Initial Distribution and Interannual Decrease of Suspended Sediment in a Two‐Basin Lake Following a Massive Mine Tailings Spill: Quesnel Lake, BC, Canada.

On 4 August 2014, a tailings dam failure at Mount Polley Mine in central British Columbia led to the largest short‐term release of mine waste into a lake ever recorded. Once released by the breach, slurry entered the smaller of Quesnel Lake's two basins, called the West Basin, from which the lake drains northwest into the Quesnel River. An estimated 38,000 ± 11,000 tonnes of fine solids remained suspended in the West Basin on 10 September 2014, 37 days postspill. This decreased to within background levels (<300 tonnes) by June 2015, by which time ∼4,000 tonnes of sediment had flowed from the West Basin into the Quesnel River, and ∼31,000 tonnes had been transported east into the main basin, the direction opposed to the mean hydraulic gradient. Here, we evaluate sediment transport in Quesnel Lake following the Mount Polley tailings dam spill. We apply conservation of mass in two ways: using data collected between 12 August 2014 and 15 October 2020 to estimate suspended sediment mass and mass flows into and out of the West Basin; and using one‐ and two‐basin completely mixed models. Suspended sediment concentrations were highly elevated through the first three seasons postspill, then fluctuated within a gradually decreasing, seasonal cycle. The observed mass trend and an analytical mass balance model of a simplified, two‐basin system suggest that suspended sediment mass will fall to below the median prespill mass by 7 ± 5 years postspill. Plain Language Summary: The largest mine waste spill into a lake to date began on 4 August 2014, at Mount Polley Mine in central British Columbia, Canada. Within the day, nearby Quesnel Lake received a volume of waste equivalent to 2% of the volume of its smaller, downstream basin, the West Basin, where the spill flowed into the lake (0.02 km3 of waste into the 1 km3 basin). Roughly 38,000 tonnes of fine solids remained in suspension 37 days postspill, and our study evaluated the movements of this suspended sediment in Quesnel Lake and the Quesnel River using water quality data collected between 12 August 2014 and 15 October 2020. We found that the net movement of suspended sediment was opposed to that of the hydraulic gradient through the first three seasons postspill, after which the amount of sediment fluctuated seasonally each year, greatest each autumn and spring. We assessed simplified one‐ and two‐basin mathematical models, the latter of which broadly describes the two‐phased response of Quesnel Lake's two basins, and both of which predict a return to background suspended sediment concentration in the West Basin by about one decade postspill. Key Points: About 38,000 Mg of sediment was suspended in the smaller of two lake basins 1 month after a spillFour fifths of this sediment went east into the larger basin and one tenth went north to the outflowTrends show a return to background suspended sediment levels by about 7 years after the spill [ABSTRACT FROM AUTHOR]