Acute and Chronic Toxicity of Oil Sands Process-Affected Water to the Water Flea ( <i>Ceriodaphnia dubia</i> ): Effect Levels and Cause

Using a toxicity identification evaluation (TIE) framework, the effects of oil sands process-affected water (OSPW) from an active settling basin were assessed with the aquatic invertebrate Ceriodaphnia dubia. Survival was not affected in two of three samples tested, but reproduction was inhibited in all samples. The IC25 estimates varied by more than 12-fold between samples (i.e., from <1.6% to 19%), and were primarily related to suspended particulates. Despite wide variations in total suspended solids (TSS) between samples, the effect threshold was relatively consistent (i.e., IC25 of approximately 12.5 mg/L TSS). There was no evidence of toxicity from sorbed organics, but particle-size analysis showed complete overlap of size ranges between OSPW particulates and the C. dubia food mixture, suggesting the effects were due to interference with food uptake. Less toxicity was attributable to total dissolved solids (TDS) and the organic fraction, with no appreciable interaction between the two. Based on conductivity, the IC25 for TDS was approximately 1,550 µS/cm. Orbitrap mass spectrometry, combined with toxicity tests on solvent gradient fractions, showed that organics toxicity was primarily a function of carbon-number-specific concentrations and potencies of classical naphthenic acids, with an estimated IC25 of 0.8 Toxic Units. Comparison of mortality and reproduction endpoints suggested an acute to chronic ratio of 2.7 for the organic fraction, and organisms exposed to untreated OSPW for 8 days recovered rapidly when transferred to treated water, with no apparent residual effects. Concurrent characterization of toxicity associated with co-occurring dissolved organic and inorganic constituents, as well as the particulate fraction, helps inform discussions related to impacts, cause, management, and remediation.