BUTTE, MT - JULY 06: A camper is parked near a contaminated slag wall near downtown Butte on July 6, 2017 in Butte, Montana. Butte is home to the toxic Berkeley Pit. Formerly an open pit copper mine, today the Berkeley Pit is part of the largest Superfund site in the United States. In its heyday, in the 1860s, the Butte area was known as the "Richest Hill on Earth" due to the bonanza in copper and other metals. (Photo by Janie Osborne/Getty Images)

Nevada wants to back out of an agreement to designate a toxic mine a priority U.S. Superfund site, a move critics warn could leave state taxpayers on the hook for hundreds of millions of dollars in cleanup costs. State regulators say a new strategy with private backing would save money and ensure faster cleanup. However, a watchdog group says the state’s unprecedented effort would further delay restoration of groundwater polluted by uranium.