The Minnesota DNR has reached a decision regarding requests made by the Fond du Lac Band of Lake Superior Chippewa, multiple environmental organizations, and others pertaining to the tailings dam permits issued to PolyMet on Nov. 1, 2018.

Minnesota Department of Natural Resources Commissioner Sarah Strommen issued the following statement on August 7:

“While we were confident in our original analysis of the PolyMet tailings dam, we have carefully examined the requests for reconsideration and related information about recent dam failures in other parts of the world,” Strommen said. “We understand people’s concerns with these dam failures and whether those events indicate a fundamental design issue with PolyMet’s dam. Our analysis demonstrates that there are significant differences in site conditions, engineering design, and operating requirements and we remain confident in the safety of the PolyMet tailings dam as permitted.”

 

According to the DNR, requests for reconsideration raised the following concerns about the approved PolyMet tailings dam permits:

  • The failure of the Brumadinho Dam in Brazil is evidence of new stability concerns regarding the upstream construction design of tailings basins.
  • The use of the “Olson Method” to analyze the strength, liquefaction, and stability of the Brumadinho dam inherently means that its use to analyze the PolyMet tailings dam was flawed.
  • Recent inspections of the existing LTV tailings basin at the PolyMet site call into question assumptions about tailings drainage and materials strength in the basin.

However, after reviewing the PolyMet and Brumadinho dams, the DNR cited critical differences that played into their ultimate decision:

  • Safety factors for the PolyMet dam were established using conservative assumptions to assess the basin’s stability under extreme conditions. These assumptions include that the entire basin had liquefied, been subjected to extreme rainfall, and been subjected to an earthquake. The analysis of the Brumadinho dam failed to include anything close to this level of assessment.
  • PolyMet’s dam will be built on flat topography, far from any community, and using ring-dike construction. The Brumadinho dam was constructed on a hillside, directly above a community, with higher-risk valley construction.
  • PolyMet’s dam will have very gradual side slopes (7:1 overall) that are inherently more stable than the Brumadinho dam which had an overall slope of 4:1.
  • PolyMet’s dam has virtually no inflow of surface waters into the basin. The Brumadinho dam had significant inflow from the adjacent watershed that necessitated the diversion of runoff from surrounding hillsides away from the tailings basin and dam. It appears this diversion system failed, resulting in heavy flows into the basin for weeks immediately prior to the dam’s failure.
  • The PolyMet tailings dam is located in an area of little or no seismic activity. The Brumadinho dam is located in an area of moderate seismic activity.
  • PolyMet’s dam is approximately eight miles from its mine site, which minimizes any risk of blasting impacts to the dam. Reports indicate that there had been mine blasting in close proximity to the Brumadinho dam on the morning of the failure.