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Simulation of Herbicide Transport in an Alluvial Plain

Herbicide transport through the vadose zone was studied in the Upper Rhône River Valley (SouthWest Switzerland). The herbicides atrazine and isoproturon were applied to instrumented field plots and the concentrations reaching the groundwater were measured. The solute transport is closely linked to precipitation. Following the first heavy rainfall after the application, the chemicals are quickly transported through the vadose zone and reach the groundwater in a short time. The transport experiments were simulated with the mechanistic deterministic model HYDRUS-1D. The mobile-immobile water concept was used to account for the rapid transport. In the study area, the shallow groundwater influences considerably the water conditions in the unsaturated zone; in such cases the use of a one-dimensional model to simulate the water flow and the chemical transport in the vadose zone is difficult because of problems in defining the lower boundary condition. Groundwater flow is typically three-dimensional and therefore, a saturated unsaturated 3-D model or the coupling of an unsaturated 1-D model to a 3-D saturated model would be more appropriate. Nevertheless, HYDRUS-1D allowed to describe qualitatively some observations and to confirm the assumption that accelerated flow occurs on the experimental plots.