Natural radioactivity in drinking water in the surroundings of a metamorphic outcrop in Hungary: the hydrogeological answer to practical problems

Groundwater quality constantly evolves through rock–water interactions, which can enrich groundwater with undesirable elements such as naturally occurring radionuclides. The aim of this study was to understand the cause of gross alpha activity exceeding the screening value of 0.1 Bq L−1 measured in groundwater-derived drinking water in the vicinity of a metamorphic outcrop in Hungary. As groundwater quality is strongly dependent on the properties of groundwater flow systems, environmental tracers (δ2H and δ18O composition, 226Ra, 222Rn, total U activity concentration, and 234U/238U ratio) and hydraulic evaluation were applied to understand groundwater dynamics. The collected groundwater samples had total U activities up to 540 mBq L−1, which translates into an indicative dose below the drinking water parametric value. However, in the presence of dissolved uranium, the δ2H (−52.6–(−83.4)) and δ18O (−7.17–(−11.96)) values led to the conclusion that local flow systems were sampled that are known to be most vulnerable to any changes in their recharge area. The results confirm that the groundwater flow system approach involving environmental tracers and hydraulic evaluation is a powerful tool for identifying the cause of natural radioactivity in groundwater-derived drinking water.