Imperial Metals Corp. operates the open-pit Mount Polley copper-gold mine near Likely, British Columbia, about 56 km north-east of Williams Lake. As was widely reported in the media, a breach of the tailings pond on August 4 released an estimated 10 million cubic meters of water and another 4.5 million cubic meters of contaminated sediment into Hazeltine Creek, which drains into Quesnel Lake.

What is a tailings pond?

Metal mines produce a large amount of tailings from the process of pulverizing the rock and treating it with chemicals and water to extract the valuable metals. Tailings ponds capture the residual slurry. Sedimentation separates the water from the particles. Clean water evaporates or is discharged into nearby streams. As the sediments accumulate, the embankment of the tailings pond needs to be raised and widened over time.

How can a tailings pond fail?

According to the US Environmental Protection Agency's report Design and Evaluation of Tailings Dams, a tailings pond may fail due to a number of reasons: foundation failure, overtopping by flood waters, embankment erosion due to heavy rainfall, subsurface erosion through seepage along pipes, and liquefaction due to earthquakes or prolonged earth movements. Failure is often preceded by "distress signals" such as cracking and wet spots. Instrumentation placed along the embankment can monitor changes and predict unstable conditions.

Which substances were deposited in Mount Polley's tailings pond?

The table below shows the disposals into the tailings pond that the Mount Polley mine disclosed to the federal National Pollutant Release Inventory in the last five years:

Source: National Pollutant Release Inventory, Facility Number 5102

The above data makes it quite apparent that the level of production increased dramatically in 2013 from the previous four years, leading to a much higher need to discharge effluent through the tailings pond.

Was this accident foreseeable or preventable?

A technical report by environmental consulting firm Brian Olding & Associates Ltd from 2011 pointed to a number of problem issues. For example, the company had not yet prepared a detailed monitoring plan or emergency contingency plan. Also, the pond levels were already getting too high, and the mine was in need of discharging 1.4 million cubic meters of water per year to remain at the same level. The report did not include a review of the structural engineering of the tailings pond and dams, however. Since 2012, the company has been warned several times that the height of the water in the pond exceeded recommended levels. While not all facts are known yet, there are early indications that the tailings pond failure was preventable if the company had acted sooner to address concerns and not increased output of the mine before appropriate and adequate effluent treatment options were in place. Part of the blame may also fall on the provincial government, as the 2011 BC Auditor General Report lamented that there was inadequate monitoring and follow-up investigations at dams and mines.

What is best practice with respect to tailings?

A tailings pond is usually the cheapest method of treating tailings. Well managed, tailings ponds can be quite effective and safe. Waterproof bottom liners can prevent seepage into the groundwater. Water collection systems can capture residual seepage. There are also more costly alternatives to tailings ponds. One method is dry stacking: tailings are dewatered through a combination of mechanical pressure systems and vacuum filtration systems. The filtered tailings can be transported by conveyor belt to a "stack" site. Dry stacking completely eliminates the risk of catastrophic failure and tailings runout, and also prevents groundwater contamination through seepage.

What happens to tailings ponds when a mine closes?

When a mine closes, it takes about 2-5 years for the water to become completely clean, at which point it can be released safely and the land reclaimed and rehabilitated. Former tailing ponds can also be developed into lakes. In British Columbia, it is common practice to require the posting of a "Reclamation Security" bond as part of the permitting process. These reclamation bonds are supposed to cover any future reclamation cost. There remain legacy sites that are not covered, however.

What happens next at Mount Polley?

Imperial Metals is liable for the clean up of the pond breach. The province of British Columbia has issued a pollution abatement order under sections 80 and 83 of the Environmental Management Act. The company is required to comply with this order and can be fined in case of non-compliance. Provincial inspectors are investigating the dam breach and will determine responsibility. Numerous water quality inspections are in progress in order to determine potential adverse effects on humans or wildlife. Preliminary tests revealed low levels of contamination that do not exceed regulatory standards. It is likely that the provincial government will increase inspections of other, similar facilities. It will also be necessary to conduct an independent investigation into potential long-term effects from the contaminated sediments that have spilled into the local environment.

What can we learn from the Mount Polley accident?

While not all facts are fully known yet about what went wrong at Mount Polley, there appear to be deficiencies in our pollution prevention and emergency response systems. We need more state-of-the-art monitoring of industrial sites as an early warning system. Accident prevention is better (and cheaper) than disaster recovery. In case of accidents, we also need better preparedness by requiring owners of industrial sites to prepare environmental emergency (E2) plans for a wider range of potential accidents, and share these plans with local, provincial, and federal authorities. When accidents occur, speedy interventions can limit and contain damage.