The Chicago River Swim is this weekend. How do you know the water is safe?

Hundreds of people will jump into the Chicago River this weekend for the Chicago River Swim raising money for ALS research and swim lessons. But how do you know it's safe to get in the water?

To ensure the river is safe, organizers are doing real time monitoring but also working with a team of researchers for testing.

Masters of Public Health candidate Will Kane walked CBS News Chicago through the steps they take at a lab at the University of Illinois Chicago, filtering concentrate bacteria, isolating the DNA, then amplifying it to measure how much is in the water.

So far, each day has come back well under the health and safety threshold.

"The river, especially in the main channel here downtown, is mostly just Lake Michigan water," he said. "I would say it's not a good idea to go swimming farther in the north or the south branches, but the area downtown is low risk."

The water comes from the same eight collection sites along the river swim course each day, from Wabash to Lake Street. It's collected by swim organizers and brought to the UIC School of Public Health.

"We are on day six today so we are testing tomorrow, Friday, and Saturday leading up to the swim on Sunday.," said Abhilasha Shrestha, research assistant professor of environmental and occupational health sciences at UIC.

Shrestha said there is one thing that worries her: the forecast. Rain runoff can push bacteria numbers higher. But even recent history is reassuring.

"This year, especially summer was wet for us. And even with that amount of rain, the beaches were quite clean. We saw relatively lower exceedances and advisors this year," she said.

Researchers estimate that even with all the rain, only 3% of testing came back over the threshold; good enough odds that the scientists we spoke to said they'd jump in.

"I would if I knew how to swim. They knew they wanted me to jump, but I don't do open swim water swimming. Only swim on the kiddie pool," Shrestha said.

If you want to know what the data says about river water safety, it's updated daily on the city's website.

Weekend events kick off Friday with "The Big Jump" before the swim on Sunday. So far, more than 500 people have signed up to do the swim. Applications are still open to join, though there are conditions to prove you are a strong enough open water swimmer.