Potential Hydrologic Pathways of Deicing Salt Chloride Transport Evaluated with SWMM

Deicing salts are a major contributor to freshwater salinization in the Northeastern United States, disrupting ecosystem function, aquatic life, and infrastructure integrity. It is often presumed that surface runoff is the dominant mode of chloride transport from impervious surfaces to surface waters despite empirical evidence for soil and groundwater transport. Our project aimed to (1) estimate the hydrologic pathways for chloride transport in a small, urbanized catchment in the Northeastern United States; and (2) provide a critical evaluation of simulation routines commonly used to guide the management of deicing salts. We investigated possible variations in urban chloride pathways by monitoring stormwater discharge and chloride across a winter season with deicing salt spreading and snow accumulation and melt. We analyzed these time series with US EPA SWMM, which presumes chloride transport occurs exclusively via surface runoff into stormwater systems. We develop two parallel models: one using the base empirical temperature-index snow routines, and a second substituting a process-based snowmelt model. Each model was calibrated via the generalized likelihood uncertainty estimation algorithm to both stormwater volumes and chloride mass. Both models adequately simulated water volumes but failed to reproduce observed chloride dynamics, suggesting substantial deicing salt movement through soils and groundwater as a major urban hydrologic pathway. The results of this study will inform the development of urban hydrologic fate and transport models and chloride best management practices.