Review of Microbial Processes in the Near-Surface Environment and Their Implications for the Chemical Fingerprinting of Hydrocarbon Fuels

Abstract

The chemical composition of middle-distillate fuels released in the subsurface environment is predominantly affected by biochemical processes resulting in selective degradation of hydrocarbons by indigenous microorganisms. The resulting progression in pattern recognition-based fingerprints depends on the composition of the hydrocarbon product, parameters of the product release (e.g., quantity and rate of release) and environmental conditions. The changes for saturated hydrocarbons follow a common sequence of removal of different compound classes: n-alkanes > isoalkanes > alkylcyclohexanes, with a systematic relationship between decreasing relative degradation rates and increasing chain length. However the timescale of these transformations is defined by the subsurface redox condition. Aerobic biodegradation of saturated hydrocarbons can be observed on a relatively short timescale of years to decades, whereas anaerobic degradation is a much slower process. The fact that aerobic biodegradation produces measurable changes in n-alkane concentrations within this timescale makes it a logical option for evaluating time elapsed since a fuel release into the subsurface environment. This paper provides the synopsis of major modes and pathways of biodegradation processes in the near-surface environment and their effect on the temporal changes in the fingerprints of middle-distillate fuels. In particular, a linear decrease in n-alkane concentrations with time allows for the time of fuel release estimates, utilizing the zero-order kinetics model for the Christensen and Larsen method. This model also allows for establishing applicability limits of this approach. The model is relevant only for hydrocarbon contamination in unsaturated (vadose) zone soil where the availability of sufficient concentration of molecular oxygen favors aerobic biodegradation.