Aqueous-Phase Diffusion in Unsaturated Geologic Media: A Review

Aqueous-phase diffusion in geologic media is very important in agricultural production, contaminant transport and remediation, risk assessment, and waste disposal. This review was undertaken to investigate (1) the components of effective diffusion coefficients, (2) diffusion as a function of water content and diffusion models, as well as the effects of temperature, compactness, and sorption on diffusion, and (3) diffusion in aggregated media. We focus on the behavior of, and relationship between, nonsorbing diffusion species and water content, particularly in porous aggregates. At low water content, aqueous-phase diffusion occurs in thin liquid films on particle surfaces, and diffusion is very slow. Diffusion is monotonically related to water content, but the relationship is not simple and depends on the range of water contents (e.g., the different forms of relationships at different water-content ranges). Data from the scientific literature further show that such relationship is related to the texture (pore-size distribution) of the geologic medium. In this article, we compile the available models that describe the relationship between diffusion coefficient and water content, with some models incorporating parameters related to medium properties. Temperature dependence of diffusion is related to the viscosity change of water at different temperatures. We note that the effects of water content on interacting sorption and diffusion deserve further investigation. Our investigation showed that, for porous aggregated media such as tuff gravel, the total water content is comprised of surface water (inter-aggregate regions) and internal water (intra-aggregate regions). Surface film water around particles and pendular water between the aggregate contacts serve as the predominant diffusion pathways. Diffusion in tuff gravel could be very slow, because water films on surfaces could be discontinuous or absent. Finally, future directions regarding the testing of aqueous-phase diffusion in complex systems (e.g., unsaturated porous aggregates) are