“It’s a disaster, culturally and environmentally,” said Taylor McKinnon of the Center for Biological Diversity, one of the plaintiffs. He worries about the impact on the delicate alpine tundra and to human health should skiers fall into the treated sewer-water snow and ingest it.

The United States Forest Service, which owns the land where the resort is, says the treated water meets the highest standards — just below drinking water — and is already used to irrigate golf courses, soccer fields and parks, according to Corbin Newman, a regional forester.