Lake Calhoun is one of Minneapolis' most popular and packed lakes during the summer.
It's also contaminated with something called PFOS.
Now, don't panic – they've been looking at levels of the chemical in the fish and water for years, and it's considered to be at pretty safe levels, well below federal recommendations. And officials know where it's been coming from and are working to address it.
But here's a look at what the problem is, whether it's dangerous if you want to get in the water, and what the next steps are.
WTF is PFOS?
PFOS is short for perfluorooctane sulfonate – it's an organic chemical that's been flourinated (literally, flouride's been mixed in) and was used to make furniture fabrics, carpets, clothing, paper food packaging, and more, the Environmental Protection Agency says.
It can accumulate in humans (and fish and the environment), and studies have indicated higher levels can cause some health effects, including cancer, developmental issues in newborns, and more.
How did it get into Lake Calhoun?
Researchers at the University of Minnesota noticed it back in 2004, the Minnesota Pollution Control Agency says.
The agency started doing testing and found it was coming from Douglas Corporation – a metal plating facility in St. Louis Park. The PFOS was used during the manufacturing process, and the fumes were let out to the roof.
The PFOS then kept building up on the roof, and during rain or melting snow, it would run off with that water into the lake and other parts of the environment.
Worth noting: PFOS has not been found in nearby drinking water supply wells.
Can I swim in Lake Calhoun? Is it dangerous?
It's fine to swim in.
The amount of this PFOS stuff in Lake Calhoun is "actually quite low" at about 35 parts per trillion, Summer Streets with the Minnesota Pollution Control Agency told BringMeTheNews
"To put that in context, the safe drinking water level set by the U.S. EPA, which takes into account daily consumption for a lifetime, is two times higher than the concentration in the lake," she said.
So bottom line: "PFOS in Lake Calhoun is not a concern for swimmers," she said. "The very small amount of water that might be ingested during swimming would not be enough to cause harm."
There is a warning about how often you should eat fish from the lake – including once a month max for some species.
Streets also noted that they don't know the long-term effects on the environment; it hasn't been studied long enough.
What's being done about?
Douglas Corporation stopped using PFOS products in 2010, and the Pollution Control Agency says the company has "taken a number of other actions" to stop the chemical from getting out, including closing roof vents, replacing the roof, and replacing plating.
The levels of PFOS coming off of it have gone down, a lot, MPCA says. The last round of testing, which came three years ago, found PFOS in fish was down, and the agency will test again in 2016 for more recent results.
But there's still some of it coming from the storm water.
So Douglas and the MPCA reached in agreement in May (just announced this week): The company will monitor storm water coming from its building, and has to design a way to capture storm water before it leaves, or treat it so there's less PFOS in it.