Vapor Transport in Unsaturated Soil

Experiments were performed to determine the desorption kinetics, diffusion, and advection of two common subsurface VOC contaminants: trichloroethylene (TCE), and chloroform (CF) both individually and as a gas mixtures. Effective diffusion coefficients of TCE were determined in the absence and presence of CF. At high relative pressure the 'effective' distribution coefficient for these vapors decreased and consequently their diffusive transport was enhanced. This result is attributed to the presence of non-linear sorption isotherms for the vapors. Advective transport of TCE and CF also displayed mild gas concentration effects suggesting that the accurate prediction of vapor transport may require consideration of reactions at the gas water interface including vapor condensation and competition. The breakthrough and subsequent desorptive release of TCE vapor from a column of aquifer sand was well predicted by a transport model that employed a Gamma distribution of mass transfer rate constants. Comparisons of experimental data and model predictions have been successfully made at two different gas flow velocities for sand exposed to low (P/Po = 10%) and high (P/Po = 90%) relative pressure of TCE.