THE PROBLEM OF PERMEABILITY: MODELING MITIGATION OF ROAD SALT CONTAMINATION AND STORMWATER RUNOFF BY RETROFITTING LOW IMPACT DEVELOPMENT IN A MOUNTAINOUS, URBAN WATERSHED

Urbanization and traditional development has resulted in higher stormwater surface runoff and degradation of water quality. The application of deicing salts has given rise to increasing salinity in streams draining urban areas that experience snowfall events. Low impact development (LID) practices have been shown to mitigate both the intensity of stormwater runoff and the concentration of pollutants in a stream, while increasing the influx of groundwater recharge. In this study, scenarios that retrofit existing impervious surfaces with LID in a mountainous, urban watershed are modeled to predict how stream conditions may change. The Soil Conservation Service Curve Number (CN) method is used to estimate direct runoff and is combined with mass balance equations to estimate groundwater flow and Cl^- concentrations. Rather than applying the 30m resolution National Land Cover Dataset (NLCD) traditionally used for the CN method, impervious layers were digitized using 6-inch aerial photography and separated into specific classes. 30 scenarios were modeled with the highest and lowest experimentally derived CN values for five types of LID systems. When all impervious surfaces in the watershed were retrofitted with LID, maximum potential benefits were estimated to decrease stormwater runoff by 30.1-50.0%, increase mean baseflow by 1.5-2.5%, decrease mean streamflow by 3.2-5.3%, decrease mean stream Cl^- concentrations by 1.0-2.6%, and decrease maximum stream Cl^- concentrations by 17.2-31.8%. Impervious surfaces in the study watershed were also reclassified using the NLCD with assumptions made by the commonly used L-THIA LID model. When compared to manually digitized impervious surfaces, the NLCD overestimated paved surface area by 2.0% and underestimated building area by 0.4%.