The Mt Polley mine disaster

At two am on the morning of 4 August, 2014, a tailings pond at Imperial Metals’ Mount Polley gold and copper mine broke through an earthen dam, sending 10 million cubic metres of tailings water and 4.5 cubic meters of tailings sediment pouring into Polley Lake and from there through Hazeltine Creek into Quesnel Lake. A complete water ban has been issued for the area, with no water use of any kind permitted.
This type of disaster is new to British Columbians, and no one really knows exactly what the impacts may be, though they seem to be downplayed by both the mining company and the BC Minister of Mines. The president of Imperial Metals has stated publicly that the tailings pond water was not contaminated and that he ‘would drink it.’ This seems to be at odds with 2013 data from Environment Canada that the tailings pond contained arsenic, mercury, and other heavy metals. Minister of Mines Bill Bennett released a statement suggesting only sand and water was released from the pond, but failing to mention contaminants.
This post avoids some of the trickier questions around politics to focus on three main areas of concern: water, sediments, and ecological impacts.
The big question is whether or not there were contaminants in solution in the tailings pond water. One source notes that, according to Imperial Metals, the pond was alkaline (pH 8.5) rather than acidic, but Imperial should ultimately know the exact composition of their wastewater. If it does contain dissolved contaminants, these could spread not only through the surface water system but into the groundwater as well, where it becomes much more difficult to monitor and remediate. The other issue with a deep lake system like Quesnel Lake is that it likely doesn’t see a lot of water turnover, thus any contaminated water inputs would be diluted only by the volume of surface water that overturns in the lake.
This is a huge one – and not only from the tailings themselves but from the debris scoured away in the path of the flood. The flood remodeled the formerly narrow Hazeltine Creek into a 150 ft wide spillway, flushing everything from fine sediments to boulders and trees ahead of it. While the coarse sediments would have been deposited soon after reaching the still waters of Quesnel Lake, the fine sediments can travel much farther in suspension. This has implications for salmon spawning in any tributaries from the lake, as these fine materials can potentially silt up spawning beds.
The other issue with sediment is that contaminants adsorb to sediment surfaces, so anywhere the sediment goes, the contaminants come along for the ride. The contaminants can even be released from stored sediment that’s disturbed at a later date.
This is one of the more difficult aspect of the spill to examine. There are the direct effect of contaminants on both aquatic organisms such as the returning salmon (which DFO predicted would peak on 11 August at 4.1 million fish in the Chilko and Quesnel sockeye runs), and on terrestrial organisms that come in contact with the water. There is high concern that toxins will move through the food web via bioaccumulation and biomagnification, with implications for First Nations wild food and medicine sources.
What might be the process moving forward?
In the short term: fieldwork to test water quality in both surface and groundwater, sample in situ sediments to determine contaminant levels, and survey stream systems to determine changes in stream morphology and potential implications for aquatic organisms. Numerical modelling to simulate the event and determine the extent of the sediment plume and potential sediment deposition areas. Ecological surveys of the salmon run to assess returns, mortality rates, and spawning success. Ideas on other ways we can get a better picture of the effects of the spill are welcomed in the comments.
In the longer term: Sampling of aquatic and terrestrial organisms to quantify bioaccumulation and/or biomagnifications of contaminants. Continued water sampling and sediment sampling to determine whether the contaminants are changing over time in both concentration and mobility.
Researchers from the University of Northern British Columbia have already begun taking samples to study this event, and I look forward to hearing a lot more about their work. I hope Imperial Metals can afford the cleanup, estimated at $200 million or more. Especially given that their stock lost 40% of its value on the news of the tailings pond breach…

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9 thoughts on “The Mt Polley mine disaster”

  1. Hi Sarah,
    From what I see on the web and from your post, it looks like a Glacial Lake Outburst Flood (GLOF) so it may have good ideas for other ways to get a better picture of the effects of the spill. In my department for example (I hope doesn’t sound like autopromotion), there Matt Westoby and Neil Glasser working on that (,,
    Maybe another way to investigate the extent of the damage is to have a look at voluntary dam breach for dismantling like in the case of the White Salmon River and Condit Dam (
    I hope it helps.
    Thanks for you blog,
    PS: I try to make people aware of that event on the side of the Atlantic (Wales) but it’s difficult.

    • Thanks for the note Pierre – you’re absolutely right in that the spill event was similar to moraine-dammed lakes breaching in glacial environments. I decided to leave that out to avoid overloading the reader with too much more information than what was already there. But definitely a natural analogue to what’s happened here!

  2. Thanks for blogging on this. It is still early but the dam of information from CA Government and Imperial seems very stable at this point. Fishermen in SE AK are watching closely the progression of these proposed mines at the headwaters of rivers that cross our countries borders. This is terrible for Canada and its fish. Will the CA Gov hold imperial accountable? Will the CA Gov hold itself accountable? Will Canadians hold themselves accountable? I hope those individuals, businesses and families effected by this avoidable disaster will somehow be made partially whole by those responsible. As for the environment, the fish, and those who depend on both we will have to wait and see…

  3. Hi Sarah, thanks for this thoughtful post.
    If glacial outburst floods are an appropriate analogue, then it seems highly relevant that 11,000 year old glacial outburst sediments from Interior BC (Upper Fraser and Thompson) are present in distinct layers at the bottom of Saanich Inlet on Vancouver Island, a distance of 300-500 km from their source. If the Mt Polley disaster is similar, then this would imply that the contaminated fine sediments in the water column could disperse downstream through the Fraser and out into the Salish Sea….
    Blais-Stevens, Andrée, John J. Clague, Rolf W. Mathewes, Richard J. Hebda and Brian D. Bornhold
    2003 Record of large, Late Pleistocene outburst floods preserved in Saanich Inlet sediments, Vancouver Island, Canada. Quaternary Science Reviews 22(21–22):2327–2334.

    • Hi Iain – that’s certainly a possibility, though we’d have to compare the volume and mechanism of the Pleistocene floods vs. the tailings pond flood in order to ensure we’re comparing apples to apples (rather than grapefruits 🙂 )


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