The steady-state and transient equations describing the process of diffusion in free (aqueous) solution are presented. Four types of free-solution diffusion coefficients are identified with respect to the systems used for their measurement. The free-solution diffusion coefficient is shown to be a function of several factors, including the temperature, viscosity, and dielectric constant of the solution; the radius and valence of the diffusing chemical species; and the equilibrium chemistry of the solution. Several modifications must be made to the free-solution diffusion equations in order to describe diffusion in soil. The modifications result from the reduced cross-sectional area and the longer and more tortuous pathways experienced by solutes diffusing in soil. In addition, some solutes may be subject to reversible sorption reactions during transport through the soil. As a a result of these effects, diffusion in soil is slower than diffusion in free solution. The equations describing diffusion in soil are used to indicate the significance of diffusive transport of contaminants through fine-grained soil barriers. For waste containment barriers, the significance of diffusive contaminant transport is described with respect to three flow conditions. The significance of diffusion increases as the seepage velocity decreases. As a result, the best waste containment barrier is one in which diffusion controls the transport process. However, the rate of contaminant transport through waste containment barriers may still be relatively rapid even if diffusion is the only active transport mechanism, especially when relatively thin barriers are built to contain relatively high concentrations of contaminants.
Abstract