MODELING OF THE GASEOUS-DIFFUSION COEFFICIENT THROUGH UNSATURATED SOIL SYSTEMS

Determination of gaseous phase diffusion through unsaturated soil beds is essential for estimating the transport of volatile contaminants an d designing remediation processes in the vadose zone. Factors controll ing gas phase diffusion include both the distribution of pore sizes an d water within the soil matrix. Diffusion column experiments were carr ied out to measure the effective diffusivity of argon gas through air as a function of soil moisture content and soil physical characteristi cs. Argon was chosen as a non-sorbing tracer because of its low aqueou s solubility and chemical inertness. Thus, direct calculation of gaseo us phase diffusion resistance was possible. A diffusion model based on parallel diffusion between interparticle pore and macropore regimes w as developed. This model incorporates pore size distribution informati on along with other readily obtainable soil bed parameters. The new mo del provided better agreement with experimental data than did other ex isting models. (C) 1997 Elsevier Science B.V.