Multi-tracers Strategy to Define a Conceptual Model for the Coastal Aquifers of Mediterranean Islands, Case Study of the Bonifacio Aquifer (Corsica, France)

Abstract

A hydrochemical and multi-isotope study was conducted to identify the flow paths, the recharge areas and the geochemical processes governing the evolution of groundwater in the Mediterranean carbonate coastal aquifer of Bonifacio (Corsica). The study is aimed at improving the hydrogeological conceptual model based on environmental tracer investigation tools to characterise and quantify the complex aquifer system. Hydrogeochemical parameters and isotope (δ²H, δ¹⁸O, ³H) surveys of rainwater and groundwater have been carried out monthly during two years. A local meteoric water line has been defined and marine, terrestrial and anthropogenic influences on the recharge water hydrochemistry have been described. Preferential recharge during autumn/winter of rainfall is observed and depletion in the isotopic signature for some groundwater samples suggests a recharge at higher altitude from the surrounding granites. A modification of the input signal during infiltration through the unsaturated zone appears and the groundwater hydrochemistry displays differential variation in time and space, with the presence of inertial water bodies in the lower aquifer mainly. Dissolved anthropogenic gases CFCs and SF₆ were used to evaluate groundwater residence time. CFCs have been relevant despite the presence of a deep unsaturated zone and the computed rate of groundwater renewal is pluriannual to multi-decadal. Natural SF₆ was found in granites and has been used as a direct tracer of groundwater origin, highlighting its role in the aquifer lateral recharge. Strontium isotopes (⁸⁷Sr/⁸⁶Sr) were used to improve the knowledge on groundwater mineralization and mixing processes, and are relevant to confirm and quantify the granitic contribution to the aquifer recharge. To improve the quantification of the aquifer water balance terms, submarine groundwater discharges were also studied using aerial infrared images in conjunction with Radon and Radium isotopes (²²²Rn, ^223,224Ra).