Mark Thompson of Montana Resources outside the Horseshoe Bend Treatment Plant in Butte, Mont., in December, says fears about the plan to treat toxic Berkeley Pit water at the treatment plant are “overblown.”

BUTTE, Mont. — It was an unusual and unfortunate confluence of events: A larger-than-normal number of geese was making a later-than-normal migration over Montana when a snowstorm blew in at the wrong time and sent them soaring to the wrong place.

The white birds splashed down in a 50 billion-gallon toxic stew that is part of the nation’s largest Superfund site. At least 3,000 died.

Residents of this mining city say the snow goose deaths this fall were a wake-up call that raises broader questions about the Berkeley Pit and whether federal regulators will be ready when the former copper mine that collects heavily acidic, metal-laden water reaches capacity.

“We need to be prepared, and they’re not prepared,” former state lawmaker-turned-activist Fritz Daily said. “I’m talking about the environmental future of this town, I’m talking about the economic future of this town, I’m talking about the social future of this town.”

The Anaconda Copper Co. mined thousands of miles of tunnels under Butte over a century, finding gold, silver, lead, zinc, manganese and especially copper. The old mine shafts started flooding when mining there ended in 1982, sending contaminated water into the Berkeley Pit. It’s been slowly filling up ever since.

The liquid is expected to reach a critical level in 2023, and environmental officials are finalizing a plan for keeping it from contaminating Butte’s groundwater and a stream at the headwaters of the Columbia River basin. The critical level of 5,410 feet, set by the Montana Bureau of Mines and Geology as the lowest elevation in Butte’s drainage system, is the point at which water from the pit and flooded mines under the city could escape into other waterways. Fifty feet above that, the pit and mine water would enter the city’s groundwater.

The U.S. Environmental Protection Agency along with Atlantic Richfield, which bought Anaconda Copper in 1977, and Montana Resources, owner of an adjacent mine, negotiated a solution that relies on a water treatment plant built near the pit in 2000. The Horseshoe Bend plant now treats more than 4 million gallons of water a day that would otherwise flow into the pit, and the plan is for it to treat another 3 million gallons a day directly from the pit starting in 2023.

Mark Thompson, Montana Resources’ environmental affairs manager, compared the plan to a sewage treatment plant that continuously discharges water that meets environmental standards. “It’s nothing to be worried about,” he said. The recent mass goose deaths were an anomaly, he said, and activists’ attempt to tie them to a threat to Butte’s human population is “well overblown.”