Swim advisories are issued for potentially hazardous weather or water quality conditions. The Chicago Park District follows United States Environmental Protection Agency (USEPA) guidance for testing of recreational waters and tests for indicator bacteria called Enterococci in the collected samples.
Although the type of Enterococci that is tested for at beaches is not itself harmful, it is used as an “indicator bacteria” for potentially harmful germs (bacteria or viruses). This means that if Enterococci is found, there is a statistical likelihood that other germs may be present.
The sheer variety of germs that could be found in the environment and their low concentrations make it very difficult and expensive to test for each individual organism. To protect public health, the Environmental Protection Agency (EPA) has conducted studies that provide a statistical link between the risk of getting sick and the presence of Enterococci above certain concentrations.
There are many unknown sources that can cause high levels of Enterococci in the water. Studies have shown that potential sources include waste from gulls, pets and wildlife, high temperatures, severe weather, stormwater runoff, bacteria from the sand, and low lake levels. Research has suggested a strong link between high Enterococci levels and fecal droppings from ring-billed gulls and other birds, which is why feeding birds at the beach is prohibited and gull harassment techniques have been implemented at some sites that have a history of high levels of Enterococci. The Park District also provides numerous lidded containers for trash and recyclable materials to decrease the available food source for these "nuisance species". Additionally, the Chicago Park District Department of Natural Resources cleans each beach daily with a beach groomer.
EPA guidance calls for municipalities to test recreational public beach waters at least once every week. The Chicago Park District tests beach waters for bacteria seven days per week using EPA’s newly established rapid test method. The rapid test method uses DNA detection of Enterococci instead of culturing live E. coli bacteria cells, and cuts the time required for test results from 18 to 24 hours to 4 hours. To help decrease Enterococci levels, the Chicago Park District uses an integrated approach of best management practices in an effort to minimize various sources of the bacteria.