The co-importance of physical mixing and biogeochemical consumption in controlling water cap oxygen levels in Base Mine Lake

Bitumen extraction in the Athabasca oil sands region of Alberta Canada has resulted in 1 billion m3 of tailings. Base Mine Lake (BML), the first commercial-scale pit lake (PL) in the Alberta oil sands, is currently being assessed as a potential fluid fine tailings (FFT) reclamation strategy. Here, field-based, BML water cap depth-dependent oxygen consumption rate (OCR) determination, geochemical characterization and modelling of physical mixing were combined to identify the key processes driving the oxygen mass balance and distribution. The highest OCR values (66.9 ± 5.9 O2/m2/d) occurred at the FFT water interface and were driven by dissolved methane and ammonia concentrations. Given the extensive mass of these reductants within the FFT, their biogeochemical redox cycling is anticipated to impact BML oxygen concentrations for decades. The integration of OCR and modelling results revealed the importance of physical mixing as a mechanism to maintain an oxic hypolimnion and FFT water interface. The oxygen mass balance indicates that without the physical movement of oxygen into the hypolimnion, the OCR values currently observed would result in anoxia in the lower 3.5 m of the BML water cap, migrating upwards of the oxic-anoxic boundary and enabling the generation of new oxygen-consuming constituents directly within the water cap through anaerobic redox transformations. Results here guide the development and application of water-capped tailings as a reclamation strategy for not only the oil sands industry, but also for other anthropogenically impacted or engineered systems with high oxygen demand.