Microbial Contaminants Associated With Urbanization of a Drinking Water Reservoir

Nonpoint source pollution is a pervasive and ubiquitous water quality issue in the United States. In North Carolina, The Research Triangle is experiencing rapid growth leading to increased nonpoint source pollution and water body impairment. Impacted waters include Jordan Lake, which is located within an urbanizing watershed and serves as a source of drinking water. Regulators and community groups have expressed a commitment to protecting and restoring these waters. However, remediation plans are controversial due to potential costs, uncertainty of achievable results and the allocation of load reduction between users and non-users of the drinking water supply. Emphasis to date has centered on nutrient loading of the reservoir, although pathogen and sediment loading are of increasing concern. The magnitude, mechanisms and timing of contaminants entering the lakes are not well understood, but are critical determinants in designing an efficient program for water quality improvement. Sound scientific support is needed to better inform management plans for source water protection, and for stormwater and land use policies that affect the watershed. This study evaluated loading of microbial contaminants into Jordan Lake as a function of rainfall and landuse. The primary objectives were to (1) analyze the magnitude, timing and distribution of microbial contaminants at sites upstream of Jordan Lake; (2) relate loading and sample concentration of microbial contaminants to specific land use and impervious surfaces; and (3) analyze samples for pathogens and human-specific microbial source tracking markers to more directly assess human health risks and to identify sources of pollution. Samples were collected bimonthly under dry-weather conditions and over the course of three storm events. Each sample was analyzed for multiple parameters, including concentrations of fecal indicator bacteria (FIB) and for markers of human-source fecal contamination. Concentrations of FIB, including fecal coliforms and Escherichia coli, were generally higher in more developed watersheds. Fecal indicator bacteria were also significantly greater during storm events than during dry-weather conditions. Dry-weather loads of fecal indicator bacteria showed considerable seasonal variation. However, the average storm event delivered contaminant loads equivalent to months of dry-weather loading. This pronounced rainfall effect dwarfed the measured landuse effect. Analysis of intra-storm loading patterns provided little evidence to support “first-flush” loading of FIB. This loading pattern suggests that traditional best management practices (BMPs) may be ineffective for protecting reservoirs against microbial contaminants during storms. Human-specific markers tested in this study proved useful for identifying areas for targeted restoration. Three study sites were consistently, although not always, positive for Methanobrevibacter smithii, a marker of human sewage. The results of this study help elucidate mechanisms by which contaminants are mobilized and transported, and introduce novel tools for the protection and management of drinking water sources. This study also helps to identify timing and conditions for public health vulnerabilities, and for effective management interventions. A better understanding of the influence of urban development on microbial contamination of NC surface waters will contribute to the knowledge needed to bring these waters into compliance with state standards. Furthermore, the knowledge gained will be applicable to watersheds nationwide.