Assessing the acute and chronic toxicity of exposure to naturally occurring oil sands deposits to aquatic organisms using Daphnia magna

Abstract

In the Athabasca region, the oil sands are located at or near the surface making open-pit mining viable. In addition, the Athabasca River and its tributaries flow through these oil sands deposits, thereby receiving bitumen-associated contaminants through natural fluvial erosional and weathering processes. A key knowledge gap has been related to understanding both the magnitude and significance of the toxicological and ecological effects on aquatic organisms exposed to naturally occurring bitumen entering fluvial systems. Using the Daphnia magna model system, this study assessed the ecotoxicological effects of exposure to bitumen-elutriate treatments that simulated the early stages of fluvial/erosional exposure conditions. No significant among-site differences were observed in the survival of D. magna after 48 h exposure to elutriates produced from a 24 h extraction cycle, and chemical analysis indicated low concentration of a complex mixture of hydrocarbon and metal contaminants. In contrast, the same elutriates impaired reproduction and growth after a 21-day chronic exposure. F1 neonates from the chronic tests were tested for sensitivity to the reference substance potassium dichromate, revealing a decrease in their sensitivity. Inter-generational effects were also observed, with a significant decrease in subsequent neonate production, when daphnids were moved to a clean medium. Supplemental acute toxicity assays using 48 and 72 h bitumen extraction cycles progressively increased daphnid mortality after a 48-h exposure to the respective elutriates. This indicates that bitumen-associated contaminants are being liberated after initial input and fluvial washing (24 h), highlighting the need for future work to assess toxicity responses and associated elutriate water chemistry of a longer fluvial exposure time-series. This work contributes to our understanding of the possible effects of natural bitumen exposure on riverine aquatic ecosystems, providing new information to inform the delineation of baseline conditions to assess environmental change and the design of future regional effects-based monitoring programs.