Geomicrobiology and geochemistry of fluid fine tailings in an oil sands end pit lake

Abstract

End pit lakes are a proposed mine closure landscape for fluid fine tailings (FFT) in the Athabasca oil sands region of northern Alberta, Canada. Oil sands end pit lakes are created when FFT are transferred into a mined-out pit and capped with water. Base Mine Lake is the first full-scale demonstration end pit lake and is located at Syncrude Canada Limited’s Mildred Lake mine. The geochemical development of Base Mine Lake has important implications for the viability of end pit lakes as a FFT reclamation strategy within the Alberta oil sands. The FFT is a source of several constituents that may negatively impact surface water quality through advective and diffusive transport mechanisms. This research examines relationships between microbiology and pore-water chemistry within FFT to inform predictions of long-term chemical mass loading from FFT to the water cap. Samples were collected from Base Mine Lake in 2016 and 2017, extending from 0.5 m above the tailings-water interface to 40 m below the interface. High-throughput amplicon sequencing of the 16S rRNA gene and a detailed analysis of FFT pore‑water chemistry was conducted to identify the spatial distribution of microbial populations and associated geochemical gradients. The sequencing results identified microbes associated with various metabolisms, notably hydrocarbon degradation, sulfate reduction and methanogenesis. The importance of microbial metabolisms shifted with depth and the greatest potential for biogeochemical cycling was exhibited near the tailings-water interface. Pore-water pH decreased sharply below the interface from above 8.1 to below 7.8, and zones of nitrogen cycling, sulfate reduction and methane oxidation were identified within the upper 2 m of the FFT deposit. Biologically-driven reactions occurring in this zone can potentially reduce the flux of dissolved methane, ammonium and hydrogen sulfide across the interface, affecting mass flux calculations and estimations of long-term mass loading to the water cap. The microbial community in deeper FFT was dominated by hydrocarbon degraders, syntrophs and methanogens. Pore-water pH gradually decreased in this area to a minimum of 6.9 and dissolved methane concentrations remained above 30 mg L‑1. Microbial methane production is likely controlled by hydrocarbon degradation and long-term methanogenic rates and pathways will be largely dependent on the availability of substrates produced by syntrophic microbes. Findings from this study were also used to refine a conceptual model to better understand long-term mass loading of dissolved constituents.