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The US Environmental Protection Agency recommends that E. coli (Escherichia coli) be used to monitor the water quality of recreational beaches of the Great Lakes in order to protect the health of swimmers. (Environmental Protection Agency, Ambient Water Quality Criteria for Bacteria - 1986, January 1986.) EPA chose E. coli because it is found in the intestines of warm blooded animals and historically been used as an indicator of the presence of animal or human water products thus, the possible presence of disease causing organisms. An epidemiology study in Santa Monica Bay, California conducted in 1996 showed that swimmers experienced fever, nausea, and gastroenteritis, as well as cold and flu-like symptoms after swimming in waters found to contain high levels of bacteria. (How Safe is it to Swim in Santa Monica Bay? Coastlines, 1, Summer 1996).
The criteria Indiana uses to evaluate full body contact for recreational use is stated by rule which says E. coli concentrations shall not exceed 125 cells per 100 ml water as a geometric mean based on not less than five samples equally spaced over a thirty day period nor exceed 235 cells per 100 ml water in any one sample. 327 IAC 2-1-6. Indiana was recognized in a report released in 1996 by the Natural Resources Defense Council as one of only five coastal states in the nation that consistently use the EPA recommended standards or something stricter to monitor beaches. The Indiana Dunes National Lakeshore and several other county and municipal health departments routinely monitor many of Indiana's Lake Michigan beaches roughly between Memorial Day and Labor Day. When samples show that E. coli counts are too high, swimming is not recommended which is often referred to as a "beach closing."
Better methods to analyze water samples for E. coli are being researched. There are many uncertainties surrounding the sources contributing to the existence of E. coli in the water. Preliminary studies associate high E. coli concentrations with heavy rainfall, and wind direction, but the data are not sufficient to confirm this association. Scientists do not understand how extensively tributaries transport E. coli to Lake Michigan. Some beaches might be closed due to high levels of E. coli while sampling at other beaches along the shoreline show no E. coli to be present.