Abstract
The aim of this thesis is to use dissolved gases to develop new tracers, which provide information on groundwater recharge and circulation conditions in karst aquifers. However, the structural characteristics of karst aquifers, such as the possible presence of an epikarst aquifer, a thick unsaturated zone, air-filled cavities at depth and rapid and significant recharge, pose scientific obstacles to the use of gaseous tracers in a karst context. These barriers raise fundamental questions about the air-water equilibrium processes during water transfer along the unsaturated zone and the formation of excess air (EA). To answer these questions, two study sites were used: the Durzon catchment area (Causse du Larzac) developed in a predominantly dolomitic lithology and the Lez catchment area (North of Montpellier) developed in a predominantly calcareous lithology and subjected to anthropogenic pressure (pumping for drinking water supply). These pilot sites are part of national observatories (SNO H+ and SNO Karst respectively) integrating the OZCAR organisation (Observatoires de la Zone Critique - Applications et Recherche). This thesis work was based on measurements made at different time and space scales: i) point measurements during hydrological cycles VS continuous measurements during a flood; ii) measurements at the spring integrating information from the whole catchment area VS more local measurements in a borehole or in karst cavities integrating information at the drain or matrix scale. The dissolved gas signatures suggest that the air in the Durzon vadose zone conduits has a similar composition to atmospheric air down to 120 m depth. In addition, dissolution of the gases appears to occur in a near-surface horizon (i.e. the sorage epikarst). The monitoring of the AE signal and its modelling at the Durzon spring suggests that the process of trapping air bubbles is mainly related to pressure variations that occur at the top of the saturated zone of the karst. On the other hand, the continuous monitoring carried out at the Lez spring in partnership with Géosciences Rennes and the CondatEau analysis platform using the CF-MIMS (Continuous Flow Membrane Inlet Mass Spectrometer) made it possible to initiate an approach for dating a deep flow from 4He data calibrated by CFC data. Monitoring of the EA signal at the Lez spring suggests that active management of the Lez karst may have an impact on the complex evolution of this signal.