The 500-year floods tested the integrity of the mine’s tailings storage structures in a way that no one at the company wanted. But the infrastructure performed exactly as engineers predicted, and none of it was compromised, according to McDowell, the U.S. region vice president for legal, environmental and government affairs at Sibanye-Stillwater.

Now the company is looking toward the next phase of production at the set of platinum-palladium mines. The company wants to build two new storage structures above the East Boulder drainage, including a pond that’s capable of holding 1.17 billion additional gallons of tailings.

The storage space is needed if the company plans to mine the J-M Reef through the next couple of decades. The rock layer within the Stillwater area holds one of the world’s richest deposits of platinum and palladium — rare metals used in catalytic converters.

To construct the new facilities, the company needs permits from the U.S. Forest Service and Montana Department of Environmental Quality. The agencies are reviewing the company’s proposal. They plan to release an environmental impact statement, then collect more public feedback in the summer.

In the meantime, two conservation groups and several landowners have concerns about the downstream risks that a breach of the tailings dam would pose. They’re asking the company and agencies to consider incorporating additional protections into plans for the site.

Specifically, Montana Trout Unlimited, Earthworks and several landowners would like the Custer Gallatin National Forest and Montana DEQ to analyze the possibility of storing tailings that have been filtered into a dry state, so that the waste can be stacked and transported away from the area.

“I live under Goliath’s spear,” said Noel Yanots, a downstream resident on the East Boulder. “A failure of the tailings pond will destroy everything I have with zero insurance coverage for that failure. I want the mine to be successful. I know that there are options, such as dry tailings, to reduce the downstream risks. I urge a close look at these options. It would certainly make us all sleep better.”

Sibanye-Stillwater would like to incorporate the technique into future plans, but McDowell claims the technology hasn’t advanced enough for the tailings produced at the East Boulder Mine.

DEQ’s Hard Rock Mining Bureau wrote in an email that the state agency, in partnership with the Custer Gallatin National Forest, is still working out many of the details behind the project, which will be shared when a draft environmental impact statement is released.

“We have received comments through the scoping process expressing interest in filtered tailings and that alternative is part of our ongoing deliberation,” the agency said. Staff noted that with the addition of the proposed facility, the tailings storage capacity at the mine is projected to last until around 2042 to 2045.

—

The Stillwater Complex has been mined since the latter portion of the 19th century, and the continuity of the ore grade and the scale of the mineral deposits there are unique, the Custer Gallatin National Forest wrote in its 2020 land management plan.

Today, the Sibanye-Stillwater Mining Company owns two large underground platinum and palladium mines within the area, both of which produce the highest-grade ore in the world.

The East Boulder Mine, about 25 miles south of Big Timber in the Beartooth Mountains, has operated since 2001. The Stillwater Mine lies about 12 miles to the west, and it has operated since the 1980s. Slowly, employees at both mines are digging toward one another, McDowell said.

According to documents from the company, 50% of the tailings from the East Boulder Mine are used to backfill the underground mine and the other 50% are deposited into the tailings storage structure, which has the capacity to support production through 2025.

To accommodate the next phase of the process, the company needs to build the Lewis Gulch Tailings Storage Facility and Dry Fork Waste Rock Storage Area, which will allow it to store more tailings and waste rock as production continues.

Platinum and palladium are among the country’s most critical minerals, and the Stillwater area’s J-M Reef holds the only known deposits in the United States.

The mining company’s latest estimates show that there’s enough ore body there to last for another couple generations of workers, and the long lifespan of the site is part of what motivates Sibayne-Stillwater to operate sustainably, McDowell said.

“It’s not like a lot of extraction with a boom cycle where there’s only a little to be produced,” she said. “Here we feel a big responsibility that we need to do everything right. Because the lifespan of the resource is long, it truly makes sense for us to get things right for our communities and neighbors as much as we can.”

McDowell pointed to a 2022 economic impact study from the University of Montana, which identifies Sibanye-Stillwater as one of the largest private sector employers in Montana, altogether contributing more than $6 billion in economic output to the state annually.

The report also notes that in 2021, the mining company provided just under 2,000 jobs, netting its employees an average compensation of about $150,000 per year.

“We truly are a huge economic driver,” McDowell said. She added that the company is legally bound to a Good Neighbor Agreement with the Northern Plains Resource Council and other local organizations. The groups work to ensure proactive steps are taken to protect water quality and the environment as mining proceeds.

—

While the economic value of the East Boulder Mine is significant, Bonnie Gestring, the Northwest program director at Earthworks, worries that the consequences of a dam failure at the proposed storage pond would be catastrophic.

Earthworks, Montana Trout Unlimited and several landowners in the Boulder River area released two new animated videos that simulate what could happen if the dam around Sibanye-Stillwater’s proposed Lewis Gulch Tailings Storage Facility fails.

The simulations, produced by the engineering firm Lynker, are based on the mining company’s own breach assessment, which identifies two possible dam breach scenarios. Sibayne-Stillwater claims that both are extremely unlikely.

Under one scenario, coincidental 1,000-year to 500-year floods would cause water to rise over the pond’s embankment, causing massive erosion and the release of mine tailings and water, according to scoping documents.

Lynker’s model simulations suggest that the wave of tailings and water could reach the East Boulder Campground within 20 minutes. By minute 26, peak flows could exceed 88,000 cubic feet per second, and the river channel’s depth would hover at around 28 feet.

The mass of water and mine waste would reach the confluence of the Boulder and West Boulder rivers within about two hours, and flows there would peak at more than 30,000 cubic feet per second about a half hour later. Depths within the river channel would rise to 20 feet.

At the confluence of the Boulder and Yellowstone rivers in Big Timber, it would take about four-and-a-half hours for the water and tailings to arrive. About an hour after they do, flows would peak at 30,000 cubic feet per second, and the depth of the river would rise to 16 feet.

The mining company points out in its assessment that properties along the Boulder River would be inundated anyway if floods of that scale were to occur. It adds that under a “sunny day scenario,” which factors in a major earthquake, downstream flows would have less volume and would travel at a slower pace.

McDowell said that Sibanye-Stillwater uses world-class designs for its tailings storage structures, and the company’s engineers construct them so they can withstand the maximum amount of rainfall or seismic activity that experts deem plausible within the area.

At the East Boulder Mine, engineers plan to design the new tailings storage structures to withstand a 7.5 magnitude earthquake, which releases over two times the energy of the 7.3 magnitude earthquake that killed 28 people near Hebgen Lake in 1959.

Engineers also plan to design the storage structures so they can withstand a rain-on-snow flood where a total of 47 inches of precipitation accumulates within a 24-hour period, according to documents provided by the company.

For comparison, the National Weather Service reported that up to 5 inches of rain fell on the Beartooth and Absaroka Mountains during the June floods in 2022. That combined with up to 5 inches of water, amounting to a maximum of about 10 total inches of water.

—

The 2014 Mount Polley Mine disaster in British Columbia showed what can happen when a dam around a tailings storage pond collapses, Gestring said. At the time, glacial movements underneath the dam caused its foundation to weaken and eventually breach.

More than 6 billion gallons of toxic waste from gold and copper tailings poured into lakes, rivers and streams throughout the area, contaminating drinking water and salmon spawning grounds.

In 2015, a panel of engineers issued a report about the event, and it recommends that the industry shift toward storing tailings below ground. It also calls on companies to use filtered tailings technology to remove water from mine waste instead of storing it in liquid form.

Even though Sibayne-Stillwater claims the chances of a failure at the Lewis Gulch Tailings Storage Facility is low, the statistics only go so far when you factor in human error, which is often the cause of serious dam failures, Gestring said.

Earthworks, Montana Trout Unlimited and several landowners would like to see the mining company, Montana DEQ and the Forest Service consider the possibility of storing filtered tailings as they analyze different alternatives for the upcoming project.

“Despite industry promises, the reality is that tailings dams fail, and severe tailings dam failures are happening more frequently,” Gestring said in a news release. “It’s essential that the agencies consider the most protective options during the mine permitting process.”

McDowell said Sibayne-Stillwater is motivated to incorporate the dry stacking technique into future plans. The problem is that technology has not advanced to a point where tailings with a fine grain, like the waste that’s produced at the East Boulder Mine, can be filtered and stacked.

Experts have been looking at ways to develop the technology for many years, but the reality is that the company needs to move ahead with the conventional method of waste storage or the mine will shut down and it won’t be able to employ people, McDowell said.

“We absolutely want to do dry stacking, primarily because it takes up less space, and our footprint and the area we operate on is very small,” she said. “We’ve been actively studying it and researching it for a number of years... but it’s not going to be developed in time.”

Dan Vermillion, owner of Sweetwater Travel Company and a former Montana Fish and Wildlife Commissioner, said the community largely agrees that the mine has run a quality operation in Sweet Grass County.

People seem to agree that the owners have done a good job maintaining water quality and protecting the downstream fishery, and while Vermillion has no short-term concerns, he is worried about the potential that the tailings dam could fail, irreparably harming ranches, fish, wildlife and the town of Big Timber.

Vermillion said he takes issue with the idea that the probability of such an event happening is low, since evidence suggests that weather patterns are only getting more volatile.

Last year’s flooding is an example. It might be an aberration, but increasingly, we’re seeing “extreme droughts and extreme wet moments,” Vermillion said.

The East Boulder River drains out of the Absaroka-Beartooth Plateau and into the Boulder River’s main stem, where some of Montana’s finest float-accessible fly-fishing lies. The campgrounds along the drainage are well-used, and the water supports Yellowstone cutthroat trout and lots of irrigation, he said.

Ultimately, the mine’s operators have been great neighbors who’ve done a great job, “but we’re asking them to do a little bit more,” he said. “When we operate in a place of great environmental quality, it behooves us, if we can, to make the extra step so we protect the environment.”