Résumé
The management of surface and groundwater resources requires to define river watershed and groundwater recharge zones. Rainfall that infiltrates the recharge zone becomes groundwater, which can feed a local or regional flow. Regional flows represent a generally larger body of groundwater, which may be totally disconnected from the local river, thus becoming a resource of major interest. In Mediterranean karst aquifers, this type of regional flow is common, firstly because the geological structures are favorable (thick carbonates, structural discontinuities that connect superimposed or juxtaposed aquifers), and secondly because karst networks have been able to develop at depth towards paleo-outlets, for example in karsts linked to the Messinian event that drained the Mediterranean Sea.When withdrawing groundwater with boreholes, it is therefore important to know what type of water the well intercepts. The challenge is to propose a method that can differentiate regional from local water flows. This is a major challenge, as we are often dealing with the same types of rock, which are therefore likely to give a similar geochemical signature as regard to major ions. Among the vast array of existing geochemical tracers, some are complex or expensive to analyze, such as strontium isotopes or water sulfate isotopes. So, although they are an interesting source of information on water-rock interaction processes, they are not systematically used. On the contrary, water isotopes (d2H and d18O) have the advantage of being easy to sample, and relatively easy to analyze. These isotopes are known to be good markers of recharge altitudes, or of mixing between water masses, or of the origin of water in relation to rainfall. In this study, we present a groundwater characterization approach based on the d2H versus d18O plot, applied to the case study of the Huveaune watershed and the Port-Miou hydrogeological catchment area in south-eastern France, covering a vast area of around 500 km². Over a two-year period, 23 water sampling points (river, spring, borehole) were monitored for a total of 97 analyses, under a variety of hydrological conditions (low-water, flood, base-water, high-water). Water groups characteristic of local and regional groundwaters were thus established. Water recharged in the highest altitudes (between 500 and 1150 m above sea level) is characterized by a depleted isotopic signature, enabling it to be identified when collected by boreholes in the plains close to sea level. Four water groups were thus characterized. The graphical tool enables water managers to locate the water withdrawn in new boreholes, so as to easily assess the water mass intercepted by the borehole. We show that long-term monitoring of water isotopes in drinking water could help improve water management.