NON-POLAR PRODUCTS OF ANAEROBIC BIODEGRADATION OF N-ALKANES UNDER METHANOGENIC CONDITIONS IN AN OIL SPILL AT BEMIDJI, MN

Anaerobic biodegradation of hydrocarbons has been the subject of debate in the last few years. Whereas pathways of aerobic weathering and degradation have been widely studied and the degradation progression well established, anaerobic biodegradation has been much slower to be recognized and studied. Knowledge of its progression, mechanism, intermediates, and products is still emerging. n-Alkanes frequently are the dominant family of constituents in spilled oils. However, it has only relatively recently been recognized that the chemically inert n-alkanes could even be anaerobically degraded.

Most reports on anaerobic biodegradation of hydrocarbons focus specifically either on the disappearance of specific analytes (e.g. hexadecane) leading to terminal degradation products, CH4 and CO2, or on polar metabolites or daughter compounds in the groundwater associated with the contaminant plume (e.g., BTEX compounds). Little attention has been paid to products from biodegradation that might remain within the oil body itself. This study of a crude oil spill in Bemidji, MN, reports on non-polar products from the biodegradation of n-alkanes found in the main oil accumulation. Compound-specific carbon isotope analysis of the homologous n-alkanes helps to delineate the degradation progression, which proceeds from the high molecular weight end of the series rather than the lower end as in aerobic weathering, and which results in an absolute concentration enhancement of lower-weight homologs. This progression, then, suggests that lower molecular weight n-alkanes are being formed from microbial degradation of higher-end n-alkanes.

The Bemidji spill provides a unique opportunity to follow the anaerobic biodegradation progression. Despite the fact that the oil was spilled and disbursed from a single incident, and that all the oil has been in the subsurface for 24 years, the degree of degradation is highly variable at different sites, allowing molecular tracking of the process. For example, in the two lobes of the oil body resting on the water table, the degradation ranges from barely degraded (downgradient) to highly degraded (upgradient) in terms of n-alkane loss. Other work in this session (Bekins et al.) reports on the physical factors that contribute to this variability.