A MULTIMODAL APPROACH TO IDENTIFYING STORMWATER POLLUTANT HOTSPOTS IN COASTAL URBAN WATERSHEDS

Nonpoint source pollution is the main source of surface water impairment in the coastal US and in Charleston, SC, where the effects of rapid urbanization are compounded by the problems of coastal flooding. The goal of this project is to create a method that helps identify stormwater pollutant hotspots within highly urbanized watersheds. This approach uses stormwater quality data collected as a function of population density (land use) and duration of rain events (antecedent moisture).

Sub-watersheds were delineated using geographic information systems (GIS); a 1-m DEM and flow-direction raster dataset was used to reshape and simplify a 1980s Charleston city basin map. Sheet flow samples were collected from five sub-basins according to standard runoff collection procedures during 9 rain events from September 2016 to July 2017. Collection times were categorized as either first flush (1+ days of no precipitation preceding the event) or mid-storm. Nutrient ions (NO₃^-, NO₂^-, SO₄^- and PO₄^-), trace metals, fecal indicator bacteria (enterococcus), and bulk parameters such as pH and TDS were measured. Multivariate analysis was used to identify relationships between analytes and parameters.

Stormwater data indicated presence of (i) enterococcus bacteria greatly exceeding USEPA standards, (ii) trace metals exceeding the USEPA’s freshwater toxicity index levels, and (iii) excess nutrient anions. Concentrations were variable between sub-drainage basins, and there was a statistical correlation between population density and concentration of Zn, Pb, and Ni. Cu was significantly more concentrated in first flush runoff, while As and Cr were mobilized mid-storm. Two sub-basins were identified as hotspots with consistently high enterococcus and nutrient concentrations.

It appears that even in a small-to-moderate-sized city, stormwater contamination is predominant and varies widely. Results from this study will help establish effective stormwater management practices that target hotspots within the Charleston peninsula as well as provide an effective template to identify contaminant hotspots that cause surface water impairments in other coastal communities.