Natural radioactivity in drinking water: application of environmental tracers and regional groundwater flow analysis in northwestern Hungary

Groundwater accounts for 92% of drinking water supply in Hungary, making natural radioactivity in aquifers a major concern for drinking water quality. This study investigates the causes of gross alpha activity exceeding EU parametric value of 0.1 Bq/L in northwestern Hungary using a regional groundwater flow system approach. Geochemical sampling of springs, drinking water wells and thermal wells was carried out in 2021 and 2024. In-situ physicochemical parameters such as pH, redox potential, temperature, and electrical conductivity were measured. Laboratory analyses were carried out wher also the activity concentrations of total U, ²²⁶Ra, and ²²²Rn were measured. Pressure–elevation profiles were used to characterize local and regional groundwater flow conditions. In drinking water samples, total U activity reached up to 540 mBq/L, which can be linked to the local geogenic sources of the Sopron Mountains and adjacent Pannonian sediments. Radionuclide-specific measurements indicated that elevated gross alpha activity in several drinking water wells is primarily due to dissolved uranium under oxidizing conditions in local flow systems and recharge areas. Activity concentrations of ²²⁶Ra and ²²²Rn were generally low, with one sample showing 301 mBq/L of ²²⁶Ra and 219 Bq/L of ²²²Rn. The presence of radium may be attributed to regional flow systems, whereas the high radon activity cannot be explained solely by in-situ decay of radium and warrants further investigation. The results demonstrate that the integration of hydrochemistry, environmental isotopes, and groundwater flow system analysis is a powerful tool for understanding the causes of natural radioactivity in groundwater-derived drinking water.