Indicator organisms themselves are not necessarily pathogenic but their presence "indicates" or suggests recent contamination by human sewage or other waste which may result in human illness. Sources of contamination include storm water runoff, sewage overflows and feces from wild and domestic animals.
The currently used bacterial indicators for recreational waters include total coliform, fecal coliform and enterococcus. Total coliform is a group of bacteria found in the intestinal tract of warm-blooded animals but some may also occur naturally in the environment. Fecal coliform is a sub-group of total coliform and serves as a more specific indicator for human and animal waste contamination. Enterococci is present in the intestines of all warm blooded animals and because it survives longer than fecal coliforms in natural waters, enterococcus is a more reliable measure of water quality.
Samples are collected in sterile 100 ml containers and transported on ice to a certified laboratory. The samples must be cultured on plates within 6 hours of the time they are collected. After a 24-hour incubation period, the number of colonies on the culture plate are counted and reported as the number of colony-forming units per 100 milliliters of sampled water, or CFU/100 ml.
During sample collection, environmental conditions are also observed and recorded, along with recent rainfall amounts.
The maximum allowable level or "action-level" of fecal coliform is 400 CFU/100 mls. The action level for enterococcus is 105 CFU/100mls for a single water sample. In addition to a single sample measure, the geometric mean for enterococci is calculated using the 5 most recent test results. This will reflect the average water quality over the past month. The action level for a geometric mean is 36 CFU/100 ml, as opposed to a single sample action level of 105, signifying a trend towards higher than normal bacterial levels in the water for this particular period. If a sample meets or exceeds any of these limits, the beach must be resampled and an advisory may be posted.
The use of bacterial indicators is limited because a very small sample (100mls, about the size of a juice glass) is used to represent a very large body of water. Environmental contamination can also be very localized or patchy, especially if the source of contamination is wildlife. Poor sample results may reflect that particular sample area, but not necessary represent the water quality for the entire beach area. Conversely, waters with indicators exceeding certain levels may be considered a potential health risk but levels within acceptable ranges are not necessarily free of risk.