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United States Department of Agriculture

Agricultural Research Service

Title: Distributions of E. Coli and Enterococci in the Surface Waters of the Upper Oconee Watershed of Georgia

Authors
item Fisher, Dwight
item Dillard, Anthony

Submitted to: Conference on Watershed Management to Meet Emerging TMDL Environmental Regulations
Publication Type: Proceedings
Publication Acceptance Date: July 24, 2001
Publication Date: November 14, 2003
Citation: Fisher, D.S., Dillard, A.L. 2003. Distributions of E. coli and enterococci in the surface waters of the Upper Oconee watershed of Georgia. Conference on Watershed Management to Meet Emerging TMDL Environmental Regulations. p. 119-122.

Interpretive Summary: The Southern Piedmont extends from Alabama to Virginia between the Appalachian Mountains to the Northwest and the Coastal Plain to the Southeast. The Upper Oconee watershed of Georgia is typical of this region and protecting water quality is a key issue as urban development proceeds in this historically agricultural region. Certain bacteria are used to indicate fecal contamination in surface water and these bacteria may be found in higher numbers than those recommended by regulatory agencies. Researchers at the J. Phil Campbell Sr., Natural Resource Conservation Center in Watkinsville estimated the numbers of two types of bacteria used to indicate fecal contamination (E. coli and enterococci) at 18 sites for 3 years (2200 observations) within the Upper Oconee watershed. At our creek and river sample sites, the land area represented ranged from approximately 6 to 150 square miles. Even though both E. coli and enterococci are used to indicate fecal contamination they were not well correlated. High numbers of enterococci relative to E. coli appeared to often indicate wildlife as sources of contamination rather than agricultural sources. We also used statistical distributions to compare various threshold levels and the percentage of observations below the threshold. For example, a level of 200 microbes per 100 milliliters of water is often used as a threshold for regulatory action. With the E. coli data pooled across sample sites, only approximately 25% of our observations were below 200 microbes per 100 ml. With the enterococci data, 38% of the samples were below 200 microbes per 100 ml. The two microbe tests had similar statistical distributions but enterococci numbers were more frequently (38% vs. 25%) below a threshold of 200 microbes per 100ml than E. coli. These results can be used by state and federal regulatory agencies to establish standards that take into account background levels of indicator bacteria in the Southern Piedmont.

Technical Abstract: The Upper Oconee watershed of Georgia is typical of the Piedmont and water quality is a key issue in the urbanization of this historically agricultural region. Fecal indicators may be found in higher numbers than regulatory targets and agricultural non-point sources are sometimes implicated. We enumerated E. coli and enterococci bacteria at 18 sites for 3 years (2200 observations) within the Upper Oconee watershed of Georgia in the Southern Piedmont of the USA. Contributing areas ranged from 1,500 to 40,000 ha. Even though both E. coli and enterococci are used as fecal indicator organisms they were not highly correlated. High numbers of enterococci relative to E. coli may indicate wildlife sources. Cumulative distributions provided a means of comparing various regulatory threshold levels and the percentage of observations below the threshold. With the E. coli data pooled, approximately 25% of the samples were below 200 MPN/100 ml, approximately 68% were below 550 MPN/100 ml, and at 1000 MPN/100 ml only 82% of the samples had lower estimates. With the enterococci data, 38% of the samples were below 200 MPN/100 ml, 72% were below 550 MPN/100 ml, and approximately 82% of the samples were below 1000 MPN/100 ml. The two assays had similar distributions but enterococci numbers were more frequently (38% vs. 25%) below an MPN of 200 /100ml than E. coli. Both assays indicate contamination but lack of correlation may be related to source and viability in the watershed.

Last Modified: 4/17/2014
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