Catastrophic Tailings Spill at Mount Polley Mine

From Mining Watch Canada    

On August 5, approximately 10 billion litres of wastewater and 5 billion litres of solid tailings waste escaped the impoundment at Imperial Metals’ Mount Polley mine in the interior of British Columbia. The creek that received the brunt of the flow was completely obliterated, some of the waste backed up into Polley Lake and some the wastes and debris from the torrent continued downstream into Quesnel Lake. A local state of emergency was called and a precautionary ban was put on using surface and groundwater in the area. The following is our effort to synthesize the many reports and commentaries that have come out during the first four days following the spill, and to answer some of the questions we’ve been getting from media and the public. For hazeltine_creek_post_spill_small background, we’ve relied on a 2011 report jointly commissioned by the Xatsul and T’exelc First Nations and Imperial Metals, the company’s website, and a 2004 technical report.     Hazeltine Creek following the spill (photo by Chris Blake).

What are tailings and what was in the impoundment?

Tailings are the wastes left over from the crushing, grinding, and processing of mineral ores. Because low-grade large-scale mines like Mount Polley are mining ores where the sought-after minerals (gold and copper in this case) are less than 1% of the ore, a lot of waste is created from processing the 20,000 tonnes of ore that went through the Mount Polley mill each day. During processing, the ground rock is mixed with water and reagents to remove the gold and copper, and the remaining slurry (mix of water and sandy or muddy solids) is pumped to the tailings impoundment for disposal. (See also our Mine Waste Primer.)

The Mount Polley tailings impoundment is no “pond” – it is nearly 2 square kilometres with a perimeter of 5 kilometres. As Iain McKechnie pointed out with this image on Twitter, that’s almost the size of Vancouver’s Stanley Park.


Tailings often contain residual minerals including lead, mercury, arsenic, cadmium, and selenium, that can be toxic if released to the environment. If a substance in the tailings is included in the toxic substances listed under the Canadian Environmental Protection Act, mine operators must report the amount of these substances in the tailings to a publicly available database – the National Pollutant Release Inventory (NPRI). (Prior to a legal challenge by MiningWatch, Great Lakes United, and Ecojustice, the mining industry was exempt from reporting.)

Below is a table with the toxics contained in the Mount Polley tailings from the last five years of reporting (this differs from earlier reported amounts, which included substances contained in waste rock as well as tailings). The extent to which these substances pose a threat to the environment depends on the geochemistry of the tailings and the surrounding conditions.

Substances in Mount Polley tailings as reported to the NPRI (in tonnes)






















































2 1 8.6






























0.7 0.5 0.4 0.6 2.6


0.01 6.8 8.2 9.0 24

Tailings also include reagents used in processing the ore. Reagents used at Mount Polley include xanthates, which are known to be toxic to aquatic organisms, but the company estimates that most of the residual xanthates leave the site with the mineral concentrate. Testing of processing chemicals in tailings and effluent is not currently required by Environment Canada or the B.C. government.

What are the effects of the spill?

The most obvious impact of the spill is the destruction of the 10 kilometre-long Hazeltine Creek watershed. A small tributary to Quesnel Lake, the creek has been completely buried in tailings and a huge swath of trees mowed down. According to the B.C. Fisheries database, Hazeltine Creek provided habitat for chinook, coho, and sockeye salmon, rainbow trout, and a number of other fish species. If it is even possible to restore the habitat for these species, it will require removing the tailings from the creek bed, re-establishing a natural creek bottom, getting vegetation to regrow along the shore and keeping the water and sediments clean enough to support a healthy ecological community.

While much of the tailings remain in the Hazeltine Creek watershed, some of the solids went into Polley Lake and Quesnel Lake, both of which are important recreational fishing areas with high quality water that are used as a drinking water sources by local residents.

Initial testing of Quesnel Lake by the B.C. government has not shown any impairment to the use of the lake as drinking water or for aquatic life according to established standards. This may be thanks to the lake’s large size and outflow that would have diluted and dispersed any contaminants from the wastewater. Much of the contaminant load from the spill will be in the sediments, which will settle out of the water column and not be captured in surface water samples. These are very preliminary findings and until the tailings upstream are stabilised, contaminant loading to the lake will continue. It is very difficult to know what the medium or long-term repercussions for Polley Lake, Quesnel Lake and aquatic communities further downstream will be. Fish – and in particular salmon – are extremely sensitive to several of the substances listed above, and much more work is needed to evaluate the risks.

The spill has had significant emotional and psychological impacts with the shock and grief of witnessing this kind of disaster and the uncertain future of a cherished watershed. Chief Bev Sellars of the Xatsull First Nation told the media that many members of her band were in tears when they learned of Monday’s release. “Because they know the destruction that’s going to happen from this breach. It’s just a real sad day.”

The local community is also stressed by the immediate loss of the 300 jobs at the mine and the uncertainty about when or if the mine will reopen.

The financial costs of cleaning up the spill will be considerable and are estimated in the hundreds of millions of dollars. Mining companies are required to post financial assurances with the province for routine clean up costs at the end of a mine’s operating life; unfortantely these costs are not made public. A report by University of Victoria Environmental Law Clinic indicates that these amounts may not be sufficient for routine mine closure – let alone a major disaster like this. Imperial Metals admitted that its insurance is not likely to cover all the costs with the President stating: “I made the commitment, to the best of my ability. If it’s $400 million, then we are going to have to get mines generating to make that money to do the cleanup. We don’t have $400 million in the bank, so we’ll have to make that to do it.”

If Imperial does not fund the clean-up citizens will be left with the financial, environmental and social costs of the spill.

What could have caused the failure?

Tailings in an impoundment like the one at Mount Polley remain saturated so they have little to hold them together, and a breach in the impoundment means the water and solids flow together out of the impoundment and then downstream. It does not require a large breach to start this process, so Imperial Metals’ statement that the portion of the impoundment that failed is relatively small compared to the total length of the dam is totally irrelevant.

Catastrophic (sudden and severe) failure is an inherent risk of large tailings dams. The risks increase with the size and height of the impoundment and the amount of water relative to the solids kept in the impoundment. As Andrew Nikiforuk points out, as we exploit lower and lower concentrations of minerals, tailings impoundments are growing larger and larger and more and more risky.

In addition to the water from processing the ore, large tailings facilities also accumulate rainwater. If it does not evaporate, excess water must be released to maintain the impoundment, usually following treatment to remove contaminants such as heavy metals and suspended solids. Excess water that flows over the top of an impoundment is extremely dangerous as it can erode the impoundment wall, creating a gully that increases rapidly in size as ever more waste flows out of it. The weight of excess water also increases the pressure on the impoundment walls and destabilise them if there are any weaknesses.

Impoundments are built to the Canadian Dam Association’s standards and are, in theory, supposed to withstand extreme weather and seismic events. The Mount Polley impoundment was built in 1997, and it was evaluated and deemed secure by Amec consultants in 2004 before it was put back into use after the mine was closed for several years. There is no indication that any extreme weather or seismic activity contributed to the failure.

There are several lines of evidence that indicate that Imperial Metals amplified the inherent risks of a tailings impoundment by storing a large volume of water and allowing the water level to go beyond the modest one-metre buffer demanded by its provincial permit. The CBC reported that the B.C. Ministry of Environment gave five warnings to the company about the amount of water in the impoundment. The Minister of Energy and Mines disputed this statement, noting that there was only one incident in May of this year and that it was dealt with quickly by the company pumping excess water into a mined out pit.

The engineering firm responsible for the initial construction and oversight of the impoundment up to 2011 issued a statement saying that it also cautioned the company and BC that the embankments and impoundment were “getting large and it is extremely important that they be monitored, constructed and operated properly to prevent problems in the future”.

A former mine employee spoke to the media, stating that the water was being kept too high and there were previous breaches to the impoundment. If high water was a reoccurring problem and regulators gave the company a series of warnings rather than taking stronger measures, a serious failure of the regulatory system occurred.

Imperial requested a discharge permit to release more water from the impoundment in 2009 but did not provide a satisfactory plan to do so. Another application to discharge was being processed at the time of the spill. The 2011 report jointly commissioned by neighbouring First Nations and Imperial focussed on the discharge issue and provided a number of recommendations to Imperial about how to proceed. From the available information it seems clear that Imperial continued building the impoundment walls higher rather than dealing with the water. The former employee also noted a failure to increase the width of the base of the impoundment to stabilize it as the height increased.

In his critical editorial in the Northern Miner, John Cumming, made a point of noting that Imperial Metals is a member of Canada’s mining establishment not some rogue fly by night operation. It’s also a member of the Mining Association of Canada and has been implementing the Association’s Tailings Management Program as part of the Towards Sustainable Mining Initiative. MiningWatch has repeatedly asserted that such voluntary management approaches are inadequate to deal with the risks associated with mining.

Mining engineer and I Think Mining blogger Jack Caldwell summed up his observations of  the available evidence this way: “I suspect it failed because there was too much water in the dam, the corner gave way, an upstream slide occurred, and the disaster ensued. They are saying nobody could have anticipated this. Rubbish. It was entirely predictable given the facts.”

Could this happen at other sites?

This is the largest tailings spill in Canadian history but certainly not the first. Just last year, on October 31, 670 million litres of coal slurry spilled from an impoundment at the Obed coal mine into the Athabasca River near Hinton, Alberta. The Coalition Québec meilleure mine has documented many smaller recent spills in that province.

Whenever we have the combination of an inherently risky waste management option, combined with lax government and a company that pushes the risk boundaries, we are likely to have another failure.

The other crucial issue about tailings impoundments is that they remain on the landscape forever. As Water Matters’ Bill Donahue pointed out to the CBC, impoundments may be portrayed as a final solution, but they are not. If left on the landscape forever, the likelihood of failure eventually approaches certainty.

What are the alternatives to tailings impoundments?

Disposing tailings slurry into an impoundment is not the only way to manage the millions of tonnes of waste generated by modern-day mining. Some mines, mostly underground mines extracting smaller volumes of ore, backfill exploited areas with tailings – a safe and sound option. Drying tailings or turning them to a paste that then hardens and dries are two other options. All these options add costs to mining operations, and so are not favoured by corporate interests focussed on the bottom line. They also have their own technical challenges. See our Mine Waste Primer for more.

An option that must also be considered is simply not mining a particular deposit. Where ecological and social risks are high and the economics don’t allow for a more secure longer-term solution, not mining remains the only guaranteed way to keep tailings out of streams, rivers, and lakes.

Addtional Links

Video: Helicopter Fly Over by Global News (2 min)

Video: Helicopter Fly Over by Cariboo Regional District (37 min)

BC Ministry of Environment Incident Page

Union of BC Indian Chiefs Statement