The effects of land use characteristics on urban stormwater quality and watershed pollutant loads.

Abstract

Urban developments can result in higher runoff and nutrient loadings transported to downstream receiving waterbodies. While much effort has been made recently in watershed restoration in the U.S., a lack of recent runoff quality data limits the prediction capability of urban watershed models. The objectives of this study was to fill an existing information gap on how rainfall and land uses interact and affect such loadings. This study instrumented six coastal urban catchments, each dominated by a single land use. We measured total nitrogen (TN), total phosphorus (TP), total suspended solids (TSS), total Kjeldahl Nitrogen (TKN), ortho-P, and nitrate concentrations in runoff from 30 storm events over one year from six urban land uses, namely commercial, industrial, transportation, open space, low density residential, and high density residential. Results indicated that the median event mean concentrations (EMCs) for TSS, TP, and TN were 30 (19-34), 0.31 (0.26-0.31), and 0.94 (0.73-1.25) mg L-1, respectively. TSS EMCs from the open space and industrial land uses were significantly greater than other land uses; there were positive correlations between TN concentrations and imperviousness and between TP concentrations and turf coverage. Both the amount and intensity of rainfall positively influenced TSS concentrations in runoff regardless of land use. Using the collected data, this study developed a generic equation for predicting the loading of a pollutant as a function of rainfall depth. This equation was verified by comparing its predictions with the simulations of a sufficiently-calibrated water quality model in terms of TSS, TP, and TN loadings from various land uses in another coastal catchment for a period of ten years. Average TSS, TN, and TP loadings from the catchment were estimated to be 0.86, 0.03, and 0.01\xa0kg\xa0ha-1\xa0cm-1, respectively.