EXAMINING CONTROLS ON CHEMICAL MASS TRANSPORT ACROSS THE TAILINGS-WATER INTERFACE OF AN OIL SANDS END PIT LAKE
End pit lakes (EPLs) are an important remediation technology for fluid fine tailings (FFT) generated during oil sands mining and upgrading. EPLs are created by pumping FFT into a mined-out pit and then capping them with water. The first commercial scale EPL is Base Mine Lake (BML) at Syncrude Canada Ltd.’s Mildred Lake Mine in the Athabasca oil sands region in northern Alberta, Canada. The long-term evolution of mass loading in BML has implications for the viability of EPLs as a remediation technology in the Alberta oil sands. Internal mass loading from FFT to the overlying water cap may impact water quality in EPLs. Mass loading in BML is driven primarily by advective-dispersive transport from FFT settlement. However, the influence of mixing on the upper FFT by methane (CH4) ebullition has not been explored. This research examines the potential for CH4 ebullition in BML and the influence it may have on mass transport. FFT porewater samples were taken from 0.5 m above the tailing-water interface (TWI) down to 40 m below the interface in 2016 and 2017. Sensors that record temperature and pressure were deployed in 2018 and 2019. Detailed FFT porewater chemistry analysis was integrated with published data to define the distribution of dissolved constituents within BML. Numerical modelling, noble gas analysis, and dissolved gas pressure analysis were used to determine the potential for CH4 ebullition within FFT. Transport modelling that included CH4 ebullition was carried out to simulate observed chemical depth profiles. The degree and distribution of CH4 saturation vary spatially throughout FFT in BML. Dissolved CH4 concentrations were at or near saturation between 1.5 to 3 m below the TWI throughout BML. Annual seasonal cycles and continually settling FFT both influence CH4 solubility. Advective-dispersive mass transport modelling with mixing by ebullition found that different locations required different amounts of mixing to simulate geochemical tracer depth profiles. Typically, locations with greater ebullition potential and settlement required more mixing by ebullition. Dissolved CH4 transport modelling showed that anaerobic oxidation limited flux rates from the FFT into the water cap. The results of this study refined the conceptual model for internal mass loading as well as developed a model for parameters that can influence CH4 ebullition in an oil sands EPL.
oil sands, tailings, geochemistry, mass transport, methane
Master of Science (M.Sc.)