A model for stripping multicomponent vapor from unsaturated soil with freeand trapped residual nonaqueous phase liquid

We present a model for the multicomponent vapor transport due to air ventin g in an unsaturated zone in the presence of free and trapped phases of resi dual nonaqueous phase liquid (NAPL). On the microscale the soil particles a re assumed to form spherical aggregates with micropores filled with immobil e water, trapped phases of NAPL and air. The interaggregate space is occupi ed with mobile air, and a thin film of free NAPL adheres on the aggregate s urface. While the free NAPL can readily be in equilibrium with macropore va por, the mass transfer from immobile phases in aggregates is rate-limited b y aqueous diffusion. This model enables us to predict the vapor concentrati ons of various chemical species and the free NAPL saturation over the macro scale, based on the detailed understanding of the aqueous concentrations of the species and the trapped NAPL saturation within the aggregates. The mod el is compared favorably with some experimental data of sparging multicompo nent vapor out of an intact core taken from a contaminated site. The distin ctive features of multicomponent transport, clearly exhibited by the data, are further examined in the simulations of a hypothetical case of three-aro matic vapor transport under a radial flow field. It is found that while the vapor concentration of the most volatile component drops monotonically wit h time, those of the less volatile may rise as their mole fractions in the NAPL increase. The vapor concentration of a heavy component may have a loca l maximum at the evaporation front of the free NAPL. In the case of radial flow the free NAPL has two receding evaporation fronts. Condensation of the heavy component downstream of the far front causes a temporary increase of its total concentration there. With trapped NAPL and soil aggregation the macroscale transport is retarded, and the effluent concentrations end up in noticeable tailing.