Abstract
Hydrodynamic dispersion coefficients, D, for an aggregated Andisol were determined for saturated and unsaturated flow conditions. The dispersivities, λ, as a function of pore-water velocities, v, and volumetric water contents, θ, were compared with those for a non-aggregated dune sand. The water retention curve for the Andisol had a stepwise shape. The water content decreased drastically at air-entry h=-15 cm and lower pressure head h=-3,160 cm. The radius of the aggregate was assumed to be approximately 0.1 mm based on the air-entry value. The estimated θ for the aggregate were 0.5 cm3 cm-3 while the saturated water content was 0.74 cm3 cm-3. The dispersivity, λ, increased linearly with v and reached more than 2 cm for saturated water flux, q, ranging from 146 to 3,085 cm d-1. This velocity dependency was due to solute exchange between inter-and intra-aggregate pores. The convection-dispersion equation (CDE) agreed well with measured breakthrough curves (BTCs) regardless of q values because of the relatively rapid solute exchange. On the other hand, λ for unsaturated conditions decreased as θ decreased from saturation, and had a constant value of 0.2 cm for θ=0.5-0.6 cm3 cm-3. Effects of water flow in inter-aggregate pores decreased and flow in intra-aggregate pores became dominant for lower θ. The CDE also agreed well with unsaturated BTCs because of relatively homogeneous flow in the soil aggregates.
