1. Fresh water lenses on saline groundwater Dispersive Behavior of the Mixing Zone between a Shallow Freshwater Lens and Upward Seeping Saline Groundwater S.Eeman, A. Leijnse, S.E.A.T.M. van der Zee Wageningen UR, Sub-department of Soil Physics, Ecohydrology and Groundwater management

2. Background Low lying areas Changing climate Non-equilibrium of ground water system Influence on land use options? Characterization of small scale system Source: G. Oude Essink, Deltares

3. Lens on field Scale Most relevant for users Mixing zone not negligible Boundary conditions well defined

4. Methods Numerical modeling using SUTRA Density dependent Unsaturated zone Analysis of results using spatial moments of concentration change with depth δc/δz Z

5. Comparison to the analytical solution by Maas (2007) Analyses of dispersion and diffusion Results

6. Numerical vs Maas(2007) First vertical moment dC/dy From initially saline towards steady state Steady state Max. thickness in time

7. Processes contributing to mixing Different processes Different circumstances Phase Location Num. calculations Local coordinate system Parallel to “interface” Less accurate near ditch Longitudinal dispersion Transverse dispersion Molecular diffusion

8. Contributions to mixing I Moving ‘interface’Steady state conditions

9. Contributions to mixing II 5m 15m 25m

10. Conclusions Steady conditions: analytical model of Maas (2007) is a very good approximation Contribution of dispersion and diffusion to mixing depend on phase and location Developing lens: longitudinal dispersion Steady state: transverse dispersion / diffusion Mid field: BC prohibits transverse dispersion Near ditch: transverse dispersivity

11. Current work Start of fieldwork in Zeeland with P. de Louw, Deltares (thanks!) Sensitivity of a fresh water lens for different climatic conditions

12. Thank You!