Prediction and experimental validation of liquid-phase diffusion resistance in unsaturated soils

Determination of liquid-phase diffusion through unsaturated soils is important for estimating contaminant transport in soils and design of remediation processes for contaminated soils and groundwater. Liquid-phase diffusion through unsaturated soils is governed by both the pore size distribution and water distribution of the soil matrix. Diffusion tube experiments were carried out using several soils packed to field densities to determine the effective diffusivity of chloride ion as a function of soil moisture content. Chloride was selected to serve as a non-volatile, non-sorbing tracer species. A transport model was developed to predict liquid-phase tortuosity as a function of readily obtained soil parameters, including density, moisture content, particle size distribution and pore size distribution. The basis of the model was parallel diffusion resistances in the inter- and intra-particle pore regimes. The new model was found to provide an accurate prediction of observed experimental results. Previously reported models did not agree with experimental results over significant portions of the experimental domain investigated.