The Lincoln Memorial Reflecting Pool is being drained after a parasite killed about 80 ducks there late last month.

The National Park Service began draining the pool Sunday after the ducks, many of them ducklings, were found to have been killed by parasites that grow and develop in snails that live in the pool, according to a NPS release. Officials said that chemical treatments alone could not sufficiently reduce the snail and parasite population.

NPS will clean and treat the pool for parasites in a process lasting about three days, according to the release. The pool is expected to be fully refilled by Monday.

Park officials said the parasites were a minimal risk to humans who could only be affected by “sustained contact” with the water in the reflecting pool. The parasites can cause “swimmer’s itch,” a noncontagious rash that rarely needs medical treatment, the release states.