Wellenberg is the proposed candidate site for LLW/ILW repository in Switzerland (Figure 1). The performance assessment of the planned repository calls for a thorough hydrogeological characterization of the site. The spatial distribution of the hydraulic conductivity within the host rock is a key input for the evaluation of the host rock performances. The spatially variable conductivity field is modelled geostatistically in 3 dimensions using a conditional simulation method. In order to account for the possible range of spatial variability, several simulations of the conductivity are generated. The conditional simulation of the conductivity - representing possible versions of the unknown reality - are used as input for numerical modelling. For each simulated conductivity field, the groundwater flow is numerically modelled using finite elements. This Monte Carlo approach allows the propagation of the input uncertainty due to spatial variability of the conductivity onto the predictions delivered by the flow model. With the help of this approach several specified performance criteria - together with their uncertainty - were estimated for the evaluation of storage capabilities of the Wellenberg host rock.