ROAD SALT IMPACTS GROUNDWATER QUALITY AND RECHARGE IN STORMWATER MANAGEMENT AREAS

It is common for modern roadway design to incorporate a stormwater management plan. Many plans include ponds or basins that are intended to delay the release of stormwater runoff to surface water. During winter months when freezing conditions occur, stormwater runoff may also contain a large amount of sodium chloride – road salt – that has been applied to roads to maintain safe driving conditions. Road salt will interact with the natural environment in ways that impact soil and groundwater chemistry and recharge. While many stormwater practices promote infiltration to reduce peak discharge to surface water and protect surface water quality, they overlook potentially negative impacts on groundwater.

We monitored groundwater levels, salinity, and quality beneath a stormwater infiltration basin in Delaware to evaluate salt transport in groundwater and the impacts of road salt on groundwater chemistry. Chloride behaves conservatively, making it an excellent tracer of salt movement in the heterogeneous aquifer beneath the basin. We found that chloride is preferentially transported through higher permeability zones and did not diffuse or disperse through the entire aquifer thickness. Sodium, however, was temporarily retained in shallow soil, leading to a lag time between chloride and sodium movement through the aquifer. Importantly, increasing chloride concentrations were correlated with greater radium concentrations, indicating salty water is mobilizing radium from the aquifer matrix to groundwater.

We have been investigating the dynamics between salt loading, sodium retention in soil, and infiltration rate of the basin surface for multiple years. Reduced infiltration rate leads to a longer residence time of standing water in the basin where salt concentrations may increase as water evaporates prior to infiltration. Our results highlight the importance of considering stormwater impacts on groundwater and the inevitable discharge of salty water to surface water when designing stormwater management infrastructure.