Exploring Surface Water-Bitumen Interactions in Athabasca Oil Sands Wetlands Using Stable and Radiocarbon Isotopes

Abstract

Naphthenic acid fraction compounds (NAFCs) have been identified as aquatic contaminants of concern associated with bitumen extraction in Canada’s Athabasca oil sands region. Previous investigations found that NAFCs occur and degrade in young (<100-year-old) wetlands, but the degree to which these NAFCs might be bitumen-derived remains unclear. To quantify contributions of modern versus fossil NAFCs, carbon isotope-based methods were applied to two intensively sampled wetlands suspected to be influenced by oil sands. Carbon-13 isotope ratios measured by pyrolytic decarboxylation (δ¹³C_pyr) fell within a narrow range (−27.5 to −26.0‰) and radiocarbon measurements (Δ¹⁴C) revealed a significant fossil carbon contribution (−795 to −290‰), presumably from bitumen. Along the flow pathway of one wetland, a positive shift of ∼1.5‰ in δ¹³C_pyr correlated with increasing distance from a suspected oil sands point source. Lower Δ¹⁴C values (i.e., less ¹⁴C) at this site corresponded to lighter δ¹³C_pyr r values – the opposite trend found in previous applications of δ¹³C_pyr analyses, whereby bitumen-derived NAFSs were around −22 to −21‰. This study shows how source attribution of NAFCs in surface water using δ¹³C_pyr may be less straightforward compared to groundwater, possibly due to differences in natural attenuation processes between these two environments.

Carbon isotope-based forensic techniques applied in the Athabasca oil sands region wetlands generated contrasting results, highlighting ambiguities in bitumen carbon mobility.