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

Abstract

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.

Improved predictive mapping of indoor radon concentrations using ensemble regression trees based on automatic clustering of geological units

“…Carbonate rocks, especially limestone, are subject to weathering, which is also called karstification. Karst formations are often characterized by large cave systems that facilitate the propagation of radon gas (Savoy et al, 2011). Few explanations exist however for high uranium or radium abundances in carbonate rock.…”

Karst water resources in a changing world: Review of hydrological modeling approaches

“…More recent monitoring approaches also include parameters such as dissolved gases [ Klump et al ., ; Savoy et al ., ], trace metals [ Vesper and White , ], turbidity, and particle size distribution [ Pronk et al ., ], as well as microbiological variables [ Pronk et al ., ]. These variables yield considerable information concerning karst aquifer dynamics and transport processes, though this knowledge has not yet been fully transferred to modeling.…”

Review: Natural tracers in fractured hard-rock aquifers in the Austrian part of the Eastern Alps—previous approaches and future perspectives for hydrogeology in mountain regions

“…The approach to using CO 2 as an indicator for recharge areas has previously been tested only in karst aquifers White 1971, 1972;Fank et al 1989;Harum et al 1992;Benischke et al 1996;Savoy et al 2011). Due to the correlation of soil thickness and pCO 2 one can assume that in fractured aquifers with a soil zone and an unsaturated zone of sufficient thickness, premise 2 (independency of seasonal influences) should be fulfilled as well.…”