Hydrologic and water quality performance of permeable pavement with internal water storage over a clay soil in Durham, North Carolina.

Permeable pavement is an effective tool for improving stormwater hydrology and water quality when sited over soils with high infiltration rates, but its efficacy over less permeable soils is uncertain. This study examined permeable pavement performance when built over a low-conductivity, clay soil. Four parking stalls (50 m2 total area) were retrofitted with permeable interlocking concrete pavement (PICP) to treat 15.2 m2 of contributing impervious area (0.3:1 run-on ratio). Using an elevated underdrain, the site incorporated a 150-mm internal water storage (IWS) zone to increase exfiltration and promote anaerobic conditions for denitrification. From March 2014-April 2015, 22% of influent runoff volume was reduced via exfiltration and evaporation. Inter-event drawdown of the IWS zone created storage to capture and exfiltrate more than 70% of the runoff volume from precipitation events less than 8 mm, and peak flows were significantly reduced (median 84%). Relative to stormwater runoff from a nearby impermeable asphalt reference watershed, the permeable pavement produced significantly lower event mean concentrations (EMCs) of all pollutants except nitrate, which was significantly higher. Permeable pavement effluent and reference watershed runoff were 99%, 68%, and 96% different for total suspended solids (TSS), total nitrogen (TN), and total phosphorus (TP), respectively. Significantly lower permeable pavement effluent EMCs for copper (Cu, 79%), lead (Pb, 92%) and zinc (Zn, 88%) were also observed. The median effluent concentrations of TN (0.52 mg/L), TP (0.02 mg/L), and TSS (7 mg/L) were all very low relative to the literature. Sampling of nitrogen species in the IWS zone 12, 36, 60, and 84 h post-rainfall was done to better understand mechanisms of nitrogen removal in permeable pavement; results indicated denitrification may be occurring in the IWS zone. Effluent pollutant load from the permeable pavement was at minimum 85% less than from nearby untreated asphalt runoff for TP, TSS, Cu, Pb, and Zn, and was 73% less for TN. Permeable pavements built over low-permeability soils with internal water storage can considerably improve long-term hydrology and water quality.