Gas And Vapor Phase Transport

Diffusion is the principal mechanism in the interchange of gases between the soil and the atmosphere. Fick's law of diffusion can describe the diffusion of gases in soils as long as the restrictive conditions for the applicability of the equation apply. The soil-gas diffusion coefficient, D-p, is the fundamental property that must be known to use Fick's law to calculate gas transport in soils. Values of D-p change with soil-air content and tortuosity. Techniques for accurately measuring D-p in soil cores are well established. However, because measured data for D-p are often not available, equations to predict D-p from easily obtainable soil-physical parameters have been developed. It is well established that different equations are needed to estimate D-p for undisturbed and for disturbed soil samples. Equations for undisturbed soil must include parameters that take into account the effects of soil type, such as texture, structure, and pore-size distribution, on D-p. Soil-type effects on D-p in sieved and repacked soils appear to be minor.