Preferential solute movement through larger soil voids. I. Some computations using simple theory

Abstract

Viscous solution flow down vertical cylindrical channels and planar cracks, with simultaneous molecular diffusion of the solute into the surrounding soil, was modelled. Chloride and phosphate were chosen as representative of non-sorbed and adsorbed ions respectively. In channels at least 0.2 mm in diameter, and cracks at least 0.1 mm wide, almost instantaneous preferential movement of both chloride and phosphate was predicted. Little or no preferential movement was predicted in smaller channels or cracks. For example phosphate was predicted to move to a depth of 200 mm within 10 min in saturated soil containing 0.2 mm diameter continuous channels. However, it would take phosphate over 2 months to reach the same depth in similar soil with the same hydraulic conductivity, but containing only 0.05 mm diameter channels. Channels and cracks permitting preferential solute movement would be solution-filled only at pressure potentials above - 0 2 m, so such movement can only occur in near saturated soil. Although highly idealized soil-void geometries were assumed, the results have a number of practical implications related to the movement of nutrients and pollutants in field soils.