The radial diffusion method .2. A Semianalytical model for the determination of effective diffusion coefficients, porosity, arid adsorption

A model for interpreting diffusional transport in porous geological materials is developed. The model is based on a laboratory method described in a companion paper [van der Kamp et al., this issue] by which radial diffusion from or into a cylindrical reservoir in a core-sized sample is measured. The model accounts for radial diffusion, mass balance in the reservoir, linear adsorption, decay or transformation, and periodic abstraction of samples. The model is derived using the Laplace transform method for both finite and semi-infinite domains. For conditions where solute concentrations equilibrate (i.e., in finite diameter samples), a simple expression is derived that can be used to interpret the results for effective porosity and a retardation factor. It is demonstrated that the method can provide independent measures of the effective diffusion coefficient, adsorption, and effective porosity when the results are interpreted using the model. Several real and hypothetical diffusion experiments are presented to illustrate the use of the model.