EVANSTON, Ill. (CBS) -- The City of Evanston has closed all of its lakefront beaches for the day, after water tests showed dangerous levels of E. coli bacteria.
The five public beaches in Evanston – at South Boulevard, Lee Street, Greenwood Street, Clark Street and Lighthouse beaches – are all closed. Only the dog beach at Church Street remains open.
Northwestern University also operates a private beach, which is also closed.
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The City of Evanston tests the water daily at all the beaches daily. When E. coli counts exceed 235 CFU, or chloroform forming units, it is likely that bacteria are forming colonies in the water.
On Wednesday, the samples showed counts to between 238 and 800 CFU at Evanston beaches.
E. coli occur naturally in the fecal waste of humans and other warm-blooded animals. Some strains can be harmful, and they can indicate the presence of other disease-causing bacteria.
A variety of factors can cause E. coli pollution, including illegal sewer connections or discharges, malfunctioning sewer systems, storm runoff after rainfall, or even swimmers pooping in the lake, the City of Evanston said.
The bacteria will dissipate as a result of natural die-off, wind and wave action, or ultraviolet light from the sun. It usually takes 24 hours, but the timeframe is unpredictable, the City of Evanston said.
The water must be resampled from both the shallow and deep areas of the water at the beaches. Bacteria levels cannot be determined until 24 hours after the samples are taken.
Chicago's beaches were all open Wednesday, but swim advisories were posted at most of them due to high E. coli levels. The advisories are in effect at the 12th Street, 31st Street, 57th Street, 63rd Street, Calumet, Fargo, Foster, Howard, Humboldt, Jarvis, Juneway, Kathy Osterman, Leone, Loyola, Montrose, Oakwood, Rainbow and Rogers beaches.
No restrictions are in place at the George A. Lane, Hartigan, North Avenue, Oak Street, Ohio Street or South Shore beaches.
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