Isotope tracing (δ15N & δ18O of NO3; δ11B) of diffuse pollution sources in complex aquifer systems

摘要: The Sevre-Niortaise watershed, upstream of the Niort City and Poitevine Marsh ("Green Venice", France) is under strong anthropogenic pressure, especially with diffuse pollution from agricultural activities. This watershed representative problem nitrate contamination on a large scale NO3 values in surface groundwaters often far exceeding quality standard 50 mgNO3.L-1. led to exemptions for drinking water supply many catchments. Nitrate concentrations can vary greatly over yearly hydrological cycle, but following mechanisms that are still poorly understood, given very complex structure terms hydrogeological exchanges between groundwater network. results close relationship rivers also used supply. specificity this area consists multilayer limestone aquifers mainly composed Infra-Toarcian Dogger layers, partly karstified, isolated by pseudo-impermeable layer, Toarcian marls, crossed numerous faults. multi-isotope approach (δ15N & δ18O NO3; δ11B 87Sr/86Sr) together classical chemical tools presented. They constitute first step project aims develop methodology assess evolution impact (agricultural / urban) waters. understanding functioning (1) relations aquifer layers all along (2) both spatially temporarily, (3) origin pollutions, according analysis local potential sources (organic mineral fertilizers, sewage effluents) (4) assessment residence time; will then be as constraints numerical modeling. Different modeling tested reproduce content cycle evidenced isotopic approach. could predictive tool evaluate reduction pressure quality. final deliverable consist based design monitoring survey, applied accurate specific geological context.