AMEC Earth and Environmental, which became the new engineer of record for the facility, issued a statement that cited the ongoing investigation and inability to answer specific questions.

It added, “While AMEC serves as the Engineer of Record on the most recent raising of the dam, implementation of the AMEC design has not been completed and some construction activity was still taking place to complete our design. Investigations at or near the breach are currently prohibited due to safety concerns and we are awaiting the results of field surveys by Imperial Metals to determine the status of dam construction at the time of the breach.”

However, W. Scott Dunbar, associate professor at the Norman B. Keevil Institute of Mining Engineering at the University of British Columbia, says “water is the ultimate culprit” in breaches of this nature.

“There was too much water in the pond (and the impoundment in general) but whether that contributed to the failure and how it did will be the subject of the investigation,” he says.

One factor, though, is weight. “The weight of the water can cause a breach, but the embankment is designed to withstand water up to the crest. However, under some circumstances too much water can also weaken the internal structure of the embankment,” Dunbar says, likening it to the effect of slowly rising tide on a sand castle.

A spill of this magnitude is rare, he says. “Worldwide, this is a major breach.”

The last major mine breach globally was the Los Frailies mine in southern Spain in April 1998, where a tailings pond failure dumped 7.2 million cu yd of water as well as 1.7 million cu yd of tailings into the surrounding agricultural lands. That mine had to shut down because of the cleanup and compensation costs.

The last breach of a similar magnitude was in 2010, when a tailings pond at an alumina plant in Hungary ruptured and sent about 845,000 cu yd of what was called “red sludge” through towns in the area, sweeping cars from roads, killing seven people and injuring 150. It also threatened the Danube River, but the flow was mostly diluted by the time it reached the famed waterway. That plant, which dumped ferric oxide, aluminum oxide, arsenic, mercury and sodium hydroxide, resumed operations shortly after the incident.

The cleanup at Los Frailies took about eight months and $200 million, Dunbar says. He would not predict time and costs for the Mount Polley cleanup, saying “it really depends on both the areal extent and on whether the tailings have to be removed.”

“If they are threatening fish or other habitat, they will have to be removed,” he says, adding, “The mitigating factor in this case is that the Mount Polley tailings are not acid-generating.” Among the minerals are nickel, arsenic, lead, copper and its compounds. The tailings at Los Frailies were acid-generating, containing sulfide minerals that produce acid after contact with air or water, he says.