Mass discharge of road salt via groundwater to surface waters in Southeastern Wisconsin

Chloride is the predominant dissolved-phase constituent of road salt and is highly mobile in runoff water, surface water, and groundwater. The concentrations of chloride in surface waters of Southeastern Wisconsin have been measured in exceedance of both chronic (395 mg/L) and acute (757 mg/L) toxicity levels for several decades during periods of road salt application and precipitation/snowmelt-driven runoff. Interestingly, higher chloride levels have also been observed during summer months along sections of the Root River when road salt applications are negligible and precipitation/snowmelt-driven runoff is relatively low. It has been hypothesized that the mass discharge of residual chloride via groundwater flow (baseflow) to the Root River can explain the high levels of chloride during summer, but this hypothesis has yet to be tested. The goal of this research is to quantify the residual groundwater component of the mass discharge of chloride to the Root River before, during, and after the summer months. The methods proposed here rely on two hydrograph separation techniques as follows: 1) flow-based separation to quantify the runoff and groundwater components of river discharge and 2) isotopic-based (18O-H2O and 2H-H2O) separation to quantity the new and old components of river discharge. Both hydrograph separation techniques will be paired with time-series chloride concentration data to quantify the mass discharge of residual (old) chloride via groundwater flow to the Root River. The research proposed here aims to better understand the influence of groundwater discharge on surface water quality and is aligned with the University of Wisconsin System (UWS) groundwater research program’s funding priorities.