Catchment Scale Hydrologic and Water Quality Impacts of Residential Stormwater Street Retrofits in Wilmington, North Carolina

Low impact development (LID) is a design approach that utilizes stormwater control measures (SCMs) to maintain and restore the natural hydrologic regime of an urban watershed through infiltration, runoff treatment at the source, and minimization of impervious surfaces. This paired watershed study evaluated the impacts of LID SCMs on hydrology and water quality at a catchment scale. In February 2012, a pair of bioretention cell (BRC) bumpouts, two permeable pavement parking stalls, and a tree filter device were installed to treat residential street runoff in Wilmington, North Carolina. In the LID-retrofit catchment, SCMs treated 52% of the directly connected impervious area (DCIA) and 69% of the total drainage area for potential mitigation of peak discharge and runoff volume. For water quality improvement, 94% of the DCIA and 91% of the total drainage area was retrofitted. Underlying soils in the study area were Baymeade Urban and Leon Urban sands. Mean peak discharge significantly decreased 28% post-retrofit and lag times in the catchment remained unchanged, while mean runoff depth significantly decreased 52%. When compared with the control catchment, runoff depths in the LID-retrofit catchment were significantly less for storms with low hourly storm intensities (<2.7 mm/hr), but significantly greater for storms with high intensities (>7.4 mm/hr). Runoff thresholds in the LID-retrofit and control catchments were 5.2 mm and 3.5 mm, respectively. The LID runoff coefficient significantly decreased by 47% from 0.22 to 0.13 and is substantially less than other runoff coefficients reported for traditional residential development. Post-retrofit concentrations of TKN, TP, TSS, Cu, Pb, and Zn significantly decreased by 62%, 38%, 82%, 55%, 89%, and 76%, respectively. Concentrations of NO2,3-N and TAN did not change. Mass exports of TKN, TAN, O-PO4-3, TP, TSS, Cu, Pb, and Zn significantly decreased by 78%, 61%, 55%, 73%, 91%, 53%, 88%, and 77%, respectively. NO2,3-N load decreased by 46%, although this was not significant. Most improvements in water quality were due to dramatic decreases of particulate and particulate-bound pollutant loads. This was attributed to first flush retention of runoff by the BRC and permeable pavement that treated 52% of the DCIA and treatment by the tree filter unit that serviced 42% of the DCIA. This study has shown that a limited number of LID SCMs installed within a medium density residential street right-of-way over sandy soils can mitigate some hydrologic and water quality impacts of existing development.