MODELING THE TRANSPORT AND FATE OF ROAD SALT IN URBAN FLOODPLAINS

Road salt application in the northeastern United States has led to increasing chloride concentrations in urban streams. Many studies have reported hydrologic storage of salt in the subsurface, which results in higher stream chloride concentrations through time, but the hydrologic processes controlling storage have not been fully investigated. Previous research has shown that surface water-groundwater (SW-GW) interaction in urban streams hydrologically connected to adjacent floodplain aquifers results in annual buffering of large swings in chloride concentrations in surface water. To investigate this process, we created a 3D numerical groundwater flow and solute transport model of an urban floodplain in Syracuse, New York, using MODFLOW and MT3DMS. The model ran for one year and was calibrated to three independent field measurements: groundwater chloride distribution in the floodplain, water table slope between the hillslope and the stream, and groundwater discharge to the stream across the 500-m reach. We used the model to evaluate the importance of different hydrologic processes on controlling road salt storage in the floodplain, including hyporheic exchange, hillslope flux, and increased recharge to the aquifer during overbank flooding events. We found that overbank events in the winter, where the stream has very high chloride concentrations (>1500 mg/L) are the main source of chloride to the subsurface. Subsurface heterogeneity results in a range of chloride concentrations in the model, and low conductivity surface layers are the most important for salt storage. Urban planners who want to use restoration techniques to decrease stream salinization via buffering should consider engineering overbank flood events to promote salt storage in the subsurface during the winter.