Whether the controversial PolyMet mine gets built in Minnesota’s Iron Range will likely come down to how it affects the environment.

Gov. Mark Dayton, who has the final say about whether PolyMet gets a state permit, said he wants to determine if the mine’s economic benefit is worth the environmental risks — risks he wants to make extremely small.

“Our responsibility is to insist upon the impossible perfection and get as absolutely close to it as humanly possible,” Dayton said in October of the PolyMet proposal.

The science surrounding the PolyMet project is under fierce dispute. Supporters point to a state report predicting PolyMet can meet all necessary environmental laws, while opponents say that report is flawed and point to other research warning of bigger dangers.

“There’s a tremendous amount of data,” said Bruce Richardson, PolyMet Mining’s vice president of corporate communications. “Groups may disagree with the findings. They’re entitled to those opinions. But the facts and the science bears out that the project meets regulations and meets the requirements of the law.

“Steve Morse, executive director of the anti-PolyMet Minnesota Environmental Partnership, disagreed.

“This is too risky for Minnesota,” Morse said recently of the state’s PolyMet environmental impact study. “There is certainly some risk that’s acceptable, but we’re not convinced that this type of mining can be done in this kind of water-rich environment. It’s really up to (PolyMet) … to make the case. And this doesn’t make that case.”

Here’s the latest about PolyMet’s potential impact to the environment in northeastern Minnesota.

What’s the big picture?

Unlike the Iron Range’s traditional iron mines, PolyMet would mine copper and nickel — the first such mine in Minnesota. In the process, it would produce toxic waste containing mercury and sulfides that are capable of damaging the environment.

PolyMet’s location along the Laurentian Divide in northeastern Minnesota highlights the scrutiny. It’s surrounded by wetlands and close to watersheds that drain eventually into the St. Louis River and then Lake Superior. Not far to the north, waters instead drain toward the Boundary Waters.

PolyMet argues that it can contain this waste and prevent almost all of it from leaching into the surrounding ecosystem.

After a review process lasting nearly 10 years, the Minnesota Department of Natural Resources concluded PolyMet would be able to limit pollutant releases to the amounts permitted by law.

But environmental groups don’t trust the DNR conclusion that PolyMet could contain all its pollution. They say too much is unknown and that the risks are too high to approve the mine without further independent review.

What kind of pollutants would PolyMet produce?

In addition to copper, nickel and platinum, the rocks dug up at the PolyMet pits also contain trace amounts of mercury and larger amounts of sulfide minerals — a form of sulfur.

When sulfides are exposed to water and air, a natural chemical reaction produces harmful sulfuric acid. In addition to being toxic to plants and animals, sulfides also contribute to soil degradation, according to a 2011 presentation from William Orem of the U.S. Geological Survey.

Sulfides also interact with mercury, already toxic, to turn it into its most toxic form: methylmercury.

What would PolyMet do to contain and treat that pollution?

After being dug up, the 553 million tons of rock PolyMet is expected to produce would be crushed and mixed with water to separate the valuable ore from waste rock. The estimated 225 million tons of ore would be shipped to market, but the 308 million tons of waste rock would remain on site in stockpiles and slurries.

Stockpiles would simply be huge piles of waste rock.

To limit runoff from escaping into the environment, the mining company would build cutoff walls reaching down to the bedrock and a drainage system to channel water to a reclamation site. Stockpiles with more potential to pollute would have the added safeguard of an impermeable liner underneath.

The slurry would be stored in a “tailings basin”: four square miles of containment area, originally built as early as the 1950s. PolyMet has agreed to update the unlined basin to restrict leakage, including buttressing the basin and adding a layer of absorptive bentonite clay. The DNR’s environmental impact statement predicts this would intercept “greater than 90 percent of all the seepage that remains in the ground as groundwater,” but acknowledges that “a portion of groundwater seepage is (assumed) to bypass the containment system and eventually emerge in surface waters.”

In real terms, that seepage is about 2,600 gallons per minute, most of which would reach the nearby Embarrass River, which flows toward Lake Superior. The DNR says wetlands around the PolyMet site could absorb most of the sulfate before it reaches the river.

“While this seepage has a high sulfate load, it appears that much of the sulfate is sequestered by natural processes in wetlands between the Tailings Basin and the Embarrass River,” the environmental impact statement says.

What happens after the mine closes?

Mining operations at PolyMet would last about 20 years, but the cleanup and containment process would last much longer.

Some of the waste rock would be returned to one of the pits from which it was taken, which would then be covered with a wetland. Another pit would be filled with water as a giant lake. One of the stockpiles would remain and be covered to prevent drainage.

Water on the site would continue to require treatment “for as long as necessary.” State officials have intentionally kept that timeframe open, though Richardson, the PolyMet vice president, said the company hopes intensive, mechanical treatment would be required for only 55 years. After that point, PolyMet hopes to shift to less intensive — and less expensive — “passive treatment.”

Environmentalists fear the cleanup process will remain intensive for far longer.

Who would pay for this treatment once the mine has stopped making money?

If PolyMet is given a permit, it would come with a condition: that the mining company set aside millions of dollars to pay for cleanup costs. That’s to prevent PolyMet from finishing operations and then declaring bankruptcy to rid itself of the expensive cleanup costs — as some mines have done in past decades.

But how much PolyMet would be required to set aside has not yet been determined. With PolyMet predicting low cleanup costs and environmentalists predicting high cleanup costs, this figure is already the subject of heated debate.

What do opponents worry could go wrong?

The environmentalist critique of PolyMet is in part based on the general worry that there’s so many things the mine has to do to contain polluted water that something is likely to go wrong, even if we don’t know what yet.

“They’re actually claiming that they would keep (99.5) percent of the water in treatment,” said Aaron Klemz, a spokesperson for the anti-PolyMet environmental group Friends of the Boundary Waters Wilderness. Fellow environmentalist Paula Maccabee of the group WaterLegacy added: “Wouldn’t you want to know what the effects would be on Minnesota drinking water … if PolyMet collected just 80 percent?”

Richardson said PolyMet had demonstrated its ability to contain the water at the mine.

“We’ve said we can meet water quality standards,” he said. “We actually built a reverse-osmosis pilot plant and processed 3 million gallons of mine water from the site and showed that we can meet that standard.”

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