The Mount Polley tailings spill occurred in the Cariboo region of central British Columbia, Canada . The spill began 4 August 2014 with a breach of the Imperial Metals-owned Mount Polley copper, gold and silver mine tailings pond, releasing its water and slurry with years worth of mining waste into Polley Lake. The spill flooded Polley Lake, creating a plug at Hazeltine Creek, and continued into nearby Quesnel Lake and Cariboo River. By 8 August the four-square-kilometre (1.5 sq mi) sized tailings pond had been emptied of the majority of supernatant (process water) that sits atop the settled crushed rock solids (mining waste, or 'tailings'). The cause of the dam breach and subsequent tailings spill has been investigated with a final report published 31 January 2015. Imperial Metals had a history of operating the pond beyond capacity since at least 2011. Remediation and reconstruction has been underway at the site since 2014. These efforts have included investigation on impacts to human health and safety and affected ecosystems while removing the tailings spill, reconstructing creek shorelines, installing fish habitats, and replanting trees and other local vegetation. Investigation by the remediation team showed elevated levels of selenium, arsenic and other metals consistent with historical tests before the dam breach. These initial reports had been concerned about the chemical impact of the tailings spill on the surrounding environment, but it was determined through subsequent investigation and remediation that the challenge posed by tailings spill was physical in nature.
Tailings are the remainder of what remains after desired minerals have been removed. The Mount Polley Mining Corporation mined copper, gold, and silver from their mine at Mount Polley. The volcanic rock from which the desired minerals are extracted contains a mixture of orthoclase (36.95%), albite (24.38%), magnetite (7.38%), calcium plagioclase(7.12%), diopside (4.48%), garnet (3.33%), biotite (3.04%), epidote (2.12%), calcite (2.01%), chalcopyrite (0.17%), and pyrite (0.04%). The tailings at Mount Polley contain a usually low amount of chalcopyrite and a high amount of calcite, making them geochemically benign. A typical concern of the tailings of metal mining such as acid rock drainage does not occur at Mount Polley due to the unique distribution of compounds in the volcanic rock. The relatively high levels of calcite allow the mineral to act as a neutralizing agent for sulphides that are constituent in the chalcopyrite and pyrite. Therefore, the crushed rock tailings of Mount Polley are virtually inert, not reactive with air or water, instead having properties like natural sand.
The Mount Polley open pit copper and gold tailings spill in the Cariboo region of British Columbia began in the early morning of 4 August 2014 with a partial breach of the tailings pond dam, releasing 10 million cubic metres (10 billion litres; 2.6 billion US gallons) of water and 4.5 million cubic metres (4.5 billion litres; 1.2 billion US gallons) of slurry into Polley Lake. Another source gives the quantity spilled as: 7.3 million cubic metres (7.3 billion litres; 1.9 billion US gallons) of tailings, 10.6 million cubic metres (10.6 billion litres; 2.8 billion US gallons) of water, and 6.5 million cubic metres (6.5 billion litres; 1.7 billion US gallons) of interstitial water. The slurry of tailings and process water carried felled trees, mud and debris and "scoured away the banks" of Hazeltine Creek which flows out of Polley Lake and continued into the nearby Quesnel Lake. The spill caused Polley Lake to rise by 1.5 metres (4.9 ft). Hazeltine Creek was transformed from a two-metre-wide (6.6 ft) stream to a 50-metre-across (160 ft) "wasteland"* and Cariboo Creek was also affected. As of 8 August 2014, the tailings continued to pour into Quesnel Lake. By the end of the day the four-square-kilometre (1.5 sq mi) sized tailings pond was "virtually empty". Mine safety experts and media articles have called the spill one of the biggest environmental disasters in modern Canadian history. British Columbia's government initially insisted the dam failure was not an environmental disaster but stated "We will have a much better idea 24 hours from now on the quality in Quesnel Lake," , a position clarified again by British Columbia's Environment Minister, Mary Polak, in November 2014 after more sampling data became available. On August 6, two days after the breach, the British Columbia Ministry of Environment issued a Pollution Abatement Order to Mount Polley Mining Corporation. The company submitted an action plan for the Preliminary Environmental Impact Assessment, environmental monitoring, stopping the flow from the "Tailings Impoundment" breach, as required. The company was required and did report weekly on the implementation of action plan measures.
On 6 August 2014 Cariboo Regional District declared a local state of emergency in several nearby communities, including Likely, British Columbia, partly because of concerns over the quality of drinking water, which affected 300 residents. On August 9, water use restrictions were removed for residents of Likely and downstream from that community based on water quality testing by the British Columbia Government. However, the government continued the advisory for points upstream of Likely. On August 12, Interior Health further rescinded the ban on drinking water, narrowing the "Do Not Use" order to only the impact zone directly affected by the breach, which includes Polley Lake, Hazeltine Creek, and an area within 100 metres (330 ft) of the shoreline sediment deposit, where Hazeltine Creek runs into Quesnel Lake. This remaining advisory is expected to stay in place indefinitely. The government advised that boiling the water would not help in the "do not use" areas. All five testing sites of the second water test run had zinc levels above chronic, or long-term, exposure limits for aquatic life.
The government described the purpose of the state of local emergency was to provide "exceptional" powers in the interest of public safety, and an equitable distribution of potable water to the residents of Likely. All tourism businesses in the affected areas remained open. Because the affected water system is salmon-bearing, there was a temporary closure of part of the Chinook salmon fishery by Fisheries and Oceans Canada. Fishing along the Fraser River was not affected. Rainbow trout toxicity test results from water collected at Quesnel Lake near the mouth of Hazeltine Creek on August 5 and 6 showed water was not toxic to rainbow trout.
The University of Victoria Environmental Law Centre filed a complaint with B.C.’s privacy commissioner regarding the government not releasing environmental assessments and dam inspection reports about Mount Polley mine; after journalists found a dam inspection report from 2010 and environmental assessments from 1992 and 1997 in public libraries, the B.C. government has withheld subsequent reports. Indigenous political activists led by Kanahus Manuel set up blockades and held numerous community based protests against Imperial Metals following the disaster.
Since the spill, Alaskan mine opponents, including environmentalists such as Trout Unlimited Alaska, aboriginal peoples, the fishing industry and politicians, point to several planned B.C. mining projects involving three major salmon-producing river systems that run downstream into Southeast Alaska. The large Red Chris gold and copper mine owned by Newcrest Mining is now in operation and located in the headwaters of the Iskut River, a major tributary of the Stikine River whose estuary is near Petersburg and Wrangell, Alaska. The KSM project owned by Seabridge Gold Inc has been approved by B.C., awaits federal approval and is located near the Unuk River system supporting one of Southeast Alaska's largest Chinook salmon population, and flows into Alaska, although its tailings facility would be located in the Nass River watershed emptying into the Pacific in B.C. A third mine is slated to reopen and expand in the Taku River near Juneau.
On 18 August 2014, the British Columbia government, with support of the Soda Creek Indian Band and Williams Lake Indian Band, ordered an independent engineering investigation into the pond breach and a third-party review of all 2014 dam safety inspections for every permitted mine's tailings pond in the province. The panel reviewing this breach was composed of Norbert Morgenstern, P.Eng., Dirk van Zyl, P.Eng., and Steven Vick. Their final report was published 31 January 2015. The investigation covered many factors, including the question of whether the piezometers measuring the water pressure on the dam had been located correctly. The last readings, 2 August 2014, did not show any changes in the water pressure. A principal finding of the panel determined that the tailings dam collapsed because of its construction on underlying earth containing a layer of glacial till, which had been unaccounted for by the company's original engineering contractor. In 2010, Mount Polley Mining Corporation's (MPMC) engineering firm reported a 10-metre (33 ft) crack in the earthen dam while working to raise it, and that piezometers were broken, which were later fixed. In 2018, the Association of Professional Engineers and Geoscientists of British Columbia charged three engineers who worked on the tailings storage facility with negligence or unprofessional conduct.
The long-term water management plan for the Mount Polley mine site has been approved by an independent statutory-decision maker from the Ministry of Environment and is expected to be fully in place by fall 2017 and will replace the short-term water management plan that has been in place since 30 November 2015.
Mount Polley Mining Corporation submitted its formal permit amendment application, which included the long-term water management plan and supporting Technical Assessment Report, in October 2016. The documents were subject to extensive public consultation, including First Nations and local communities. The application also underwent a full technical review from the Cariboo Mine Development Review Committee (CMDRC), which includes representatives from provincial and federal agencies, First Nations, local governments (City of Williams Lake and Cariboo Regional District), and the community of Likely.
The Mount Polley Mining Corporation (MPMC) treats mine site water with water treatment plant technology by Veolia prior to release into Quesnel lake. The water is monitored for turbidity at 15 second intervals and water quality is assessed at Quesnel lake as part of MPMC's Comprehensive Environmental Monitoring Plan. About 15,000 cubic meters of site water is discharged into Quesnel lake per day. This is below the 29,000 cubic meters threshold allowed under the mining corporation's permit.. The water at Quesnel lake, Quesnel river, Polley lake, and Hazeltine creek are regularly monitored by the Ministry of Environment.