Indicator Bacteria Removal by Stormwater BMPs in Coastal North Carolina

Stormwater runoff is a transport mechanism for indicator bacteria to receiving waters, resulting in an increased risk to public health through consumption of contaminated shellfish or ingestion by swimmers. The resulting economic and public safety concerns are common throughout the United States, particularly in coastal areas. Urban stormwater is commonly treated by stormwater Best Management Practices (BMPs), each of which provides some combination of natural treatment mechanisms and fosters certain environmental conditions. Although BMPs have been studied in detail for many pollutants, there is still a relatively limited understanding of their ability to remove or inactivate indicator bacteria. Geographical differences may also result in varied performance for indicator bacteria removal, as particle association is an important component of indicator bacteria transport and may vary based on soil type. The North Carolina State University Biological and Agricultural Engineering Department monitored 6 stormwater BMPs in Wilmington, NC, to evaluate their efficiency with respect to indicator bacteria removal. The study locations included two bioretention cells, two stormwater wetlands, and two wet ponds. Results indicated that the bioretention cell with a 1-foot media depth did not perform well in comparison to the 2-foot media depth cell. Increased soil water flux in the shallow cell was believed to contribute to these results. Data collected from these studies also indicates that positive removal of indicator bacteria is possible in many types of BMPs; however, removal can be highly variable from BMP to BMP and from storm to storm. Further, stormwater BMPs may promote environments where indicator bacteria can persist, leading to export during some storm events. Finally, even if positive reductions in indicator bacteria are noted, research indicates that achieving effluent concentrations of indicator bacteria consistent with surface water standards may not be possible in many cases.