Characterization of the boron, lithium, and strontium isotopic variations of oil sands process-affected water in Alberta, Canada

Oil sands operations in Alberta, Canada generate large volumes (>840 × 10⁶ m³) of oil sands process-affected water (OSPW) that contains contaminants that are toxic to aquatic life. OSPW is stored in open tailings ponds and leakage from the ponds presents a potential long-term environmental risk. However, the presence of naturally occurring saline water in the oil sands region in Alberta presents a challenge for delineating the impact of OSPW in cases where OSPW is leaked to the environment. Here, we characterize the inorganic chemistry and isotopic variations of boron (δ¹¹B), lithium (δ⁷Li), and strontium (ε_Sr^SW) of OSPW from tailings ponds in order to evaluate the source of salinity in OSPW and the potential utilization of these isotope ratios as forensic tracers of OSPW migration in the environment. In addition to generating new data, we compiled published data to build a comprehensive dataset of the geochemical composition of different water sources in the oil sands region of Alberta. OSPW is brackish (TDS ∼ 1800 mgL⁻¹), with elevated chloride (392 ± 184 mg L⁻¹), boron (2.2 ± 0.4 mg L⁻¹), and lithium (0.12 ± 0.11 mg L⁻¹) concentrations relative to the Athabasca River that is used for bitumen extraction. OSPW is characterized by narrow ranges of δ¹¹B (23.7 ± 1.8‰), δ⁷Li (16.3 ± 1.7‰), and ⁸⁷Sr/⁸⁶Sr ratios (ε_Sr^SW=-3.9 ± 19.2). The geochemical and isotope ratios in OSPW reflect mixing of residual, saline formation water from Lower Cretaceous units with fresh surface water that has been modified by interactions with the solid oil sands. The elevated boron and lithium concentrations and δ¹¹B, δ⁷Li and ε_Sr^SW variations we observed are distinct from the compositions of a shallow freshwater aquifer in the area (8.1 ± 5.0‰ 11.3 ± 0.9‰ and 9.7 ± 5.6, respectively) and the local rivers (15.6 ± 4.7‰ 15.1 ± 1.4‰ and 30.3 ± 16.0). However, groundwater and saline springs in the region presents a wider range of geochemical and isotopic values that are not always distinguishable from OSPW. The integration of boron, lithium, and strontium isotopes provides a potential monitoring tool for tracing OSPW release to local freshwater sources.