| Summary: | Extraction processes in the Athabasca Oil Sands Region of Canada (AOSRC) produce large volumes of tailings waste that require management and reclamation. Water capped tailings technology (WCTT) is currently being assessed as a reclamation strategy by Syncrude Canada Limited in the form of Base Mine Lake (BML), as it is the first AOSRC demonstration WCTT project (Fort McMurray, AB). BML is a pit lake (PL) consisting of a ~40 m layer of fluid fine tailings (FFT) that is capped with a ~10 m mixture of fresh and oil sands process waters. For BML to be considered a reclamation success, it should be able to support macrofauna; i.e. an oxic zone must develop and persist within the water cap. It is important to identify redox biogeochemical processes that influence oxygen concentrations in this pilot PL demonstration system to inform the design of the ~30 pit lakes (PL) that are currently planned across the AOSRC to ensure reclamation success. To identify the oxygen consuming constituents (OCC) and the biogeochemical processes impacting BML oxygen concentrations, this study comprehensively characterized the water cap geochemistry using standard aquatic sampling methods over the summer of 2016, as well as using a specialized sampling device capable of capturing simultaneous samples every 10 cm over a 2-meter zone (n=20). The Fixed-Interval Sampler (FIS) was used to retrieve simultaneous samples in identified biogeochemically active zones of the metalimnion and hypolimnion- FFT-water interface (FWI) during the summer of 2016. Seasonal results identified depth dependent concentration trends consistent with the FFT acting as a source of oxygen consuming constituents (OCC) to the overlying water cap. Characterized concentrations of CH4, NH4+ and ΣH2S were highest in the surface FFT pore water at ~450µM, ~250µM and ~25µM, respectively, subsequently decreasing by the metalimnetic region to concentrations <20% of their respective hypolimnion-FWI concentrations, consistent with redox biogeochemical cycling and the observed profile of decreasing oxygen concentrations with increasing depth. Characterization of FIS samples provided greater resolution of trends specifically within the FWI region and the metalimnion. Results show that ΣH2S extinguishes rapidly above the FFT, reaching non-detectable concentrations by 1m above the FWI. Methane concentrations also decreased rapidly from ~450µM in the FFT to ~100µM by 0.3m above the FWI, consistent with methanotrophy. Characterization of FIS samples for nitrogen species concentrations within both the FWI and metalimnetic regions identified a near 1:1 conversion from NH4+ to NO3- by the metalimnion consistent with microbial nitrification. This process has not been shown to occur in oil sands tailings ponds, indicating that WCTT conditions can enable divergent biogeochemical cycling from that occurring in active tailings ponds. Evidence suggests that the relative concentrations of CH4 and O2 in the lower BML hypolimnion limit the ability of ammonia oxidizing microbes to carry out the full two-step nitrification process. This study has revealed the key oxygen consuming processes within the metalimnion and hypolimnion of BML and provides evidence that BML exhibits a difference in the relevance and magnitude of biogeochemical processes than those observed in tailings ponds. As such, tailings pond research cannot be solely relied upon to determine the viability of PL as a successful reclamation strategy and further PL research is required to develop robust WCTT reclamation strategies. Thesis Master of Science (MSc) Operators in the Athabasca Oil Sands Region of Canada (AOSR) produce millions of m3 of waste known as fluid fine tailings (FFT). A wet reclamation strategy currently being piloted by Syncrude Canada Ltd. to deal with these large volumes of tailings is water capped tailings technology (WCTT) in the form of a pit lake (PL), Base Mine Lake (BML). BML was built in an exhausted open pit mine, covering a layer (~40m) of FFT with a ~10m freshwater cap. For WCTT to be successful, a freshwater ecosystem capable of supporting life must develop within the water cap. As FFT contains compounds which can consume oxygen if mobilized into the water cap, there is uncertainty as to whether WCTT will be a successful FFT reclamation strategy. This thesis aimed to characterize oxygen consuming processes occurring throughout the water cap and revealed depth dependent importance of methane, ammonia and hydrogen sulfide in determining water cap oxygen concentrations.
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