Identifying the Origins and Attachment Behavior of Non-point Source Microbial Contaminants

Elevated microbial concentrations are the single largest cause of water impairment in North Carolina, however, many questions regarding the origins and behavior of microbial contaminants remain unanswered. While rainstorms generally result in substantial increases in the concentrations of indicator organisms (e.g., fecal coliforms) in receiving waters, there is considerable uncertainty regarding the source of these contaminants. In most cases, the common assumption is that human and/or animal sources dominate, and that microbes are transported directly over the land surface to receiving waters via stormwater runoff. But, there is evidence to suggest that some portion of the microbes measured in receiving waters originate from receiving water sediments and are mobilized by the more rapid and turbulent flows that occur during storms. The fraction of total organisms originating from these two sources will have important implications for the design of water quality improvement strategies. Microbial loads transported directly from upland sources (e.g., parking lots, lawns) to receiving waters via runoff can be intercepted and treated by a variety of Best Management Practices (BMPs), while loads originating in receiving water sediments may involve some very different management strategies.

Another critical consideration when managing microbial contaminants, regardless of the source, is microbial partitioning between the particulate (attached) and “free” (unattached) phases. Microbes attached to larger, heavier particles generally settle out quickly, while the lighter free phase organisms remain more mobile in the environment and are often transported much greater distances downstream. As a result, partitioning has important implications for both BMP and TMDL development, yet there are few quantitative estimates of microbial attachment behavior. The absence of such information makes it difficult to evaluate the effectiveness of microbial-related BMPs (e.g., detention basins), while also making it impossible to develop models that provide reliable estimates of microbial fate and transport in receiving waters (models that often form the basis of TMDLs). Research underway at UNC (published in Water Research), indicates that 40-55% of indicator organisms in Triangle area receiving waters are attached to “settleable” particles during a storm. This is an important and relevant finding, but additional information regarding when and where attachment takes place will be necessary before conclusive statements regarding BMP and TMDL development can be made. If, for instance, a large fraction of the microbes in runoff from upland sources is attached to particles, then detention basins will be an effective means of reducing microbial loads to receiving waters. If, on the other hand, microbes in stormwater are primarily in the free phase, and only attach to particles (either in the water column or in the sediments) after they have entered a receiving water, then other mitigation approaches will need to be considered.

This proposal seeks to provide insight into these issues by undertaking the following tasks:

• Evaluate the concentrations and partitioning behavior of indicator organisms (Fecal coliforms, E. Coli, Entercocci) at three separate points in the transport chain for two separate land use types, including
  • upland source (e.g., parking lot, gutter);
  • the terminal point prior to receiving water (e.g., storm sewer outfall, detention basin);
  • in the receiving water;
  • in receiving water sediments.
• Perform the above analyses under both dry weather and storm conditions;
• Track the concentration and partitioning behavior of indicator organisms throughout the progression of individual storms;
• Characterize water quality parameters (e.g., organic carbon, nutrients) related to the survival and/or regrowth of indicator organisms in receiving waters, BMPs and storm sewers.

Results will provide an assessment of several important issues with direct relevance to the management and mitigation of microbial loading to impaired waters, including

• Identification of the relative fraction of indicator organisms attributable to upstream sources (e.g., lawns, parking lots) vs. those mobilized during transport to receiving waters (e.g., storm sewer) vs. those mobilized from receiving water sediments;
• Assessment of the fraction of attached vs. unattached organisms at each point in the chain;
• Evaluation of BMP effectiveness, primarily detention basins;
• Improved understanding of the transport and fate of indicator organisms once they enter a receiving water.

The level of funding available from the Urban Water Consortium’s Stormwater Group will allow us to address the issues above, but will also provide for leveraging opportunities involving the State’s 319 program which is capable of funding the exploration of a number of related (but much more expensive) questions, including

• Studies comparing the presence and partitioning behavior of indicator organisms with actual pathogenic organisms (e.g., salmonella, cryptosporidium);
• Analysis of the microbial removal effectiveness of additional BMPs (e.g., buffer strips);
• Bacterial source tracking using molecular techniques capable of distinguishing the fraction of organisms, both indicator and pathogen, attributable to human, animal and other sources;
• Studies to evaluate the survival and regrowth of indicator organisms and pathogens under conditions representative of receiving water sediments, storm sewers and some BMPs.