A Multimodal Approach to Identifying Stormwater Pollutant Hotspots in Coastal Urban Watersheds

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Kirker, Ashleigh N
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Stormwater runoff is a primary concern for water quality in the Southeastern US and beyond. Surface water near the Charleston Harbor in SC is impaired by their low dissolved oxygen concentrations, and by urban contaminants flushed off of impervious surfaces of the nearby Charleston peninsula. The goal of this study is to create a methodology to determine the areas of highest concern for contaminating stormwater in Charleston. It was expected that landuse, antecedent rainfall, and traffic flow would be related to variations in contaminant concentrations, and allow us to pinpoint hotspots. We sampled stormwater over a sixteen month period to locate these contaminant hotspots. Enterococci, a fecal indicator bacteria, was measured with a nutrient indicator system. Common urban trace metals were determined with ICP-MS, and nutrients were quantified and tracked. Enterococci consistently exceeded USEPA guidelines by two orders of magnitude. The most important relationship involving enterococci in stormwater was not a spatial one, but involved antecedent rainfall; there is significantly more enterococci in stormwater when the three days before sampling had been relatively dry. Most stormwater samples contained some of the nine trace metals, which were related to one another and to antecedent rainfall and traffic flow. Like enterococci, antecedent rainfall appears to relate to a decrease in metal concentrations, possibly because of a 'flushing out' effect. Cu, Ni, Pb, V, and Zn occur in high concentrations in areas of heavy traffic. A relationship was noted between residential landuse and nutrient concentration in runoff. Hotspots for stormwater contamination in Charleston are heavily trafficked roads (for Cu, Ni, Pb, V, and Zn) and dense residential areas for nutrients. Reduced contaminant concentrations are observed when there is significant antecedent rainfall. Sampling urban stormwater is an effective means of finding hotspots of primary concern for surface water quality. From a management perspective, this means that targeted practices can reduce contamination in runoff. Furthermore, this method allows the sources and hotspots of nominally nonpoint source pollutants to be determined.
stormwater, contamination, coastal watershed