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A dam collapse in Brazil has some worried about PolyMet’s plans. Why the DNR says it won’t happen here

The disaster in Brazil raised eyebrows among opponents of the PolyMet copper-nickel mine planned near Hoyt Lakes. That’s because the design of the dam in Brumadinho was similar to one PolyMet hopes to use for its project. 

Brumadinho dam break
A view of the Vale dam that burst, in Brumadinho, Brazil, on January 25.
REUTERS/Washington Alves

In January, the tailings dam at a Brazilian iron ore mine collapsed, killing nearly 250 people. The wave of toxic waste and mud also wrecked two dozen buildings and polluted water for five miles.

In Minnesota, the disaster raised eyebrows among opponents of a copper-nickel mine planned near Hoyt Lakes. That’s because the design of the dam in Brumadinho was similar to one PolyMet Mining hopes to build. In fact, the Vale mining company had used a method to judge dam safety created by a PolyMet adviser. And the tragedy in Brazil embodied the worst fears of some Minnesota environmental activists and the Fond du Lac Band of Lake Superior Chippewa, who warn PolyMet could pollute the St. Louis River.

Those groups, and some DFL state legislators, recently asked state regulators to reconsider authorizing PolyMet’s mining plan. “The DNR must modify the tailings … storage permit to require best practices, like dry tailings storage,” which don’t require a pond or dam, says a letter last month signed by 18 Democratic lawmakers. “Minnesota should accept nothing less after the tragic collapse of a similar tailings dam in Brazil that killed 250 people.”

The state Department of Natural Resources, however, says any similarities between PolyMet and the Brumadinho dam are limited. DNR Commissioner Sarah Strommen announced last week the agency would not change two critical permits granted to PolyMet last year after researching what happened in Brazil.

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“We understand people’s concerns with these dam failures and whether those events indicate a fundamental design issue with PolyMet’s dam,” Strommen said in a news release. “Our analysis demonstrates that there are significant differences in site conditions, engineering design, and operating requirements and we remain confident in the safety of the PolyMet tailings dam as permitted.”

Here’s what we know about the criticism of the PolyMet’s tailings dam — and why state regulators approved it:

Same construction method as Brazil dam

Those who aim to stop PolyMet because it carries a risk of acidic runoff and water pollution have long criticized the company’s tailings dam. PolyMet expects to repurpose an existing tailings basin once owned by LTV Steel Mining for processing taconite. Waste from the mine is covered in water in a tailings pond, and the slurry is held back by a dam. 

A consultant hired by the DNR to research the state’s financial protections with PolyMet — but not dam safety specifically — has said the setup approved by DNR was “inherently unstable and irresponsible” and “will eventually fail.” Environmental groups have often used those words to recommend PolyMet switch to “dry stack” tailings, in which the company drains water from tailings and piles the sand-like remains.

three types of dam construction
Minnesota DNR
A graphic in a state DNR dam safety permit for PolyMet, showing three types of dam construction. PolyMet's use of the 'upstream' method is controversial, as the 'upstream' method was used by Vale mining company at the Brumadinho mine.
Twin Metals Minnesota, a company hoping to build a copper-nickel mine near the Boundary Waters Canoe Area Wilderness, recently announced it would use dry stack technology for much of its tailings, rather than covering the waste in water.

After the collapse in Brazil, PolyMet’s opponents once again raised questions about the possibility of a similar disaster in Minnesota.

The PolyMet dam will be built using what’s known as the “upstream” construction method, a common design in which the dam is built upward in step-like stages, slanting inward toward the center of the tailings basin. Upstream construction was used at the Brumadinho dam, and Brazil banned the method after the collapse. “It is shocking that Minnesota would permit PolyMet to open a new mine using such a risky system, when even Brazil now prohibits them,” the letter from DFLers says.

PolyMet also used the Olson Method — a safety assessment used to analyze the strength and stability of a tailings dam — and hired its creator, Scott Olson, as a consultant. The method was also used in Brazil for the Brumadinho dam.

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In response, environmental groups such as WaterLegacy and Duluth for Clean Water, as well as the Fond du Lac Band, asked the DNR to reconsider PolyMet’s Dam Safety Permit and a Permit to Mine.

DNR: PolyMet is different

The DNR has dismissed the idea of PolyMet using dry stacking. The tailings storage is less feasible in wet environments, the agency says, and re-using the LTV Steel facilities would have less impact on wetlands, since the tailings basin PolyMet would repurpose already exists. 

Twin Metals plans to put about half of its tailings back into its underground mine, especially during cold and wet conditions. Since PolyMet is an open-pit mine, that is not an option and would risk pollution from a wet tailings pile, said Jess Richards, an assistant commissioner at DNR. PolyMet also intends to collect and treat water from the tailings basin to stop polluted seepage currently happening at the old LTV site.

“So it’s not as simple as to say, ‘let’s just build a different tailings basin,’ ” Richards said in an interview.

Before the collapse in Brazil, the DNR’s regulators approved the idea of a tailings dam and basin. Their team included a member of panel that investigated a 2014 dam failure at the Mount Polley mine in British Columbia (which did not use an upstream design for its dam).

Still, Richards said the DNR “took a hard look” at concerns from the Fond du Lac Band and the environmental groups over several months. Their conclusion: PolyMet’s proposed dam is much different from the Brumadinho one in Brazil, Richards said.

First, the dam in Brazil was steeper and perched on the side of a mountain. PolyMet’s dam will be flatter and “on relatively flat ground,” says Strommen’s order affirming the permits. The Brumadinho dam also had a significant amount of water flowing into the tailings basin that was piped away from the area, the letter says. The order notes reports that the pipeline breached, causing water to spill into the basin for several weeks before the dam collapsed.

The dam in Brazil was also more at risk of earthquakes and near active mining operations and mine blasting that “threatened the stability of the dam,” Strommen wrote. In Minnesota, actual mining will be done about eight miles from the tailings basin.

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Lastly, Strommen’s letter says the DNR independently reviewed Olson’s work. Olson himself did not review the Brumadinho dam’s stability analysis, and told the state it was used incorrectly in Brazil. Richards said the DNR checked whether the dam could fail in extreme circumstances, such as if there was “full liquefaction of the basin,” extreme rainfall or an earthquake.

“There’s a real big difference in the level of rigor and conservative assumptions that were put into the PolyMet analysis compared to the Brumadinho dam in Brazil,” he said.