Chemical mass transport between fluid fine tailings and the overlying water cover of an oil sands end pit lake

Abstract Fluid fine tailings (FFT) are a principal by‐product of the bitumen extraction process at oil sands mines. Base Mine Lake (BML)—the first full‐scale demonstration oil sands end pit lake (EPL)—contains approximately 1.9 × 10 8 m 3 of FFT stored under a water cover within a decommissioned mine pit. Chemical mass transfer from the FFT to the water cover can occur via two key processes: (1) advection‐dispersion driven by tailings settlement; and (2) FFT disturbance due to fluid movement in the water cover. Dissolved chloride (Cl) was used to evaluate the water cover mass balance and to track mass transport within the underlying FFT based on field sampling and numerical modeling. Results indicated that FFT was the dominant Cl source to the water cover and that the FFT is exhibiting a transient advection‐dispersion mass transport regime with intermittent disturbance near the FFT‐water interface. The advective pore water flux was estimated by the mass balance to be 0.002 m 3 m −2 d −1 , which represents 0.73 m of FFT settlement per year. However, the FFT pore water Cl concentrations and corresponding mass transport simulations indicated that advection rates and disturbance depths vary between sample locations. The disturbance depth was estimated to vary with location between 0.75 and 0.95 m. This investigation provides valuable insight for assessing the geochemical evolution of the water cover and performance of EPLs as an oil sands reclamation strategy. Key Points First study of solute release from fluid fine tailings (FFT) stored under water cover in first oil sands end pit lake Mass balance confirmed constituents in FFT pore water largest mass input to water cover Numerical models show advection‐dispersion during FFT settlement controls mass transport