Nearly 100 billion gallons of flood water was set free into Lake Michigan during the weekend's historic rainfall.
But those who rely on Lake Michigan drinking water have nothing to fear from contamination, officials say.
"We have stepped up our monitoring," noted Chicago Department of Water Management spokesman Tom LaPorte. "But we have no reason to change our purification process."
Intake stations in Highland Park and Evanston also are reporting no negative shifts in water quality, but the Evanston plant did add extra chlorine as a precaution.
"We did that just in case we did see something, but we haven't seen an additional need," said Dave Stoneback, spokesman for the Evanston water department.
Highland Park water superintendent Don Jensen said the amount of added flood water into Lake Michigan may sound high, but it is minuscule in such a massive lake. "This has not had any impact on us," he said.
Evanston pumps in water for other suburbs, including Arlington Heights, Palatine, Buffalo Grove and Wheeling. Highland Park serves Deerfield and Glenbrook.
Most other suburbs, including much of DuPage County, get lake water through Chicago.
Officials up and down Lake Michigan were forced to open flood gates into the lake to prevent added flooding as the region was swamped with days of rain that sent rivers and streams over their banks.
Lake Michigan water, which millions of suburbanites rely on, is sucked from the lake's floor miles offshore.