Solute movement through undisturbed soil columns under pasture during unsaturated flow

Abstract

Previous studies of solute movement concerning the influence of initial soil water content have led to apparently contradictory results. Here we describe some experiments which aimed to determine the effect of both pasture and initial water content on solute movement. Solid SrCl₂, CaCl₂, and Ca(NO₃)₂ were surface-applied to undisturbed columns of a fine sandy loam under short pasture. The soil columns were 300 mm in both diameter and length. A rotating rainfall simulator delivered steady-state rainfall at about 10 mm/h. The leachate at the base was collected under suction and analysed, and one column was analysed for resident concentrations of strontium. Solute transport could be accurately described by coupling Richards’ equation with the convection dispersion equation, when ion exclusion or exchange were taken into account. The dispersivity was about 70 mm, only slightly higher than found previously for the same soil without vegetation. There was no significant difference in intrinsic behaviour when solute was applied to either an initially wet or a dry topsoil. The contrasting results from earlier published studies were probably due to incipient ponding and macropore flow. This will not usually occur in New Zealand pasture soils under typical rainfall intensities, but might under irrigation or when the soil structure is degraded. It is suggested that soil cores need to have dimensions at least as large as the dispersivity if they are to encompass most of the local variation in solute concentration.