Experimental and Modeling Study of Solute Transport and Hydrodynamic Dispersion in Japanese Volcanic Ash Soils.

Japanese volcanic ash soils, Andisols, have unique physical properties. For example, the water retention curve of an Andisol generally has a dual-porosity shape resulting from separate macropore (inter-aggregate) and micropore (intra-aggregate) contributions, and a very high saturated water content (often as high as 0.85 cm³/cm³) because of the inherent nature of soil aggregation in these soils. A large number of laboratory solute transport experiments were carried out to determine the hydrodynamic dispersion coefficient, D, in the convection-dispersion equation (CDE) for several Japanese Andisols under saturated and unsaturated flow conditions. The dispersivity, λ, for saturated flow was found to increased linearly with the pore-water velocity, v, indicating that solute mixing between the inter- and intra-aggregate pores was the dominant process determining solute dispersion. Effective dispersion coefficients in the mobile-immobile transport model (MIM) were determined based on the observed λ-v relationships, while assuming that the intra-aggregate (micropore) region remained stagnant. The obtained MIM parameter values imply that solutes mixed well between the macropore and micropore regions, and reached equilibrium within a relatively short period of time when the soil is saturated. The dispersivity was found to decrease as the water content decreased since flow and transport became increasingly restricted to only the inter-aggregate (micropore) region.