DRY AND WET WEATHER FLOW NUTRIENT LOADS FROM A LOS ANGELES WATERSHED

Historical data, results from field sampling, and modeling were used to characterize the long-term dry- and wet-weather flow discharges from the Ballona Creek Watershed in southern California for the 1991 96 water years. The contaminants of interest were total suspended solids, BOD, nitrate nitrogen, nitrite N, ammonia N, total Kjeldahl N, and total phosphorus. The dry- and wet-weather flow loads were compared on an annual water-year basis, a seasonal basis, and a catchment basis. Stormwater runoff models for the watershed were developed using the EPA Storm Water Management Model and used, along with an event mean concentration procedure, to simulate annual and seasonal runoff quality. Results indicated that wet-weather flow contributed at least 67% of the average annual pollutant load for each contaminant considered, but dry-weather flow from Ballona Creek Watershed was as important a source of N as wet-weather flow from other catchments. Depending on the parameter studied, the wet- to dry-weather flow load ratio ranged 2 54.