Products and distribution patterns of hydrocarbons undergoing biodegradation after spillage into the environment differ according to the type of degradation that is occurring. The progression of degradative loss of hydrocarbon families has been well documented in the literature. Ian Kaplan has been one of the pioneers in this field of geochemistry. Our study follows from Kaplan’s work; we show that in long-term anaerobic spill systems, the progression of biodegradative loss of aliphatic constituents is different from that in systems impacted by physical weathering or aerobic degradation. Two case studies of spilled oil products demonstrate the distribution patterns resulting from long-term anaerobic degradation. These spills are the well-studied 1979 crude-oil spill in Bemidji, Minnesota, where anaerobic biodegradation in the oil body is known to occur, and a chronic diesel-fuel spillage from 1953-1991 at Mandan, North Dakota. The generic order of loss of aliphatic families in these spills is the same as in aerobic/weathering systems (i.e., n-alkanes > n-alkylcyclohexanes > branched-chain (isoprenoidal) alkanes). However, in these long-term anaerobic systems, losses within the aliphatic homologous series or families occur from the high molecular weight end of the distributions rather than from the low-end as is characteristic of aerobic/weathered systems.

Anaerobic degradation of the n-alkane and n-alkylcyclohexane homologs thus results in an enhancement of the lower-end members. This enhancement reflects both a relative and an absolute concentration increase. The degradative progression causes a broadened homolog distribution pattern at the lower molecular weight range, a gradual lowering in carbon number of the homolog maximum, and a gradual decrease in the total homolog range. If not recognized, these degradative alterations in distributions can be misinterpreted as lower distillation-range fuels or fuel mixtures in forensic fingerprinting investigations. In particular, the alteration due to anaerobic degradation in the n-alkylcyclohexane distribution and range, both of which are widely used to characterize fuel-type, can confound fuel-type identifications.