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This paper discusses major environmental alteration processes and describes a set of chemical tests that have been developed to monitor compositional changes in hydrocarbon fuels released into the environment. The methods examine various homologous series of hydrocarbons including straight chain (paraffins or n‐alkanes), branched chain (isoparaffins or isoprenoids), alicyclic (naphthenes or alkylated cyclohexanes), polycyclic (steranes and terpanes), and aromatic structures (benzene, toluene, ethylbenzene. xylenes, alkylated benzenes, polycyclic aromatic hydrocarbons, and aromatic steranes). Each one of these groups of hydrocarbons has a different tolerance to environmental alteration by evaporation, dissolution (water washing), and biodegradation. When used as an analytical system on environmental samples, the data obtained provide information on fuel type recognition patterns and on degradation levels of the various fuels, allowing for an estimate of residence time.

Estimating time of a middle distillate fuel release in soil can be performed under certain restricting environmental conditions using the Christensen‐Larsen method (CLM). This method is based on the linear correlation between the time since a diesel fuel release and the corresponding value of n‐heptadecane to pristane ratio (n‐C17/Pr) but requires knowledge of the initial ratio value. The empirical nature of this method does not, however, allow accounting for variance in the initial fuel value used by CLM. Based on the zero‐order approximation of the Monod model, we have deduced a generalized equation that can be used for estimating release ages of middle distillate fuels with different initial values. When combined with other site‐specific factors, this equation provides a useful tool for the time of release estimates.

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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 nalkane 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.