DIFFUSION IN SATURATED SOIL .1. BACKGROUND

Recent studies suggest that diffusion may be an important, if not dominant, mechanism of contaminant transport through waste containment barriers. This paper represents the first of two papers pertaining to the measurement of diffusion coefficients of inorganic chemicals diffusing in saturated soil. In this paper, both steady-state and transient equations describing the diffusive transport of inorganic chemicals are presented. Several factors affecting diffusion coefficients are identified. A method for measuring diffusion coefficients for compacted clay soil is described. The definition for the diffusion coefficient for diffusion in soil (known as the effective diffusion coefficient, D*) is shown to vary widely. In general, variations in the definition of D* result from consideration of the different factors that influence diffusion of solutes in soil and the different ways of including the volumetric water content in the governing equations. As a result of the variation in the definition of D*, errors in interpretation and comparison of D* values can result if the appropriate definition for D* is not used.