SOLUTE TRANSPORT IN A LAYERED FIELD SOIL - EXPERIMENTS AND MODELING USING THE CONVECTION-DISPERSION APPROACH

Greater understanding of the processes affecting solute transport in field soils is required to meet the ever-increasing demand for improved management of field-applied chemicals. In this study, we sought to elucidate the effect of distinct interfaces between horizons of strongly-contrasting texture on solute transport. Field experiments of solute transport were performed on a soil consisting of three layers of different texture in which porous cup samplers had been installed at four depths in twenty sites. Similar experiments were done in a lysimeter of area 2m(2) and 1m deep. Convection-dispersion modelling was first attempted using the lysimeter data. This was successful, provided that the surface 250 mm of soil were not used to calibrate the model coefficients. Layering within the profile appeared to have little effect on solute transport. The transport porosity was found to be just two-thirds of the water-filled porosity. However, convection-dispersion modelling of the field data was not particularly successful, probably due to the spatially-variable nature of solute transport coupled with variation in the water application pattern. Textural differences in the soil seemed to be overwhelmed by both small-scale heterogeneity of water application and local variation of solute movement through the soil, especially near to the soil surface. It appears that the hydraulic processes occurring in the surface soil require more attention by modellers of solute transport than they have been afforded in the past.