The U.S. National Park Service said Friday that it would drain a reflecting pool on the National Mall in Washington to clear it of a parasite they think has killed more than 80 ducks since the first death was discovered May 20.

The pool sits in front of the famous Lincoln Memorial and near the long, low, black wall of the Vietnam Veterans Memorial. Experts said they had detected high levels of the parasites, which grow in snails that live in the pool.

The experts said they weren’t sure why the levels are higher this year than usual. It could be related to the recent spell of hot weather, or the fact that the pool was not drained and cleaned during the off-season this year, as has been done on the past.

“We’re probably really never going to know why now, or what the cause of this was,” said Park Service spokesman Mike Litterst.

Park officials did try treating the water in the pool, which helped, but about 30 more ducks died anyway.

So now the service plans to drain the pool next Tuesday so workers can completely clean the bottom of it. The work is expected to be finished by next Friday.

There was no word on what would become of the ducks who usually swim in the reflecting pool. There are several other bodies of water close by, including a pool near the U.S. Capitol that has a newly installed “duck ramp” to allow its web-footed visitors to enter and exit the pool more easily.