PolyMet Mine: Threat of Tailings Dam Failure

Tailings dam failure is one of the most serious – and avoidable – threats from the proposed PolyMet sulfide mine. A PolyMet tailings dam collapse would release polluted wastewater, tailings slurry, and sediment – harming water quality and aquatic life in the Partridge and Embarrass Rivers. Increased mercury contamination of fish resulting from release of mercury and sulfate would pose a grave danger to human health from the Partridge and Embarrass Rivers downstream to the St. Louis River and, potentially, all the way to Lake Superior.

Mount Polley “upstream” tailings dam failure in British Columbia, Canada (2014). Photo by Jonathan Hayward/Canadian Press.

PolyMet Tailings Design Flaws

Tailings and wastewater from the PolyMet sulfide mine would be contained by dams made mostly of tailings the consistency of sand. The PolyMet tailings mound would rise more than 200 feet (roughly 18 stories or two-thirds the height of the Statue of Liberty) above the ground surface. The proposed PolyMet tailings dam creates unnecessary threats of pollution and catastrophic failure:

  • PolyMet tailings would be very wet; PolyMet tailings slurry would contain 86 percent water by volume. Tailings holding less water – referred to as “filtered tailings, “paste tailings,” or “dry stack tailings” depending on how much water is removed – are more stable and produce much less polluted drainage.
  • The PolyMet tailings dam would be built on top of the old LTV Steel Mining Company taconite tailings dam, which hasn’t been used for decades. The foundation for PolyMet’s dam includes compacted wetlands, peat as thick as 20 feet in places, and fine tailings and tailings slimes, which as their name would suggest are slippery like wet clay. This foundation, shown in a drawing from PolyMet’s Application for a Permit to Mine, also decreases the stability of the PolyMet tailings dam.
Diagram from PolyMet’s permit to mine application.
  • The PolyMet tailings dam uses “upstream” construction, which is the cheapest, but riskiest, way to build a tailings dam. Most of the serious dam failures in recent decades have been associated with “upstream” dams, including several that were built or approved by the same consultants and methods used to plan PolyMet’s tailings dam for Minnesota.
  • The danger of PolyMet tailings dam failure is compounded by PolyMet’s plan to pump wastewater back to a “pond” – really a polluted lake as big as 685 NFL football fields –increasing the risk that the tailings would liquefy and the structure of the tailings dam would fail.
  • Dan Sutton, one of the consultants reviewing the PolyMet tailings dam, wrote to the Minnesota Department of Natural Resources (DNR):
The FTB [tailings] plan gives me severe indigestion because a lake on top of a pile of sand is inherently unstable and irresponsible.

Deb Allert, president of the Minnesota Academy of Family Physicians, Lake Superior Chapter told Minnesota commissioners in a meeting on September 25, 2015:

In the event of dam failures or breaches, highly toxic substances will be released into nearby watersheds including lead, arsenic, and mercury and sulfates, which produce methylmercury in bogs and wetlands . . . Methylmercury is highly toxic to humans. Even small amounts bioaccumulate in the food chain to toxic levels.

Catastrophic Failure of Similar “Upstream” Dams

From 2014 through October 2017, eight failures significant enough to make global news occurred in Canada, Mexico, Brazil, China, USA, and Israel.

Mount Polley Tailings Dam (British Columbia, Canada)

On Monday, August 4, 2014, a sunny summer day, the “upstream” tailings dam at the Imperial Metals Mount Polley copper-nickel mine collapsed, resulting in the worst mining disaster in Canada’s history. The breach released an estimated 6.3 billion gallons of tailings and wastewater.

A month later, residents on British Columbia’s Quesnel Lake, pictured below, reported “a plume of sediments in the lake that shows up and disappears” and tests in water containing these sediments showed “elevated levels of metals that exceed drinking and aquatic life guidelines.”

Waste material and water from the Mount Polley mine tailings pond spills from Hazeltine Creek into Quesnel Lake on Aug. 5. Photo by Jonathan Hayward/Canadian Press.

The Mount Polley dam was designed by Knight Piesold, the same engineers who designed the PolyMet tailings disposal facility. The Independent Expert Panel Report on the Mount Polley Tailings Storage Facility Breach found that the unfiltered wet tailings at Mount Polley were a major factor in the dam’s catastrophic failure. The experts recommended dry stack tailings as Best Available Technology to avoid dam failure. PolyMet and the DNR rejected dry stack tailings.

Fundão Tailings Dam (Samarco, Brazil)

On November 6, 2015, an “upstream” iron ore tailings dam collapsed at the Samarco mine in Brazil. The dam collapse started a mudslide that flattened a village, killing 17 people, injuring 50 and requiring that 600 people be evacuated.

The “upstream” tailings dam failure in Samarco, Brazil in 2015. Photos by Romerito Pontes.

Nearly 16 billion gallons of mine waste was released. Laboratory testing in downstream samples of water from the impacted Rio (River) Doce detected mercury, aluminum, lead, copper, arsenic, and other chemicals. Arsenic in sampling after the dam breach was 2,639.4 micrograms per liter – more than 200 times Brazil’s 10 micrograms per liter standard.

A United Nations Environment Report on mine tailings failure concluded:

Large tailings dam failures can release a tsunami-like wave of mine waste capable of destroying everything in its path.

This UN Report also concluded that tailings slurry from the Fundão Dam failure in Samarco Brazil traveled 620 kilometers (385 miles) downriver, despite 4 dams along the way, ending up in the Atlantic Ocean.

Image from the UN Environmental Report on Mine Tailings Storage.

Córrego do Feijão Tailings Dam (Brumadinho, Brazil)

On January 25, 2019, the catastrophic failure of the Córrego do Feijão “upstream” dam in the town of Brumadinho in Minas Gerais, Brazil, resulted in approximately 300 deaths.

Córrego do Feijão “upstream” tailings dam catastrophic failure in Brumadinho, Brazil (2019). Photo by BBC – EPA.
Less than a month after the Feijão dam collapsed in Brumadinho, to prevent the very threat that DNR has discounted without even allowing a contested case hearing, the nation of Brazil prohibited construction of mining dams built with the “upstream” method and ordered that all existing dams constructed by the “upstream” method be decommissioned.
On February 28, 2019, WaterLegacy and our allies petitioned the Minnesota DNR to reconsider PolyMet tailings dam safety permit and permit to mine for the following reasons:
  • The same design: DNR approved an upstream design for PolyMet’s tailings dam. As a result of the most recent dam failure, Brazil has banned all new upstream dams and ordered that all existing upstream dams be decommissioned by August 2021.
  • The same consulting engineer: PolyMet and the owner of the collapsed Brazilian dam both hired Dr. Scott Olson to review dam stability and advise them of the risk that the tailings would liquefy and dams collapse.
  • The same method to predict the risk of collapse: In 2013, Dr. Olson’s method of calculating the risk of a dam collapse determined that the deadly Feijão Dam was safe. The same Olson Method is the foundation for DNR’s prediction that the PolyMet dam is safe from collapse.
Although the DNR denied the request to reconsider its PolyMet permits, concerns about tailings dam safety were an important factor in the Minnesota Court of Appeals’ Order to suspend the PolyMet permits on September 18, 2019. The Court held:
Relators raise serious, justifiable concerns about the ongoing regulation of the NorthMet project, and we agree that the post-permit circumstances that relators have identified require close attention, review, and appropriate action by the DNR and other permitting authorities. Pursuant to its statutory obligations, the DNR could reasonably be expected to undertake a full-scale review of the Brumadinho dam failure and its impact on the analysis underlying the decision to issue the NorthMet dam-safety permits.