Combining residence time and isotopic tracers for a better understanding of the fresh and marine water recharge and groundwater flux of a karstic thermal aquifer

Karst hydrosystems are complex systems where the use of water resources is often shared by many actors. The addition of thermal and/or marine components in these systems makes classical methods of investigation difficult to apply. The combined use of geochemical tracers can allow a better understanding of the complex hydrogeological system and application of integrated water resources management. The thermal karst aquifer of Thau (South of France) illustrates the complexity of karst hydrosystems since it is a zone of convergence of young and shallow karst water, hot and mineralized thermal water and marine water (Thau Lagoon and/or seawater). Moreover, recurring phenomena of saltwater intrusion from the Thau Lagoon (i.e. inversac) into the freshwater aquifer can occur, modifying the type of circulation. It is proposed in this study, supported by the Dem'Eaux Thau project (2017-2022), to combine, with classical geochemical tools, tracers of water-rock interaction processes (Sr, Li) and residence time tracers (CFC, SF₆, ³H, ⁴He, ¹⁴C, ³⁶Cl) to better understand the origin of fluxes and to characterize the mixing that takes place in the system. 
Isotopic tools (Sr, Li) allowed to better constrain the thermal reservoir: thermal water flows mainly in a carbonate aquifer (⁸⁷Sr/⁸⁶Sr), fed by deeper flows from bed-rock ([Li]). The residence time of this water estimated by the different tracers (⁴He, ¹⁴C, ³⁶Cl) is consistent and indicates ages of several thousands of years (10 000 to 50 000 years). This water subsequently feed a shallow reservoir (100-300m) through local fractures and mix with variable proportions of recent karst flows. These flows are mixtures of regional flows of about 30 years affected by thermonuclear tests (CFC, SF₆, ³H, ¹⁴C, ³⁶Cl) and more recent local flows contaminated by local anthropic activity (SF₆, NO₃^-). During the inversac phenomenon, fluxes and mixtures are modified bringing a saltwater component to the system. Geochemical tools allow us to quantify the impact of the inversac and to characterize the spatial impact of such contamination by highlighting the connections between the intrusion zone and the different monitoring points.