Radon and CO₂ as natural tracers to investigate the recharge dynamics of karst aquifers

摘要

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