The relationships between stratigraphic and tectonic setting, recharge processes and underground drainage of the glacierised karst aquifer system 'Tsanfleuron-Sanetsch' in the Swiss Alps have been studied by means of various methods, particularly tracer tests (19 injections). The area belongs to the Helvetic nappes and consists of Jurassic to Palaeogene sedimentary rocks. Strata are folded and form a regional anticlinorium. Cretaceous Urgonian limestone constitutes the main karst aquifer, overlain by a retreating glacier in its upper part. Polished limestone surfaces are exposed between the glacier front and the end moraine of 1855/1860 (Little Ice Age); typical alpine karrenfields can be observed further below. Results show that (1) large parts of the area are drained by the Glarey spring, which is used as a drinking water source, while marginal parts belong to the catchments of other springs; (2) groundwater flow towards the Glarey spring occurs in the main aquifer, parallel to stratification, while flow towards another spring crosses the entire stratigraphic sequence, consisting of about 800m of marl and limestone, along deep faults that were probably enlarged by mass movements; (3) the variability of glacial meltwater production influences the shape of the tracer breakthrough curves and, consequently, flow and transport in the aquifer.

Keywords:Karst hydrogeology Soil Epikarst Radon Carbon dioxide Unsaturated zone s u m m a r y This study investigated the use of radon ( 222 Rn), a radioactive isotope with a half-life of 3.8 days, and CO 2 as natural tracers to evaluate the recharge dynamics of karst aquifer under varying hydrological conditions. Dissolved 222 Rn and carbon dioxide (CO 2 ) were measured continuously in an underground stream of the Milandre test site, Switzerland. Estimated soil water 222 Rn activities were higher than baseflow 222 Rn activities, indicating elevated 222 Rn production in the soil zone compared to limestone, consistent with a 226 Ra enrichment in the soil zone compared to limestone. During small flood events, 222 Rn activities did not vary while an immediate increase of the CO 2 concentration was observed. During medium and large flood events, an immediate CO 2 increase and a delayed 222 Rn activity increase to up to 4.9 Bq/L and 11 Bq/L, respectively occurred. The detection of elevated 222 Rn activities during medium and large flood events indicate that soil water participates to the flood event. A soil origin of the 222 Rn is consistent with its delayed increase compared to discharge reflecting the travel time of 222 Rn from the soil to the saturated zone of the system via the epikarst. A three-component mixing model suggested that soil water may contribute 4-6% of the discharge during medium flood events and 25-43% during large flood events. For small flood events, the water must have resided at least 25 days below the soil zone to explain the background 222 Rn activities, taking into account the half-life of 222 Rn (3.8 days). In contrast to 222 Rn, the CO 2 increase occurred simultaneously with the discharge increase. This observation as well as the CO 2 increase during small flood events, suggests that the elevated CO 2 level is not due to the arrival of soil water as for 222 Rn. A possible explanation for the CO 2 trend is that baseflow water in the stream has lower CO 2 levels due to gas loss compared to water stored in low permeability zones. During flood event, the stored water is more rapidly mobilised than during baseflow with less time for gas loss. The study demonstrates that 222 Rn and CO 2 provides value information on the dynamics of groundwater recharge of karst aquifer, which can be of high interest when evaluating the vulnerability of such systems to contamination.

Dans le cadre de l 'établissement des zones de protection du captage de Glarey, l 'une de principale source pour l 'alimentation en eau potable de la commune de Conthey (Valais, Suisse j, une série de 12 essais de traçage a été réalisée sur le bassin d'alimentation de la source entre 2005 et 2006. Le but de ces essais était de délimiter le bassin d 'alimentation de la source sur le karst de Tsanfleuron. D'une structure géologique relativement simple -le pendage des calcaires urgoniens de 5° à 20° en direction de l'est et sud-est, implique un drainage des eaux en direction de la source de Glarey -le karst de Tsanfleuron et pourtant entouré de plusieurs sources présentant des débits importants (Sarine, Tschoetre, Marnes) dont les bassins d 'alimentation étaient jusqu 'à maintenant inconnus. Les essais de traçage ont permis de mettre en évidence la contribution du karst de Tsanfleuron dans l 'alimentation de ces différentes sources et ainsi de délimiter la surface du bassin d 'alimentation de la source de Glarey.

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