Biogeochemical characterization of froth treatment tailings

Froth treatment tailings (FTT) are byproducts of bitumen extraction at oil sands mines in northern Alberta. Produced during froth treatment, where diluent such as naphtha is added to separate bitumen from water and solids, FTT contain residual hydrocarbons and sulfide minerals like pyrite, potentially posing reclamation challenges. This study investigated the spatial and vertical distribution of hydrocarbons and microbial communities across a transect of an FTT deposit at Syncrude's Mildred Lake Settling Basin. Residual naphtha and petroleum hydrocarbon (PHC) concentrations reflected deposition history, with higher concentrations near the pond and in deeper, older tailings at the dyke. Microbial diversity was lower in FTT than in underlying coarse tailings, with the lowest diversity observed at ∼30 m depth, likely due to moderate to high PHC concentrations, anoxic conditions, and nutrient limitations. Microbial community composition varied with depth, material type, and location, and FTT were enriched in taxa involved in hydrocarbon degradation (Pseudomonas), sulfur cycling (Thiobacillus, Desulfovibrio, Desulfotomaculales), and methanogenesis (Methanosaeta). Among hydrocarbons, toluene and ethylbenzene were strong predictors of microbial variation. Pyrite content also emerged as an important driver, likely due to its role in redox processes. These results highlight the close links between residual diluent, tailings geochemistry, and microbial ecology, emphasizing the importance of accurate FTT characterization to support closure landform design and inform future reclamation monitoring.