Evaluation of a filtration/dispersion method for enumeration of particle-associated Escherichia coli.

The transport processes of microorganisms in storm-generated flows have not been clearly elucidated, limiting the ability of computational models to effectively design and evaluate watershed remediation plans. Although several studies have identified association with particulates as a critical factor in predicting microbial transport and fate, no generally accepted method exists for the identification of the planktonic and particle-associated fractions of microorganisms in water samples. In this study, a filtration/dispersion method proposed for quantification of these fractions was verified using laboratory-composed samples of bovine Escherichia coli and sterile topsoil. A statistical experimental design allowed quantification of potential sources of experimental error (e.g., filter retention, die-off), although no significant sources of methodological error were identified. On average, 78% of E. coli cells were particle associated after 1 h of contact time. Further application of the method was illustrated by an isotherm experiment examining the association of a bovine strain of E. coli with sterile topsoil. Further examination of this method in laboratory or field-based studies of microbial partitioning between the planktonic and particulate phases in surface runoff appears justified.

Development of bacteria and benthic total maximum daily loads: a case study, Linville Creek, Virginia.

Two total maximum daily load (TMDL) studies were performed for Linville Creek in Rockingham County, Virginia, to address bacterial and benthic impairments. The TMDL program is an integrated watershed management approach required by the Clean Water Act. This paper describes the procedures used by the Center for TMDL and Watershed Studies at Virginia Tech to develop the Linville Creek TMDLs and discusses the key lessons learned from and the ramifications of the procedures used in these and other similar TMDL studies. The bacterial impairment TMDL was developed using the Hydrological Simulation Program-Fortran (HSPF). Fecal coliform loads were estimated through an intensive source characterization process. The benthic impairment TMDL was developed using the Generalized Watershed Loading Function (GWLF) model and the reference watershed approach. The bacterial TMDL allocation scenario requires a 100% reduction in cattle manure direct-deposits to the stream, a 96% reduction in nonpoint-source loadings to the land surface, and a 95% reduction in wildlife direct-deposits to the stream. Sediment was identified as the primary benthic stressor. The TMDL allocation scenario for the benthic impairment requires an overall reduction of 12.3% of the existing sediment loads. Despite the many drawbacks associated with using watershed-scale models like HSPF and GWLF to develop TMDLs, the detailed watershed and pollutant-source characterization required to use these and similar models creates information that stakeholders need to select appropriate corrective measures to address the cause of the water quality impairment when implementing the TMDL.