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A Tiered Approach to Countywide Bacteria Monitoring and Assessment in Nashville, Tennessee

William P. Hamilton 1, Edward L. Thackston 1, Scott Woodard 2, and Butch Bryant 3

1 Vanderbilt University, Nashville, Tennessee,
2 Consoer Townsend Envirodyne Engineers, Inc., Nashville, Tennessee,
3 Metropolitan Nashville Department of Water and Sewerage Services, Nashville, Tennessee

Since 2001, a countywide stream monitoring and assessment program for pathogens has been conducted as part of the Metropolitan Nashville Combined Sewer Overflow Abatement Program. The program, which represents a partnership between the Metropolitan Nashville Department of Sewerage Services, academia (Vanderbilt University), and the private sector (Consoer Townsend Envirodyne Engineers), provides a quarterly depiction of the background bacterial quality at approximately 65 sites on 36 streams in Davidson County, Tennessee, and helps identify segments where more intensive investigation is warranted.

Based on 303(d) listings and known potential impacts, sampling sites were identified and are monitored for Escherichia coli and Enterococcus spp. during dry weather (operationally defined as at least 5 days post rain). Sites with either elevated E. coli densities (50% of the state single sample maximum E. coli density) or an elevated E. coli: Enterococcus ratio (EC/Ent > 2.0) are resampled. In addition to E. coli and Enterococcus, sites are resampled for fecal coliform and fecal streptococcus to provide information on probable source. Resampled E. coli isolates are subjected to Antibiotic Resistance Analysis (ARA) using a regionally specific database (3000+ isolates, ARCC = 80%).

Sites with chronic problems undergo more intensive and extensive sampling. This tiered approach efficiently identifies problem sites and rationally concentrates efforts, leading to the identification of several pollutant sources. Results were used by state regulators to remove 28.9 pathogen stream miles from the final 2002 303(d) list (179.9 pathogen miles originally) and to properly classify 89.3 pathogen stream miles as "urban runoff" instead of "collection system failure."

Keywords: pathogens, bacteria, dry weather, E. coli, monitoring, antibiotic resistance, 303(d), Tennessee

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