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 coefficien t was determined as a function of air-filled porosity, using a pore mo del based on two tortuous tubes of different radii joined in series. T his model was fitted to measured relative diffusion coefficients on un disturbed 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 porosi ty, and relative diffusion coefficient were made to evaluate the effec ts of water distribution in the soil pores on the relative diffusion c oefficient. 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 bloc king by soil water. At pressure heads corresponding to the air-entry v alue in the water retention function, air-filled porosity ranged from 0.095 to 0.256 m3 m-3 and the relative diffusion coefficient ranged fr om 0.002 to 0.013. From this point, with increasing air-filled porosit y, the relative diffusion coefficient increased rapidly to values rang ing from 0.165 to 0.383 at zero water content. It was concluded that t he air-entry value is an important parameter, which indicates a priori the water content at which pore blocking becomes relevant.