GROUNDWATER FLOW IN AN OIL SANDS TAILINGS STRUCTURE: ONE TOTHIAN SCALE DRAINAGE BASIN AND ONE LARGE ENVIRONMENTAL LEGACY

Management of the enormous volumes of tailings derived through the process of open pit oil sands mining involves their long term storage in large dyke and pond structures. The structures are almost entirely constructed of processed sand material and can be nearly 100 m in height resulting in the development of an extremely transient, man-made groundwater flow system. The tailings ponds evolve into small-scale drainage basins with the main components of a classic Toth system including local recharge and discharge areas and multiple scales of groundwater flow. Contaminated waters from the oil extraction process are released to the ponds and represent a potential environmental threat. Monitoring of hydraulic head patterns, groundwater seepage rates and sediment water content illustrate the flow system within one such pond-dyke structure is continuing to evolve for decades after decommissioning as drainage of process waters proceeds within the highly permeable structure. Numerical simulations are used to investigate the long-term environmental impacts of this tailings facility on both groundwater and the adjacent Athabasca River based on the records of transient hydraulic head and seepage face drainage collected over a 20-year period. The results indicate that because of the internal drainage process, the majority of the saturated finer grained tailings within the pond have become isolated from the overall groundwater flow system as a result of the development of unsaturated conditions in the underlying coarser materials. Predictive modelling suggests that the environmental risk from seepage releases from the tailings complex will progressively decrease over time and become negligible while at the same time, the generation of partially saturated conditions in the sand dyke structure continually increases the strength and security against catastrophic failure. The perched conditions beneath the pond, however, result in a significant reduction on the drainage rates from the saturated fine tailings creating in a massive environmental legacy associated with the eventual reclamation of the tailings complex. The case illustrates the importance of the Toth basin flow theory for large earthen structures.