CALIBRATION OF JOINTED TUBE MODEL FOR THE GAS-DIFFUSION COEFFICIENT IN SOILS

Modeling of gas and vapor transport in soils requires knowledge of the relative gas diffusion coefficient. The relative diffusion coefficient was determined as a function of air-filled porosity, using a pore model based on two tortuous tubes of different radii joined in series. This model was fitted to measured relative diffusion coefficients on undisturbed samples of nine soil types at a wide range of water contents. It was found that model parameters vary with the soil types studied. Combined measurements of the water retention curve, air-filled porosity, and relative diffusion coefficient were made to evaluate the effects of water distribution in the soil pores on the relative diffusion coefficient. After the samples were saturated with water, the residual air-filled porosity ranged from 0.051 to 0.167 m3 m-3 and the relative diffusion coefficient was practically zero, which is due to pore blocking by soil water. At pressure heads corresponding to the air-entry value in the water retention function, air-filled porosity ranged from 0.095 to 0.256 m3 m-3 and the relative diffusion coefficient ranged from 0.002 to 0.013. From this point, with increasing air-filled porosity, the relative diffusion coefficient increased rapidly to values ranging from 0.165 to 0.383 at zero water content. It was concluded that the air-entry value is an important parameter, which indicates a priori the water content at which pore blocking becomes relevant.