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Bacteria Criteria for the Protection of Recreational Uses – 2019

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The Virginia State Water Control Board recently adopted nationally recommended bacteria criteria published by EPA in 2012. Bacteria criteria are intended to protect people from organisms such as viruses and bacteria in water bodies (e.g., lakes, rivers, beaches).  Swimming and other recreational activities in contaminated water can make people ill. The criteria are intended to be protective of recreation designated uses, such as swimming, wading, and boating. The criteria are required to be updated periodically by the Beaches Environmental Assessment and Coastal Health Act of 2000 (BEACH Act). The BEACH Act states that when EPA publishes new or revised criteria, States having coastal recreation waters shall adopt and submit to the EPA Administrator new or revised water quality standards for the coastal recreation waters of the State for all pathogens and pathogen indicators to which the new or revised water quality criteria are applicable. The new or revised criteria must be as protective of human health as the criteria published by EPA. 

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The criteria – what are they?

 The criteria rely on the latest research and science, including studies that show a link between illness and fecal contamination in recreational waters. They are based on the use of two bacterial indicators of fecal contamination, E. coli and enterococci. E. coli is the indicator for freshwater and enterococci for saltwater. They are the same indicator organisms used for the bacteria criteria Virginia adopted in the early 2000s. The new criteria are designed to protect primary contact recreation, including swimming, bathing, surfing, water skiing, tubing, water play by children, and similar water contact activities where a high degree of bodily contact with the water, immersion and ingestion are likely. All Virginia waters are designated for primary contact recreation.

Fact Sheet: Why and How DEQ Evaluates Bacteria in Recreational Waters. 

The criteria include three components:

  1. A number of culturable colony counts of either the bacteria E.coli or enterococci,
  2. a duration of 90 days as an averaging period for a measure of central tendency called a geometric mean (GM), and
  3. an allowable excursion rate of no more than 10% of samples allowed to be greater than a Statistical Threshold Value (STV).

The magnitude of the bacterial indicators is described by both the GM and the STV for the bacteria samples. The GM is a never-to-be-exceeded value in a 90-day period and STV is intended to be a value that should not be exceeded by more than 10 percent of the samples taken within the 90-day period. Waters are assessed against the GM and STV. Waters for which monitoring data are at, or below, the criteria values pose a low risk of illness to the public and meet the recreational designated use.

The Criteria

The GM is a "never-to-be-exceeded" value and no more than 10% of the data in a 90-day period may exceed the STV. The value of the STV is higher than the geometric mean and is set at the 90th percentile of all values observed in the dataset that supports the criteria. For example; for E.coli the GM value is 126 bacteria colony counts per 100 milliliters (counts/100 ml) and a STV of 410 counts/100 ml. This is because in the data set of bacteria samples and the epidemiological studies used to establish the criteria, the geometric mean was calculated to be 126 and 90% of the bacteria counts were at or below 410 counts/ml. The STV is intended to avoid incorrectly classifying a water body as posing an excessive risk to swimmers, when in fact it is likely just demonstrating the same level of variability in bacteria values that would be consistent with a geometric mean of 126 counts/ml. That is, given the normal variability of bacteria counts in surface waters, we expect about one half of the samples to be above the mean value, but as long as the higher counts are below the 90% STV, this is deemed to be acceptable. The below table summarizes the criteria. 




E. coli


126 counts/100mL

410 counts/100mL



35 counts/100mL

130 counts/100mL


Additional Information:

Virginia Department of Environmental Quality – Designated Uses

Virginia Department of Environmental Quality - Water Quality Monitoring Programs

Virginia Department of Health – Beach Monitoring and Swimming Advisories

EPA web site - Recreational Water Quality Criteria
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Virginia Department of
Environmental Quality
P.O. Box 1105
Richmond, VA 23218
(804) 698-4000

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