A Traditional Analysis of the First Flush Effect for Nutrients in Stormwater Runoff from Two Small Urban Catchments
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Nonpoint stormwater runoff remains a major threat to surface water quality in the USA. More effective stormwater control measures can be designed by understanding patterns in pollutant export with respect to the runoff hydrograph. In particular, nutrient concentrations in urban stormwater can cause deleterious effects in sensitive watersheds in the Southeast and Mid-Atlantic USA. A year-long study captured stormwater samples from 36 storm events at two catchments (one primarily impermeable and the other substantially wooded) and analyzed for total suspended solids and various nutrient species. Using these data, the first flush effect (the assumption that the initial portion of a rainfall-runoff event is more polluted than the later portions) was evaluated based on several published methods and definitions. Based on an analysis of multiple methodologies, the ranking of first flush strength among the pollutants was total suspended solids (TSS) > ammonia (NH3) > total Kjeldahl nitrogen > NO2-NO3 > total phosphorus > orthophosphate (O-PO4). Nitrogen species generally displayed a stronger first flush than phosphorus species, with O-PO4 showing the weakest first flush effect. Various relationships ° climate, land use, and the first flush strength were also explored. Of the rainfall characteristics analyzed, total rainfall and runoff volume each inversely affected the first flush strength of TSS on the more impervious catchment. Although orthophosphate did not have a strong first flush effect, the relative first flush strength for O-PO4 increased with increasing rainfall or runoff. Land use did not influence the first flush strength of the pollutants. On average, most pollutants exhibited a slight first flush effect, but substantial pollutant loading still occurred in the latter portion of the storm’s total runoff volume. Thus, treating the majority of a storm’s total pollutant load requires capturing a commensurate fraction of runoff volume.
KeywordsStormwater Runoff First flush Watershed Catchment TSS Nutrients Nitrogen Phosphorus
The authors would like to thank the North Carolina State University for funding this project. The authors acknowledge the contributions of Dan Line and James Blackwell toward assistance with monitoring design, monitoring equipment installation, and sample collection. The authors thank to Dr. Aziz Amoozegar for project review, Jenny James and the NCSU Center for Applied Aquatic Ecology for sample analysis and bottle washing, and Dr. Wayne Robarge and the NCSU Soil Science Analytical Services Laboratory for the flexible metal analysis protocol. Lastly, the authors recognize the North Carolina State University Facilities Division for design and construction of the stormwater outfalls analyzed herein.
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