『Abstract
　This study investigated the use of radon (²²²Rn), a radioactive isotope with a half-life of 3.8 days, and CO2 as natural tracers to evaluate the recharge dynamics of karst aquifer under varying hydrological conditions. Dissolved ²²²Rn and carbon dioxide (CO2) were measured continuously in an underground stream of the Milandre test site, Switzerland. Estimated soil water ²²²Rn activities were higher than baseflow ²²²Rn activities, indicating elevated ²²²Rn production in the soil zone compared to limestone, consistent with a ²²⁶Ra enrichment in the soil zone compared to limestone. During small flood events, ²²²Rn activities did not vary while an immediate increase of the CO2 concentration was observed. During medium and large flood events, an immediate CO2 increase and a delayed ²²²Rn activity increase to up to 4.9 Bq/L and 11 Bq/L, respectively occurred. The detection of elevated ²²²Rn activities during medium and large flood events indicate that soil water participates to the flood event. A soil origin of the ²²²Rn is consistent with its delayed increase compared to discharge reflecting the travel time of ²²²Rn 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 ²²²Rn activities, taking into account the half-life of ²²²Rn (3.8 days). In contrast to ²²²Rn, the CO2 increase occurred simultaneously with the discharge increase. This observation as well as the CO2 increase during small flood events, suggests that the elevated CO2 level is not due to the arrival of soil water as for ²²²Rn. A possible explanation for the CO2 trend is that baseflow water in the stream has lower CO2 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 ²²²Rn and CO2 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.

Keywords: Karst hydrogeology; Soil; Epikarst; Radon; Carbon dioxide; Unsaturated zone』

1. Introduction
2. Study area
3. Sampling and data acquisition
4. Calculations
5. Results
　5.1. Radon and CO2 in soils
　5.2. ²²²Rn and CO2 during base flow and flood events in the underground stream
　　5.2.1. Base flow
　　5.2.2. Small flood events
　　5.2.3. Medium flood events
　　5.2.4. Large flood events
6. Discussion
　6.1. Origin and behaviour of ²²²Rn
　6.2. Origin and behaviour of CO2
　6.3. Quantitative data evaluation
　6.4. Conceptual model
7. Conclusion
Acknowledgements
References