Vapor transport in unsaturated soil columns: Implications for vapor extraction

A mathematical model was derived to examine the impact of gas advection, gas diffusion, gas-water mass transfer, gas-water partitioning, sorption, and intraaggregate diffusion on subsurface movement of organic vapors. Laboratory experiments were performed to determine the validity of the model and to investigate the impact of the various mechanisms on vapor transport. Columns were packed with a uniform Ottawa sand and an aggregated porous soil material (APSM) to compare transport in different soil structures. Toluene vapor transport was observed in the sand under dry and wet (27% water saturation) conditions. The experiments with the APSM were performed dry and at 67% water saturation. In all the sand and the dry APSM experiments, gas advection and diffusion had the greatest impact. In a wet APSM experiment, intraaggregate (liquid) diffusion was also important to consider for gas velocities greater than approximately 0.05 cm s−1. For both soil materials, sorption of toluene vapors occurred for dry conditions, while vapor sorption was negligible when liquid water was present. These findings imply that vapor extraction performance in moist, aggregated soils will be affected by nonequilibrium transport. Therefore models that are developed for predicting the complete removal of contaminants by vapor extraction must account for nonequilibrium.