Transfer of dissolved gases through a thick karstic vadose zone – implications for recharge characterisation and groundwater age dating in karstic aquifers

This study analyses the impact of a thick vadose zone on gaseous transfers and Excess Air (EA) formation using dissolved noble and anthropogenic gas tracers (Ne, Ar, N2, CFC, SF6, O2 and CO2). Understanding gas transfers integrates the issues of equilibrium and water/air exchanges along the vadose zone. In this study, the noble and anthropogenic gases were analysed in air and water over vadose zone depths of 120 m and over two contrasted hydrodynamic periods therefore enabling a rare insight into the spatial and seasonal variations of equilibrium and water/air exchanges in a heterogeneous karst environment. The results highlight that both noble and anthropogenic gases of the caves are in equilibrium with the atmosphere up to at least depths of 120 m. In water, the Noble Gas Temperature (NGT) remains stable at 8°C whilst some EA is formed (up to 1.3 cm3 STP/kg at 120 m) during transfers in the vadose zone. The high EA value measured at the spring outlet cannot be explained by an increase during transfers via the vadose zone. This high EA value is due to rapid and high water level variations during recharge. The implications of these results are important to advance our understanding in the use of noble and anthropogenic gases in characterising recharge and quantifying groundwater ages. In particular, these results help to identify the parts of the aquifer actively involved in dissolved gas transfers, highlight the futility of using a time-lag factor for groundwater age dating in karstic systems, and improve our understanding of the EA formation processes.