Monitoring and modeling of pollutant mass in urban runoff: Washoff, buildup and litter

Stormwater pollution has displaced wastewater pollution in many cases as the major polluter of public waters. Paved areas such as highways and streets in urban areas are “stormwater intensive” land uses since they are highly impervious, and have high pollutant mass emissions from vehicular activity. To help manage this source of water pollution, a multiyear monitoring program was instituted at eight Southern California highway sites over two years. From the monitoring data, washoff and buildup models were developed.; The new model is semi-empirical and uses four fitting parameters. It is capable of fitting first flush events. The model was fit to more than 40 events for 8 pollutants, and the parameters were correlated to runoff conditions, such as total runoff, antecedent dry days, runoff coefficient and average runoff velocity. The model can be used in selecting best management practices. The model can also be used for estimating event mean concentrations for events with sparse data. First flush was noted in most storm events and an improved definition of first flush is presented. Using the criteria of “high” first flush and “medium” first flush, more than 30% of the storms showed high first flush and more than 80% showed a medium or high first flush.; The second goal of the research is to determine the pollutant mass accumulation during dry periods. Mass accumulation rates were determined for total suspended solids, chemical oxygen demand, oil and grease, total Kjeldahl nitrogen and total phosphorus, and are reported in g/m2-day. A new two parameter model was developed, which can be used to predict pollutant buildup during dry days between storms.; Litter was also measured in the monitoring program. Litter production rates and event mean concentrations are reported. Vegetation composed approximately 90% of the gross pollutants (>0.5 cm) from highways. Litter production is highly variable and few significant correlations were noted. A decreasing trend in event mean concentration was noted with total runoff volume or total rainfall. An increasing trend in event mean concentration was noted with antecedent dry days.