The Chicago Park District tests the water for E. coli bacteria. E. coli is not harmful itself and is naturally occurring in the environment. However, this bacteria is an indicator of the presence of other germs that could make you sick. US Environmental Protection Agency (EPA) beach policy recommends notifying the public when E. coli bacteria levels are above the federal water quality standard, which is 235*. This standard is used at beaches throughout the Great Lakes region. According to the EPA, the number 235 corresponds to a risk level of 0.8% of swimmers becoming sick to their stomachs - or 8 out of 1000 people. For comparison, a bacteria level of 1000 corresponds to a 1.4 % risk, and a bacteria level of 2000 corresponds to a 1.8% risk.

in partnership with the US Geological Survey, has also developed statistical models that use weather data to predict the bacteria levels in real-time. These models provide information about water quality in real-time, compared to 18-24 hours to get results from a lab for traditional water quality testing.

The models were developed with grant funding from the EPA's Great Lakes Restoration Initiative. When bacteria levels are detected (or predicted to be) above 235, CPD posts an advisory at the beach.

*The unit of measurement for E. coli is "colony forming units per 100 millileters of water."

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