Characterizing annual changes in the chemistry and toxicity of surface water from Base Mine Lake, an Alberta oil sands end pit lake

Abstract

Base Mine Lake (BML) is an end pit lake (EPL) located within the Alberta oil sands containing fluid fine tailings (FFT) capped with oil sands process-affected water (OSPW) and freshwater. EPLs have been proposed as a method for the long-term reclamation of oil sands surface mining activity, however, they contain a complex mixture of dissolved organic acids, metals, and salts that are toxic to aquatic organisms. The purpose of this thesis was to characterize annual changes in the inorganic chemistry and toxicity of BML surface water to more adequately assess the sustainability and environmental risks of EPLs, and to predict the time required to reach acceptable surface water quality necessary for the development of a functional aquatic ecosystem.

Annual changes in inorganic surface water concentrations (2014 to 2016) were strongly correlated with competing processes previously described in BML: influx from FFT pore water into the overlying surface water via advective-diffusive transport offset by efflux via continuous manual pumping by Syncrude (freshwater in, process water out). Toxicological risk was assessed using a variety of benchmarks and was consistent with the consensus that salinity (as Na+, Cl-, HCO3-) is the primary driver of inorganic toxicity to sensitive aquatic organisms, while dissolved metals are of negligible concern. This was confirmed by toxicity tests using the freshwater invertebrates Chironomus dilutus and Ceriodaphnia dubia which indicated limited effects on survival, growth, development, and reproduction even after chronic exposure to whole BML surface water, and the discovery of wild Daphnia pulex in August 2016 surface water.

Although BML will continue to pose risk to some aquatic organisms, current surface water quality conditions are now sufficient to support certain salt-tolerant freshwater aquatic organisms. As long as Syncrude continues pumping water, these studies indicate that surface water quality will continue to slowly improve over time, promoting colonization and ecological succession of BML into a functional aquatic ecosystem likely dominated by brackish conditions. However, if water pumping were to be reduced or stopped entirely, the concentrations of many inorganic constituents within BML surface water would likely begin to rise again, leading to increased toxicity and reduced surface water quality.