MODELING DIFFUSION AND REACTION IN SOILS .3. PREDICTING GAS DIFFUSIVITY FROM THE CAMPBELL SOIL-WATER RETENTION MODEL

Improved prediction of gas diffusivity in soils is essential to the de velopment of better gas transport and fate models, Empirical equations analogous to three well known capillary tube models for unsaturated h ydraulic conductivity, based on the Campbell soil-water retention func tion, were used to predict gas diffusivity as a function of soil-air c ontent, Using the measured gas diffusivity at the highest soil-air con tent as a matching point value, good agreement between predicted and m easured gas diffusion coefficients as a function of soil-air content w as found for 16 undisturbed soils of varying texture, An improved pred iction was obtained using a Burdine-Campbell type of equation but with a reduced tortuosity factor, All four suggested retention-dependent d iffusivity equations gave overall better predictions (smaller root mea n square error) compared with using a simple power function model, If neither the soil-water characteristic curve nor a single measurement o f gas diffusivity at a high soil-air content are available, it is sugg ested that the Campbell model parameter, b, be estimated from soil tex ture and the matching-point value of diffusivity be estimated at maxim um air-filled porosity from the Millington-Quirk model, Using this con cept, the new b-dependent diffusivity equations gave an overall better prediction of gas diffusion coefficients for an additional 27 undistu rbed soils compared with the traditional Millington-Quirk model.