Swim Water MPN

The bacteriological examination of recreational water (water used for swimming and other water contact activities) is designed to evaluate the disease-producing potential of the water by identifying the presence and abundance in a water sample of “indicator” bacteria.  Most of these bacteria are not themselves a health hazard, but indicate the degree of contamination of the water by surface run-off, decaying vegetation, animal feces, or sewage.Three groups of bacteria are typically used to assess recreational water quality: Total Coliform, Fecal Coliform (E. coli), and Enterococcus bacteria.

Total coliform refers to the entire group of bacteria known as coliform bacteria.  Many of these bacteria are naturally occurring species that harmlessly feed on dead plant and animal matter.

Fecal coliform is a sub-group of the total coliform group, and is made up of bacteria found principally in the intestines and feces of warm-blooded animals, including humans.

E. coli is the principal fecal coliform species.  Most strains of E. coli are harmless, except for rare strains that periodically make headlines when they contaminate meat products.

Enterococcus bacteria are also found in feces of warm-blooded animals.  They have been found to be a better indicator of salt water contamination than coliform bacteria because of their ability to tolerate salt water.  Their widespread application to fresh water evaluation is still being investigated.

The Department of Health Services recommends that public beaches meet the following guidelines:

  • Single sample MPN/100 mL* results should not exceed 10,000 for total coliform, 400 for fecal coliform, 235 for E. coli, or 61 for Enterococcus.
  • The average (based on the log mean) MPN/100 mL* of at least 5 evenly spaced samples in a 30-day period should not exceed 1,000 for total coliform, 200 for fecal coliform, 126 for E. coli, or 33 for Enterococcus.
  • MPN/100 mL = Most Probable Number of bacteria per 100 milliliters of water (about 31⁄2 oz).

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