The presence of residual LNAPL (light non aqueous phase liquid) in the subsurface is a key environmental issue in most of sites contaminated by hydrocarbon compounds. Residual LNAPL indeed remains in the soil after mobile LNAPL is recovered and can serve as a long-term source of dissolved hydrocarbon plumes. The aim of this work is to assess the dissolution behavior of residual LNAPL entrapped in different porous media. To this end, lab-scale column tests (12.7 x 2.9 cm) with soils artificially-contaminated by toluene were carried out. Specifically, two types of soil, composed of medium-fine sands with different percentage of silt (9% and 14%) and clay (1% and 2%), characterized by a total porosity of about 38-39% and a total organic carbon of about 0.2%, were used. Furthermore, control tests with columns packed with glass spheres of 6 mm diameter were also carried out. In each experiment, columns were first saturated with demineralized water from the bottom and then with toluene from the top to maintain stable displacement. The column saturated with toluene was then flushed with water at different flow rates (0,5–1 ml/min) in order to displace the free phase of toluene. The residual toluene saturation in the column was then calculated as difference between the volume of toluene injected and the one displaced. Afterwards, to assess the dissolution kinetics of toluene from the residual phase entrapped in the column, demineralized water was flushed in the column at different flow rates (0.2–2 ml/min) and the effluent samples were collected in vials and measured by HSS-GC-MS analysis. The obtained results revealed, as expected, that the residual saturation of the two tested soils was significantly higher than the one observed for the column packed with the glass spheres. The time-series dissolution profiles for toluene, expressed as a function of the pore volume flushed in the column, highlighted initially high aqueous contaminant concentrations (in the order of the solubility values) which were followed by a period of rapid decline and an asymptotic time when concentrations declined very slowly. The dissolution profiles were also described by a modified first order kinetic model allowing to identify the dissolution rates for the tested different porous media and to provide some insights about the long-term impacts of residual LNAPL
