Developing Strategies for Reducing Inputs of Microbial Contaminants to Receiving Waters

This project focuses specifically on the development of a restoration plan for Northeast Creek, a 303(d)-listed stream in the Jordan Lake watershed that has been identified as "high priority" for TMDL development due to high fecal coliform counts arising from urban/stormwater runoff. This is not an isolated problem, as elevated fecal indicator concentrations (used to "indicate" fecal contamination and/or the presence of pathogenic organisms) are the single largest cause of water quality impairment in North Carolina. Somewhat ironically, however, is the fact that there is almost no information on the effectiveness of detention basins, the most common type of Best Management Practice (BMP), in reducing microbial loading to receiving waters. The primary reason for this knowledge gap is the poor understanding of microbial attachment behavior. Microbes in aquatic environments can be classified in one of three forms (Fig. 1): (i). unattached or "free"; (ii). attached to lighter particles (e.g., algae, leafy matter), or; (iii). attached to denser particles (e.g. sand, clays). While the low density of the first two classifications means that these microbes will not be effectively removed by settling (Fig. 2), organisms in the third class often will. As detention basins are the dominant means of stormwater treatment in the Northeast Creek watershed (Fig. 3), identifying the fraction of indicator organisms attached to "settleable" particles, and where/when that attachment occurs will be critical for the development of a watershed restoration plan. In addition, since most BMPs involve some degree of sedimentation, estimates of microbial settling characteristics will also be useful in other areas of the state. Similarly, estimates of microbial attachment will be very useful in accurately modeling microbial fate and transport in receiving waters, an important consideration as the results of these models often act as a basis for TMDL development.

Initial research on North Carolina receiving waters shows that 40-50% of fecal indicator organisms are attached to settleable particles (Fig. 4), but where this attachment occurs is also important (Fig. 5). If indicators are associating with particles in runoff from upland sources (e.g., lawns, parking lots), then detention basins intercepting this flow prior to its entering a receiving water will be successful in reducing microbial loads. However, if microbes in upland runoff are in the free phase, and attach to particles only after they enter a receiving water, different management approaches will be required. In response to these questions, the proposed project will evaluate microbial loads at several points in the transport chain (e.g., upland sources, sewer outfall) during dry- and wet-weather conditions. A range of six indicator organisms will be considered, including fecal coliforms, E. coli, enterococci, C. perfringens spores, somatic and male-specific coliphage. Microbial loadings at each point will also be characterized with respect to the fractions of microbes attached to settleable particles, with measurements taken throughout individual storms (Fig. 6). Flows from different land use types will be monitored, and the performance of detention basins will be compared with that of other BMPs (e.g., wetlands) to evaluate their relative effectiveness. Lastly, the sources and attachment behavior of indicator organisms will be compared with that of several key pathogens (Cryptosporidium, Salmonella). These comparisons will provide an understanding of the circumstances under which "indicator" organisms truly provide an accurate indication of threats to public health. The results from this project will not only allow for the development of an improved restoration plan for Northeast Creek, but will also be applicable to impaired waters across the state of North Carolina, as such, both the city of Durham and the Urban Water Consortium’s Stormwater Group will be providing funding and/or support to this project. This project will lead to measurable improvements in the water quality of Northeast Creek by providing a blueprint for locating BMPs throughout the watershed that will maximize the cost effectiveness of reducing microbial loadings.

Figure 1 Microbial partitioning between particulate matter and the "free" phase

Figure 2 Detention basin effectiveness in reducing microbial loading to receiving waters

Figure Not Available
Figure 3 Northeast Creek and current BMP locations ( = wet detention basins)

Figure 4 Fraction of indicator organisms associated with denser "settleable" particles

Figure 5 Microbial transport chain: A.) upland sources, B.) intermediate interception points, C.) receiving waters and D.) sediment

Figure 6 Concentration of fecal coliforms attached to denser particles ("settled") vs. those in suspension over the course of a storm (cms = m3/s)