PolyMet Mine Overview

Northern Minnesota wetland and forest site proposed for PolyMet sulfide mine. Photo by Ron Levine.

The proposed PolyMet NorthMet open pit copper-nickel mine would be Minnesota’s first non-ferrous (that is, not iron or taconite) mine. The PolyMet mine would be located in the Duluth Complex, a massive geological formation that begins near the city of Duluth and continues northeast to the Canadian border. Metals, like copper and nickel, found in the Duluth Complex are chemically bound with sulfur in sulfide ores. That’s why mines like PolyMet are called “sulfide” mines.

Excerpt from DNR map of Duluth Complex. Click on image to see complete map.
Copper-nickel mining exposes sulfide ores to water and air, releasing sulfuric acid. Sulfide mining in a water-rich environment, like that of northern Minnesota, has a track record of 100 percent failure to protect water quality. PolyMet — which has proposed cheap and outmoded designs for its operations — will be no different.

The PolyMet sulfide mine would threaten clean water, destroy wetland and forest habitats, impair the health of downstream communities, exacerbate climate change, and create financial liabilities for taxpayers throughout Minnesota.

PolyMet proposes to blast and dig over 500 million tons of waste rock and ore from the ground over 20 years. The PolyMet deposit is a low-grade deposit, with a higher concentration of sulfur than of copper and nickel combined. Over 99% of what would be dug out of the ground from the PolyMet mine would be waste. The mine would leave behind two huge contaminated mine pits, a 526-acre permanent waste rock site, a toxic hydrometallurgical waste disposal site and a huge tailings waste basin.

If the PolyMet mine is not stopped, other sulfide mines proposed in the Boundary Waters watershed, the Lake Superior watershed and the Mississippi River watershed could endanger water resources, habitats and communities throughout Minnesota.

Click to image to download fact sheet: PolyMet Mine Overview.

Environmental Review

As with many other projects where there is strong economic or political pressure, the environmental review process for the PolyMet mine was seriously flawed. WaterLegacy challenged the process for almost a decade until 2018 when our request for a Supplemental Environmental Impact Statement (SEIS), which was based on PolyMet’s plan for expansion, was denied. Lawsuits in federal court can still challenge the PolyMet Final EIS as well as agency approvals for the project.


The PolyMet sulfide mine could not be built without major permits and approvals by both state and federal agencies, including a Minnesota state Water Pollution Permit, a state Permit to Mine, state Dam Safety Permits, and a federal Clean Water Act Section 404 wetlands dredge and fill permit. WaterLegacy and out allies have challenged these permits, making a compelling record of the dangers posed to Minnesotans by PolyMet pollution and destruction of habitats.

Water Pollution Permit

The PolyMet NorthMet water pollution was issued by the Minnesota Pollution Control Agency (MPCA) in December 2018. WaterLegacy has challenged the PolyMet sulfide mine in both the federal and state courts. Starting in the spring of 2018, WaterLegacy exposed evidence of a PolyMet permitting scandal. The MPCA destroyed documents and successfully lobbied the U.S. Environmental Protection Agency (EPA) to suppress comments criticizing weaknesses in the PolyMet permit. After several years of litigation, the courts not only found that MPCA has issued the PolyMet water pollution permit using irregular procedures, but reversed the permit decision as “arbitrary and capricious” and due to failure to protect either surface water or groundwater from pollution.

PolyMet Federal Wetlands Permit

Under the Clean Water Act, destruction of wetlands that are waters of the United States is prohibited unless the U.S. Army Corps of Engineers (Army Corps) grants a Section 404 permit to destroy (“dredge and fill”) wetlands. The U.S. Army Corps granted a Section 404 wetlands destruction permit to PolyMet for the NorthMet mine on March 21, 2019. The U.S. Army Corps revoked that PolyMet NorthMet mine permit on June 6, 2023, in an unprecedented decision upholding Tribal sovereignty to object to a federal permit that would cause violations of downstream tribal water quality standards.

PolyMet Permit to Mine

The Minnesota Department of Natural Resources (DNR) issued the PolyMet NorthMet permit to mine and dam safety permits on November 1, 2018.

WaterLegacy and our allies succeeded in suspending the permit to mine and dam safety permits during our appeals to the courts, so PolyMet momentum has stalled, and no pollution or destruction of resources has occurred while the courts review the legality of the permits. The Minnesota Supreme Court reversed the PolyMet permit to mine for lack of a term limit and ordered a contested case hearing on whether PolyMet’s plan for reactive tailings waste storage violated Minnesota rules. An administrative law judge has recommended that the permit to mine be rejected, since PolyMet’s plan would not comply with Minnesota’s reactive mine waste rule. The DNR has not made a decision on whether to reissue a permit to mine for the PolyMet NorthMet project.

Threat of Tailings Dam Failure

Mine waste tailings can be as large as grains of sand or as small as talcum powder (slimes). PolyMet plans to store its waste tailings in an above-ground tailings basin without a liner on top of an existing basin and dam that contains old tailings from an iron ore mine, along with peat and slimes. The PolyMet tailings basin and dam would be located in the headwaters of the St. Louis River, the largest U.S. tributary to Lake Superior.

PolyMet’s “upstream” dam design for its tailings basin is similar to the design of a tailings dam in Brazil, which collapsed in January 2019, killing over 250 people. The PolyMet design is the cheapest, most dangerous design for tailings dams. Seepage from PolyMet’s tailings waste would require water quality treatment for more than 500 years – potentially paid for by Minnesota taxpayers.

Mercury & Health Risks

When sulfur-bearing ore, like that which would be mined by PolyMet, is exposed to air and water, sulfuric acid forms, resulting in acid mine drainage and leaching toxic metals that can permanently contaminate water.

Pollution from the PolyMet mine would degrade Minnesota’s streams and rivers in the Lake Superior and Boundary Waters watersheds, damage wild rice, and contaminate fish with toxic methylmercury, harming the developing brains of fetuses, babies, and children.

Environmental Injustice

The proposed PolyMet mine and processing plant would be located in tribal Ceded Territories where the Ojibwe/Chippewa have Treaty-reserved Rights to hunt, fish, and gather plants. In addition, since the PolyMet mine project would be constructed in the headwaters of the St. Louis River, its sulfate and mercury pollution would affect downstream communities, including the Reservation of the Fond du Lac Band of Lake Superior Chippewa and the City of Duluth.

Impact on Climate Change

The PolyMet mine project would exacerbate climate crisis due to its combustion of massive quantities of fossil fuels, resulting in millions of tons of carbon dioxide equivalent air pollution. In addition, the mine would destroy approximately 1,000 acres of wetlands and drain water from thousands more, releasing sequestered carbon and causing additional climate damage.

Glencore & PolyMet

PolyMet is now wholly owned by the Glencore multinational corporation, a Swiss-based company with a history of bribery, corruption, labor abuses, and toxic pollution across several continents. Glencore is not responsible for the costs of cleanup or pollution treatment under any PolyMet permits – even if PolyMet declares bankruptcy. That cost would fall to Minnesota communities and taxpayers. The ownership of the NorthMet project, should it be resurrected, would be divided between Glencore and Teck Resources, both of which are foreign corporations.