Modelling Effects of Cover Material and Cover Depth on Hydrological Regime and Salt Redistribution in Reclaimed Oil Sand Landscapes in Northern Alberta

Abstract:

Large-Scale surface mining is continuing in the Athabasca oil sands region in northern Alberta, Canada, causing significant ecosystem disturbances and changes in hydrology. Reclamation efforts in this region require understanding processes that control water, nutrient and salt fluxes through reclaimed landscapes which is critical to restoring their productivity. These processes were tested in a comprehensive mathematical model, ecosys, which was used to determine the effect of different cover thicknesses on water balance, water buffering capacity, salinity and the productivity in the South Bison Hills reclamation site of Syncrude Canada (SCL). This site was constructed in 1999 by capping peat mineral mix and secondary (glacial till) soil over saline sodic overburden. The site was constructed with three different soil cover thicknesses: 35 cm (thin), 50 cm (intermediate) and 100 cm (thick) along a 20% north facing slope. Model outputs were validated with field measured volumetric water content, runoff, snow data, electrical conductivity (EC) and plant productivity data recorded from 1999 to 2013. Model and field results show differences in horizontal and vertical water transport among the three reclaimed prototype covers. Lower water retention capacity in the 35 cm cover compared to the 50 cm and 100 cm covers caused greater soil moisture variation so that permanent wilting point was reached during dry years, decreasing plant growth due to water stress. In addition, the modeled and field-measured EC values indicated some upward salt movement from overburden to cover material over the time. This movement caused higher EC values (6 – 8 dS m^-1) to be reached in the shallow rooting zone of the 35 cm and 50 cm covers than of the 100 cm cover several years after the covers were established. The determination of cost effective but ecologically sustainable cover depth is a challenge and will be a focus in future simulations.