Simulation of vertical water flow and bromide transport under temporarily very dry soil conditions

The exchange of old and new water in the unsaturated zone depends mainly on soil physical conditions and the antecedent soil moisture conditions. A tracer experiment with bromide in a large undisturbed weighing lysimeter under field conditions with temporarily very dry soil conditions, allows detailed process studies on the solute transport in a homogeneous soil. The process of drying by transpiration causes transport of bromide (Br) from large to small pores. During wetting periods a remobilization of the Br over a long period can be observed. On the basis of our results, we propose a multi-region mobile/immobile water-solute transport model, in which mobilities in different bounded regions of the soil water are calculated similarly to the unsaturated hydraulic conductivity, and the exchange between the regions is calculated as a first order process.