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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 3 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 22 d 21 , 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. End pit lakes (EPLs) offer long-term FFT containment with relatively low initial costs [Cumulative Environmental Management Association (CEMA), 2012]. These mine closure landscapes contain FFT deposits under a water cover within decommissioned mine pits. In the future, EPLs will be incorporated into the larger mine 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 advectiondispersion during FFT settlement controls mass transport

Fluid fine tailings (FFT) are soft tailings produced by the bitumen extraction process associated with open-pit oil sands mining. Oil sands mine operators have proposed the use of end pit lakes (EPLs) to contain soft tailings and Syncrude Canada Ltd. has developed the first EPL in the industry. This EPL, referred to as Base Mine Lake, contains FFT transferred from an above-ground tailings facility in a mined-out pit. The FFT was placed at elevated temperatures relative to natural groundwater temperatures in the region, so the FFT will act as a long-term source of heat. Evaluation of the thermal regime within the EPL requires the characterization of the thermal properties of FFT. Laboratory testing was undertaken to measure the thermal properties (thermal conductivity and volumetric heat capacity) of the FFT over a range of water contents and to evaluate the effect of bitumen content on the thermal properties. Field testing was also undertaken to verify that these properties were similar in the undisturbed FFT, measured at a larger scale.