Water resources management is now recognized as a multidisciplinary task that has to be performed in an integrated way, within the natural boundaries of the hydrological basin or of the aquifers. Policy makers and water managers express a need to have tools able at this regional scale to help in the management of the water resources. Until now, few methodologies and tools were available to assess and model the fate of diffuse contaminants in groundwater at the regional scale. In this context, the objective of this research was to develop a pragmatic tool to assess and to model groundwater flow and solute transport at the regional scale. A general methodology including the acquisition and the management of data and a new flexible numerical approach was developed. This numerical approach called Hybrid Finite Element Mixing Cell (HFEMC) was implemented in the SUFT3D simulator developed by the Hydrogeology Group of the University of Liège. A first application of this methodology was performed on the Geer basin. The chalk aquifer of the Geer basin is an important resource of groundwater for the city of Liège and its suburbs. The quality of this groundwater resource is threatened by diffuse nitrate contamination mostly resulting from agricultural practices. New field investigations were performed in the basin to better understand the spatial distribution of the nitrate contamination. Samples were taken for environmental tracers (tritium, CFC’s and SF6) analysis. The spatial distribution of environmental tracers concentrations is in concordance with the spatial distribution of nitrates. This allows proposing a coherent interpretative schema of the groundwater flow and solute transport at the regional scale. These new data and the results of a statistical nitrate trend analysis were used to calibrate the groundwater model developed with the HFEMC approach. This groundwater flow and solute transport model was used to forecast the evolution of nitrate concentrations in groundwater under a realistic scenario of nitrate ...