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Park district officials also have an explicit warning to stay out of the water.
You can see that stern warning on signs all across Lake Opeka that display, "STAY AWAY FROM THE WATER."
Park district officials posted the signs because of toxic blue-green algae found blooming earlier this week. It's shut down all lake activity, including paddle boating, kayaking, boating, swimming and fishing.
Don Miletic with the Des Plaines Park District said the IEPA is instructing the district to "ride it out and it will take its natural course."
Miletic said it could be a week or more before the algae toxins dissipate and the waters reopen.
Here's what's behind it:
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According to the Illinois Environmental Protection Agency, it's what's called "cyanobacteria" that naturally occurs in lakes. When fresh water has the right nutrients in it like nitrogen and phosphorous, and receives a lot of sunlight when it's warm, toxic algae blooms can happen.
They can cause diarrhea and nausea in people, and can be fatal to pets. Pets like Stacey Block's 12-year-old dog Zero.
"He has on occasion gone up to the water and drank out of it, so I had to be real careful with him [Wednesday]," Block said. "I'm not even taking a chance."
The Park District said they don't want to take any chances either. It cancelled a Fall Fest fishing derby set for this weekend.
"We didn't want to put anyone at risk," Miletic said.
Now his team is investigating whether storm runoff could have contributed to the toxic bloom.