Mapping and source identification of groundwater pollution by nitrate : theory and application to the Brusselian sand groundwater body

(eng) This thesis aimed to improve our understanding of groundwater pollution by nitrate at the regional scale of the Brusselian sand groundwater body (Belgium). Despite all the nitrate reduction actions that were undertaken to meet European directives, this study area still presents one of the highest groundwater nitrate concentrations of the Walloon Region. Further assessing the pollution levels of groundwater and identifying the contamination sources are therefore needed for designing sustainable water management strategies. Our first main objective was to assess groundwater contamination at the regional scale by mapping the nitrate concentrations. The locations where groundwater quality properties were measured are scarce and sparsely distributed over space. We therefore used a data fusion method in order to improve the mapping quality of groundwater nitrate concentrations in areas located far away from monitoring stations. The transfer time of nitrate to the aquifer is one of the reasons that could explain the lack of improvement in water quality. We estimated the travel time of percolating water through the vadose zone. Our second main objective was to identify sources of groundwater nitrate pollution in the study area. Two statistical methods were used to link the measured nitrate concentrations with land and land use attributes. The isotopic signature of groundwater nitrate allowed to identify the source of pollution observed in some monitoring stations, and gave insight about processes affecting the contamination.