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