Microbial Methane Cycling in Oil Sands Pit Lakes

Abstract

Canada’s oil sands represent the third largest proven oil reserve globally and are the largest contributor to Canadian oil production. Surface mining of oil sands generates substantial volumes of tailings. The disposal of tailings and restoration of post-mining landscapes are regulatory obligations and essential for the sustainable development of oil sands resources. Water-capped tailings technology (WCTT) is an emerging strategy for oil sands mine reclamation. WCTT involves capping fluid fine tailings with a water layer to form a pit lake, which is expected to evolve into a functional boreal lake ecosystem with water quality improvement driven by freshwater input and in situ biodegradation. Base Mine Lake (BML), commissioned by Syncrude Ltd. in 2012, is the first commercial-scale demonstration of WCTT. An adaptive approach has been adopted for the management of BML to monitor and intervene in its performance. In this process, methane has been identified as a key component in the biogeochemical cycling of petroleum-related contaminants and the overall development of the lake. Methane is produced from methanogenic biodegradation of petroleum hydrocarbons in the tailings and can be transported to the water column through diffusion, advection, and ebullition. In the water column, methane can consume oxygen via oxidation by aerobic methanotrophs, which hinders the development of a persistent oxic zone necessary to support aquatic life. Methane ebullition may also facilitate the transfer of organic compounds from tailings to the water column and contributes to greenhouse gas emission. Given these challenges, a comprehensive understanding of methane production, transport, oxidation, and its environmental implications is critical for predicting BML’s future development. This dissertation provides insights into the influence of carbon sources, temperature, and microbial communities on methane dynamics using phospholipid fatty acid (PLFA) biomarkers and multiple carbon isotope tools. It quantifies methane production in BML and estimates its total annual production. This study also compares BML with Demonstration Pond, a smaller-scale pit lake with over 30 years of operation, which offers a possible scenario for methane dynamics and carbon cycling in the future BML. In addition, this study found evidence of dynamic water exchange at the tailings-water interface potentially driven by ebullition, as indicated by PLFA isotope biosignatures. As part of the ongoing monitoring and research program of BML, the findings in this dissertation provide valuable information to assess the impact of methane production on the evolution of BML and contribute to enhanced management practices for BML and other proposed pit lakes in Alberta’s oil sands region. They also improve the understanding of biogeochemical processes in other petroleum hydrocarbon impacted and methane generating sediments.