Tracer transport modelling for unsaturated soil zone - effects of anion exclusion and immobile water

The injected tracer method for the evaluation of mass flow of moisture in the unsaturated zone is based on the 'piston-flow' model. The assumptions inherent in the concept are that the dynamic behaviour of the tracer is similar to the moisture in transport and that there is no 'immobile' component of soil moisture. The studies indicate that the assumptions can be verified by suitable mathematical simulation of the experimental data. The multi-cell mixing model and the advection-diffusion model have been used to simulate the laboratory and field tracer profiles. The faster movement of the [Co(CN)6]3- ion in clayey soils as compared to tritium is attributed to the anion exclusion of the trivalent anion. The simulation of tracer-profiles using the modified advection-diffusion model taking into account the mobile and immobile components of soil moisture is also discussed. (author)