Fine sediment dynamics in Quesnel Lake: the regulation of contaminant and nutrient transfers from the Mt Polley mine spill

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In August 2014, Canada's largest mining disaster occurred at the Mt Polley copper-gold mine, near Likely, British Columbia. The tailings pond wall breached, releasing 25 Mm3 of tailings, wastewater and construction materials. Further scouring of vegetation and surface materials occurred along a 9 km reach before entering the west basin of Quesnel Lake. Polley Lake, a smaller lake adjacent to the tailings pond was also dosed with tailings but remained off limits to researchers for the first year. This proposal aims to assess fine sediment processes regulating contaminant (metals/metalloids) and phosphorus (P) dynamics and their medium-term effects on foodwebs in both Quesnel and Polley Lakes. Observations from a 2016 core sampling program along a lake transect incorporating the tailings deposit and further afield indicated a layer, several millimeters to tens of centimeters thick, of flocculated surface material that remained unconsolidated above either natural sediments or tailings material. This material was experimentally found to be very mobile and therefore likely to be reintroduced into the water column at various times of the year, potentially during seiching (wind-mixing) events and fall and winter turnovers. The quality of this resuspended sediment will have an impact on the lake functioning in that it contains a large mass of phosphorus (~5% wt) as well as metals - both of which can be problematic for aquatic organisms in excess. The bioavailability of these contaminants will be dependent upon the chemical and erosive conditions at the sediment-water interface. Seasonal changes in both of these variables will regulate if sediment-associated nutrients are available for in-lake primary production and if sediment-associated contaminants are incorporated by filter feeding invertebrates, which are a main food source for the sport and salmonid fishes in the lake. The uniqueness of this proposed project is the evaluation of a catastrophic event providing full-lake fertilization, but delivered in concert with high loads of metal and metalloids. While the phosphorus introduced in the tailings waste may act as a fertilizer increasing primary production in this lake, these organisms may also incorporate metals, thereby magnifying their impact within the foodweb. As most of the metals from the Mt Polley spill have been identified as being sediment-associated, the fate of these unconsolidated flocculated sediments at the sediment-water interface is in question. A series of field and laboratory projects will be undertaken by Petticrew and her HQP over the next five years to address these issues. This project will also provide valuable information on the resiliency of large and medium-sized lake ecosystems to massive disturbances such as those associated with mining disasters, and other similar events in pristine watersheds, thereby helping to protect Canada's aquatic ecosystems.