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Predicting Chemical Fingerprints of Vadose Zone Soil Gas and Indoor Air from Non-Aqueous Phase Liquid Composition

Complex mixtures of volatile organic chemical (VOC) vapors can exist over subsurface accumulations of non-aqueous phase liquids (NAPLs) and contaminated soils. The ability to predict the relative soil gas chemical composition arising from such NAPLs is relevant to studies of the sources and fate of soil gas, and in assessing the possible intrusion of soil gas chemical constituents to indoor air. However, it is difficult to directly compare the chemical ‘fingerprints’ of NAPL and vapor because the differences are significant in liquid-gas partitioning coefficients among the many volatile hydrocarbons that compose the liquid NAPL. In this article, detailed chemical characterization data from analysis of liquid NAPLs are used to calculate equilibrium vapor phase distributions of 66 diagnostic paraffin, isoparaffin, aromatic, naphthene, and olefin (PIANO) compounds. Measured NAPL and predicted vapor phase chemical composition are presented for a diverse group of 10 hydrocarbon products that include crude oil, solvents, gasoline, distillate fuels, and coal tar. The laboratory investigation and field study described in this article demonstrate that chemical fingerprints of such predicted vapor phase hydrocarbons can be used as compositional benchmarks for reconciling source(s) of soil gas and indoor air-borne hydrocarbons.