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 and designing remediation processes in the vadose zone. Factors controlling gas phase diffusion include both the distribution of pore sizes and water within the soil matrix. Diffusion column experiments were carried out to measure the effective diffusivity of argon gas through air as a function of soil moisture content and soil physical characteristics. Argon was chosen as a non-sorbing tracer because of its low aqueous solubility and chemical inertness. Thus, direct calculation of gaseous phase diffusion resistance was possible. A diffusion model based on parallel diffusion between interparticle pore and macropore regimes was developed. This model incorporates pore size distribution information along with other readily obtainable soil bed parameters. The new model provided better agreement with experimental data than did other existing models.