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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.

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All Evanston Beaches Closed Due To High Bacteria Levels