Understanding complex volcanic hydrosystems using a multi-tracer approach.

Climate change affects groundwater availability and residence times, necessitating a thorough understanding of aquifer characteristics to define sustainable yields, particularly in regions where water is heavily exploited. This study focuses on the Volvic volcanic aquifer (Chaîne des Puys, France), where groundwater recharge has decreased due to climate change, raising concerns about water use sustainability. To address these challenges, this work proposes a multi-tracer approach, based on hydrogeological monitoring, including the estimation of groundwater ages, major elements chemistry and water stable isotopes to better characterise this resource decrease and more peculiarly its origin and its impact on the environment that has never been addressed. Relative fractions of ancient and modern water contributions (up to 20 %) to the aquifer have been thus estimated as well as the apparent ages of groundwaters (34 years). We highlight the complementarity of tracers used, allowing a better definition of recharge sources and transit times of groundwaters within the aquifer. These results led to the proposal of a hydrogeological conceptual model, highlighting a bi-modal recharge, distinguishing between a long-term recharge upon 30 years, supplemented by a recent component (≃ 1 year) related to annual precipitation. This study provides valuable information on groundwater circulation and the response of volcanic aquifers systems to climate change, while highlighting the importance of assessing residence times. By addressing the challenges posed by systems with contrasting permeability and recharge gradients, it improves understanding of volcanic hydrology and provides a basis for the development of (numerical) hydrological models to assess the impacts of global change.