PROFILING STABLE ISOTOPES OF WATER SIGNATURES TO DEFINE MASS TRANSPORT MECHANISMS FROM WATER CAPPED FLUID FINE TAILINGS IN THE OIL SANDS INDUSTRY

Thirty End Pit Lakes (EPLs) are planned for remediation of open pit oil sands mines in the Athabasca Oil Sands region, however their feasibility as a reclamation feature has yet to be fully evaluated. End Pit Lakes are constructed within mined-out pits and many will contain fluid fine tailings (FFT). The main objectives of these lakes are to: (1) provide an effective means of isolating these soft, consolidating tailings within the closure landscape; (2) passively treat mine site water, including expressed FFT pore water; and (3) support the development of a sustainable biological system. Mass transport processes from the FFT to the overlying lake will have considerable influence on EPL performance. Physical mass transport from the FFT to the lake will occur due to diffusion, but will also be affected by advective transport due to tailings self-weight consolidation. Defining physical mass transport mechanisms and rates through the FFT provides insight on geochemical conditions at the FFT – lake water interface, and offers a better understanding of mass balance in the EPL.

Syncrude Canada Ltd. has initiated the first full-scale EPL (Base Mine Lake), and established a monitoring program to characterize the physical, geochemical, and biological processes occurring in this new system. The stable isotopes of water (δ²H and δ¹⁸O) signature of oil sands process affected water has been shown to be highly distinct from that of freshwater associated with snow melt or rainfall (Baer, 2014). As a result, the isotope composition of the lake water and FFT pore-water can potentially be used as a conservative tracer for mass transport. Samples were collected every 0.1 m across the FFT – lake water interface (2 m above, to 2 m below), at three locations in Base Mine Lake. Water from each sample was analyzed with a Picarro L-2120-i Cavity Ring Down Spectrometer based on the vapour equilibration technique used by Wassenaar et al. (2008) to determine δ²H and δ¹⁸O concentrations. These results were compared to δ²H and δ¹⁸O concentrations of the lake at multiple locations, as well as inflow and outflow water. Numerical modelling was used to interpret the measured isotope profiles across the FFT – lake water interface and elucidate mass transport mechanisms from the FFT to the lake water.