Résumé
Found in the atmosphere, biosphere and geosphere, water is a link for all compartments in the Critical Zone. Therefore a better understanding of each part of water cycle is necessary, only the study of groundwater may be sometimes complex. Mechanisms that occur during recharge and transfer through the vadose zone remain difficult to understand. Depending on the type of aquifers and its degree of heterogeneity, some classical methods used in hydrogeology can be difficult to apply, then new methods based on hydrodynamic or natural tracing still need to be developed. Among these tracers, excess air (EA) derived from dissolved gases (Ne, Ar, N (sub 2) ) could give key information about processes occurring in the vadose zone. EA is formed by entrapped air bubbles in the vadose zone after a rapid increase in groundwater levels. Hydrostatic pressure caused by the water level on air bubbles forces gas dissolution, which increases the gas concentration compared to natural equilibration from the simple application of Henry's law. Quantification and study of EA could thus provide information on recharge and mechanisms that occur during transfers from the surface to the discharge area. Application of this tracer in karstic aquifers is ideal, since groundwater levels can undergo rapid increases during recharge events. This project aims to improve our comprehension of EA formation and evolution along the vadose zone in karst systems to use EA as a proxy of recharge processes. We develop a quantitative EA approach by coupling gas data with hydrodynamic data. We focused on the Durzon system located in SE France and characterized by a thick vadose zone developed in dolomite. Monthly noble gas measurements of the Durzon spring were conducted between 2010 and 2011. In addition, the evolution of noble gases and EA in the vadose zone was investigated at depths, ranging from -5m to -390m depth for high and low flow conditions. The results show that EA increases with depth through the vadose zone from 0 cm (super 3) STP/g at the near-surface to 2 cm (super 3) STP/g at -120m deep in the Durzon system. However, maximum values of EA at the spring can reach 10 cm (super 3) STP/g during low flow periods. Anti-correlation between EA and spring flow has been observed and suggests links between the hydrological cycle and EA. This EA evolution is linked to hydrodynamic processes using lumped parameters model.