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Article Excerpt
ABSTRACT
Escherichia coli and enterococci were enumerated in water collected bi-weekly for one year at ten stations in the Alapahoochee River system. The concentrations of E. coli were usually below a standard of 576 colony forming units (cfu) per 100 ml, except after heavy rain. At five stations the concentrations of enterococci were generally above a standard of 151 cfu/100 ml. Concentrations of both groups showed a direct but weak correlation with pH. For quality control, 772 colonies typical of E. coli and 773 colonies typical of enterococcus were tested by the appropriate verification procedures. Sixty-six (8.5%) of the former, and 57 (7.4%) of the latter, appeared to be false positives. Further study of 51 apparent false positive E. coli isolates showed that only 15.7% did not contain E. coli, indicating an actual false positive rate of about 1.3%. Further study of 18 false positive enterococcus isolates showed that 16.7% were Enterococcus species and 83.3% were Streptococcus species. Thus the actual false positive rate was estimated to be 6.2%.
Keywords: Escherichia coli, enterococci, bacteria, fecal coliforms, indicator, water quality, verification, false positive, pH, Alapahoochee River, Georgia
INTRODUCTION
The Alapahoochee River, a major tributary of the Alapaha River in southern Georgia, is formed from the confluence of Mud Creek and Grand Bay Creek on the border of Echols and Lowndes counties. Parts of this system have been listed as impaired by Georgia's Environmental Protection Division (1, 2). As part of a larger project aimed at mitigating aquatic pollution in the watershed, the water quality of the Alapahoochee system was evaluated during a one year period beginning in May 2005 (3). The basic physicochemical properties of the water as well as biological and chemical indicators of water pollution were studied (3). Here we report on some aspects of the bacteriological portion of the study.
Currently the State of Georgia classifies its surface waters according to their designated use as follows: drinking water supplies, recreation, fishing, wild river, scenic river, and coastal fishing (4, 5). Most rivers and streams, including those in the Alapahoochee watershed, are designated as fishing. According to State guidelines, water quality is monitored using fecal coliform concentrations as the standard, with fishing streams listed as impaired if the geometric mean of four water samples in a thirty day period exceeds 200 colony forming units (cfu) per 100 ml between May and October or 1,000 cfu/100 ml between November and April; or if a single water sample exceeds 4,000 cfu/100 ml between November and April (4, 5).
In a 1976 publication, the United States Environmental Protection Agency (EPA) recommended using fecal coliforms (which can grow at 44.5[degrees]C; produce acid and gas during lactose fermentation; and include Escherichia coli) as indicators of fecal contamination, and suggested that a geometric mean of 200 cfu/100 ml not be exceeded in fresh water that is used for swimming (6). The EPA later estimated that this geometric mean concentration of fecal coliforms in fresh water would be associated with an illness rate of approximately 8 per 1,000 swimmers (7). In the mid-1980s, however, the EPA published documents indicating that the concentrations of E. coli and enterococci in fresh water at bathing beaches, but not the concentrations of fecal coliforms, were directly correlated with the occurrence of gastrointestinal illness in swimmers (7, 8). Therefore the EPA has recommended E. coli and/or enterococci (rather than fecal coliforms) as indicator bacteria for evaluating the quality of fresh water (7, 9).
In 1986, the EPA published specific criteria based on concentrations of E. coli and enterococci for evaluating fresh water that is used for swimming or other activities in which body immersion is considered likely; water used in these ways is referred to as primary contact recreational water (7, 9). These 1986 criteria were developed to protect swimmers from developing gastrointestinal illness as a result of ingesting contaminated water (7, 9). For primary contact recreational fresh water, the EPA has suggested using E. coli and/or enterococcus criteria corresponding to an illness rate of no more than 14 per 1,000 swimmers, and has recommended using criteria corresponding to an illness rate of 8 per 1,000 swimmers for fresh water that is heavily used for swimming (9). Two types of bacterial criteria are generally recommended for evaluating water quality: (i) the geometric mean of the concentrations of the indicator bacteria in several water samples and (ii) a single water sample maximum allowable concentration (SSM) of the indicator bacteria (7, 9). The EPA proposed different SSM criteria depending on the frequency of primary contact recreation. For infrequently used water, a SSM criterion based on the 95% confidence interval for the appropriate geometric mean can be applied; but for heavily used water, the confidence interval is reduced to 75% (7, 9).
Water used for activities in which body immersion is unlikely (e.g., fishing) is considered secondary contact recreational water. Although the EPA has not published bacterial standards specific for secondary contact recreational water, one of its publications notes that "... states must adopt primary contact recreation wherever attainable for all surface waters within their jurisdiction ..." (9). The EPA has also suggested possible approaches for evaluating the quality of secondary contact recreational water when the bacterial criteria for primary contact recreation cannot be met. Such approaches include using a criterion that is five times that used for primary contact recreational waters (9). Several issues have been raised concerning the EPA's 1986 bacterial criteria for evaluating water quality (10). For example, many States have questioned whether these criteria can be appropriately applied to all waters in the United States (10).
In the present study, the concentrations of E. coli and enterococcus bacteria in aseptically collected water samples were determined using membrane filtration methods approved by the EPA (11, 12). According to the published 1986 EPA criteria (7, 9), the concentrations of both groups of bacteria indicate...
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