Experimental and Modeling Study of Flow and Transport in Aggregated 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. Infiltration experiments into variable-saturated Andisols were conducted using calcium chloride solutions. Water content as well as the soil electrical conductivity, ECa, were monitored using TDR probes inserted horizontally at different depths. Water flow and solute transport through the soil columns were evaluated based on the Richards equation and the mobile-immobile transport model (MIM) assuming stagnant flow in the inter-aggregate (micropore) region. The Durner (dual-porosity van Genuchten) model was used for the hydraulic property. The considerable impacts of the aggregated structure on flow and transport, and the applicability of the MIM for transient flow conditions, are shown and discussed.