"The zero discharge policy of the Alberta oil sands operation means that oil sands process-affected water (OSPW) is currently stored in tailings ponds due to its known toxicity to aquatic organisms. However, several treatment technologies are underway to sufficiently remove OSPW-derived contaminants including naphthenic acids to allow for the potential discharge of treated OSPW into receiving waterbodies. Hence, the potential ecotoxicological effects of the treated OSPW in the aquatic environment should be assessed. Samples were collected from 15 sites along the Athabasca River. The organic compounds were extracted from these samples using solid phase extraction. Then these extracts were analyzed through Fourier-transform infrared spectroscopy (FTIR) and a battery of in vitro bioassays. In this study, the chosen toxicity pathways for OSPW contaminants are (1) non-specific toxicity: cytotoxicity; (2) specific toxicity: activation of xenobiotic metabolism endpoints and activation of endocrine estrogen receptor; and (3) reactive modes of action: genotoxicity and mutagenicity. Cytotoxicity results are reported as EC10 (relative enrichment factor [REF]). For OSPW, this is 22 ± 13 REF. However, results for the river suggest that there are no major cytotoxic effects in these samples. The preliminary results for the YES Assay imply that as expected the river sites estrogenicity is much lower than OSPW. Additional in vitro bioassays are ongoing to establish a more comprehensive baseline conditions of the Athabasca River prior to potential OSPW effluent discharge. Although the data still requires validation, the preliminary results show the potential differences in cell toxicity pathways between OSPW and the aquatic environment."