The Geochemical Evolution of Oil Sands Tailings Pond Seepage, Resulting from Diffusive Ingress Through Underlying Glacial Till Sediments.

摘要

Oil sands tailings are comprised of sands, silts, clays, and process-affected water (OSPW). The latter includes high concentrations of dissolved ions, as well as organic contaminants, making the water toxic to aquatic organisms. In Northern Alberta, tailings ponds are being constructed on glacial till, overlying sand channel aquifers, establishing a potential hydraulic connection between the pond and downstream water systems. However, to date, no targeted attempts have been made to characterize the biogeochemical evolution and end products as tailings pond OSPW infiltrates into glacial till prior to reaching these aquifers, thus overlooking a key component of the contaminant transport pathway. Addressing this knowledge gap is a critical step towards protecting aquatic resources. Cation exchange capacity, exchangeable cation, batch sorption and radial diffusion cell experiments and supporting geochemical simulations were conducted: a) to assess the potential for release (or attenuation) of trace elements and major ions from glacial tills when exposed to OSPW; and b) to identify the principal geochemical processes involved in controlling pore water and sediment chemistry. The experiments revealed that sediment-bound cations available for exchange, consisted of Ca>Mg>K>NH4>Na; while the mean cation exchange capacity in the till (Methylene Blue method) was 4.7±2.7meq 100g-1. Results further indicate that the ingress and interaction of OSPW with the glacial till sediment-pore water system will result in: the mitigation of incoming sodium by ion exchange with sediment-bound calcium and magnesium, followed by limited precipitation of calcium and magnesium carbonates; sulfate reduction and subsequent sulfide precipitation; and biodegradation of organic carbon. High concentrations of OSPW chloride (∼375mg L-1) are expected to persist. Ion exchange, oxidation-reduction, and mineral phase reactions including reductive dissolution of metal oxyhydroxides influenced trace metal mobility, which is similar to previous observations within sandy aquifer settings. Furthermore, though several trace elements showed the potential for release, large-scale mobilization is not supported. Understanding the environmental impact of tailings seepage is of great importance in managing water resources in Alberta. The present research offers a scientific basis to guide future remediation and reclamation strategies, seepage management schemes, and development of compliance legislation, and is therefore anticipated to have industry-wide benefit.