Since the 1980s, several techniques have been developed to estimate the year a petroleum hydrocarbon release occurred. In this article, we evaluate and expand on the model of Christensen and Larsen, who proposed that the degradation of normal heptadecane relative to pristane (i.e., nC17/Pr ratios) could be used to estimate the age of diesel fuels released into the environment. Linear regression analyses of nC17/Pr ratios from known subsurface releases of crude oil, middle distillate, fuel oil, and lubricating oil in diverse climatic settings (Subarctic, temperate, and tropical) define a statistically significant, negative linear correlation termed the middle distillate degradation (MDD) model, in which, like the Christensen-Larsen model, nC17 is almost entirely degraded within about 20 yr. By comparison, our investigations indicate that degradation of nC17 relative to Pr in aerobic, surface environments is also systematic, following a first-order kinetic relationship in which nC17 degrades about 5–6 yr subsequent to the release. As observed by others, the timescale of degradation under aerobic conditions is accelerated.
We also present analyses of average initial (nC17/Pr)o ratios of about 4500 worldwide crude oils and 90 domestic refined products (diesel jet A, fuel oils) to evaluate how variations in this parameter impact MDD model ages. As stipulated in debates surrounding the original Christensen-Larsen model, applications of the MDD model should be evaluated carefully on a case-by-case basis and not in an ad hoc fashion. Our results not only provide a database for evaluating the significance of geographic variations in (nC17/Pr)o ratios but also allow experienced investigators to estimate MDD model age uncertainties (3–10-yr window of uncertainty under optimal to worst case conditions, respectively) at sites where it is determined that such models are applicable.