Incident raises questions about how Trump’s administration would the country’s largest Superfund site from leaking its poisonous mix into other waters

At least 3,000 geese were killed by a toxic stew formed by a former copper mine in Butte, Montana, last fall, raising questions about how the new Trump administration will handle the largest Superfund site in the country.

In November, a large flock of geese making a late migration over Montana was blown off course by a snowstorm, which sent the birds toward the abandoned copper mine. They splashed down into the 50bn-gallon pool, polluted heavily by acidic chemicals and metals, and died en masse.

Before the Trump administration took office last Friday, federal regulators were struggling to make a plan to keep the poisonous mix from leaking into other waters and seeping into the town’s groundwater system.

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“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, and earning the city of 30,000 the nickname “The Richest Hill on Earth”. The old mine shafts started flooding when mining there ended in 1982, sending contaminated water into the Berkeley pit. It has 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 Silver Bow creek, a stream at the headwaters of the Columbia river basin.

The critical level of 5,410ft was set by the Montana bureau of mines and geology as the lowest elevation in Butte’s drainage system. Above that, water from the pit and flooded mines under the city could escape into Silver Bow creek. Fifty feet higher, the pit and mine water would enter the city’s groundwater.

The Environmental Protection Agency (EPA), Anaconda’s owner Atlantic Richfield 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 4m gallons of water a day that would otherwise flow into the pit, and the plan is for it to treat another 3m gallons a day directly from the pit starting in 2023.

The plant has never handled 7m gallons at once, but EPA officials say there is enough time for a thorough review to ensure the contaminated water will never escape into the city’s Silver Bow creek, whose banks have yet to be cleaned of the mine waste that was dumped there for decades.

“We’re going to really start digging into pilot studies and performance testing,” said EPA project manager Nikia Greene, “to determine what kind of upgrades, what kind of redundancies do we need.”

Atlantic Richfield and Montana Resources, which operates Horseshoe Bend, are confident the treatment plant will work as designed. Atlantic Richfield spokesman Brett Clanton said the plant received $1m worth of upgrades in 2015, and additional improvements will begin by 2019.

Facebook Twitter Pinterest Old mining frames dominate the skyline of Butte, Montana. Photograph: Matt Volz/AP

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”.

But community activists worry the treatment-plant option is being favored because it’s the cheapest technology, not necessarily the best. They also noted that only last month, thousands more geese died at the pit under similar circumstances.

They also suspect Atlantic Richfield will seek a waiver from the EPA allowing the company to discharge pit water into Silver Bow creek, even when that water does not meet Montana’s clean water standards.

If that happens, Silver Bow creek will be devastated, and the hundreds of millions of dollars spent on cleaning waterways downstream will be erased, according to a community coalition, Restore Our Creek.

“When you think about the Berkeley pit water, whatever form that takes, it’s going to have to go through this one mile,” member Evan Barrett said of the creek running through the city. “I think the community understands this mile is critical to our future and also the future of the whole watershed.”

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Greene acknowledged the EPA is studying a possible waiver of the state’s water quality standards, but said he could not elaborate because it is part of confidential negotiations with Atlantic Richfield. Donald Trump’s pick to lead the EPA, Scott Pruitt, has not yet been confirmed by the Senate; activists have called Pruitt, a state attorney general with a history of suing the EPA in favor of corporations, an “unprecedented disaster” for the environment.

“I’m very confident that whatever discharged water, however it’s treated, will not be allowed to be discharged if it affects Silver Bow creek,” Greene said.

Perhaps the community’s biggest concern is that there is no strategy for the Berkeley pit beyond keeping the water from reaching the critical level.

Daily and the Restore Our Creek coalition members said the plan only ensures that Butte will remain a Superfund site forever. That would be unusual for a program meant to permanently clean places polluted by hazardous pollutants. Cleanups of nearly 400 sites across the nation have been completed and removed from the Superfund list as of December, according to the EPA.

The EPA’s Greene could not say whether Butte will always be a Superfund site, though he said there will always be some form of operation maintenance for the treatment plant plan.

“That’s a tough question,” he said. “I guess Superfund is going to be there for a long time.”