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Natural mixing of lake waters may resuspend contaminants deposited in Quesnel Lake by the Mount Polley mine spill, according to scientists who have been studying the lake since the spill in 2014.

The new paper, , found that turbidity, or cloudiness, in portions of B.C.’s Quesnel Lake increases each spring and fall, when the lake mixes from top to bottom in a natural process called turnover.

The source of the turbidity appears to be the resuspension of spill-related material from the bottom of the lake, raising concerns about contaminants being reintroduced to the water column.

“During the first turnover after the spill, mixing brought suspended sediment to the surface, turning the previous clear-blue lake to bright green. Twelve months after the spill, the suspended sediment loads had substantially reduced, and the lake waters had appeared to return to their pre-spill state—but it turns out this was only temporary,” said researcher Andrew Hamilton, who did the work as a postdoctoral fellow at UBC and at the University of Alberta.

“Over the next three years, we saw an increase in turbidity above the spill deposits at the bottom of the West Basin every spring and fall when winds drove large basin-scale wave motions, like the sloshing of water in your bathtub,” he added. “As turnover occurred, this sediment was then mixed from depths below 100 metres over the whole water column.”

The Mount Polley mine spill was the largest mine-related spill ever documented at the time, dumping 18 million cubic metres of waste water and sediment into Quesnel Lake.

The tailings contain elevated concentrations of some metals, such as copper, and if these metals are being seasonally resuspended from the lake bed they could make their way into the food web, says Hamilton.

“Understanding all factors affecting the long-term water quality of Quesnel Lake is critical as its watershed supports substantial recreation, trout fisheries and Fraser River Pacific salmon stocks that are vital to Indigenous, recreational and commercial fisheries,” he said. “Inevitably these spills end up flowing downstream into lakes or the ocean where they can disappear from view, yet that doesn’t mean the impact is over.” 

The multi-year research is led by Dr. Ellen Petticrew, a geography professor at the University of Northern British Columbia.

Since 2014, Petticrew and �������� Environmental Science Professors Dr. Phil Owens and Dr. Stephen Déry, along with Hamilton and an interdisciplinary group of scientists from UBC, the University of Lethbridge, and Fisheries and Oceans Canada (DFO) have monitored the effects of the spill.

This study presents results from 2014 through 2017.

“We have seen a seasonal increase in some metal concentrations in the Quesnel River that corresponds to the timing of the turbidity increase observed in the lake,” said Petticrew. “This paper explains some of these other processes we are observing, and helps build a more complete understanding of the long-term impact of the spill on the lake.”

Study co-author Bernard Laval, a professor of civil engineering at UBC, adds: “While the immediate environmental devastation of other catastrophic mine waste spills around the world may have been more apparent, our research shows that the environmental impacts of the spill in Quesnel Lake are long-term. Investigating these subtle impacts over time will help inform if and how Quesnel Lake recovers, and the potential for lasting impacts of other tailings spills which are increasingly frequent globally.”

This research was supported by funding from , the , ��������’s Landscape Ecology Endowment, UBC’s , as well as in-kind support of instrumentation from Fisheries and Oceans Canada and the Mount Polley Mining Corporation.