Solute dispersion during unsteady leaching as affected by aggregate size and soil water content

Abstract

Abstract Solute dispersion during unsteady leaching of chloride which had been applied as a solution was investigated in soil columns differing in aggregate size (<1 and <4mm), initial soil water content, and the amount of leaching water applied. The approximate analytical solution of De Smedt and Wierenga (Soil Sci. Soc. Am. J., 42, 407-412, 1978) for solute flow during infiltration and redistribution was extended to describe solute concentration profiles in which the water content at the solute front was different from that at the front of subsequently infiltrating leaching water. The magnitude of the solute dispersion was evaluated as the dispersivity in the solution that gave the best fit of the calculated concentration profiles to those measured. The dispersivities thus determined were consistently larger for the coarse-aggregate columns than for the fine-aggregate columns irrespective of the initial soil wetness, reflecting the more efficient leaching observed in the latter columns. In the coarse-aggregate columns chloride dispersion was more enhanced when the soil water content was lower, while the reverse was true in the fine-aggregate columns. The inefficient leaching and larger dispersivity in the unsaturated coarse-aggregate columns suggest that the solute dispersion in aggregated soils during unsaturated unsteady water flow is due mainly to the restriction of the diffusion process within the aggregates. The implication of these results on the solute leaching in field soils is also discussed.