Estimation of groundwater ages, recharge and transfers times in volcanic aquifers: Advantages and interests of multi-tracer approaches (3H, CFC-SF6, 18O/2H) coupled to hydrogeological data in the management of water resource of the Volvic watershed (FR).

Understanding the hydrogeological functioning of aquifers is essential in contexts where water resources are intensively used. Moreover, climate change can have long-term effects on groundwater in terms of availability, residence and transit times. Thus, careful management of groundwater resources require the understanding of the aquifer’s characteristics that can allow then the setting of sustainable yields values in contexts where water is exploited. This understanding requires in particular the estimation of the age of the groundwaters as well as the transfers/transit times within the aquifers. Our study focuses on the Volvic volcanic aquifer (Chaîne des Puys, France), where the question of water use has increasingly raised for several years, given the significant use of drinking water, both for the public drinking water network and bottled water, and the decrease of precipitations (and groundwater recharge) over the watershed due to climate change.

Previous studies on Volvic watershed allow defining the overall functioning of the system and comparing withdrawals and recharge on an annual scale, but groundwater ages have been only roughly defined even if they appear as a key point for addressing the question of the resource decrease. We propose then a multi-tracers approach, based on hydrogeological monitoring (hydrodynamical and meteorological data’s), including the estimation of groundwater ages (CFCs, tritium (³H)), major and traces elements chemistry and water stable isotopes (¹⁸O/²H) to better characterise this resource decrease and more peculiarly its origin and its impact on the environment that has never been addressed.  The relative fractions of modern and ancient water contributions to the Volvic aquifer will thus be estimated as well as the apparent ages of groundwaters. We highlight here the complementarity of tracers used in the dating of waters, which allows a better definition of recharge sources and flow paths within the aquifer.

This will provide key information about the time of the recharge and the time when the decrease began due to increase of abstraction, climate change or a combination of both of these effects.