Bacterial Water Quality


Table of Contents



The presence of Escherichia coli (E. coli) in surface waters is often attributed to fecal contamination from agricultural and urban/residential areas.  However, variation in E. coli concentrations from site to site and the contribution of human vs. agricultural sources are not readily understood.  In addition, E. coli concentrations at a particular site may vary depending on the baseline bacteria level already in the river, inputs from other sources, dilution with precipitation events, and die-off or multiplication of the organism within the river water and sediments.  The concentration of E. coli in surface water depends for the most part on the runoff from various sources of contamination and is thus related to the land use and hydrology of the contributing watersheds.

Sediments may affect the survival and often act as a reservoir of E. coli in streams.  Sedimentation and adsorption, which offer protection from bacteriophages and microbial toxicants, can lead to higher concentrations of E. coli in sediments than in the overlying water column (Burton and others 1987).  Thus, the sediment often acts as a reservoir for E. coli in the stream.  In addition, fecal bacteria may persist in stream sediments and contribute to concentrations in overlying waters for months after initial contamination (Sherer and others 1992).


To help maintain water that is safe for drinking and swimming, routine monitoring for enteropathogens (which cause gastrointestinal diseases and are disseminated through fecal contamination of water) is necessary. Routine monitoring of enteropathogens, which can cause serious diseases such as cholera, typhoid, salmonellosis, and dysentery, is unreliable since these organisms are difficult to detect (Atlas and Bartha 1993). Instead, an indicator organism, such as E. coli, is used to determine fecal contamination.  The presence of E. coli, a normally non-pathogenic intestinal organism of warm-blooded animals, is easy to test for and is relatively more abundant than the enteropathogens thus leaving a safety margin for the detection of disease-causing organisms.

E. coli is considered a more specific indicator of fecal contamination than fecal coliforms since the more general test for fecal coliforms also detects thermotolerant non-fecal coliform bacteria (Francy and others 1993). The E. coli test recommended by the United States Environmental Protection Agency (EPA) confirms presumptive fecal coliforms by testing for the lack of an enzyme which is selective for the E. coli organism. This test separates E. coli from non-fecal thermotolerant coliforms. It should be noted that E. coli may not be an appropriate indicator for protozoan and viral diseases caused by such organisms as Cryptosporidium, Giardia, and the hepatitis virus due to their lower numbers in water and lower infectious doses.


Escherichia coli (E. coli) is the most reliable indicator of fecal bacterial contamination of surface waters in the U.S. according to water quality standards set by the EPA.  Although E. coli bacteria are not typically pathogenic in and of themselves, an extensive epidemiological study (Dufour 1984) demonstrated that E. coli concentrations are the best predictor of swimming-associated gastrointestinal illness. EPA bacterial water quality standards are thus based on a threshold concentration of E. coli in water above which the health risk from waterborne illness is unacceptably high.

The EPA recommended recreational water quality standard for E. coli is based on two criteria:  1) a geometric mean of 126 organisms/100 ml based on several samples collected during dry weather conditions or 2)  235 organisms/100 ml for any single water sample (EPA 1986).  The geometric mean is calculated by the equation: geometric mean of y = nth root of y1 * y2 * y3...yn.  If either criterion is exceeded, the site is not in compliance with water quality standards and not recommended for swimming. The current EPA water quality standard for E. coli corresponds to approximately 8 gastrointestinal illnesses per 1000 swimmers (Dufour 1984).


Current water quality standards in Vermont also use E. coli as an indicator organism.  The Vermont Water Quality Standards (Vermont Water Resources Board 1996) separates water into 2 classes--A and B. Class A waters are drinkable and class B waters are suitable for direct contact, such as swimming.  The standards also allow for waste management zones, which have a mixing zone of 200 feet, within class B waters.

The water quality standards currently adopted by the state of Vermont for class B recreational waters are based on a threshold concentration of 77 organisms/100 ml water for any single sample (Vermont Water Resources Board 1996). Any sample that exceeds this threshold will be in violation of the standard. This criterion corresponds to approximately 4 expected illnesses per 1000 swimmers (Dufour 1984). Unlike the EPA standards that are based on a geometric mean of several samples over time, the Vermont standards are based on a threshold concentration for any single water sample. The current threshold standard remains unchanged in the Vermont Water Quality Standards adopted June 10, 1999 that becomes effective on July 2, 2000 (Vermont Water Resources Board 1999).

Revision of Vermont's recreational water quality standards, however, is currently under discussion by the Vermont Water Resources Board. At least three possible scenarios are under consideration: 1) maintain current water quality standards, i.e. a threshold of 77 organisms/100 ml water for any single sample, 2) adopt EPA water quality standards, i.e. a geometric mean of 126 organisms/100 ml water based on several samples or a threshold of 235 organisms/100 ml water sample for any single sample, or 3) adopt a new standard based on a geometric mean of 77 organisms/100 ml water for several samples or a threshold of 143 organisms/100 ml water for any single sample as proposed in Vermont Water Quality Standards Recommended Revisions (Lawson 1998).  Results of a comparison of the proposed standards applied to observed E. coli concentrations from the Mad River watershed are summarized on this web site.



Atlas, Ronald M. and Richard Bartha. 1993. Microbial Ecology: Fundamentals and Applications. Benjamin/Cummings.  Redwood City, CA: .

Burton, G. A., D. Gunnison, and G. R. Lanza. 1987. Survival of pathogenic bacteria in various freshwater sediments. Applied and Environmental Microbiology 53 (4): 633-638.

Dufour, Alfred P. 1984. Health effects criteria for fresh recreational waters. EPA-600/1-84-004. Office of Research and Development, USEPA, Washington, DC.

EPA. 1986. Ambient water quality criteria for bacteria-1986. EPA/440/5-84-002. Office of Water Regulations and Standards, USEPA, Washington, DC.

Francy, D. S., Donna N. Myers, and Kevin D. Metzker. 1993. Escherichia coil and fecal coliform bacteria as indicators of recreational water quality. U.S. Geological Survey. Water Resources Investigations Report 
93-4083. Columbus, Ohio.

Lawson, Barry R.  1998.  Vermont Water Quality Standards Recommended Revisions.  Water Quality Standards Task Group for the Vermont Water Resources Board.  Montpelier, VT.

Sherer, Brett M., J. Ronald Miner, James A. Moore, and John C. Buckhouse. 1992.  Indicator bacterial survival in stream sediments. Journal of Environmental Quality 21: 591-595.

Vermont Water Resources Board. 1996. Vermont water quality standards. Montpelier, VT.

Vermont Water Resources Board. 1999. Vermont water quality standards. Montpelier, VT.


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created by Deb Sargent under the direction of Dr. Leslie Morrissey
School of Natural Resources, University of Vermont
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