Transport of stable isotopes of water and sulphate within reclaimed oil sands saline–sodic mine overburden

Abstract

The reclamation of shale overburden dumps from oil sands mining requires the placement of reclamation covers comprised of salvaged organic and mineral soils. The primary issues associated with the long-term performance of these covers are their ability to store sufficient water to meet transpiration demands and the potential threat of salt ingress into the cover from the underlying shale. The first issue has been addressed in previous studies, so the objective of this study was to evaluate controls on salt ingress through observations and modelling of the transport of the stable isotopes of water and salt within reclaimed profiles at the South Bison Hills overburden dump located north of Fort McMurray, Alberta. The water flow model was based on a dual porosity soil–vegetation–atmosphere model calibrated to observed field data. This model was then used to simulate deuterium and sulphate transport within the soil profiles. The optimized transport model for deuterium was used to estimate net percolation rates through the cover soil into the underlying shale. This model was then used to assess the controls on sulphate migration, including the rate of sulphate generation as a result of ongoing oxidation of the pyritic shale. The model results indicate that the average net percolation rate is a function of topographic location, ranging from 2.2 × 10−5 m/d at slope locations to 20.8 × 10−5 m/d at plateau locations during the unfrozen days. These rates of net percolation should have produced observable patterns of salt flushing from the cover and upper shale. However, the observed sulphate levels could only be simulated by including a production term related to pyrite oxidation of the shale. The simulated oxidation rates ranged from 0.4 to 5.65 mg/L/d, similar to those estimated from previous laboratory and field investigations.