Mount Polley Mine (“the Mine”) is a copper and gold mine located in the Central Interior of British Columbia, approximately 65 km northeast of Williams Lake, near the community of Likely and on the traditional territories of the Xat’sull (Soda Creek) and T’exelc (Williams Lake Indian Band) Nations.
Mount Polley Mining Corporation (MPMC) is a subsidiary of Imperial Metals Corporation, a Canadian mining company with its corporate head office in Vancouver, British Columbia.
The Mine commenced production on June 13, 1997, and remained in operation until September 2001, when operations were temporarily halted. Mining operations resumed in March 2005 and continued until August 2014, when the dam breach. It resumed partial production in mid-2015 and full production in mid-2016.
The Mine stored its tailings in a Tailings Storage Facility (TSF) located 3km southeast of the mill site. The TSF was a U-shaped earthen dam over 4km long and 50m high, and consisted of three embankments: the Main Embankment (ME), the South Embankment (SE), and the Perimeter Embankment (PE). The TSF served as a collection pond for mill tailings as well as mine runoff water.
The TSF was located above Hazeltine Creek, a local watercourse connected to Polley Lake at its upstream end and Quesnel Lake at its downstream end, as well as other connected watercourses such as Edney Creek.
Hazeltine Creek, Edney Creek, Polley Lake, Quesnel Lake and Quesnel River all contain fish habitat for one or more commercially significant species, such as Rainbow Trout and Chinook, Coho and Sockeye Salmon.
On the night of August 3-4, 2014, the dam enclosing the TSF at Mount Polley Mine failed. Over the next 16 hours, the failure led to a progressive breach of the PE, releasing over 24 million cubic metres of water and mine tailings into the surrounding environment.
The debris flow entered Hazletine Creek, scouring the channel and floodplain, and flowing upstream to Polley Lake and 9.4km downstream to Quesnel Lake. A thick deposit of tailings plugged the entrance to Polley Lake, blocking the flow of water – this obstruction has come to be known as the “Polley Plug”.
Approximately 18.6 million m3 of water and materials flowed into Quesnel Lake, some of which settled to the lake bottom and some of which remained suspended as “cloudy water” that returned to the surface of the lake in the late fall.
In December 2015, an investigation by the Chief Inspector of Mines concluded that the TSF failed because the embankment slid on a layer of lightly overconsolidated glaciolacustrine clay (the “Upper Glaciolacustrine Unit”, or “UGLU”) located approximately ten metres into the dam’s foundation. The UGLU had never been properly characterized by MPMC or its engineers.
The Chief Inspector also found that several other proximate causes contributed to the dam breach: the downstream slope of the dam was over-steep for its height; there was an unfilled excavation at the toe of the embankment; and there was an excess of supernatant water in the TSF combined with inadequate tailings beaches (distance between the wastewater reservoir and the dam crest).
An investigation by the Independent Expert Panel Review came to similar conclusions in its January 2015 report. The Expert Panel said ‘Business as usual is not acceptable’ and recommended extensive changes in mining policies and practices in B.C.
In May 2016, “An Audit of Compliance and Enforcement of the Mining Sector” prepared by BC’s Auditor General state “[MEM] must ensure the mine is designed, built, operated and reclaimed to an acceptable standard.” BC’s Auditor General found that MEM failed to enforce the law or to apply its policies in connection to the slope design and water level of the TSF.
In Octobre 2016, MiningWatch Canada, with the support of other organizations, filed a private prosecution against the B.C. government and the Mount Polley Mining Corporation for violations of the federal Fisheries Act.