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 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

Temporal variations in turbidity in an oil sands pit lake

“…Note that towards the end of summer this assumption is violated, because the epilimnion deepens rapidly.The fresh water inflow during summer does not affect the turbidity of the epilimnion. The maximum fresh water inflow is 1.8 × 10 6 m 3 /month (see [5], Fig. 3), which for a 5 m deep epilimnion corresponds to a maximum turbidity decrease of 0.03%/day, much less than the observed decrease.…”

Putative Mixotrophic Nitrifying-Denitrifying Gammaproteobacteria Implicated in Nitrogen Cycling Within the Ammonia/Oxygen Transition Zone of an Oil Sands Pit Lake

“…The FFT samples were collected into 250 mL high density polyethylene bottles using a custom-built fixed interval sampler for collecting FFT at 0.10 m vertical spacing over the 2 m interval positioned immediately below the FFT-water interface. Additional details on the design and operation of this sampler are provided by Dompierre et al (2017). Temperature and pH were measured immediately on the boat using a combined electrode (model 8172BNWP; Thermo Fisher Scientific, Waltham, MA, United States).…”

Impact of atmospheric pressure variations on methane ebullition and lake turbidity during ice‐cover

“…The same process probably occurs in Base Mine Lake, enhancing the exchange of heat and dissolved contaminants between the mud layer and water column. This process provides an explanation for the enhanced mixing hypothesized by Dompierre et al (2017) in modeling exchange between the mud and water column. The rising bubbles increase dissolved methane concentration in the water column, which leads to methane oxidation and contributes to decreasing dissolved oxygen concentration (Risacher et al 2018).…”