Effect of Density‐Driven Advection on Trichloroethylene Vapor Diffusion in a Porous Medium

Abstract

To quantify the vertical diffusive mass transfer of trichloroethylene (TCE) in a low water‐saturated porous medium, diffusion experiments were conducted in soil columns using a medium‐size quartz sand at two different water saturations; namely, completely dry and at irreducible water saturation. Advective transport of TCE vapors was mainly generated by density gradients in the soil gas during diffusion of TCE vapors in the vertical direction. The maximum stationary TCE vapor concentrations measured at the column outlet attained only 70 to 75% of the prescribed inlet vapor concentration, indicating a strong influence of density‐driven advective mass transport on vertical diffusion of TCE. Without taking into account the density of the vapors, the measured apparent TCE diffusion coefficient was about seven and five times smaller than the coefficients predicted by the model for dry sand and sand at irreducible water saturation, respectively. A numerical transport model was used to interpret the experimental results. The numerical results fit the experimentally measured TCE breakthrough curves well. A sensitivity study demonstrated that the simulated vapor breakthrough curves were not very sensitive to variations in the dimensionless Henry's constant, the gas–solid or water–solid phase partitioning coefficients but were sensitive to modification of the intrinsic permeability and the gas tortuosity–connectivity parameter. Two series of numerical simulations were also conducted to analyze the steady‐state conditions of vertical vapor transport. The results highlighted how both the relative density of TCE vapors and the height of the soil column, expressed as a dimensionless Rayleigh number, might reduce the vertical diffusive mass flux.