The U.S. Fish and Wildlife Service says Cheyenne Bottoms, a central Kansas site that includes internationally recognized wetlands, aren’t fully functioning and could be threatened by oil drilling.

Heather Whitlaw, field supervisor for the Fish and Wildlife Service in Manhattan, says her agency opposes a recent request by a Kansas company to drill for oil in Cheyenne Bottoms, a 41,000-acre land sink that’s also the largest interior marsh in the U.S. and where about 250,000 waterfowl stop during seasonal migrations.

The Army Corps of Engineers is expected to have a decision by the end of the year.

The Fish and Wildlife Service also has concerns about the impact of nearby oil production outside the Bottoms, which is on the state’s list of impaired waters because of siltation and oxygen depletion.