Asphalt parking lot runoff nutrient quality: characterization and pollutant removal by bioretention cells

The objectives of this study were to characterize asphalt parking lot (APL) runoff quality, to determine influent factors to predict nutrient concentrations and loads from eight sites in North Carolina and to assess two grassed bioretention cell efficiencies to remove APL runoff pollutants. From the eight APL sites, event mean concentrations (EMCs) and loads were measured and statistically analyzed for six nutrient forms, TN, TKN, NH4-N, NO2,3-N, TP and OPO4-P, whose average EMCs were 1.57, 1.19, 0.32, 0.36, 0.19 and 0.07 mg/L, respectively. Nitrogen EMCs were slightly lower than those from highway runoff; whereas, phosphorus EMCs were not very different. Current load prediction models, generally based on highway or roadway nutrient concentrations, are therefore expected to over-estimate nitrogen loads from asphalt parking lots. Spring and summer presented the highest EMCs and loads, respectively. Significant seasonal differences (p<0.05) were found mainly between spring and both fall and winter for concentrations and between summer and fall and winter for most loads. In an attempt to determine the factors affecting EMCs and loads, Pearson correlation tests and multiple linear regression analyses were performed. Rainfall depth, catchment area, the percentage of asphalt and natural land use were good predictors of nutrient EMCs and loads. However, the factors were not all significant simultaneously. Results indicated that nutrients accumulated before being washed-off from impervious surfaces and that pollutant build-up rate was greater than the rate of runoff production. Two grassed bioretention cells including internal storage zones (ISZs) were monitored for 12 months in central North Carolina. Fill media depths were 0.75 and 1.05 m for the north (N) and the south (S) cells, respectively and in-situ underdrain soils were clay (N) and sandy loam (S). Asphalt parking lot runoff and outflows were analyzed for TN, TKN, NH4-N, NO2,3-N, TP, OPO4-P and fecal coliforms (FC). Outflow volumes and peak flows were generally lower than those of inflow runoff. Overall, effluent nitrogen species concentrations and loads were significantly (α=0.05) lower than those of the runoff, except for the south cell EMCs and loads for NO2,3-N. Except for NO2,3-N, nitrogen species load reductions ranged from 34 to 85 %. Apart from fall and winter during which a longer hydraulic contact time seemed to be needed, the ISZs appeared to improve denitrification. TP EMC and load reductions (from 31 to 54 %) were generally not significant and OPO4-P EMCs increased (-35 (N) and -45 (S) %). However, effluent concentrations for both phosphorous species were quite low. The best nutrient EMC and load reductions occurred during spring and summer. When considering effluent EMCs in addition to removal rates, the grassed cells showed promising efficiencies for FC and nutrient pollution abatement when compared to conventionally vegetated bioretention cells (tress, shrubs and mulch) previously studied in North Carolina. Finally, it appeared that a bioretention system comprising a 0.75 m fill media depth over a clayey soil was more efficient than a 1.05 m fill media depth over a sandy loam soil.