13C and 14C evidence of pollution of a soil by fossil fuel and reconstruction of the composition of the pollutant

n-Alkanes from soils in which C3 plants have been cultivated were analysed for their distributions and individual δ13C ratios. For long-chain n-alkanes from unpolluted soils, both the strong odd-carbon number predominance, with a carbon preference index (CPI) of 9.2, and the light isotope composition, −35‰, are typical of modern C3-plant waxes. In contrast, long-chain n-alkanes from another soil sample show a weak odd-carbon number predominance, with a CPI of 1.2, indicating a large anthropogenic contribution from ancient and thermally mature organic matter. This derivation from ancient organic matter is strongly supported by (1) the δ13C ratios of those alkanes, −30‰, which does not match reported values for C3 plant n-alkanes and which falls within the range of values reported for fossil-fuel n-alkanes, and (2) the 14C analysis of the alkane fraction which gives an age of 8770 yrs BP. Moreover, assuming that each soil n-alkane is a mixture of C3-plant n-alkane and fossil-fuel n-alkane, the isotopic composition provides a means of reconstructing the composition of the fossil-fuel pollution. This method, using both isotopic and quantitative analyses of individual substances, represents a powerful means to resolve multiple sources in environmental and geochemical investigations.