Solute transport in a layered field soil : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science at Massey University

Abstract

Although concern about the effects of movement of chemicals through soil has brought about a need for greater understanding of solute transport, the question as to where best to focus the research effort remains open. Initially a philosophical fram ework was presented that described in a general sense how research into solute transport has been conducted. It was argued that we must combine modelling with experimentation for effective progress in understanding, and that the efforts in field versus lab experimentation and process- versus nonm echanistic modelling should be balanced. Currently there is a need for more field experimentation, but the preferred direction of the modelling effort is less clear. Both process-based and non-mechanistic models are considered in order to deduce the effect of soil layering on solute transport. Field experiments were carried out on a soil consisting of three layers of distinct texture. This soil was instrumented with porous cup samplers at four depths at twenty sites. There was also a 2 m2 lysimeter within the plot. In the first experiment irrigation was used to supplement rainfall in order to leach a surface application of solid KCl through the soil. Porous cup samples of the soil solution were collected on numerous occasions and soil cores less often . The experiment of the following year was similar in design except that no irrigation was used. Finally, in the third year, the Iysimeter was instrumented with porous cup samplers and the same experimental design repeated on a smaller scale. A convection-dispersion (CDE) model was applied to the lysimeter data. This was successful, provided that the surface soil and assumed Dirac delta solute input were not included in the calibration. Layering within the profile appeared to have little effect on solute transport. The transport porosity was revealed to be two-thirds of the water-filled porosity, thus a substantial part of the water-filled porosity did not transport solute. The CDE modelling of the field data was not particularly successful, probably due to the spatially variable nature of solute transport and water application. The Aggregated Mixing Zone (AMZ) model was also used. This model subdivided the transport porosity i nto convective and dispersive components, and also allowed for non-interacting flow paths. Although the AMZ model was conceptually appealing, parameterisation of the model was found to lack discrimination. Little further understanding of solute transport was gained from this model. Textural differences in the soil seem to be overwhelmed by both small-scale heterogeneity of water application and solute movement in the soil, especially near to the surface. It was apparent that processes occurring in the surface soil require much more attention than they have been afforded in the past. Both process-based modelling and field experimentation will increase our understanding of solute transport. It also seems that an increased effort in improving measurement techniques will be advantageous.