Understanding Complex Volcanic Hydrosystems in the Light of Climate Change and Water Usages Through Multiproxy Approaches Coupling Environmental Tracers to Geological and Hydrogeological Investigations

Abstract

Climate change can have long-term effects on groundwater in terms of availability and residence 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 intensively exploited. This understanding requires in particular the estimation of the age of the groundwaters as well as the transit times within the aquifers. Focusing on the Volvic volcanic aquifer (Chaîne des Puys, France), where the question of water use has increasingly raised for several years given it significant uses and the decrease of groundwater recharge over the watershed due to climate change, this study aims at highlighting the importance of defining groundwater residence times in complex volcanic aquifers facing global change. 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 modern and ancient water contributions to the aquifer have been thus estimated as well as the apparent ages of groundwaters. 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 distinction between long-term recharge, supplemented by a recent component. Looking beyond the Volvic aquifer system, the study provides new insights on groundwater circulations in volcanic systems in continental areas and the impact of residence time on the reactivity of this type of aquifer to climate change. In this way, the study constitutes a baseline for prospective hydrological models in relation to the effects of global change.

Keywords: Hydrochemistry, Chlorofluorocarbons (CFCs), Tritium, Mean residence time, Volcanic aquifers, climate change