Characterizing Residual Hydrocarbons and Microbial Dynamics in Froth Treatment Tailings for Reclamation Planning

Froth treatment tailings (FTT) are a byproduct of bitumen extraction in Alberta’s oil sands and, while the smallest tailings stream by volume, they present disproportionate challenges for closure and reclamation. 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. These constituents can influence redox conditions, microbial activity, and hydrocarbon degradation processes, ultimately affecting long-term deposit behavior. Despite their importance, FTT remain understudied compared with other tailings types, especially in the context of terrestrial beach deposits targeted for reclamation.

This study investigated the spatial and vertical distribution of hydrocarbons and microbial communities across a 1.4 km transect of a naphtha-based FTT beach deposit at Syncrude’s Mildred Lake Settling Basin. Samples were collected from six locations spanning the pond edge to a reclamation dyke, with depths ranging from 0.15 to 46 m. Chemical analysis revealed that petroleum hydrocarbon (PHC) and residual naphtha concentrations reflected deposition history, with higher concentrations found in deeper, older FTT near the dyke and at shallower depths adjacent to the pond. Naphtha concentrations were most strongly correlated with heavier PHC fractions (F2–F4), while toluene and ethylbenzene emerged as key indicators of microbial variation.

Distinct microbial communities were observed in FTT relative to the underlying coarse tailings, with reduced diversity at depths greater than ~30 m. FTT were enriched in hydrocarbon degraders (e.g., Pseudomonas), sulfur-cycling taxa (Thiobacillus, Desulfovibrio, Desulfotomaculales), and methanogens (Methanosaeta). Community composition varied with depth, distance from the pond, and presence of FTT, with the strongest drivers being PHC concentrations and pyrite content. These findings suggest that residual hydrocarbons act both as substrates and stressors, shaping microbial ecology while interacting with geochemical processes such as sulfur reduction and oxidation as well as methanogenesis.

Together, this work provides one of the first spatially resolved assessments of FTT deposits illustrating how residual diluent, hydrocarbons, and microbial processes interact to influence subsurface conditions. Accurate characterization of FTT is essential for predicting long-term behavior, guiding the design of closure landforms, and informing reclamation monitoring programs.