Anyone who’s visited a shore of Lake Washington probably guessed it.

When parks officials announced Sunday that six beaches on King County’s largest lake were closed due to high bacteria levels in water samples, locals’ minds pegged a possible culprit immediately: A little goose poop.

Or a lot of goose poop, as the case may be.

It’s hard to say for sure, though, said Daniel Nidzgorski, an ecologist with the King County Department of Natural Resources and Parks.

First off, Nidzgorski wants you to know it’s not unusual or alarming for E. coli — most of which comprises “good gut bacteria and does not cause illness” — to be found at beaches in the Seattle area. That’s what happens when people and animals live by water, he said Monday.

But some types of E. coli can make people sick when they’re transmitted through contaminated water or food. So, when the count of E. coli bacteria gets above a certain level at any of the almost 30 King County beaches tested weekly, public health officials are notified and they decide whether to close the waterfronts to all swimmers and waders.

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That’s what happened Sunday, resulting in the closures of Matthews, Mount Baker and Madrona beaches in Seattle, Enatai and Newcastle beaches in Bellevue, and Gene Coulon Beach in Renton. All six have been deemed unsafe for people and pets.

Now, as to who or what is to blame for the presence of that bacteria, Nidzgorski cannot definitely say.

“All the bacterium tells us is, there is poop in that water,” he said, “and it could come from any warm-blooded animal — people, pets and wildlife.”

Tests to determine the source of the fecal bacteria have not been conducted. They usually take a couple of weeks, he said.

But when tests have been done in the past to determine whether people or pets are the problem, they come up negative, Nidzgorski said.

“We think, but can’t really prove, that geese and ducks are responsible for many of the beach closures around here,” he said, explaining that the tests to determine the presence of E. coli from people and pets are accurate but not those for waterfowl.

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“Our parks tend to be very attractive to geese and ducks,” he said. “Geese love the same things people do: an open, grassy field with an unobstructed beach into the water.”

He said some parks have had some success discouraging geese by re-landscaping and planting bushes and shrubs between the fields and the water. However, that solution “blocks people, too.”

Water samples from closed beaches are tested twice a week until the levels are deemed safe enough for them to reopen, according to King County.

The beaches had been on a shrinking list of lakeside sites that are still open as others shut down throughout the region because of concerns related to the COVID-19 pandemic. Seattle closed Magnuson Park Beach, Seward Park Beach and East Green Lake Beach for the entire summer because of budget cuts.

Mercer Island closed Groveland Beach Park, and in Kirkland, the pier at David Brink Park and the pier and parking lot at Houghton Beach Park were closed to avert large gatherings.  

Seattle Times staff reporter Paige Cornwell contributed to this report.

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