Evaluating in situ biodegradation of 13C-labelled naphthenic acids in groundwater near oil sands tailings ponds

Jason M E Ahad; Hooshang Pakdel; Paul R Gammon; Tariq Siddique; Alsu Kuznetsova; Martine M Savard

doi:10.1016/j.scitotenv.2018.06.159

Evaluating in situ biodegradation of ¹³C-labelled naphthenic acids in groundwater near oil sands tailings ponds

Sci Total Environ. 2018 Dec 1:643:392-399. doi: 10.1016/j.scitotenv.2018.06.159. Epub 2018 Jun 22.

Authors

Jason M E Ahad¹, Hooshang Pakdel², Paul R Gammon³, Tariq Siddique⁴, Alsu Kuznetsova⁴, Martine M Savard⁵

Affiliations

¹ Geological Survey of Canada, Natural Resources Canada, Québec, QC G1K 9A9, Canada. Electronic address: jason.ahad@canada.ca.
² INRS, Centre Eau Terre Environnement, Québec, QC G1K 9A9, Canada.
³ Geological Survey of Canada, Natural Resources Canada, Ottawa, ON K1A 0E8, Canada.
⁴ Department of Renewable Resources, University of Alberta, Edmonton, AB T6G 2P5, Canada.
⁵ Geological Survey of Canada, Natural Resources Canada, Québec, QC G1K 9A9, Canada.

PMID: 29940450
DOI: 10.1016/j.scitotenv.2018.06.159

Abstract

Potential 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 glacio-fluvial 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 (δ¹³C) analysis. At Site 1, highly ¹³C-enriched δ¹³C values (up to ~+7100‰) confirmed the in situ microbial breakdown of CHCA and CHDCA. At Site 2, δ¹³C-PLFA values from -60.6 to -24.5‰ 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.

Keywords: 16s RNA; Groundwater; Naphthenic acids; Phospholipid fatty acids; Stable isotope probing; Tailings ponds.