SOURCES OF ESCHERICHIA COLI
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
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
E. COLI AS INDICATOR ORGANISM
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.
EPA WATER QUALITY STANDARD
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
VERMONT STANDARDS FOR E. COLI
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.