Abstract
The design of clay/clayey barriers for the containment of buried wastes conventionally has been based on the assumption that the
hydraulic conductivity controls the rate of leachate percolation. However, recent studies show that diffusion is a controlling
mechanism of solute transport in many fine-grained soils. Although the measurement of the hydraulic conductivity of finegrained
soils is comparatively a common practice in soil engineering, the measurement of diffusion coefficients is not. As such, it
is becoming increasingly essential to assess the movement of chemicals through soil barriers due to diffusion. Studies indicate
that diffusion may be an important, if not dominant mechanism of contaminant transport through waste containment barriers.
This paper is therefore in line with the efforts made in the measurement of diffusion coefficients of inorganic chemicals passing
through saturated soils. Herein, both steady-state and transient equations relating to the diffusive transport of inorganic chemicals
are presented. A number of factors affecting diffusion coefficients are identified and a simple method for measuring diffusion
coefficients for a compacted barrier is defined. The definition for the diffusion coefficient of soil called the effective diffusion
coefficient, D* is seen to vary widely. Generally, the variations are due to the different factors affecting diffusion of solutes in
soil and the various ways of including the volumetric water content in the governing equations. Hence, errors in interpretation
and comparison of D* values can occur if the appropriate definition is not used. In a nutshell, the concept of diffusion may be
unfamiliar to many soil specialists, worsened by the myriad terminologies linked to the study of diffusion in soils. Thus the study
attempts to acquaint soil engineers with vital information for the measurement of diffusion coefficients for barrier designs.