The dam debate: PolyMet tailings basin dams are key point in upcoming permits

HOYT LAKES -- Of the bad things that could happen once PolyMet starts running Minnesota's first-ever copper mine, critics say, among the worst would be a catastrophic breach of the tailings basin dam.

Aaron Grosser, senior geotechnical engineer at Barr Engineering, talks about the plans to build dams for the PolyMet project near Hoyt Lakes. A dam would be built in the distance to raise the tailings basin on the right to the level of the former LTV Steel tailings basin on the left. Steve Kuchera /
Aaron Grosser, senior geotechnical engineer at Barr Engineering, talks about the plans to build dams for the PolyMet project near Hoyt Lakes. A dam would be built in the distance to raise the tailings basin on the right to the level of the former LTV Steel tailings basin on the left. Steve Kuchera /

HOYT LAKES - Of the bad things that could happen once PolyMet starts running Minnesota's first-ever copper mine, critics say, among the worst would be a catastrophic breach of the tailings basin dam.

A hulking, man-made earthen dike that will stretch for miles and reach 252 feet high when finished, the dam will hold back millions of gallons of water mixed in a slurry with finely ground rock left over after crushing and processing - after the copper, nickel and other valuable metals are extracted.

Much of that waste rock will be as small as grains of beach sand. In theory, the stuff will settle into the basin, and as more is pumped in, the dams will be raised in steps, 20 feet at a time, over the 20-year life of the mine.

Most of the water will be re-used to keep the slurry system carrying waste rock from the processing plant to the tailings basin. Any water that leaves the basin will be treated by PolyMet with a reverse osmosis filter system (to remove sulfates), leaving clean water flow into the Partridge and Embarrass rivers and on into the St. Louis River and Lake Superior.

PolyMet needs a dam safety permit from the Minnesota Department of Natural Resources to start work adding on to the tailings basin that was once used by Erie/LTV Steel Mining Co. in its taconite processing operations. (PolyMet also has applied for a second dam permit for a possible hydrometallurgical processing facility where acidic waste might be stored.)


Those draft dam permits are likely to be released for public comment by the DNR within weeks, along with other permits that will set rules on mine waste and other key details of mining and processing operations.

Preventing a mess

The old LTV basin here - which the PolyMet dam will eventually match - is a massive structure, high enough to offer vistas for 25 miles or more, towering above all but a few hills in this otherwise boggy forest of Northeastern Minnesota.

Eventually the PolyMet tailings basin will cover 2ยฝ square miles with 10 million cubic yards of mine waste pumped in each year for 20 years.

Any breach of the dam would be a mess to be sure, especially after several years of mining, with water and muck spread for miles. In one scenario presented as part of the environmental impact statement process, a breach of PolyMet's north dam would overwhelm a downstream wetland, potentially inundate two dozen structures and reach the Embarrass River within a few hours.

Disastrous tailings basin dam failures happened in 2014, at the Mount Polley copper/gold mine in British Columbia, flooding downstream lakes with mine waste; and in 2015 at an iron ore mine in Brazil, killing 19 people and destroying downstream villages.

PolyMet officials say such disasters simply couldn't happen here. Their dam will have far less slope - just 13 percent compared to 70 percent for Mount Polley - with less elevation in play (the Mount Polley and Brazil dam breaks were in mountainous regions.)

That flatter slope means less stress on the dam base and walls and adds a far higher "factor of safety," said Aaron Grosser, senior geotechnical engineer for Barr Engineering, PolyMet's lead consultant on the dam project.


The Minnesota DNR agrees, saying a Mount Polley-like disaster is unlikely here because the "PolyMet tailings basin would be constructed to a significantly higher engineering and construction standard than was used at Mount Polley."

Grosser said that, as the basin fills, the dam wall will be raised using mid-sized mine waste rock and buttressed with large rocks on the outside. Bentonite clay will be added in areas to help keep the basin more waterproof.

Both internal and outside engineers, including consultants for the DNR, all have suggested modifications to the design that will make it stronger and better, said Brad Moore, PolyMet's executive vice president of environmental and governmental affairs. That includes dropping a plan to use concrete pumped underground to shore-up the base of the dam and instead use rock on the outside of the dam to make it easier to monitor how the buttressing is working and allow more to be added if needed.

"The tailings basin design was one of the most-studied aspects of our project,'' Moore said.

Even if the dam did fail, PolyMet notes, their tailings waste will be non-toxic and non-acidic and won't cause the kind of downstream acid mine pollution that critics of the mine fear. (Copper is generally attached to high-sulfur rock that, when exposed to air and water, can cause acidic runoff.)

"The pH will be essentially neutral,'' Moore said, with the tailings basin water expected to be in the 7.8 to 8.1 level, on the base rather than acidic side of the spectrum.

New on top of old

PolyMet says they are building on to the half-century record of success of the former LTV tailings basins. Those dams have held back taconite mining waste with no major problems, including one basin that is successfully closed and reclaimed as grassland wildlife habitat. Dozens of similar tailings basin dams are holding across the Iron Range.


Next to the closed-and-capped LTV tailings basin, PolyMet plans to build on top of two other adjacent LTV tailings basins left partially full when LTV shut down in bankruptcy in 2001. PolyMet's planners say they will make the dams and basins stronger with updated engineering.

"They had a great system out here before, and we're making it better,'' said Grosser, who also worked on the tailings basins here while LTV was running them.

Grosser said the basin is being designed to handle a massive 1,000-year-type rainfall, in excess of 20 inches in a short period, with added wave action factored in for storms. That means building-in excess capacity to prevent "overtopping'' when water spills over a dam, often causing erosion or a break.

But Kevin Lee, staff attorney for the Minnesota Center for Environmental Advocacy, said building a new tailings basin on top of old mine waste is a bad idea from the start. Engineers have warned that the base of the LTV basin includes peat and "slimes" that may not be a strong footing.

"The DNR's consultants, outside consultants, other engineers ... all have said that it may not be a great idea to put this new kind of mine waste on top of something that's 50 years old to start. They shouldn't count on a structure that was built before environmental rules were even in place," Lee said.

Lee and others also aren't as sure that the ground rock and water inside the tailings basin will be as non-acidic and clean as PolyMet predicts. While PolyMet officials say they will leave the most acid-generating rock at the mine pit, and the remaining higher-sulfide rock will stay with the copper, not the waste rock, Lee said that type of precision isn't easy.

"They aren't using a scalpel. They're using blasting and giant equipment. They can't be perfect,'' Lee said. "There are all sorts of comments on this by experts who say it's not that simple."

Others are concerned that water already is leaking out of the old LTV basins and that it will be tough for PolyMet to make sure the new basins don't also leak. But now, with more sulfur-bearing and potentially acidic rock in play, the consequences of leakage are much higher.


PolyMet said a barrier will be built around the entire tailings basin, down to bedrock, to prevent any groundwater from getting in and any tailings basin water from getting out without being treated.

The company's plan also calls for water from the mine pit to be pumped eight miles to the tailings basin to be filtered by the same reverse osmosis treatment plant.

Wet or dry storage?

PolyMet also plans to leave a permanent pond on top of the mine waste basin even after the mine is closed. The hope is the water will act as a barrier to rob any acid-generating rock from a key element - oxygen - needed to cause pollution. Because most of the new tailings will be deposited underwater in the basin "the amount of tailings that will be exposed to oxygen and thus have the ability to release heavy metals will be limited," the company says.

It's likely the water on the mine waste will need to be contained forever and any water leaving the site will need to be filtered in some manner, either ongoing reverse osmosis or some new technology.

But Lee and others also disagree with PolyMet's plan for wet storage and wet "closure." In the final report on the Mount Polley disaster, experts said dry disposal of mine waste is a better long-term solution to prevent dam failures and mine waste from escaping - that there is simply less chance for catastrophe when dealing with a pile of dry waste rock.

PolyMet and the DNR say dry storage was briefly considered, but was dismissed early in the environmental review because the waste would have to be transported to a new location - likely some nearby, undisturbed area - and thus offered higher costs with no real environmental benefit over reusing the old LTV site that's already disturbed.

Some PolyMet critics still disagree.


"The problem is that while everyone agrees dry storage is better and safer, no one is actually saying that's what PolyMet has to do because it might cost a little more up front,'' said Paula Maccabee, attorney for the group Water Legacy. "The DNR appears ready to let them build on top of an unstable base of peat and slimes and let the chips fall where they may. ... Instead of making them start over with a better plan, the DNR is letting (PolyMet) try to fit a square peg into a round hole."

On background

Toronto-based PolyMet is proposing to build an open pit copper-nickel mine and processing plant north of Hoyt Lakes that will employ about 300 people for 20 years or more - the company's first and only mining project. The proposal has cleared the state environmental review process and now awaits more than 20 state and federal permits before work can start. The company, which is now one-third owned by Swiss commodities giant Glencore, also must raise money to pay for construction, likely $500 million or more.

Supporters say the copper project will help diversify the Iron Range economy now heavily dependent on cyclical iron ore mining while critics say the high-sulfur rock where copper is located threatens to cause acidic pollution to the region's pristine waters.

READ MORE: Two examples of when dams failed

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