Toxicity of surface water and pore water from an oil sands pit lake (Lake Miwasin) to Daphnia

Canada holds approximately one-third of the world's confirmed crude oil reserves, primarily concentrated in the Alberta Oil Sands (AOS) region, in the form of bitumen. However, the extraction process of oil sands generates significant volumes of tailings and process water, which raises concerns about their potential effects on aquatic life and the need for remediation. Lake Miwasin, constructed in 2018, serves as a pilot-scale pit lake and a model for this context. It features treated fluid tailings overlaid by a mixture of oil sands process-affected water (OSPW) and freshwater, exhibiting seasonal stratification. Since its establishment, Lake Miwasin has been colonized by various organisms, including the Crustacean, Daphnia pulex. Monitoring data from Lake Miwasin has shown the presence of compounds in both water and sediment that could be detrimental to aquatic organisms. Therefore, the primary goals of this research were to assess surface water and pore water toxicity, including an evaluation of selenium (Se) bioaccumulation in Daphnia species exposed to the conditions representative of Lake Miwasin at its early development stages and possibly other future pit lakes in the AOS region. This study examined the toxicity of Lake Miwasin surface water (LMW) and pore water (LMP) using lab-cultured and native Daphnia species. Interestingly, LMW exhibited no acute or chronic toxicity towards D. magna and D. pulex (lab strains) and the native Daphnia sp. (collected from Humboldt Lake, SK, Canada). However, LMP demonstrated acute toxicity to both lab strains and the native D. pulex (collected from Lake Miwasin, AB, Canada). Lake Miwasin pore water also negatively impacted lab D. pulex reproduction, leading to a reduced number of offspring. Salinity emerged as a significant stressor in LMP, and a Toxicity Identification Evaluation (TIE) phase I suggested ammonia and metals in LMP as potential contributors to the observed toxicity in the tested organisms. On a related point, results showed that concentrations of dissolved Se in LMW from 2019 to 2021 exceeded the Canadian Council of Ministers of Environment (CCME) water quality guidelines for long-term aquatic life protection (1 µg/L), and the British Columbia Ministry of Environment (BC MoE) water guideline alert concentration for the protection of aquatic life (1 µg/L). The latter water guideline was also adopted by the Government of Alberta. Further, another experiment assessed Se bioaccumulation in D. pulex through dissolved and dietary exposure routes, drawing comparisons to native specimens collected from Lake Miwasin. In semi-static tests (12 days), lab strain D. pulex exposed to selenate [Se(VI)] showed a Se concentration-dependent increase from days 5 to 12 for most treatments, whereas a lower bioaccumulation occurred at higher Se concentrations, which suggests a potentially internal regulatory mechanism. Native D. pulex exposed to LMW in laboratory conditions showed that Se bioaccumulation levels were similar to those of D. pulex collected directly from Lake Miwasin. Despite these findings, the Se concentrations in D. pulex from both lab exposures and Lake Miwasin collections remained below available regulatory guidelines for invertebrate tissue (4 µg/g), suggesting that D. pulex appears to pose minimal risk as a food source in the Lake Miwasin ecosystem. These findings provide useful insights into the toxicity of Lake Miwasin water to aquatic invertebrates, the potential for Se bioaccumulation in daphnids in Lake Miwasin, the potential enhancement of pit lake monitoring programs, and the decision-making regarding the use of end pit lakes in the reclamation of OSPW and oil sands tailings