Review of Existing Stormwater Monitoring Programs for Estimating Bight-wide Mass Emissions from Urban Runoff

Urban runoff is perceived as a large source of pollutant inputs to the ocean, but no mass emission monitoring programs have been established to assess this discharge. Recently, however, National Pollutant Discharge Elimination System permits for urban runoff discharges were issued to stormwater management agencies on a regional (county-wide) basis and the 1994-95 water year represents the first period in which urban runoff water quality measurements have been monitored bightwide. The goal of this study was to use the data generated by these monitoring programs to estimate mass emissions of pollutants to the Southern California Bight from urban runoff. After documenting the sampling design of each stormwater monitoring program, then collating information on rainfall, watershed area, runoff volume, and water quality measurements, we estimated the mass emissions of total suspended solids, nutrients (ammonia, nitrate, phosphorous), and five trace metals (chromium, copper, nickel, lead, zinc). Although the mass emissions of these constituents appeared substantial relative to other sources, there was tremendous uncertainty in the load estimates. Less than 5% of the watershed areas and less than 2% of the annual runoff volumes were actually monitored during 1994-95 water year. Extrapolation of water quality data to these unmonitored channels and flows, which is necessary to develop bight-wide emission estimates, are hampered by the tremendous variability in contaminant concentrations among the different watersheds and storm events. This variability in water quality measurements from urban runoff are not well understood. This lack of understanding adds to our uncertainty in the load estimates we provide.