A mine tailings pond at the Imperial Metals operated Mount Polley Mine, an open-pit copper mine in the Cariboo Region of British Columbia was breached on Monday 4 August 2014 after excessive rainfall led to the failure of a retaining wall. This in turn led to around 14 500 000 cubic meters of water escaping from the pond, into nearby Hazeltine Creek, turning the creek from a 1-2 m wide stream into a 50 m wide torrent, and severely souring much of the surrounding landscape. The incident further resulted in water from the pond entering a number of lakes, ponds and rivers in the area, resulting in a major pollution scare, and leading to a ban on the use of water from many waterways in the area.
Damage caused by water from the Mount Polley tailings pond breach in British Columbia on Monday 4 August 2014. Jonathan Hayward/The Canadian Press.
Tailings ponds are used to store sediment-laden waters from mines; such waters typically contain a high proportion of fine silt and clay particles, which take time to settle out of the water. The resulting water may be fairly clean, or may contain other pollutants (typically acids, either generated by the local geology or used in the mining process), and need further treatment. The waters from the Mount Polley tailings pond were reportedly not acidic, though the mineral tailings do apparently contain heavy metals, which will be toxic to the environment. Heavy metals tend to precipitate out of the water column fairly quickly, so that most of those present will have remained in the pond (which was not emptied by the breach), and any metals which did escape will hopefully be deposited close to the mine itself, although the pressure of 14 500 000 cubic meters of water escaping rapidly will have carried anything in it a fair way.
The approximate location of the Mount Polley Mine. Google Maps.
The size of the spill means that a considerable cleanup operation is likely to be necessary. Water monitoring by the Environment Ministry of British Columbia has not raised any concern about the chemical content of the spill water at this time, but the physical damage caused by a spill of this size will case considerable problems, and the Assembly of First Nations has called for immediate action to deal with the crisis, which they believe presents a major threat to a number of Salmon spawning grounds, which are both important economic resources and an integral part of the traditional way of life of people living in the area.
The breach in the retaining wall at the Mount Polley tailings pond. The Vancouver Sun.
The Environment Ministry has reported that it has issued a number of warnings about the Mount Polley tailings pond, the most recent coming in May this year. The warnings related to the speed at which water was entering the pond, which was considerably higher than the rate at which it was leaving, and which had resulted in the height of a retaining dam being raised several times. In 2012 Imperial successfully applied to have the discharge level from the pond into Hazeltine Creek raised to 1.4 million cubic meters per year (i.e. slightly less than 10% of the total lost on 4 August 2014), and had applied to be able to release a further 3 million cubic meters per year into nearby Polley Lake.
The pond has reportedly already suffered a number of smaller breaches, the most recent in April this year, when spring meltwater overwhelmed the pump system at the site. Investigations by the Ministry at the site also found that the company had failed to voluntarily report when water levels in the pond had exceeded agreed levels, had failed to cary out adequate monitoring of waterways in the discharge area, had failed to develop a contingency plan to be used in the event of a major spill, and had failed to cary out an analysis of the dam's structural integrity. A number of substances were known to be at levels higher than approved for drinking water in British Columbia, though at the time of writing none of these has been found to be a problem in the areas affected by discharge from the pond, where they would have been further diluted by mixing with water from other sources.
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