A simple analytical solution for unsaturated solute migration under dynamic water movement conditions and root zone effects

Vulnerability mapping involves an assessment of the potential for solute migration to the water table. Existing procedures rely on simple qualitative indices to calculate this migration potential. As an alternative to these vulnerability indices this paper presents a simple, analytical solution to the advection-dispersion equation. This solution is physically representative, in that it allows for the effects of root zone processes and climate-induced water movement variation on solute transport. The basis of this analytical solution is a spatially transformed advection-dispersion equation. The root zone is treated in a lumped fashion where a relation is developed for the solute concentration of water that has passed through the root zone that includes retardation, degradation and transpiration. This lumped root zone treatment is used in an existing analytical solution that is adapted so that it can be used for a transformed unsaturated advection-dispersion equation. The procedure is tested against hypothetical simulations with dynamic water movement conditions calculated using SWIMv2. The accuracy of the approach is found to be a function of the soil permeability. Where the dynamic variation is averaged out by the water movement process (such as in soils with moderate permeability) the method can accurately represent the transport process. For soils where surface conditions lead to significant variation in water content throughout the profile the technique is more approximate.