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What is "enterococcus?"
Enterococcus (plural "enterococci") is a bacterial organism commonly found in the intestinal tracts of warm-blooded animals. The N.C. Recreational Water Quality Program's staff measure levels of enterococci in water samples and issue swimming advisories when those levels exceed established limits.
What is a "beach tier?"
In accordance with the federal BEACH act and associated guidance provided by U.S. Environmental Protection Agency, the program ranks beaches into tiers according to average public use. This ranking determines the criteria for whether an advisory is issued based on sampling results. North Carolina's tier structure is summarized in the table below:
Classification Average use Frequency Single-sample max Geometric mean # of sites
(enterococci per 100mL of water)
Tier 11 Daily 104 35 137
Tier 2 3 times / week 276 -- 41
Tier 3 4 times / month2 500 -- 31
What is a "single sample maximum?"
The program's laboratory analyzes each water sample collected so that an estimate of the number of enterococci cells contained therein can be determined. This estimate is defined as a single sample most probable number, or MPN. A swimming advisory is issued when the single sample MPN exceeds the single sample maximum, as determined by beach tier. A colony forming unit, or CFU, is a direct count of the enterococci organism and is not an estimate.
What is a "geometric mean?"
A geometric mean is a kind of mathematical average. The program's staff maintains a running monthly geometric mean of five sample MPN values collected in 30 days. At the present time, both the monthly geometric mean and the single sample maximum are used as criteria for Tier 1 beaches only. For other tier classifications, the single sample maximum is the sole criterion.