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

The following video includes an overview of the water-quality testing program:

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.

Predicting 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 buoys in the lake 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.

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

• Green – Swimming is permitted. Weather conditions are fair and water quality is good.

• Yellow – Swimming is permitted, but caution is advised. Weather conditions are unpredictable or bacteria levels are higher than the water quality criteria set by EPA for notifying the public.

• Red – Swimming is not permitted. Weather conditions are dangerous or water quality conditions are dangerous.

Flags showing the swim status at each beach can change based on weather or water quality.

• Weather:  Lifeguards stationed at each beach monitor weather conditions. We aim to provide the most current conditions at the beach as accurately as possible. However, weather conditions such as lightning can change rapidly. Due to safety requirements, there may be some delay between when lifeguards post an advisory or a ban at the beach and when they are able to update the website.

• Water quality: The Chicago Park District follows United States Environmental Protection Agency (EPA) guidelines for water quality monitoring and changes the flag status accordingly.

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

Swim advisories (yellow flags) and swim bans (red flags) are issued for potentially hazardous weather or water quality conditions.

Hazardous weather advisories and bans

The weather, unfortunately, is beyond our control. Lifeguards continuously assess weather conditions and change the advisory status at each beach as needed for the safety of the public.

Water quality advisories and bans

Many factors can cause high levels of bacteria in the water, including droppings from seagulls and other wildlife, pet waste, rain that washes bacteria into the water, high waves or winds that stir up the sand on the lake bottom, or even swimmers themselves.  Many of these sources are influenced by the behavior of beach goers. Doing your part to keep our beaches clean can help prevent water quality advisories.  Sewer overflows can also cause high levels of bacteria in the water.  If a sewer overflow occurs in Chicago, the Park District issues swim bans (red flags) at every beach until water quality tests show the beach is safe for swimming.

Please do not feed the birds

Research has suggested a strong link between high E. coli levels and fecal droppings from ring-billed gulls and other birds, which is why feeding birds at the beach is prohibited. The Park District also provides numerous lidded containers for trash and recyclable materials to decrease the available food source for these "nuisance species."

Keep things clean

In addition to removing trash and debris from the beach, Chicago Park District beach groomers expose wet sand to the sun — which kills harmful bacteria.

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

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.

The Chicago Park District

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."