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TitleEvaluating in situ biodegradation of 13C-labelled naphthenic acids in groundwater near oil sands tailings ponds
AuthorAhad, J M EORCID logo; Pakdel, H; Gammon, P R; Siddique, T; Kuznetsova, A; Savard, M MORCID logo
SourceScience of the Total Environment vol. 643, 2018 p. 392-399,
Alt SeriesNatural Resources Canada, Contribution Series 20180046
PublisherElsevier BV
Mediapaper; on-line; digital
File formatpdf; html
Subjectsfossil fuels; environmental geology; hydrogeology; geochemistry; surficial geology/geomorphology; petroleum resources; petroleum industry; oil sands; tailings; pollutants; groundwater resources; aquifers; glaciofluvial deposits; wetlands; groundwater pollution; water quality; biodegradation; microorganisms; stable isotope studies; carbon isotopes; biogenic gas; methane; Biology
Illustrationsdiagrams; photographs; tables; schematic diagrams; bar graphs; plots
ProgramEnvironmental Geoscience Sources
Released2018 06 22
AbstractPotential seepage of naphthenic acids (NAs) from tailings ponds into surface water and groundwater is one of the main environmental concerns associated with the Canadian Athabasca oil sands mining operations. Here we report the application of 13C-labelled NA surrogate compounds to evaluate intrinsic biodegradation along groundwater flow-paths originating from oil sands tailings ponds at two different sites: a glaciofluvial aquifer (Site 1) and a low-lying wetland (Site 2). Microcosms containing the carboxyl group labelled (99%) NA surrogates (cyclohexanecarboxylic acid, CHCA; 1,2-cyclohexanedicarboxylic acid, CHDCA; 1-adamantanecarboxylic acid, ACA) were lowered into monitoring wells for several months to allow sufficient time for substrate degradation and formation of a biofilm in conditions characteristic of the local aquifer. Phospholipid fatty acids (PLFAs), biomarkers for the active microbial population, were extracted from the biofilms for stable carbon isotope (delta-13C) analysis. At Site 1, highly 13C-enriched delta-13C values (up to ~ +7100 per mille) confirmed the in situ microbial breakdown of CHCA and CHDCA. At Site 2, delta-13C-PLFA values from -60.6 to -24.5per mille indicated uptake of a 13C-depleted substrate such as biogenic methane and not 13C-labelled ACA. Determination of the microbial community using 16s RNA sequencing confirmed the presence of methane-oxidizing bacteria in the subsurface at Site 2. The in situ biodegradation of NAs at Site 1 demonstrates that the indigenous microbial population in the shallow subsurface near tailings ponds can readily break down some of these compounds prior to surface water discharge. The lack of evidence for microbial uptake of 13C-labelled ACA at Site 2 demonstrates that other NAs, in particular tricyclic diamondoid acids, may persist in the environment following seepage from tailings ponds or natural sources.
Summary(Plain Language Summary, not published)
Naphthenic acids (NAs) occur naturally in bitumen and become concentrated in waters used in oil sands mining operations. Under the framework of NRCan's SOURCES Project, this study used 13C-labelled NA surrogates to examine their biodegradation in groundwater near tailings ponds. The determination of highly 13C-enriched carbon isotope values (d13C) in phospholipid fatty acids (PLFAs) - biomarkers for the active microbial population - confirmed the in situ microbial breakdown of two NA surrogates at one site. This result implies a possible reduction in toxicity in impacted groundwater prior to surface water discharge. On the other hand, the lack of evidence for microbial uptake of a 13C-labeled surrogate at another site demonstrates that certain NAs may persist in the environment following seepage from tailings ponds or natural sources.

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