Boron, lithium, and strontium isotopes as tracers for migration of oil sands process affected water from tailings ponds in Alberta, Canada.

English Abstract:

Oil sands operations generate a large volume (~840x106 m3) of oil sands process-affected water (OSPW) that is stored in open tailings ponds in Northern Alberta, Canada. A potential long-term environmental risk associated with the tailings ponds is the seepage and migration of OSPW into underlying aquifers. We measured the inorganic chemistry and variations of dissolved boron (δ11B), lithium (δ7Li), and strontium (87Sr/86Sr) isotope ratios in OSPWs and groundwater underlying the South Tailings Pond (STP) site in Alberta, Canada. OSPWs are characterized by elevated boron, and lithium with distinctive δ11B (26.0±1‰), δ7Li (19±2‰), and 87Sr/86Sr (0.7086±0.0004) relative to the low-saline groundwater (δ11B =9.5‰±5‰, δ7Li =11‰±0.9‰, 87Sr/86Sr =0.70983±0.0004) from the underlying Wood Creek Sand Channel aquifer. Shallow groundwater underlying STP is elevated in salinity relative to low-saline groundwater. Diffusion is thought to be the dominant mechanism for migration of contaminants from tailings ponds to the shallow, clay till aquifers. However, variations in boron and lithium isotopes are significantly smaller than expected for diffusion alone. A more likely mechanism is mixing between OSPW and low-saline local groundwater along sand layers with higher hydraulic conductivity. The distinct boron and lithium isotopic ratios in OSPW and apparent conservative behavior of boron and lithium in shallow groundwater indicate boron and lithium can be used as sensitive tracers for monitoring and quantifying OSPW seepage into local groundwater and subsequent migration.