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

This study investigated the use of radon (Rn-222), Km 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 Rn-222 and carbon dioxide (CO2) were measured continuously in an underground stream of the Milandre test site, Switzerland. Estimated soil water Rn-222 activities were higher than baseflow Rn-222 activities, indicating elevated Rn-222 production in the soil zone compared to limestone, consistent with a Ra-226 enrichment in the soil zone compared to limestone. During small flood events, Rn-222 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 Rn-222 activity increase to up to 4.9 Bq/L and 11 Bq/L, respectively occurred. The detection of elevated Rn-222 activities during medium and large flood events indicate that soil water participates to the flood event. A soil origin of the Rn-222 is consistent with its delayed increase compared to discharge reflecting the travel time of Rn-222 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-222 activities, taking into account the half-life of Rn-222 (3.8 days). In contrast to Rn-222, 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 Rn-222. 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 Rn-222 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. (C) 2011 Published by Elsevier B.V.