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A aerial view shows the debris going into Quesnel Lake caused by a tailings pond breach near the town of Likely, B.C. Tuesday, August, 5, 2014.JONATHAN HAYWARD/The Canadian Press

Months after a dam at Mount Polley mine collapsed, releasing more than 25 million cubic metres of tailings and water, a polluted plume of sediments continued to circulate through Quesnel Lake.

Now a study by researchers from two universities and the federal Department of Fisheries and Oceans is raising concerns about the long-term impact of that plume, which is still exposing fish, plants and insects to copper, selenium and other metals.

Ellen Petticrew, a University of Northern British Columbia geology professor, said a study that took place over the two months immediately after the spill found that although the sediment sank in the lake, it created a layer that could be stirred back to the surface by wind and wave action.

Long after the initial spill, the plume was observed at times pulsing out into the Quesnel River.

"Because of the shape of the lake, the long skinny arms oriented in an east-west direction, when a strong wind blows, it can pile up water at one end of the lake and generate what's called a seiche, or rocking movement," said Dr. Petticrew. "And if you think about your layer cake if you push down on one end then the cold water comes up at the other end and it rocks back and forth … [until] the cold turbid water with the mine particles in it gets pushed out and empties into Quesnel River."

Dr. Petticrew said it's not yet clear how the pollutants in the plume are affecting fish in the lake or river and it might take years of study to determine that.

She said sediment was stirred up last fall during a seasonal event known as lake turnover. Data haven't been collected yet on this spring's turnover.

"We know some of the sediment settled over winter. … We just don't know how much of it will stay on the bottom and how much will get into the water column," she said. "So the concern for the future is that these particles have higher copper levels than the ambient water column, and we're concerned about the effect this can have on the biota and how well the lake can handle this load of contaminants. It's a big lake; it's a deep lake, but it's a lot of material to be dumped into it."

The tailings dam at Imperial Metals Corp.'s Mount Polley mine breached last Aug. 4, in the biggest spill in Canadian mining history. The accident occurred as more than 800,000 adult sockeye were in the Quesnel system on the annual spawning run.

Dr. Petticrew said those fish appeared to have passed through the plume without problem, but researchers are now watching to see how the eggs survived.

There are also concerns about pollutants moving up the food web. Dr. Petticrew said copper is the biggest concern because it is showing up in the highest concentrations and is known to be harmful to fish, among other things impairing growth and homing abilities.

Five researchers from UNBC, three from the University of British Columbia and four scientists from DFO were involved in the study, which was published by the journal Geophysical Research Letters.

At a mining conference Tuesday in Vancouver, B.C. Energy and Mines Minister Bill Bennett said the province is committed to strengthening safety guidelines for companies that operate dams.

"We need to lead the rest of the country, and I think we need to lead in the world in terms of adopting the best available technologies and practices that will make sure this doesn't happen again," said Mr. Bennett at the event, where PricewaterhouseCoopers noted that low commodity prices eroded industry profits last year. "We also need to make sure that what we do as government doesn't unduly hamstring the mining industry and doesn't go further than it needs to go to decrease that risk."

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