FACTSHEET ON WATER QUALITY PARAMETERS
E. coli (Escherichia coli)
£ coli (Escherichia coli) is a type of bacteria that lives in the intestines of people and animals. £ coli is commonly found
in human and animal feces. Most strains of £ coli are harmless, but some can make people sick.
Why do we measure E. coli?
Water samples are collected to measure £ coli (Figure
1) to make sure water is safe for public recreation,
such as swimming, fishing or canoeing. £ coli is
considered an indicator organism, used to identify fecai
contamination in freshwater and indicate the possible
presence of disease-causing bacteria and viruses
(pathogens). Individuals who swim or come in contact
with water with elevated levels of £ coli and other fecal
indicator organisms are at an increased risk of getting
sick because of potential exposure to fecai pathogens.
Common symptoms of ingesting a pathogenic strain of
£ coli include vomiting and diarrhea. High numbers of £
coli (and other) bacteria may contribute to cloudy water,
unpleasant odors, and increased oxygen demand (which
may reduce levels of dissolved oxygen in the water).
£ coli concentrations may be linked with other
parameters such as high totai suspended soiids (TSS)
and turbidity concentrations because the bacteria tend to
higure 1. E. coli under a microscope. Credit: Photo courtesy of
National Institute of Allergy and Infectious Diseases
be found with particles. £ coli concentrations may also
be linked with high phosphorus, nitrate, and biological
oxygen demand (BOD) concentrations.
What affects E. coli levels?
£ coli is found in the feces of warm-blooded animals.
£ coli from humans can reach surface water via
wastewater treatment plant effluent, broken or leaky
sewer pipes, and failing or poorly sited septic systems.
£ coli from animals can enter waterbodies in stormwater
runoff from feedlots, manure storage areas, or areas
where there is wildlife. Livestock that are allowed to get
into or near streams can introduce £ coli directly into
surface water.
For factsheets on other water quality parameters, visit:
epa.gov/awma/factsheets-water-quality-parameters.
For more information about the Clean Water Act Section 106 Grants Program, visit:
epa.qov/water-pollution-control-section-106-q rants.

-------
E coli
What are EPA's recommended criteria for E coli?
£ coli criteria are expressed as the number of colony-
forming units (cfu) per 100 ml_. The two sets of criteria
shown in Table 1 were developed using different
methods for calculating illness rates from scientific data.
They are based on studies that show a link between
illness and fecal contamination in recreational waters.
Both are considered protective of human health, and
either can be used to assess recreational water quality.
Recommendation 1 A geometric mean (GM) of 126 cfu
per 100 mL and a statistical threshold value (STV) of 410
cfu per 100 mL measured.
Recommendation 2 A GM of 100 cfu per 100 mL and an
STV of 320 cfu per 100 mL measured.
The GM is a statistic often used for bacterial counts in
federal and state water quality standards. The GM of
the monitoring samples should not exceed whichever
criterion is selected from the two recommendations
in any 30-day interval. The STV is similar to a 90th
percentile, meaning that no more than 10% of samples
should exceed it.
EPA recommends weekly sampling to evaluate the GM
and STV over a 30-day period and encourages more
frequent sampling at more densely populated beaches.
Table I. Two sets of E. coli criteria based on two different estimated illness rates.

Recommendation 1
Estimated Illness Rate: 36 per 1,000
Indicator
Organism
Geometric Mean
(cfu/100 mL)
Statistical
Threshold
Value (STV- 90th
percentile)
(cfu/100 mL)
£ coli (freshwater)
126
410

Recommendation 2
Estimated Illness Rate: 32 per 1,000
Indicator
Organism
Geometric Mean
(cfu/100 mL)
Statistical
Threshold
Value (STV- 90th
percentile)
(cfu/100 mL)
£ coli (freshwater)
100
320
Source: Anderson and Rounds (2003)
How do we measure E. coli?
£ coli levels are measured by analyzing bacterial growth
in laboratory analyses. This is commonly done by the
membrane filter procedure, although color test kits have
also been EPA-approved. Care must be taken when
collecting water samples because all of the sampling
containers must be sterile. More information on
measuring £ coli can be found in EPA's Volunteer Stream
Monitoring: A Methods Manual (1997).
What are the challenges of using E. coli as a water quality parameter?
Bacteria levels in waterbodies can be highly variable. In	rainfall. This can make it challenging to pinpoint sources of
streams, natural bacteria levels vary and change with	elevated £ coli concentrations.
2

-------
E. coli
What if there is an exceedance of the water quality criteria?
ADVISORY
.'ass vsassS"™"4"""
Figure 2. Public advisory warns of high
bacteria levels in water. Public advisories can
include additional information such as the
date of the posting and contact information
to check on the status of the posting. Credit:
Photo courtesy of USEPA
When elevated £ coli concentrations are observed, it is important to
respond in a timely manner by collecting additional data, posting a public
notice (Figure 2), and/or closing the waterbody to recreational activities.
Figure 3 summarizes general actions to take when responding to elevated
concentrations.
RESPOND
QUICKLY
u
Take action immediately in the event of an exceedance to
prevent human exposure to £ coli. Exposure can cause
infection, diarrhea, and other illness in humans. Additional
data may be needed to understand the cause of the
exceedance.
SHORTTERM
ACTION
Issue a public notice and post advisories to
notify the public that the waterbody is closed to
recreational activities.
LONG TERM
ACTION
To prevent future E. coli exceedances,
identify the source of the bacteria.
Depending on the source, different steps
will be necessary to remediate the problem
and reduce the likelihood of future events.
Figure 3. General actions to take when a sampling result shows an elevated
concentration of E. coli.
EPA 841F21007F | July 2021

-------