Identifying and explaining trend reversals of groundwater nitrate concentrations in Wallonia (Belgium)

Various efforts to reverse the increasing concentration of nitrates and pesticides in our groundwaters such as the Programme de Gestion Durable de l’Azote (PGDA) have been implemented in Wallonia (Belgium), driven by the EU Nitrates Directive (91/676/CEE), the Water Framework Directive (2000/60/EC) and the Groundwater Directive (2006/118/EC). 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. The objective of our research is to define the environmental and anthropogenic factors affecting the effectiveness of the regulations and management practices. We use the monitoring data of 52 groundwater wells and galleries to assess the evolution of nitrate and pesticide concentration in four different water bodies. We use as indicators the contamination levels before and after the introduction of regulations and good management practices, as well as the actual trend. We then seek to predict these indicators and explain their variability by looking into characteristics which could support or undermine the regulations and good practices such as water depth, soil type, crop area 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