Stormwater Monitoring of Innovative Street Retrofits in Urban Wilmington, NC

Abstract

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 a watershed by promoting infiltration of runoff into the soil, treating runoff at its source, and minimizing impervious surface coverage. Peer-reviewed literature is lacking on the application of bioretention, permeable pavement and tree filter boxes to treat urban street runoff on a catchment-scale. A pre- and post-retrofit study has been designed to evaluate the impacts of catchment-scale retrofits on hydrology and water quality. The project will be implemented on two midblock street sections in a residential neighborhood in Wilmington, North Carolina. It includes construction of permeable pavement parking stalls, tree filter boxes and bioretention areas that extend into the street right-of-way, thereby reducing impervious surface area and increasing pedestrian safety. Runoff volumes and pollutant concentrations of bacteria, heavy metals, nutrients, polycyclic aromatic hydrocarbons (PAHs) and suspended solids were monitored from June 2011 to October 2011 at the retrofit site and an additional control site located four blocks to the West. Preliminary results from the pre-retrofit monitoring show mean concentrations of total Kjeldahl nitrogen (TKN) from the control and retrofit watersheds were 2.02 mg/L and 1.52 mg/L, respectively. Total phosphorus (TP) mean concentrations from the control and retrofit watersheds were similar at 0.31 mg/L and 0.29 mg/L, respectively. TKN and TP concentrations observed in this study at both monitoring sites were less than TKN and TP mean concentrations from National Urban Runoff Program (NURP) urban sites and NURP streets. Copper (Cu), lead (Pb) and zinc (Zn) mean concentrations from the control and retrofit watersheds were less than those reported from NURP urban sites and NURP streets. Total nitrogen (TN) and TP yearly loads were greater in the control watershed, which may be due to the greater fraction of impervious surface coverage and fertilizer use in the control watershed. The results and conclusions from this study will help refine street retrofit design standards to meet runoff volume reduction, peak discharge mitigation and water quality goals while increasing pedestrian safety and improving aesthetics in the neighborhood.