Bimodal probability law model for unified description of water retention, air and water permeability, and gas diffusivity in variably saturated soil

Air and water permeabilities and gas diffusivity as functions of soil fluid phase (air or water) contents are governing chemical transport and fate processes in the vadose zone, and have frequently been identified as the three main transport parameters determining time and efficiency during soil vapor extraction (soil venting) at polluted soil sites. A mathematically flexible function that can accurately describe data for both soil water retention and all three transport parameters as functions of fluid phase contents in undisturbed soil with bi- or multimodal pore structure is required for numerical simulation studies. In this study, a bimodal probability law (BPL) model with a total of six fitting parameters is compared with new and literature data for soil water retention and the three transport parameters measured on undisturbed soils. Saturated and unsaturated hydraulic conductivity (K) at six matric potentials (pF) were measured on four volcanic ash soils (Andisols) where data for soil water retention, air permeability (ka), and relative gas diffusivity (Dp/D0) were already available. The BPL model accurately described data for soil water retention and the three transport parameters (K, ka, Dp/D0) for the four Andisols and other soils with bimodal porosity behavior. BPL model parameters did not correlate well between transport processes, however, an existing model for relating saturated hydraulic conductivity to air permeability at 2100 cm H2O matric potential performed well for 10 bimodal Andisols. Results indicate that relations between fluid phase transport parameters at given matric potentials may be valid across soil types and useful in the BPL model.