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Damage from a tailings pond breach at the Mount Polley mine is seen near Likely, B.C., on Aug. 5, 2014.Jonathan Hayward/The Canadian Press

The owners of the Mount Polley mine say a crack in their tailings dam found in 2010 was almost a kilometre away from the spot where the dam containing toxic waste failed this summer, and the company "fully complied" with a series of recommendations to improve safety in response to that initial fissure.

But NDP Leader John Horgan is calling for the release of technical documents to show just what the company and the province knew about the safety of the dam prior to the Aug. 4 breach in the dam that flushed 24 million cubic metres of water and mine tailings into Quesnel Lake in central B.C.

The last geotechnical inspection by the ministry of mines at Mount Polley took place in September of 2013 and resulted in no orders related to the tailings storage facility, according to ministry officials.

The government has not opened its inspection files, saying it must "protect the integrity and independence" of an independent engineering investigation and inquiry into the tailings pond breach that is expected to be completed in January.

"The most horrific environmental disaster in B.C.'s history wouldn't have happened if everything was fine," Mr. Horgan said Sunday. "They are trying to say everything that could be done, was done, but they won't release the documents to show what they did."

He said the government's role as the regulator is under a shadow because the public can't be confident that the province's inspection regime is adequate.

"The relationship between the company and the regulator has to be transparent." Instead, he said the opposition party has been denied technical briefings and access to reports.

Bill Bennett, Minister of Energy and Mines, said in an interview Sunday that his government's engineers were aware of the 2010 fissure and believed concerns had been properly addressed by the company.

"The advice from my staff is that the 2011 inspection by the ministry and by the engineers indicated the deficiencies found in 2010 had been dealt with," he said. Mr. Bennett repeated that he cannot release the reports at this time, on the advice of the Ministry of Justice.

Imperial Metals Corp., the Vancouver-based company that operates the gold and copper mine, has mostly retreated from the public eye since the dam failure.

In a rare statement from the company dated Oct. 3, the company acknowledged that a crack at least 10 metres in length had been observed in the earthen dam while work was under way to raise the dam in 2010, and that a number of instruments required to measure water pressure behind the dam were in a state of disrepair.

Those details were cited by the company's engineering firm of record at the time, Knight Piesold, in it final report before Imperial Metals switched to a new engineering firm, AMEC.

The company said it acted on a string of recommendations from AMEC. More sand was packed around the dam and a "significant instrument replacement program was carried out." A stability assessment was then undertaken in 2011 that "confirmed that the factor of safety in the vicinity of the crack was more than adequate, exceeding the required safety standards." The company "received no indication that there remained any issues of concern."

This summer, crews were working to once again raise the tailings dam at the Mount Polley mine when the structure failed and sent a torrent of waste and debris into surrounding waterways, raising fears about drinking water and the long-term impact on wildlife.

Imperial Metals had asked the Ministry of Environment for a permit to release more treated effluent from its tailings pond. That permit was pending when the dam gave way.

Editors' Note: A previous version of this story incorrectly – due to an editing error – paraphrased Bill Bennett, the B.C. Minister of Energy and Mines, as saying that the crack in the tailings dam that caused a massive leak of toxic waste in August was far from the crack detected in 2010. In fact, the cause of the August breach is not yet precisely known. This version has been corrected.

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