Origin of high radon levels in karst spring mixed waters – the casestudy of the Capodifiume spring group, National Park of the Cilento and Vallo di Diano -European Geopark (Southern Italy)

Based on results of previous geological, hydrogeological and geomorphological surveys, hydrochemical and isotopic tracer techniques were applied for investigating the origin of significantly elevated radon (222Rn) activity concentrations detected at the highly NaCl / CaCO3 mineralized Capodifiume karst spring group, located near the archaeological site of Poseidon-Paestum, Southern Italy. Compared to other karst springs in the Cilento and Vallo di Diano Geopark both, radon concentration and salinity, show remarkably high values as well as a good correlation. Radon concentrations range between 367±20 and 111±8 Bq/l with an average of 230 Bq/l. The high radon levels of the spring water can neither be explained by the composition of the karst aquifer rocks or the attached travertine deposits (limestone and dolomite), which both show low radium (226Ra) concentrations, nor by the dissolved radium content of the spring water. In the study it was shown that there must be a separate radon source inside the aquifer system close to the spring complex. The results suggest that radon source to be mainly represented by the Miocene, paleo-karst “red clays” (laterites) of the Alburni-Cervati Unit, a local discontinuous aquitard that is rich in iron and manganese oxides / hydroxides. The geological unit, which is widely outcropping in the area surrounding the spring complex, has accumulated radium by adsorption, thus generating radon, which is transported to the spring with the fast flowing groundwater. The temporal and local variability of both, the radon content and the salinity of the spring water, can be explained by a variable mixing process of waters from two different aquifer sub-systems: (i) a “normal” karst aquifer with conduits by-passing the “red clay” deposits and (ii) a “red clay” aquifer that is hydrologically connected to the laterites and that is discharging waters of higher salinity and radon concentration.