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“We had called for an effluent management strategy that I don’t think was in place at this point in time.

“More water was coming in over the year than they could deal with,” Mr. Olding said. “They just kept building the walls up higher and higher every year, and it got to the point where that was untenable.”

The firm was seeking a permit to treat and release some of the water to keep the size of the pond in check at the mine, Mr. Olding told The Canadian Press.

Bev Sellers, chief of the Xatsull First Nation, also known as the Soda Creek Indian Band, said many members of her band were in tears when they learned of the contamination.

“Because they know the destruction that’s going to happen from this breach. It’s just a real sad day,” Ms. Sellers told the wire service.

Mr. Kynoch said the breach is a major setback for the mine that was entering “a promising phase of extended operations.”

But the president downplayed water contamination of waterways, noting that the “water discharged by the event already almost meets drinking water standards.”

“Mount Polley mine has been placed on care and maintenance, and business interruption and physical damage insurers have been notified,” the company said. “While the damaged area is relatively small compared to the overall size of the dam, it is not known at this time how long it will take to restore operations.”

A BMO Nesbitt Burns Inc. analyst pegged the cost to the company at $200-million, apart from legal damages that could double that amount — more than half the company’s market cap of $763.7 million. Mount Polley contributed about 36¢ or 83% to the company’s 2014 earnings per share, BMO analyst Aleksandra Bukacheva said in a note to clients, calling the event “extremely negative” for the company.

Critics are calling the dam breach a failure both for the industry and government.

“This crisis should never have been allowed to happen and highlights systematic failures of B.C.’s environmental laws and standards, monitoring and enforcement protocols, as well as crisis-readiness,” said Jens Wieting, forest and climate campaigner at the Sierra Club B.C.