The Geological Survey of the Netherlands makes available 3D models of the subsurface at various resolutions. GeoTOP is a high resolution voxel model of the upper tens of meters of the subsurface. It consists of voxels of 100m x 100m x 0.5m for which the stratigraphy, lithology, and hydrogeological parameters are identified with their uncertainties. The first step in the creation of GeoTOP is a geological schematization of the borehole descriptions into lithostratigraphical units. During the second step, maps are created of the top and base of each unit based on the elevation in the boreholes. The third step is a 3D stochastic interpolation of lithofacies and lithology within each unit, resulting in probabilities for each voxel. In the fourth step, voxels get hydraulic conductivities from stochastic upscaling of values from laboratory tests on undisturbed samples from high quality cores from boreholes. Hydraulic parametrization of GeoTOP has been carried out for the Province of Zeeland in the Netherlands. The resulting hydraulic resistance of the Holocene confining layer compared reasonably well to existing information. However, no data was available for an area with very high resistances according to GeoTOP. Therefor, a pumping test has been designed to obtain data for the validation of these high resistances. The integration of the geology in GeoTOP gives better a priori hydraulic parametrization of (complex) confining layers for groundwater modeling. Still unresolved is how the 3D voxel model can be improved with the a posteriori values of groundwater model calibration. Corresponding email: [email protected]@en