Groundwater resources vulnerability assessment in an alluvial aquifer subjected to strong anthropogenic contamination: geochemical and multi-isotopic fingerprinting approach

Anthropogenic contamination, such as agricultural or urban activities, has led in less than a hundred years to the degradation of the quality of water resources. To assess the vulnerability of groundwater regarding organic and inorganic pollution (i.e. nitrates, pesticides, and pharmaceutical compounds) isotopic tracers may be used. However, in areas where multiple pollution sources occur, isotopic tracers may not be sufficiently discriminant, making source identification difficult. To overcome this problem, a combination of different tracers, including pharmaceutical compounds, along with the multi-isotopic approach is considered. Here, we present the preliminary results of the interpretation of a multi-tracer approach that show a mixed origin of NO₃^- contamination from agriculture and urban origin as well as the interaction between surface water (Waste-Water Treatment Plant effluent) and groundwater. The multi-tracer approach was applied in the Vistrenque basin area (Gard, France) to characterize NO₃^- sources. A combination of natural and anthropogenic tracers is implemented, including tracers of water origin (major ions, trace elements (Br, Li, and Sr)), nitrogen and oxygen stable isotopes as a tracer of NO₃^- (δ₁₅N/δ₁₈O-NO₃^-), boron stable isotopes (δ₁₁B), water stable isotopes (δ₁₈O/δ₂H-H₂O) and strontium isotopes (⁸⁷Sr/⁸⁶Sr). In addition, a suite of 80 organic molecules including pesticides and pharmaceutical compounds are trialed as tracers of the origin of the contamination. Analyses were carried out on groundwater samples, surface water, and soil samples to characterize the geochemical and isotopic signature of end-members. The dual-isotope approach δ₁₅N/δ₁₈O-NO₃^- for nitrate sources highlights differences between nitrate influenced by nitrification of NH₄⁺ in fertilizer & precipitation origin on one hand, and, nitrates nitrate influenced by manure and septic waste origin on the other hand. However, little contrast between agricultural contamination (i.e. manure) and urban contamination (i.e. sewage) was seen with this method. In parallel, a mixing trend between groundwater and WWTP effluent is evidenced by the combination between δ¹⁵N-NO₃^- and δ₁₁B. Moreover, this trend is confirmed with the occurrence of pharmaceutical compounds and a positive anomaly in Gadolinium (i.e. contrast-agent) in groundwater. As pharmaceuticals are found in several water supply boreholes the potential of pharmaceutic to discriminate the origin of organic nitrate will be investigated.