Characterisation of the input signal to aquifers in the French Basque Country: Emphasis on parameters influencing the chemical and isotopic composition of recharge waters

International audienceA hydrochemical and isotopic survey of rainwater and groundwater has been carried out during almost two years on the Ursuya Mount in the northern Basque Country (southwestern France) with the aim of enhancing the understanding of the behaviour of this aquifer and more peculiarly the recharge mode of groundwater. The input signal of this aquifer is defined thanks to 112 rainwater samples. The computed meteoric water line (δD = 7.3 δ18O + 5.1; r = 0.96) is consistent with that defined in the European IAEA/WMO network stations. The weighted mean deuterium excess about 9.7‰ is really close to the value obtained for Atlantic precipitations and clearly demonstrates an oceanic major origin. The computations conducted on the chemical dataset show that the rainwater composition is controlled by four major factors: (1) a mixed source of anthropogenic pollution and crustal material; (2) a marine source; (3) an urban source; (4) an acid source. Further, the quantification of neutralizing potentials clearly revealed below cloud processes in which crustal and anthropogenic components are responsible for the neutralization of anions. Besides the major Atlantic origin of the recharge water, the chemical and isotopic content of the samples coupled with the corresponding air mass back trajectories revealed four major geographical origins of the components: (1) northwestern part of Atlantic Ocean and (2) Southwestern part of Atlantic Ocean. The oceanic influence in airmasses from these origins is highlighted by the stable isotopic content (weighted mean close to the Atlantic Ocean signature) and by the chemical concentrations dominated by sea salt elements. (3) Northern Europe with a continental influence shown by a light depletion on isotopic signal besides purely oceanic origin and a higher concentration of crustal and anthropogenic components. 4) Southeastern area (Southeastern Europe, Northern Africa and Mediterranean Sea) with an isotopic signature consistent with the Mediterranean one and a chemical composition clearly influenced by the industrial activities of this region. The dataset is confronted with the chemical and isotopic characteristics of groundwater in order to assess the recharge water signature of the aquifer of the Ursuya Mount. The groundwater isotopic signal is close to the defined LMWL, showing actual and local recharge. Nevertheless, a slightly difference is observed between the mean value of stable isotopic content in groundwater and in rainwater. The slightly depleted values of groundwater besides rainwater can be attributed to a preferential winter/autumn recharge. An altitudinal gradient about −0.11‰ δ18O/100 m is defined and both water-rock interactions and physicochemical processes fingerprint on the groundwater composition are characterised