The state Department of Natural Resources has released a draft permit for the massive earthen dam PolyMet Mining is proposing to build to hold back millions of gallons of waste rock and water at its proposed copper-nickel mine in northeast Minnesota.
The release of the Dam Safety permit is another step in the long regulatory process PolyMet is navigating in its bid to open the state's first-ever mine for copper, nickel and precious metals.
The permit lays out PolyMet's plans to build and maintain a dam that would be several miles long and about 200 feet high, designed to hold a 2 1/2 square mile basin of tailings — essentially the finely ground rock left over after extracting the metals from the ore, mixed with water.
PolyMet plans to slurry more than 11 million tons of tailings to the basin every year over the 20-year life of the mine.
The company also plans to build containment walls down to bedrock to capture water that seeps through the dam, which would then either be returned back to the tailings basin, or be treated using reverse osmosis technology and returned to the environment to replenish neighboring streams and wetlands--treatment that's expected to be required indefinitely after the mine closes.
The dam is designed to be able to withstand a catastrophic rain event of more than 20 inches without overtopping.
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"The tailings basin was one of the most studied aspects of the NorthMet Project during the comprehensive state and federal environmental review of the project that concluded in 2016," PolyMet CEO Jon Cherry said in a statement.
"The science shows that not only can we be protective of water and other natural resources, but we will make a substantial contribution to addressing legacy reclamation issues at the site," he added.
The public now has 30 days to comment on the permit. While the release of the draft document does not grant final approval to build the tailings basin dam, it does signal the DNR is comfortable with the company's plans to this point.
State regulators hired external consultants to help assess PolyMet's permit application.
The group included a member of the review panel that investigated a massive tailings dam failure at the Mt. Polley copper and gold mine in British Columbia in 2014.
PolyMet plans to build its dam and tailings basin on top of the existing basin left behind by LTV Steel when it closed its taconite mine and processing plant outside Hoyt Lakes, Minn. in 2001.
Critics of the mine proposal, including Kathryn Hoffman, CEO of the Minnesota Center for Environmental Advocacy, say the proposed mine waste dam is one of the riskiest parts of their proposals.
"PolyMet has failed to use the best available technology, and proposes to store millions of gallons of mine waste mixed with water behind a 40-year-old dam," Hoffman said.
An expert panel assembled to study the Mt. Polley accident recommended removing water from tailings before storing them, a process known as dry-stack tailings.
That option was studied during the environmental review process, PolyMet officials said, but was eventually discarded because they determined the environmental benefits of reusing an old industrial site outweighed the damage caused by creating a new basin.
The company has also added rock buttressing to strengthen the dam, and testing performed at the site indicates the ground underneath the basin is much more stable than at Mt. Polley, company officials said.
Furthermore, the company argues, the slope of the dam it proposes to build is only 13 percent, compared to a 77 percent slope at Mt. Polley, making a dam failure much less likely. "It's a much more stable structure," said Aaron Grosser, senior geotechnical engineer for Barr Engineering who's working on the PolyMet project.
Still, environmental groups and tribal commenters have argued the state should pursue safer ways to storing mine waste than mixing it with water and storing it behind a dam.
"If that dam fails, the risk is so catastrophic downstream, that we don't think that's a risk that Minnesota should take," said Aaron Klemz, communications director for the Minnesota Center for Environmental Advocacy.
Klemz added there are other brownfield sites near PolyMet that could be used for such a facility.
The DNR also released a second draft dam safety permit that covers a proposed hydrometallurgical residue facility at the proposed mine site, that would recover additional precious metals from the processed ore.
The dam safety permits are two of more than 20 permits PolyMet needs from local, state and federal agencies to gain final approval to open the mine. A draft of the most significant of those permits — the Permit to Mine from the Minnesota DNR--is expected to be released later this year.
If the company ultimately wins final approval from state and federal agencies, PolyMet has said it could complete construction within 12-18 months of receiving its permits.