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UBC Theses and Dissertations

Suspended sediment in Quesnel Lake following the Mount Polley Mine tailings spill Granger, Brody


In the four years that followed the 4 August 2014 tailings dam failure at Mount Polley Mine, British Columbia, Canada, water quality data indicated an ongoing sediment loading to Quesnel Lake and the Quesnel River. Within one day of the tailings dam failure, a flood of slurry entered the smaller, downstream basin of Quesnel Lake, called the West Basin. Previous studies had shown highly elevated turbidity in the West Basin through the first autumn and winter after the spill, and above background turbidity each autumn and spring from 2015 onwards. It remained unclear how long this seasonally elevated turbidity would last. In this thesis, we evalutate sediment transport in Quesnel Lake following the rapid inflow of a vast quantity of material. This thesis applies conservation of mass in two ways: first, using data collected between 10 September 2014 and 21 December 2018 to estimate suspended sediment mass and mass flows into and out of the West Basin; and second, using an analytical model. On 10 September, 37 days post-spill, an estimated 38000 $\pm$ 11000 Mg of solids remained suspended in the West Basin; this decreased to within background levels ($<$300 Mg) by early June 2015. Between 10 September 2014 and 3 June 2015, 4000 $\pm$ 1200 Mg of sediment flowed from the West Basin into the Quesnel River, and $\sim 31000$ Mg entered the main basin of Quesnel Lake. A gradually decaying, seasonal cycle emerged thereafter: near background each summer, somewhat elevated during winter and spring, and above background each autumn, with an interannual decrease in magnitude. Remobilization of a turbid, bottom layer by internal wave motions during each autumn of 2015-2018 contributed to an increased mass of suspended sediment in the West Basin. Together with the observed mass trend, an analyticial mass balance model of a simplified, two basin system suggests a return to background suspended sediment levels by one decade post-spill.

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