Come on in, the water's fine-at least 90 percent of the time.
Beyond the North Avenue Beach sunbathers, just below Lake Michigan's blue surface, lie countless bacteria that could quickly cause an upset stomach or ear infection. That's enough to put some Chicagoans off taking a dip altogether.
"I probably wouldn't want to put my head under the water," said Jennifer Heist, 28, an Old Town resident who enjoys jogging along the beachfront path, but rarely swims.
"I know [the water] is far from what it was, but it's still not as clean as you'd want it to be," said Celina Reinhardtsen, 18, of Kankakee, Ill., who was walking along the beach on a recent afternoon. "It looks pretty dirty."
Many of the thousands of people who flock to the city's beaches each summer have trouble shaking the idea that the lake water is dirty, even after five years of water-quality improvements and new testing. Chicago Park District officials have been issuing fewer swim bans in recent years over poor water quality, so is there really anything to worry about?
The answer, officials say, is yes-and no. No one suggests guzzling lake water, but the water usually is safe for swimming.
"There's always some level of risk in drinking untreated water," said Lyman Welch, director of the Alliance for the Great Lakes' water quality program. "But I would not panic if you swallow some water. It's generally fairly low-risk."
It all depends on the level of E.coli, the main illness culprit, and other bacteria levels in the lake water near the beaches, officials said. Levels fluctuate daily. On most days, all of the beaches will be open. But when bacteria levels cross the acceptable threshold, the Park District imposes swim advisories, or bans swimming altogether. In 2011 the Park District saw a five-year low in swim bans at the five beaches, issuing just 36-down from 41 in 2010. And in 2012 officials tightened the threshold for swim bans, driving down the number of ban days.
"Chicago's beaches are pretty lucky in terms of water quality," said Cathy Breitenbach, the park district's director of green initiatives who oversees water testing. "We don't have sewage contamination in the lake. The bacteria in the water tends to come from wildlife and pets. Which isn't to say there's no risk, but we're lucky."
The water's relative cleanliness isn't due to luck so much as a century-old feat of civil engineering that has allowed city planners to reverse the flow of the Chicago River's sewage-filled waters away from the lake and into the Metropolitan Water Reclamation District in northern Illinois. But occasionally a bad rainstorm will cause the Chicago River to flow back into the lake again. When that happens, the Park District typically issues a swim ban for a few days, Breitenbach said, and that's all it takes for things to return to normal.
The Park District has been testing Lake Michigan's beaches for bacteria almost daily since the late 1990s, when national legislation prompted beach officials around the U.S. to pay more attention to water quality. In the past year, city officials have been using a new system to predict water quality in real time-as opposed to waiting a full day for results.
Park District employees visit the beaches at least five days a week to collect samples from the water. They bring those samples to a lab at UIC, where testers grow cultures of bacteria and count them.
Two hundred and thirty-five is the magic number of colony-forming units of E.coli per 100 milliliters of water. More than that, and the beach gets slapped with a water quality advisory for the day, and tested again the next day. Breitenbach estimated that some of the city's beaches have lower-than-ideal water quality for 8 percent to 10 percent of the season. Otherwise, beachgoers get the green light.
"People hear 'E.coli,' and sometimes it sounds scary," she said, "But this is something that is just found in nature. Some level of it is natural."
To test the levels, the Park District sends employees up and down the city's 26 miles of public beaches to collect water samples, usually between 8 a.m. and 11 a.m.
Predictive factors that can turn a clean lake into a dirty one are weather, waves and animal waste, Breitenbach said. Those factors change daily, but the Park District uses tools such as buoys, weather data and yesterday's results to predict the levels for the day.
The Park District also has been able to take some pre-emptive measures to improve water safety, she said. For example, in 2008 the Park District began using trained border collies to shoo away the seagulls that were making 63rd Street beach inhospitable on many summer days. They've also been encouraging beachgoers to pick up after their dogs and throw trash in waste bins.
63rd Street "is a lovely beach, and it no longer has the water quality problems from the birds that it once had," Breitenbach said.
Some of the city's most hardcore open-water swimmers will climb into the lake no matter what. But Steve Hernan, 46, the founder of the Open Water Chicago swimming group, says it is always smart to check with the city beforehand.
"It is an issue as a swimmer, because you're right there in it," he said. "We always monitor, especially after a storm. But the drawback to that is that you'll only find out how bad the water was after you swam in it."