Anaerobic Microbial Activity May Affect Development and Sustainability of End-Pit Lakes: A Laboratory Study of Biogeochemical Aspects of Oil Sands Mine Tailings

Reclamation of fluid fine tailings (FFT) produced by mined oil sands ore processing is an environmental and technical challenge. End-pit lakes (EPLs) are a prospective reclamation strategy needing comprehensive evaluation. We investigated biogeochemical changes in water quality and sediments by using 140 L columns containing 50 L of FFT, either unamended or amended with hydrocarbons, and capped with 20 L of process water to simulate an EPL. The columns were incubated up to 3 years anaerobically at 10, 20, or 30 degrees C. Microbial metabolism of hydrocarbons in FFT produced methane (CH4) and carbon dioxide (CO2), accelerated FFT settling by expressing more porewater to the surface, and increased turbidity in cap water. Gas ebullition caused bitumen release and chemical flux from the underlying FFT to cap water. Dissolution and biotransformation of carbonate and Fe bearing minerals in FFT during incubation led to the positive flux of major cations (Ca2+ and Mg2+), anions (HCO3-), and some trace elements (primarily Ba and Sr), thereby influencing the chemistry of overlying cap water. No discernible trend in flux of dissolved organic carbon and naphthenic acids was observed. The results suggest that microbial anaerobic activity stimulated by residual hydrocarbons in FFT may influence the progression of EPLs into viable aquatic ecosystems.