Contrasts in concentrations and loads of conventional and alternative indicators of fecal contamination in coastal stormwater

Abstract

Fecal contamination in stormwater is often complex. Because conventional fecal indicator bacteria (FIB) cannot be used to ascertain source of fecal contamination, alternative indicators are being explored to partition these sources. As they are assessed for future use, it is critical to compare alternative indicators to conventional FIB under a range of stormwater delivery conditions. In this study, conventional FIB and fecal Bacteroides spp. were monitored throughout the duration of five storm events from coastal stormwater outfalls in Dare County, North Carolina, USA to characterize relationships among FIB concentrations, alternative fecal markers, and loading of contaminants. Water samples were collected multiple times during each storm and analyzed for Enterococcus sp. and Escherichia coli using enzymatic tests and fecal Bacteroides spp. by QPCR. Both conventional FIB and fecal Bacteroides spp. concentrations in stormwater were generally high and extremely variable over the course of the storm events. Over the very short distances between sites, we observed statistically significant spatial and temporal variability, indicating that stormwater monitoring based on single grab-samples is inappropriate. Loading of FIB and fecal Bacteroides spp. appeared to be affected differently by various hydrologic factors. Specifically, Spearman correlations between fecal Bacteroides spp. and drainage area and antecedent rainfall were lower than those between conventional FIB and these hydrologic factors. Furthermore, the patterns of fecal Bacteroides spp. concentrations generally increased over the duration of the storms, whereas E. coli and Enterococcus sp. concentrations generally followed the patterns of the hydrograph, peaking early and tailing off. Given the greater source-specificity and limited persistence of fecal Bacteroides spp. in oxygenated environments, differences in these patterns suggest multiple delivery modes of fecal contamination (i.e. landscape scouring versus groundwater discharge).