A multidisciplinary study to evaluate the sustainability of a volcanic hydrosystem: Chaîne des Puys’s watersheds use by Clermont Auvergne Métropole for drinking water supply

Clermont Auvergne Métropole is an agglomeration of 300,000 inhabitants. Two types of aquifers are exploited for the drinking water supply of this population: the alluvial aquifer of Allier River (70%) and three volcanic water basins located in the Chaîne des Puys (30%). Recent studies have shown that the quantity of water in the Allier alluvial aquifer decreases drastically during drought periods and that this decrease will amplified in the future due to climate changes (2022 for example). Water managers of Clermont Auvergne Metropole are thus interested in identifying the potential of their volcanic resources to secure the water supply of the population. In this purpose, a multidisciplinary study, using existing data and providing new ones has been projected. This study includes: 1) the use of the geological model previously established by Aumar (2022) at the Chaîne des Puys scale, that allows to constrain the geometry of aquifers; 2) the acquisition of hydrochemical data that help to better define the functioning of the volcanic watersheds; 3) the measurement of hydrodynamic and meteorological data to better define the terms of the hydrological balance. Natural tracers such as stable water isotopes (¹⁸O and ²H) or major and trace elements give information on the origin of those groundwaters (local or remote) and in particular their infiltration zone (average altitudes), their flow paths or the impact of various external processes (environmental or anthropic). Groundwater dating methods (CFC, Tritium) brings a constraint on the residence time of water within the aquifer. Yet, groundwater ages are still unknown in all watersheds of the Chaîne des Puys, but it remains essential to understand groundwater flow and storage. The coupling between hydrodynamic monitoring, a large panel of hydrochemical tracers and groundwater residence time data associated with a well constrained geological model allows to provide relevant management tools to stakeholders both in terms of quantification and protection of their resources.