Modeling effects of low impact development on road salt transport at watershed scale

Low impact development (LID) practices can mitigate adverse impacts of urbanization on hydrology and water quality by using decentralized designs to control stormwater runoff at the source. Most previous research has focused on LID impacts on stormwater runoff with little consideration given to baseflow or road salt pollution. This study describes a modeling approach to assess how effectively retrofitted LID technologies can reduce salt contamination. The model coupled the curve number approach with completely mixed reservoirs to simulate watershed hydrology and solute transport. The model was tested in Boone Creek Watershed (BCW), an urban headwater watershed in the Southern Appalachians. The model proves able to capture daily stream discharge and stream chloride (Cl-) concentrations satisfactorily. The model simulation with and without LID showed LID can decrease surface runoff and increase recharge, baseflow, and interflow. Furthermore, LID led to increased Cl- load in baseflow, interflow, and recharge. Cl- load in runoff and maximum stream Cl- concentration decreased when LID is implemented. The model was run with retrofitting scenarios of various LID practices including pervious pavement, swales, rain gardens/cisterns, bioretention systems, green roofs, disconnecting conveyances from impervious surfaces, and all LID practices combined in BCW. Implementing LID reduced storm runoff by 0.21-27.8% and increased groundwater recharge by 0.05-6.26%. Mean stream Cl- concentrations stayed unchanged while the maximum stream Cl- concentration decreased by 0.03-4.78% after LID practices were implemented. The results also suggest that pervious pavement might be the best retrofitting option, particularly in commercial and high-density residential areas in BCW and other similar urban watersheds. Overall, our study suggests that LID practices could mitigate stream salt contamination by dilution from enhanced groundwater recharge. Our study further suggests this model could be used as a quick screening tool to determine an effective strategy for implementing LID practices in urban areas.