Catchment-Scale Evaluation of the Hydrologic and Water Quality Impacts of Residential Street Retrofits in Wilmington, NC

Many urban watersheds suffer from degraded water quality caused by stormwater runoff from rooftops, parking lots, streets, and other impervious surfaces. Low Impact Development (LID) is a design approach that utilizes stormwater control measures (SCMs) to maintain and restore the natural hydrologic features of an urban watershed through infiltration, runoff treatment at the source, and minimization of impervious surfaces. Limited peer-reviewed literature is available on impacts of multiple LID SCMs at a catchment or watershed scale. A paired watershed study with calibration and treatment monitoring periods has been designed to evaluate the hydrologic and water quality impacts of residential street SCMs at a catchment scale in Wilmington, North Carolina. Calibration monitoring of the control (0.35 ha) and retrofit (0.53 ha) catchments was completed from May 2011 to October 2011 (nine water quality samples, 14 rainfall events). In February 2012 bioretention bumpouts, permeable pavement parking stalls, and a tree filter device were installed in the retrofit catchment. Treatment monitoring commenced in June 2012 and will continue through February 2013 (10 water quality samples, 15 rainfall events through November 2012). Water quality, peak discharge, and flow volume are being recorded at the catchment outlets (existing catch basins). Water quality samples will be analyzed for TSS, TKN, NH₄-N, NO_2-3-N, TP, Ortho-P, Cu, Pb, and Zn. Preliminary results indicate a 14% reduction in mean runoff volume. TSS, TN, and TP mass loads at the retrofit outlet decreased by 83%, 49%, and 63%, respectively, during treatment monitoring. Preliminary water quality and runoff volume results are promising, and results from this project will help refine street retrofit design standards to meet runoff volume reduction and water quality goals.