A nutrient characterization of dry weather urban runoff was performed in four representative areas of the Newport Bay watershed in Orange County, California in the summer/fall of 2004. The study was performed to determine the contribution of dry weather urban runoff to nutrient loading
to the Bay, and to identify the extent to which specific urban activities contribute to urban nutrient loading. Characterization involved continuous flow measurement, intermittent composite water quality sampling of collective discharge from the study areas, and collection of 160 "curbside"
samples, where runoff from individual urban activities entered the public rightof-way. Flow and composite sample data was used to calculate areal nutrient mass loading rates for dry weather flows for each of the study areas. Findings in two study areas were found to be significantly influenced
by groundwater infiltration and groundwater remediation discharge. Available data from these additional sources was used to quantify their contribution and to adjust observed flow and composite sample data to represent the urban runoff component. Mean areal urban runoff flow rates for the
four study areas ranged between 8 – 85 gpd/acre. Mean annual areal loading rates for the study areas ranged between 0.029 - 0.415 lb/acre-yr for total inorganic nitrogen (TIN); 0.242 – 1.769 lb/acre-yr total nitrogen (TN), and 0.019 – 0.232 lb/acre-yr for total phosphorus
(TP). No meaningful nutrient loading differences could be discerned between residential and business urban areas. Watershed-specific loadings observed were substantially lower than the literature loading rates used in the development of the Newport Bay Nutrient TMDL baseline allocation for
urban areas. Irrigation overspray/overwatering was the most frequent runoff-generating urban activity observed, constituting over 70% of curbside events and 49% of runoff volume observed. Car washing and residential washdowns were second tier activities in both number and volume. Review of
runoff nutrient concentrations by activity did not identify a single category requiring priority management attention; field observations suggested there were a small number of highnutrient incidents within each runoff-generating category.
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