The toxicity of organic fractions from aged oil sands process-affected water to aquatic species.

The process of surface mining and extracting bitumen from oil sand produces large quantities of tailings and oil sands process-affected water (OSPW). The industry is currently storing OSPW on-site while investigating strategies for their detoxification. One such strategy relies on the biodegradation of organic compounds by indigenous microbes, resulting in aged tailings waters with reduced toxicity. This study assessed the toxicity of OSPW aged statically for approximately 18 years. Dissolved organics in aged OSPW were fractionated using a preparative solid-phase extraction method that generated three organic fractions (F1-F3) of increasing polarity. Eight aquatic species from different trophic levels were exposed to whole OSPW (WW) and the derived OSPW organic fractions to assess toxicity: Pimephales promelas, Oryzias latipes, Vibrio fischeri, Daphnia magna, Lampsilis cardium, Hyalella azteca, Ceriodaphnia dubia, and Hexagenia spp. Broad comparisons revealed that P. promelas and H. azteca were most sensitive to dissolved organics within aged OSPW, while WW was most toxic to L. cardium and H. azteca. Three cases of possible contaminant interactions occurred within whole OSPW treatments, as toxicity was higher than organic fractions for H. azteca and L. cardium, and lower for P. promelas. As such, the drivers of toxicity appeared to be dependent on the species exposed. Of the organic fractions assessed, F3 (most polar) was the most toxic overall while F2 (intermediate polarity) displayed little toxicity to all species evaluated. This presents strong evidence that classical mono-carboxylic naphthenic acids, mostly present in F1 (least polar), are not primarily responsible for the toxicity in aged tailings. The current study indicates that although the aged tailings source (≥18 years) did not display acute toxicity to the majority of organisms assessed, inorganic components and polyoxygenated organics may pose a persistent concern to some aquatic organisms.