Mount Polley: Unacceptable Risk

Image credit: Mining.com

Image credit: Mining.com

On August 4, 2014, a tragedy called “the worst disaster in Canadian mining history” occurred at the Mount Polley copper-gold sulfide mine in British Columbia. Over 1.3 billion gallons of mine waste were released into pristine waterways when a tailings dam failed. These tailings are high in heavy metals and polluted Quesnel Lake, one of the deepest lakes in British Columbia and part of the Fraser River system. The Fraser River is one of the most important salmon rivers in Canada. Until this disaster occurred, the Mount Polley mine was touted by industry as an example of how sulfide ore mining could co-exist with the clean water needed for a healthy salmon fishery.

On January 30, 2015, an independent panel investigating the collapse released its report on the causes of the Mount Polley disaster. Three independent experts found the initial design of the tailings dam was faulty because it failed to account for a weak layer of rock under the dam. These experts called the design “a loaded gun” that was exacerbated by building the dam more steeply than was safe.


The damage to the environment is irreparable, and will go on for decades as tailings continue to leach out heavy metals. The cost of an incomplete cleanup is estimated at up to $500 million but the Mount Polley mine had only $14.5 million in reclamation bonds posted with the British Columbia government at the time of the disaster.

The Mount Polley report recommends all new mines stop the practice of storing tailings mixed with water and using an approach called “dry stacking” which filters water from tailing before disposing of them. It acknowledges that this technology is more expensive but concludes “[Best Available Technology] should be actively encouraged for new tailings facilities at existing and proposed mines. Safety attributes should be evaluated separately from economic considerations, and cost should not be the determining factor.” Proposed Minnesota sulfide mines use the old technology. PolyMet compounds the problem by building its tailings facility on top of an existing taconite tailings facility.

The firms connected to the Mount Polley tailings dam also have significant ties to Minnesota proposals. The contractor that designed and built the tailings facility is Knight Piesold. They remained the “engineer of record” for the Mount Polley tailings dam until 2011. Knight Piesold is listed as a consultant on the PolyMet environmental impact statement, with expertise in tailings impoundments. In 2011, AMEC took over as “engineer of record” at Mount Polley. AMEC was the lead contractor for the Twin Metals prefeasibility study.

The Mount Polley disaster holds important lessons for Minnesota as we consider sulfide mine proposals.

  • The risk to drinking water, recreational lakes, and wilderness from a tailings dam break should be factored into a decision of whether to grant a permit to a sulfide mine proposal
  • The damage deposit (known as financial assurance) for any proposed sulfide mine should include foreseeable catastrophic costs and be posted before a mine opens
  • The recommendations of the Mount Polley panel regarding best available technology and best available practices should be reviewed and implemented
  • The work of contractors associated with the Mount Polley tailings dam on Minnesota sulfide mine proposals should be reexamined by independent experts