Isotopic and microbial evidence for biodegradation of diluted bitumen in the unsaturated zone

The oil sands region in Western Canada is one of the world's largest proven oil reserves. To facilitate pipeline transport, highly viscous oil sands bitumen is blended with lighter hydrocarbon fractions to produce diluted bitumen (dilbit). Anticipated increases in dilbit production and transport raise the risk of inland spills. To un-derstand the behaviour of dilbit in the unsaturated or vadose zone following a surface spill, we ran parallel dilbit and conventional heavy crude exposures, along with an untreated control, using large soil-filled columns over 104 days. Phospholipid fatty acids (PLFAs), biomarkers for the active microbial population, were extracted from column soil cores. Stable carbon isotope contents (delta 13C) of individual PLFAs and radiocarbon contents (Delta 14C) of bulk PLFAs were characterized over the course of the experiment. The Delta 14CPLFA values in soils impacted by dilbit (-221.1 to -54.7%o) and conventional heavy crude (-259.4 to-97.9%o) indicated similar levels of microbial uptake of fossil carbon. In contrast, Delta 14CPLFA values in the control column (-46.1 to +53.7%o) reflected assimilation of more recently fixed organic carbon. Sequencing of 16S ribosomal RNA genes extracted from soil cores revealed a significant increase in the relative abundance of Polaromonas, a known hydrocarbon-degrader, following exposure to both types of oil. This study demonstrates that in the first several months following a surface spill, dilbit has a similar potential for biodegradation by a native shallow subsurface microbial com-munity as conventional heavy crude oil.

Automated summary

The oil sands region in Western Canada has a large amount of proven oil reserves, and to facilitate transportation, viscous oil sands bitumen is blended with lighter hydrocarbon fractions to produce diluted bitumen. This study examines the behavior of diluted bitumen in the unsaturated zone following a surface spill, and compares it with conventional heavy crude oil. The results show that both diluted bitumen and conventional heavy crude oil can be biodegraded by the native microbial community in the shallow subsurface, indicating similar potential for cleanup in the event of a spill.