Material properties and cover options using HYDRUS (2D/3D) for a large undrained sand dam in northern Alberta, Canada.

Abstract

A flow and transport model was built to simulate the Southwest Sands Storage facility, which is a large oil sands tailings dam and impoundment located on the Mildred Lake oil sands lease in northern Alberta, Canada. Studying the flushing behavior and advancing the material characterization was achieved by building a two-dimensional cross section of 3000 m long and 40 m high (approx. 58,500 m2 in area) using HYDRUS (2D/3D). Building upon more than a decade’s worth of data and work by others, this study synthesised the existing material characteristics, corroborated and refined them, and performed future simulations. The use of a variable (transient) boundary condition was invaluable to gaining insights into the material characteristics of oil sands tailings; a unique material. The horizontal hydraulic conductivity ranged between 0.137 m/d to 1.27 m/d and the anisotropy ratio was restrained between 1 and 20, in line with reference values. The porosity ranged between 0.35 and 0.40 which was also inline with previous works. The residual saturation ranged between 0.13 and 0.2 which was double to triple the reference values, however an evaluation of the reference Soil Water Characteristics Charts revealed curves without distinct inflection points and therefore difficult to determine a precise residual saturation value. These tailings curves start to break between 0.05 and 0.15 then gradually decline with increasing suction. The van Genuchten unsaturated parameters (alpha = 1.24, n = 1.7) were unique in comparison to those in the agricultural soils databases but within the range of previously reported values for dyke and tailings sand. The parallel use of both a constant and a variable water flux boundary condition allowed for comparison of the future simulation results, which were of high fidelity. Under the as-is scenario (Future I) the TDS concentration of the dam will be greater than 1000 mg/L until around 2075, but because of the presence of the pond, two-thirds of the model section does not attenuate. Future simulations with remedial covers and landscaping (Futures II and III) had more holistic TDS attenuation distributions with dilution proportional to the amount of recharge. However, even the most restrictive recharge produces TDS concentrations of less than 1000 mg/L at the perimeter ditch after about 60 years due to the local flow system on Benches B and A and the influence of the Toe C ditch. Recharge rates are very important to reclamation success with topography and vegetation playing key roles. The competition for precipitation, which averages between 450 mm and 500 mm per year, in an area with high evaporation rates, means that the ideal cover should capture and transmit as much precipitation as possible. The transmissivity of the cover will be key to meeting the specific remedial goals and timelines.