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MINNEAPOLIS — The Minnesota Supreme Court will hear arguments Wednesday on an attempt by environmental groups to cancel a key permit for a long-stalled copper-nickel mine.

Opponents of PolyMet Mining Corp.’s project say state regulators should have included “end-of-pipe” limits on discharges of mercury, sulfates and other pollutants in the water quality permit. They also say the state improperly tried to suppress the concerns of the federal Environmental Protection Agency. The Minnesota Pollution Control Agency and PolyMet counter that the permit meets the legal requirements and that the project won’t violate clean water standards.

PolyMet plans to build the open pit mine near Babbitt and reuse a former iron ore processing plant a few miles away near Hoyt Lakes. The sites are in the upper St. Louis River watershed of northeastern Minnesota, which eventually flows into Lake Superior.

The water quality permit is one of three major permits issued to the company four years ago that are on hold due to legal challenges. The others are the project’s overall permit to mine and a federal wetlands destruction permit. PolyMet, whose majority shareholder is Swiss-based commodities giant Glencore, has been unable to proceed with construction of the $1 billion project without them.

Paula Maccabee, an attorney for WaterLegacy, one of several groups that will be arguing against the permit Wednesday, said the case is an important test “of both whether the courts will help us defend clean water and human health, and whether the courts will help us defend regulatory integrity in Minnesota.”

PolyMet’s project is separate from the proposed Twin Metals mine near Ely, which the Biden administration is trying to kill because it’s in a watershed that flows into the pristine Boundary Waters Canoe Area Wilderness.

Maccabee said the water quality permit issued to PolyMet won’t protect humans, fish and other aquatic life, and wild rice that grows in downstream waters, from the harmful effects of mercury and sulfate discharges. But PolyMet and the MPCA say the project will result in net reductions of those discharges because the mine would capture and treat seepage from an old iron mine waste disposal pond that PolyMet would repurpose for its own waste.

The opponents — which also include the Friends of the Boundary Waters Wilderness, the Fond du Lac Band of Lake Superior Chippewa, the Minnesota Center for Environmental Advocacy and the Center for Biological Diversity — also argue that the MPCA improperly tried to keep the EPA’s criticisms of the draft permit out of the public record, resulting in a weaker permit. Lower courts disagreed.

PolyMet spokesman Bruce Richardson said developments over the last four years mean the mine is needed even more.

Demand has soared for domestic copper, nickel, cobalt and other metals that the mine would produce, because automakers need them for electric cars. Responsible production of these metals is also crucial to the transition to other clean energy technologies such as wind and solar, he added. And the war in Ukraine “has cast an ugly light” on the perils of relying on foreign sources of raw materials and energy, he said.

“Minnesota and the U.S. need to embrace the safe production of these metals if there is any hope of meeting carbon reduction goals domestically and globally,” Richardson said.