After a dam holding mining waste burst last January in Brazil, releasing a wall of sludge and killing more than 240 people, environmental advocates asked the state of Minnesota to stay permits they had granted to PolyMet Mining Corp. for a proposed copper-nickel mine with a similar storage dam.

Now, in support of that request, they have filed court documents showing that PolyMet used the same geotechnical engineer who helped the owner of the Brazilian mine evaluate its waste dams.

Through a PolyMet subcontractor, Illinois engineer Scott Olson conducted a peer review of PolyMet’s storage dam for mine processing waste, known as tailings.

“This is literally the same person and the same method being used to evaluate a similar dam ... and it’s failed and it killed 250 people,” said Aaron Klemz, a spokesman for the Minnesota Center for Environmental Advocacy (MCEA). “He somehow failed to mention that he had walked on the dam itself four months before it collapsed.”

The dispute is playing out in the Minnesota Court of Appeals, where a coalition of advocacy groups have filed a legal challenge to mining permits granted to PolyMet by Minnesota regulators.

PolyMet and Olson have argued that nothing about the collapse in Brazil affects the safety calculations of PolyMet’s dam. The slope of the wall on the Córrego do Feijão mine dam was steeper than the one for PolyMet’s dam, they said in March court documents. Olson said his method for assessing risk failure was “misused” in a 2013 evaluation of the Brazilian dam. If it had been used correctly, he said, it would have predicted a catastrophic failure.

Olson, who teaches at the University of Illinois at Urbana-Champaign, said in an interview that the Minnesota mine opponents are “mischaracterizing” his role.

New documents filed in the case on Friday, however, renew questions about the assessments of the two dams.

Olson was part of an international panel of experts hired last year by the Brazilian iron ore company Vale SA to advise it on the stability of its many mine-waste storage facilities. As part of that study, the panel briefly examined a new safety report on Vale’s Córrego do Feijão mine-waste dam, conducted by the German engineering firm TÜV SÜD, which certified the dam as stable.

The German firm used Olson’s method, although it called it “unorthodox and unusual” in its 2018 certification. The report filed by Olson and the panel of experts is dated October 17, 2018, just months before the Brazilian dam ruptured and collapsed.

The German company’s safety analysis, and the expert panel report for Vale, were both filed Friday by the MCEA, the nonprofit WaterLegacy and the Fond du Lac Band, whose reservation is downstream from PolyMet’s site.

The reports were not available to Minnesota’s lead regulator, the state Department of Natural Resources, before it approved PolyMet’s permit to mine and dam safety permit in 2018. Klemz said a mining consultant in Utah provided copies of the two reports.

Klemz argued that Minnesota needs to review the new information and determine whether the safety analysis for PolyMet’s dam was flawed.

PolyMet spokesman Bruce Richardson called the filings “recycled arguments” that the company will address in court.

A DNR official said the agency is reviewing the advocates’ request and expects to release a decision soon. “This review includes analysis of the group’s assertions about Scott Olson and the ‘Olson Method’of dam safety analysis,” Assistant Commissioner Jess Richards said in a statement.

“While Mr. Olson submitted materials to the DNR on behalf of PolyMet, all of the dam safety plans and analysis were independently reviewed by the DNR’s own dam safety engineers as well as additional Minnesota and international experts in dam safety” Richards said.

Risky design

The Brazilian dam and PolyMet’s mine-waste dam both use what is called an upstream design, where the wall of the dam is formed from the mine waste itself, stacked over time in a fashion that slopes in toward the slurry. The design is now considered too risky. Brazil has banned the construction of new upstream tailings dams and ordered existing ones to be decommissioned.

Olson said the expert panel hired by Vale was advising the company on its waste dams systemwide, not peer-reviewing the German company’s certification of the Córrego do Feijão dam. The panel never got a copy of the German company’s audit, he said, but only watched a one-hour presentation summarizing it. The panel visited the mines involved, including the Córrego do Feijão dam, but only a few pages of the panel’s resulting report are devoted to the dam that collapsed.

“The panel was specifically not convened to review or evaluate any specific tailings dam for Vale,” he said, but rather to help them develop an approach to evaluate “their entire tailings dam portfolio.’’

Olson also said the German company did not use his method correctly, skipping a crucial third step considering what would happen if the tailings liquefied. The panel did not green-light the dam before it failed, Olson said: “Our conclusions were not that the structure was safe.”

According to Olson, the panel made only a general recommendation that Vale should resume pumping water from the tailings before decommissioning the dam. They concluded “no other action” was needed until results of other assessments were done.

Olson also said he would have given the undrained dam a riskier rating than the one conferred by the German firm.

Paula Maccabee, a lawyer for WaterLegacy, said that Minnesota needs to know if PolyMet performed the right kind of analysis on the tailings dam “before it’s too late.”