The Webber Natural Swimming Pool in north Minneapolis is back open after it was closed for nearly two weeks because someone threw an unknown substance into the water.

Over the years, the pool has had issues with staying open — usually because of high levels of bacteria in the water — and that’s something the staff at the Minneapolis Park and Recreation Board has been hard at work trying to solve.

Staff say they face new challenges because of just how unique this pool is, the only natural public swimming pool in the United States.

“It’s hard to say what’s coming next because we’ve had a lot of unusual challenges,” said Rachael Crabb, water resources expert with the Minneapolis Park and Recreation Board.

Crabb says because the pool is treated using a natural process instead of chlorine, they get plenty of unwanted visitors like waterfowl.

“They’re a big source of bacteria for us,” Crabb said.

But if you take a look around you can see several steps they’ve taken to keep them out like black netting on the fence outside the pool. They’ve also removed nests in the spring. But this summer, it’s the turtles causing problems.

“These turtles were born in old Webber pond and when they are old enough to reproduce they try to come back to where they were born and nest,” Crabb said.

Flooding along Webber Parkway used to be an issue, but they’ve since built a floodwall.

“If that water gets in the pool and the [regeneration area], we would definitely be closed because stormwater is very dirty,” Crabb said.

So why not just convert it to a chlorine pool? Crabb stresses it’s not that simple.

“There would need to be some significant redesign, and it would be a significant cost to do that,” Crabb said.

Crabb agrees it’s no fun closing for high levels of bacteria, but she fully believes the work they put in greatly reduces the days they are forced to close.

“Every year is different and when we find problems we figure out ways to solve them,” Crabb said.

Webber Natural Swimming Pool will shift to modified hours on Aug. 19 and close for the season after Labor Day.