PurposeAlthough steam injection has been studied extensively as a potential technology for the remediation of soils polluted by hydrocarbons, there is still an ambiguity concerning the applicability of the method to unsaturated and low permeability soils. The goal of the present work is to identify the dominant mechanisms of pollutant removal and evaluate the effectiveness of steam injection for a mixture of semi-volatile and non-volatile hydrocarbons, and the unsaturated zone of low permeability soils. Such information is helpful for the interpretation of the results from relevant field-scale experiments.Materials and methodsAn unsaturated and low permeability soil is polluted with a synthetic light non-aqueous phase liquid (LNAPL) which is a mixture of non-volatile and semi-volatile hydrocarbons: n-dodecane (n-C12), n-decane (n-C10), 1,2,4-trimethylbenzene, methylcyclohexane. The soil is packed in a tank (0.5 m × 0.55 m × 0.12 m) of poly-methyl-methacrylate, and two lenses of coarse-grained sand act as high-permeability hydraulic fractures, with the steam being injected through the lower one and effluent (water/non-aqueous phase liquid (NAPL)/gas) being extracted from the upper one. After the completion of the test, a great number of soil samples are analyzed with gas chromatography–flame ionization detection to map the spatial distribution of the concentration of NAPL-compounds and evaluate the NAPL-removal efficiency.Results and discussionThe condensation of water in the pores of unsaturated soil may block the upward convective steam flow and alters locally the pore structure of the soil by creating high-permeability pathways. Thanks to these pathways, the upward flow of steam at high rates sweeps the vapors of NAPL (steam stripping) formed at temperatures lower than 100°C (steam distillation). The NAPL removal efficiency is satisfactory but non-uniform throughout the soil and is maximized in areas dominated by preferential flow paths and for the most volatile compounds.ConclusionsIn the unsaturated zone of a low-permeability soil, the removal of LNAPL with steam injection is enhanced in areas dominated by high-permeability preferential flow paths. The vaporization of semi-volatile and non-volatile compounds at temperatures lower than 100°C (steam distillation) coupled with steam stripping are the dominant mechanisms of LNAPL remediation.