Preferential solute movement through larger soil voids. Ii. Experiments with saturated soil

Abstract

Miscible displacement experiments, with chloride and phosphate ions in the displacing solution, were conducted on soil material manipulated to contain different void geometries. In packed 50 mm long columns of 0.2-0.5 mm aggregates the displacing solution moved fairly uniformly through the soil, with the relative chloride concentration in the effluent reaching 0.5 approximately 8 min after chloride was applied to the surface. The relative phosphate concentration reached 0.5 after approximately 7 h. In 50 mm long cast-columns of the same soil material containing a single cylindrical channel 0.5 mm in diameter, or a slit 12 mm by 0.2 mm, both chloride and phosphate moved almost instantaneously through the columns, reaching a relative concentration of 0.5 in less than a minute. The cast columns and packed aggregate columns all had similar hydraulic conductivities. These results are in general agreement with a previously developed theoretical model. An 'undisturbed' core of the same soil showed similar pronounced preferential movement of chloride and phosphate, indicating the presence of a continuous void greater than the critical minimum size. However, in contrast a duplicate core gave results more like those found for the columns of aggregates. Soil water retentivity curves are shown to be of very limited value for predicting preferential solute movement.