Abstract
Water flow and solute transport through undisturbed Andisols in Kumamoto, Japan, having aggregated soil structure were investigated on the basis of solute dispersivity, λ, for saturated and unsaturated flow conditions. The dispersivities, λ, for the saturated undisturbed soil were considerably greater than for the disturbed soil, ranging from approximately 3 cm for the surface soil column (25-55 cm depth) to 38 cm for the sublayer soil column (60-90cm depth). The λ increased with the travel distance from 2 cm to the maximum value for the sublayer column because of preferential bypass flow through narrow pore space. On the other hand, the scale dependency did not appear for the surface column. Since plant roots distributed uniformly in the surface soil, solutes mixed well laterally in the surface column, resulting in the relatively constant dispersivity with the travel distance. The λ, for unsaturated conditions was sufficiently lower (< 1.4cm) than for saturated conditions. Furthermore, the difference between the surface and sublayer columns became small. As water content decreased, macropore flow could no longer be dominant and solute transport became uniform in the undisturbed column. Hence, the λ for the undisturbed soil became close to those for the disturbed soil for unsaturated flow conditions.
