Over the last decades, various regulations and good management practices have been implemented to reverse the increasing concentration of nitrates and pesticides in our groundwaters. The effects of these efforts on groundwater quality are complex to assess because of the non-linearity and the spatial heterogeneity of the hydrological fluxes, the numerous potential pollution sources, and the temporal delays due to the infiltration time in the vadose zone. We use the information encoded in the monitoring data of 52 groundwater wells and galleries in Wallonia (Belgium) to assess the effectiveness of the regulations and management practices. We analyze the trends of the time series and subsequently seek to explain their spatial and temporal variability by looking into the characteristics likely to support or undermine the regulations and good practices such as water depth or urban development. Our results should contribute to a better understanding and prediction of the groundwater quality trends at the catchment scale in Wallonia, providing guidance to catchment management.