The water has tested positive for toxic blue green algae in numerous locations. Water samples have been sent to a state lab and a federal EPA lab to determine just how dangerous the levels are.
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Cyanobacteria are disrupting the leisurely waterfront lifestyle in Discovery Bay.
"Residents are concerned because dogs play in the water, kids swim in the water, they're waterskiing, wakeboarding, and jet skiing," said boat charter business owner Frank Morgan.
And exposure to the blue green algae toxins is potentially dangerous.
"You can have skin irritation, you can have G.I. gastrointestinal problems, you can have neurological concerns such as foaming at the mouth for the dogs," said environmental health director Dr. Marilyn Underwood.
Last year, three dogs died after drinking contaminated water from Lake Chabot in Alameda County.
The Contra Costa County Environmental Health Division tested the waters of Discovery Bay again Tuesday. Once results come back from state and federal labs they are likely to post warning signs throughout the waterfront neighborhoods.
"I was just talking with another customer of mine earlier and she was talking about having her grandkids over for the weekend and I said just make sure that they take a good shower afterwards, don't go to bed with Delta water on you," said Delta diver Nick Chamberlain.
And the hot weather and the hot shallow water only exacerbate the problem.