Chlorine Stable Isotope and Helium Isotope Studies Contributing to the Understanding of the Hydrogeochemical Characteristics of Old Groundwater

Abstract Chlorine stable isotope and helium isotope data were integrated in order to study the chemical and isotopic characteristics of old groundwater in the Pannonian Basin. Former studies showed groundwater is of meteoric origin, based on oxygen and deuterium isotopes, which have isotopic signatures of formation waters from the deeper part of the basin. Depletion of the heavier 37 Cl isotope can be observed along the flow paths. Despite the fact that most of the sampled groundwater contains high helium levels, indicating a crustal helium origin, their 3 He content is also high. The 3 He/ 4 He ratios (expressed in R/R ac ) with values between 0.4 and 1.2 shows that helium of mantle origin must also be a component in the groundwater along the flow paths. The presence of mantle origin helium could be detected not just in the deeper part of the basin, but even in the Quaternary sediments.