Predicting the gas diffusion coefficient in repacked soil: Water-induced linear reduction model

Investigations of gas transport and fate processes in packed soil systems require knowledge of the gas diffusion coefficient, D-P, as a function of air-filled porosity, epsilon. On the basis of the literature, data from sis studies over the porosity range of 0.1 to nearly 1.0, it is reconfirmed that the Marshall (1959) model better predicts D-P(epsilon) in completely dry, repacked porous media than do the Penman (1940) and Millington (1959) models. The smaller D-P value in wet soil, as compared with dry soil at the same air filled porosity, is accounted for by introducing a water-induced linear reduction (WLR) term, equal to the ratio of air-filled porosity to total porosity, in the D-P(epsilon) model. By adding the WLR term in each of the three D-P(epsilon) models for dry porous medial the so-called WLR(Marshall), WLR(Penman), and WLR(Millington) D-P(epsilon) models for wet soil are developed. To test the three WLR models, D-P was measured at different soil-water contents in sis differently textured (6-38% clay) repacked soils. The WLR (Marshall) model accurately and best described D-P(epsilon) for all sis soils and additional soils from the literature. All three WLR models performed better than previous D-P(epsilon) models, This study implies that the smaller D-P in a wet soil, which is due to mater-induced changes in air tilled pore shape and pore connectivity, ran be described by a simple, linear function of relative air-filled porosity. The WLR(Marshall) model represents a conceptual and accurate model to predict D-P(epsilon) in sieved, repacked soil.