Coupling geochemical and isotopic tracers (δ18O, δ2H and 87Sr/86Sr) to quantify groundwater mixing in carbonate environment

Carbonate aquifers are known as a major source for drinking water in the Mediterranean region. Therefore, qualitative and quantitative estimation of the groundwater resource are crucial, especially in area with densely populated areas.

In this study, geochemical and isotopic tracers (δ¹⁸O, δ²H and ⁸⁷Sr/⁸⁶Sr) have been used to discriminate different origins of groundwater and evaluate water-mass mixing in a Mediterranean carbonate environment. The case study, located in south-eastern France, offers an attractive geological context to explore strontium isotopic tracers: trias to cretaceous carbonate rocks, that have been eroded and locally resedimented in fluvial deposits during Oligocene extension and Quaternary periods. Moreover, many karst features act as sinking zones, generating fast infiltration. The 500 km² zone investigated has also a large range of elevation, from the sea to 1148m, giving an expected significant contrast in water isotopes. Monthly water samples have been collected in-situ for two years on ground and surface waters, in rivers, boreholes as well as karstic springs. Monthly rainwater samples provide the isotopic signal of the recharge at two distinct elevations and distance from the sea.

The hydrogeochemical signatures allow to discriminate the end-members of two main waterbodies as well as a slight marine influence on one borehole. Nitrates concentrations (NO₃^-) highlight anthropogenic influence on groundwater resources. A multi-tracer approach taking in account water-rock interaction, mixing processes and recharge, coupled with hydrogeological conditions and dye-tracing tests, leads to a conceptual model of this complex hydrosystem.