Characterization of nonpoint source microbial contamination in an urbanizing watershed serving as a municipal water supply

Inland watersheds in the southeastern United States are transitioning from agricultural and forested land uses to urban and exurban uses at a rate greater than the national average. This study sampled creeks representing a variety of land use factors in a rapidly urbanizing watershed that also serves as a drinking water supply. Samples were collected bimonthly under dry-weather conditions and four times during each of three storm events and assessed for microbial indicators of water quality. Concentrations of fecal indicator bacteria (FIB) including fecal coliforms and Escherichia coli were measured using standard membrane filtration techniques. Results showed that FIB concentrations varied between 100 and 104 colony forming units (CFU) per 100 mL. An analysis of variance (ANOVA) showed that FIB were generally higher in more developed watersheds (p < 0.01). Concentrations were also significantly greater during storm events than during dry-weather conditions (p < 0.02), although concentrations demonstrated both intra and inter-storm variability. These results indicate that the magnitude of microbial contamination is influenced by intensity of watershed development, streamflow and antecedent precipitation. Dry-weather FIB loads showed considerable seasonal variation, but the average storm event delivered contaminant loads equivalent to months of dry-weather loading. Analysis of intra-storm loading patterns provided little evidence to support “first-flush” loading of either FIB, results that are consistent with environmental reservoirs of FIB. These findings demonstrate that single sampling monitoring efforts are inadequate to capture the variability of microbial contaminants in a watershed, particularly if sampling is conducted during dry weather. This study also helps to identify timing and conditions for public health vulnerabilities, and for effective management interventions.

► Fecal indicator bacteria (FIB) were measured from creeks with various land uses.
► More developed watersheds were associated with higher concentrations of FIB.
► Seasonal dry-weather loading patterns were dwarfed by storm event loading.
► Intra-storm loading patterns of FIB were not consistent with a “first-flush”.
► Single-sample monitoring efforts may inadequately describe stormwater impacts.