The Minnesota Supreme Court denied a request from the Minnesota Department of Natural Resources to add several documents into the record that would have supported a clay liner for the tailings basin of PolyMet’s proposed copper-nickel mine.

The documents could have helped bolster the state agency’s argument that a bentonite liner would prevent pollution, but the court on Monday said it was too late to add to the record and noted DNR itself had already considered the record complete.

Without the documents, the DNR may not have enough evidence to prove the liner works, again putting the project's permit to mine and dam safety permits, which are in front of the Supreme Court, in question.

After justices in last month’s oral arguments asked where studies cited by the DNR actually appeared in the record, the DNR filed a motion to supplement the administrative record with documents it said it used when considering whether bentonite, a clay, could line the tailings basin and effectively prevent water seepage and oxidation.

Even though the studies were not in the permitting record — information the agency relies on to make a permitting decision — the agency said additions should be made now because they were used in the project’s final environmental impact statement, which in turn was used in the permitting decision.

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But a coalition of environmental groups and the Fond du Lac Band of Lake Superior Chippewa opposed adding the two literature reviews and one study into the record.

On Monday, Chief Justice Lorie Gildea sided with opponents, writing in an order that at the Court of Appeals level, the DNR had considered its permitting record complete and actually opposed a similar move when opponents sought to add documents to the record.

“We believe the same fundamental principle applies with equal force here,” Gildea wrote. “Thus, we hold the DNR to its word that the record is complete and contains all the documents that it considered in making findings on the effectiveness of bentonite.”

In January, the Court of Appeals reversed the dam safety permits and permit to mine awarded by the DNR to PolyMet in late 2018 and said the agency must hold a contested-case hearing over concerns on the upstream tailings dam construction; the basin's bentonite liner; alternative methods of tailings storage; the financial assurance package; and if PolyMet's majority shareholder Glencore should be on the permit. The Supreme Court took up the case after PolyMet and DNR appealed the decision.

PolyMet, which is planning the state's first copper-nickel mine near Babbitt and Hoyt Lakes, would line and cap its tailings basin — a dam that would indefinitely store waste rock left behind after processing out copper, nickel, cobalt and other metals — with bentonite to prevent oxygen from contacting the tailings and creating acid, the type of pollution environmentalists fear most with copper-nickel mining. Bentonite would also be mixed in with the tailings.

But the method is largely unproven and if the Supreme Court determines that as well, it could play a role into whether it upholds the lower court's decision.

In its January opinion, the Court of Appeals said: “there has been disagreement, even among DNR’s own consultants, as to whether the bentonite amendment would be effective, and whether it would have other, negative effects."

It also cited DNR’s own consultant, who said: “The bentonite seal is a Hail Mary type of concept in my opinion. I believe it will exacerbate erosion and slope failure and will eventually fail.”

Additionally, the opinion noted that environmental groups presented evidence that bentonite was unproven or ineffective.

In its brief opposing the additional documents last month, the environmental groups and Fond du Lac Band urged the Supreme Court to order a contested-case hearing on bentonite.

“The court is left with one conclusion: DNR’s decision to issue the permit approving the bentonite scheme is unsupported by substantial evidence and its decision should be reversed,” the groups wrote. “At very least, this court should order a contested case hearing to resolve disputed material facts regarding the efficacy and safety of PolyMet’s bentonite scheme.”

Justices have expressed doubt that evidence showing bentonite works too.

During last month’s oral arguments, Justice Natalie Hudson asked DNR if there was actually “substantial evidence” on whether bentonite would work.

“Some of those conditions essentially allow, at least as I understand them, allow PolyMet to test the bentonite amendment for a number of years to see if it works, which would suggest that it might not work,” Hudson said. “And then they can do a study later on years down the road, years after the mine has been in effect.”

DNR attorney Jon Katchen defended the permits and said the agency “imposed a rigorous monitoring and testing program” on bentonite and that it working is a condition of a dam permit and the permit to mine, which is standard in the mining industry.

“There can be no construction (of the tailings basin) until PolyMet proves the effectiveness of bentonite,” Katchen said. “So there’s no deposition of tailings into the basin without DNR determining bentonite will, in fact, be effective.”

The Minnesota Supreme Court usually makes a decision on a case several months after oral arguments.

A number of other PolyMet permits are also working through the court system, including a separate Supreme Court case for the mine’s air permits and a Court of Appeals case on the water permits.