Following prescriptions of the recent European Water Framework Directive, a groundwater quality evaluation system must be adopted for checking the groundwater status with respect to different contaminants. A screening evaluation system (based on a system developed by the French Water Agencies) has been adapted to the specific conditions in the Walloon Region of Belgium. Groundwater quality data are aggregated into indicators with respect to different water uses such as drinking water standards, thresholds values for preserving dependent surface ecosystems, or the groundwater ‘natural or patrimonial state’. A global groundwater quality indicator can also be calculated. Different aggregation techniques are discussed with their respective influence on the final indicator. In relation to this evaluation, the monitoring network must be adapted for being (as far as possible) representative of the global quality of water in each groundwater body. Existing knowledge and understanding of the actual hydrogeological conditions were used in priority for choosing an adequate network of monitoring points. At the same time, the spatial density of points was checked in order to obtain a statistically representative network. Applications were performed in five different GWBodies belonging to the hydrographic district of the Meuse River in the Walloon Region and with different contrasted geological conditions: Cretaceous chalks, Carboniferous limestones and Pleistocene gravels of the alluvial plain of the River Meuse. These examples provide a good opportunity for further discussion and work about the main related issues: optimzation procedures, aggregation methods and estimation of the reliability of indicators.