How Will Climate Change Affect Road Salt Export from Watersheds?

Salt applied to roads for de-icing can persist for decades in watersheds, and understanding rates of salt accumulation and export is important for anticipating the magnitude and duration of its environmental impacts. Salt removal is affected by storm frequency and intensity, both of which are projected to change as a result of anthropogenic climate change. To examine the potential outcomes of changing storm regimes on rates of salt export from watersheds, we studied chloride concentrations in baseflow and surface runoff in 5 years of streamflow data (taken at 20-min intervals) in a headwater tributary of the Hudson River. Baseflow, with chloride concentrations of 200–230 mg/L year-round, dominated streamflow, except during and after large storms, and accounted for about 90% of chloride export from the watershed. Extreme precipitation events (e.g., tropical storms) increased salt export mainly by increasing baseflow discharge, which remained elevated long after storms ended. While intense events accomplished more salt export per storm, they did not produce disproportionately higher export per volume of precipitation, compared to smaller rain events. Chloride export in a wet year was almost twice that in a dry year, despite less than a doubling of rainfall. Under future climate conditions, if winter precipitation shifts toward a higher proportion of rainfall, or summers experience a greater frequency of tropical storms, then long-term residence time of road salt in watersheds may be shorter than previously anticipated. Conversely, if climate becomes drier, with increased evapotranspiration, reduced infiltration and baseflow discharge may prolong salt storage, which could exacerbate biotic stresses from high chloride concentrations in streams.