(Photo by Jason Vines)

It turns out, not all D.C. duck news is good duck news.

About 80 ducklings have been found dead in the Lincoln Memorial Reflecting Pool since May 20, the victims of “high levels of parasites that develop and grow in snails that live in the pool,” according to the National Park Service. The Reflecting Pool will be fully drained beginning on Tuesday so that crews can completely clean out the bottom. Work should be completed by next Friday.

The snails aren’t new, but the high levels of the parasite certainly are.

“There have been snails in there for as long as anyone can remember. The aquatic environment is a perfect habitat for them,” says NPS spokesperson Mike Litterst. “But why now as opposed to any other time? We’re probably really never going to know why now or what the cause of this was.”

The dead ducklings first turned up on May 20. Over the course of two days, 54 were found floating in the pool. (This is not the reflecting pool that recently added a duck ramp to help mallards in and out of the water—that’d be the Capitol Reflecting Pool.)

NPS tried treating the water, but it turns out that not even chlorine and other chemicals were enough to stop the water-borne parasite. Still, their efforts managed to slow down the die-off rate; only about 30 more birds were found dead in the following weeks, the last one on June 6.

Necropsy results recently pointed to the parasite as the cause. It also causes cercarial dermatitis, otherwise known as “swimmer’s itch,” in humans, though no cases have been reported. “To get it, you really have to have some sustained contact in the water,” Litterst says, so people who might have gotten splashed or briefly dipped a foot in the pool are at very low risk.

One of the theories floated is that the high parasite levels might somehow be related to the hot spell that the city experienced a few days before the ducks died.

NPS also didn’t drain and clean the Reflecting Pool this winter, as has been the case in years past. The thinking was that recent efforts to mitigate the geese population (via border collies chasing them away) also lessened the amount of droppings in the water, rendering an annual cleaning unnecessary. The plan was to do the cleanings every other year, though Litterst says they may rethink that in the wake of the mass duckling die-off.

It’s not the first time that an extraordinary event has forced NPS to drain the pool. In 2012, a massive algae bloom built up in the circulation system, and outside contractors had to be brought in to clean it up. This time, NPS officials will able to handle the work in-house. It is not yet clear how much it will cost.