) at the urban karst spring, indicating deicing events triggered trace element releases to groundwater. Overall, we detected road salt-driven trace element release from soil material to soil porewater and groundwater that was exacerbated by urbanization.