Natural radioactivity in drinking water in the surroundings of a metamorphic outcrop in Hungary: interpretation of practical problems in groundwater flow system context

Our study aimed to understand the origin of elevated (>100 mBqL–1) gross alpha activity measured in groundwater-derived drinking water in the vicinity of the Sopron Mountains and Lake Fertő (Neusiedl). Water samples from 10 springs and 7 water wells were analyzed for major ions and trace elements. Total U and 226Ra activity concentrations were determined by alpha spectrometry using Nucfilm discs, and 222Rn activity was measured by liquid scintillation counting. 234U/238U ratio was determined by ICP-MS and alpha spectrometry. Additionally, δ2H and δ18O measurements were performed. To get an insight into the dynamics of the groundwater flow system and to better understand the radionuclide mobilization and transport processes, the geochemical results were evaluated in the groundwater flow system context. Uranium activity was measured up to 540 mBqL–1, thus it can be concluded that dissolved uranium causes the previously measured elevated gross alpha values, though no health risk arises from drinking water consumption. The occurrence of dissolved uranium can be explained by oxidizing conditions that are prevalent along local flow systems and in recharge areas. The relatively short residence time of water, thus the presence of local flow systems is indicated by δ18O (-11.96 to -7.17‰) and δ2H values (-83.4 to -52.6‰). Spring samples have lower uranium activity (up to 93 mBqL–1) than groundwater samples (up to 540 mBqL–1) which can be explained by the longer residence time of water. Uranium is transported along flow paths under oxidizing conditions and the longer the flow route the higher the uranium concentration. This topic is part of a project that has received funding from the European Union’s Horizon 2020 Research and Innovation Programme under grant agreement No. 810980. Besides, the research was funded by the National Multidisciplinary Laboratory for Climate Change, RRF-2.3.1-21-2022-00014 project project. Some radioactivity measurements were supported by the European Commission’s Joint Research Centre (JRC) – Research Infrastructure Access Agreement No. 36227-1/2021-1-RD-EUFRAT-RADMET.