Water- and air-filled pore networks and transport parameters under drying and wetting processes

The connectivity and tortuosity of fluid-filled pore networks in the water and air phases strongly influence the mass transport in porous media. Moisture conditions (water content and distribution) alter water- or air-filled pore networks. In this study, using a sand column with variable saturated conditions, water- and air-filled pore networks were analyzed using X-ray computed tomography (CT). Water and air transport parameters, including hydraulic conductivity, gas diffusion coefficient, and air permeability, were measured. The objectives were (a) to identify the effects of entrapped air on the water-filled pore network and hydraulic conductivity and (b) to understand the water- and air-filled pore networks and relevant transport parameters in the sand column during the drying and wetting processes. Measurements of hydraulic conductivity using quasisaturated samples showed that hydraulic conductivity was drastically reduced when smaller in situ air bubbles were present inside the sand column. At the same air-filled porosity, higher gas diffusivity and air permeability were obtained under wetting than those during drying. X-ray CT image analysis revealed that the air-filled pore network connectivity during wetting was higher than that during drying, resulting in enhanced gas transport parameters during the wetting process. The observed differences in water- and air-filled pore networks during drying and wetting processes are highly promising for future multiphase mass transport models in soils.