Chemical and Toxicological Characterization of Dissolved Organics from Oil Sands Waters

Abstract

The surface mining of oil sands from the Athabasca deposit north of Fort McMurray, Alberta produces considerable tailings waste which is stored in large tailings ponds on industry lease sites. With the advent of oil sands end-pit lakes and decommissioned tailings ponds, viable strategies for the detoxification of oil sands process affected water (OSPW) are under investigation. One such strategy relies on the biodegradation of toxic organic compounds by indigenous microbes, resulting in aged tailings waters with potentially reduced toxicity. Determining drivers of toxicity within OSPW poses a great challenge because differences in ore quality and bitumen extraction methods influence organic and inorganic chemistry, and therefore, toxicity. In order to assess the toxic potential of the suite of dissolved organics in OSPW, a method for extraction and fractionation was developed. This was achieved with a liquid chromatography approach using reversed-phase solid phase extraction coupled with soxhlet extraction. The method successfully separated organic compounds from 180 L of an aged OSPW source into three fractions (F1-F3) with increasing polarities. Chemical characterization of the generated fractions included electrospray ionization high-resolution mass spectrometry, liquid chromatography quadrupole time-of-flight mass spectrometry, gas chromatography triple quadrupole time-of-flight mass spectrometry, and synchronous fluorescence spectroscopy. Method validation included fractionations with surrogate reference standards and labelled standards, which also confirmed separation according to polarity and verified high recovery of dissolved organics. This method was designed to generate bulk quantities of extract which provide enough material for a suite of toxicity bioassays. Using this novel method, aged OSPW and four bitumen-influenced groundwater sites (2 influenced by natural bitumen; 2 influenced by a mixture of natural bitumen and OSPW sources) were fractionated. The whole water and isolated fractions were then exposed to seven different aquatic species; Pimephales promelas (embryo), Oryzias latipes (embryo), Vibrio fischeri, Daphnia magna (neonates), Lampsilis cardium (glochidia), Lampsilis siliquiodea (glochidia) and Hyalella azteca (juveniles). Chemically, bitumen-influenced groundwater sites were predominantly composed of O2 and O4 species while aged OSPW was dominated by O4 species. Analysis also revealed a high variability in composition and abundance of organic and inorganic constituents across groundwater sites. Of the organic fractions assessed, F1 (least polar) and F3 (most polar) appeared most toxic overall while F2 displayed little toxicity to all species evaluated. Organisms were identified as differentially more sensitive to whole waters, likely as a result of inorganics (D. magna and L. siliquiodea), or dissolved organics (P. promelas and H. azteca).The present study indicates that although an aged tailings source (≥18 years) displayed low toxicity overall, inorganic and polyoxygenated organic components may pose a persistent concern to aquatic organisms. A general comparison of groundwater sites containing OSPW-derived constituents vs. natural bitumen-derived constituents revealed that whole water toxicities were quite similar. It is therefore likely that toxicity associated with tailings seepage into groundwater is mitigated by chemical changes as a result of soil composition and groundwater mixing. Finally, an ecotoxicological risk assessment of OS acid-extractable organics (AEO) produced a joint probability curve which predicted that the probability of producing an effect in 10% of fish and invertebrates species was 100% and 97.7%, respectively. In general, at AEO exposures in the range of 17 – 104 mg/L, an acute species sensitivity distribution revealed vertebrates (embryonic) to be more sensitive than invertebrate organisms. The risk assessment recommends a monitoring program that accounts for current anthropogenic dissolved organic input from tailings seepage, and its effect on particularly sensitive fish species. Additionally, future efforts regarding the wet landscape strategy should account for changes in dissolved organic concentrations and reduction in toxicity over time. In summation, for those organisms that display sensitivity to dissolved organics in oil sands waters, aging by natural biodegradation appears to be a viable strategy. Moreover, industrially-influenced groundwaters do not appear to pose a greater risk to aquatic organisms than groundwaters influenced by naturally-derived bitumen. Nonetheless, due to possible invertebrate sensitivities to inorganic components within whole waters, a strategy to deal with these bio-persistent compounds warrants investigation. In order to identify and characterize OSPW-derived dissolved organics that pose the greatest environmental risk, the bioactive fractions (F1 and F3) will be further fractionated and assessed toxicologically. Finally, it is recommended that organisms identified as being most acutely sensitive to OSPW-derived organic and inorganic constituents be the focus of future effects-directed analysis of OSPW toxicity, as well as impact assessment monitoring and future remediation/reclamation of industrial lease sites.