What does bacteria level mean?

Water quality information posted on this website includes both

  • the most recent water quality lab test result by the Chicago Park District for E. coli bacteria

  • the predicted real-time water quality, which is based on a computer model that uses weather data to predict water quality

What are we testing for in the water?

E. coli is used as a fecal indicator bacteria for human health risk. Although some strains can make you sick, most E. coli is not harmful. However, this bacteria is used as an indicator for the presence of other germs that could make you sick. For more information about indicator bacteria and their use in water quality monitoring, visit the U.S. Environmental Protection Agency's (EPA) website. EPA beach policy recommends notifying the public when E. coli bacteria levels are above the "single sample maximum" outlined in the federal criteria for recreational waters, which is 235 CFU/100 ml. (The unit of measurement for water quality testing is CFU/100 ml; which stands for colony forming units of E. coli per 100 milliliters of water.) This standard is used at beaches throughout the Great Lakes region. When bacteria levels are detected (or predicted to be) above 235, the Chicago Park District posts an advisory at the beach and changes the swim status.

According to the EPA criteria, the number 235 corresponds to a risk level of 0.8% of swimmers developing gastrointestinal illness - or 8 out of 1,000 people. For comparison, a bacteria level of 1000 corresponds to a 1.4 % risk, and a bacteria level of 2,000 corresponds to a 1.8% risk.

In November of 2012, the U.S. EPA published recommendations for new recreational water quality criteria. All coastal states, including Illinois, are expected to adopt the new recommendations into law by 2015. The new criteria are based on updated studies that expand on the science linking indicator bacteria such as E. coli to human health.The recommendations do include some changes to the standards for monitoring bacteria, but still recommend the use of 235 cfu/100ml as the "Beach Action Value" for notifying the public of a water quality risk. For more information on the current regulations and the 2012 recommended criteria, click here. The recommendations also include guidance for additional monitoring methods such as predictive modeling, described below, for improving the time it takes to notify the public of water quality conditions.

The Chicago Park District tests beach waters for E. coli a minimum of five days per week. Tests are conducted on weekends when high levels of E. coli are found.

redicting Current Water Quality

The Chicago Park District, in partnership with the U.S. Geological Survey, has also developed statistical models that use weather data to predict the bacteria levels in real-time. Monitoring buoysin thelake and weather stations on the lakefront continuously collect data, which then get plugged into a mathematical equation that provides the predicted bacteria level.

These models provide information about predicted water quality on the same day, compared to the 18-24 hours needed to get results from a lab for traditional water quality testing. The Park District uses the results from daily testing to make sure that the models are functioning properly

The predictive models were developed with grant funding from the EPA's Great Lakes Restoration Initiative.