Investigation of naturally occurring radionuclides in a riverbank filtered drinking water supply system

Riverbank filtered drinking water supply systems are strongly dependent on the river stage. Climate change-induced extremely low or high river stage may cause water quantity and quality problems. In this study, a riverbank-filtered drinking water supply system along the Danube River was investigated from a radioactivity point of view: we aimed to understand the origin of elevated (>100 mBq L-1) gross alpha activity measured in some wells and the variation in water quality with river level fluctuation. 10 producing, 2 monitoring wells, and the Danube were sampled at lower and higher river stages. The water samples were analyzed for major ions and trace components. Total U (234U+235U+238U) and 226Ra activity concentration were determined by alpha spectrometry using Nucfilm discs, and 222Rn activity was measured by liquid scintillation counting. Total uranium activity was measured in the highest concentration (up to 334 mBq L-1). Radium and radon activities were barely above the detection limit. Based on our results the previously measured elevated gross alpha activity is most likely caused by dissolved uranium in the groundwater. Uranium activity concentrations show increasing values from N to S which corresponds well to the occurrence of organic matter-rich, clayey floodplain deposits underlying the aquifer. Besides spatial variation, a temporal change can also be observed: lower uranium activity was measured at a lower river stage (32-248 mBq L-1) compared to a higher river stage (26-334 mBq L-1). This phenomenon could be explained by the dynamic relationship between the groundwater and the river. At the low river stage, oxygen-rich (ground)water flows from the river toward the inland and may cause the remobilization of uranium from the clayey basement layers. This process will be more and more dominant by extremely low river stages during long-lasting drought periods in the future causing water quality problems. The research was funded by the National Multidisciplinary Laboratory for Climate Change, RRF-2.3.1-21-2022-00014 project.