CHICAGO (CBS) — The effects of Monday’s heavy rains were being felt Tuesday at the city’s beaches, where officials have banned swimming, due to high bacteria levels caused by runoff.

When the Chicago area’s water treatment system is maxed out from heavy rains, the overflow pours into the Metropolitan Water Reclamation District’s Deep Tunnel system. On Monday, hours of heavy rainfall filled 109 miles of rock tunnels.

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“All the tunnels combined, it’s 2.3 billion gallons of capacity that we maxed out,” MWRD supervising civil engineer Ed Staudacher said.

After that, because the system was at capacity, a combination of rain water and sewage was released into Lake Michigan, creating unsafe water conditions at all the city’s beaches.

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While all 26 beaches on the lakefront were open to the public on Tuesday, swimming was not allowed, due to the water quality.

In DuPage County, the Stormwater Management agency’s 17 reservoirs kicked into action.

“I would say you’re talking millions of gallons right now that we’re storing at this point,” Chairman Jim Zay said.

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He said part of the problem is all the rain over the past couple weeks has saturated the ground, so rain has nowhere to go but the sewers.