Radionuclide content of groundwater in hydrogeological approach. Case study in the adjacent area of a granitic complex

Groundwater is a very common drinking water source. In Hungary, 88% of the water supply is sourced from groundwater. Radioactivity of groundwater, as a possible threat and its hydrogeological background, had not been widely investigated until recently. Following the Euratom Drinking Water Directive the radioactivity of drinking water is screened in Hungary in the last three years by gross alpha and gross beta activity measurements. Whenever the measured concentrations surpass the limit values the long-term consumption of the water can lead to health issues. Based on data provided by the National Public Health Institute high values of gross alpha activity can be found in the southern foreland of Lake Velence. Previous studies already showed high uranium concentration values (compared to average crust values) related to the Velence Granite Formation in Velence Hills and to the carbonatic and organic-rich beds of the Ujfalu Formation in the southern foreland of Lake Velence. Until recently no observations and measurements were made regarding the radioactivity of the groundwater. Therefore, uranium, radium and radon concentration measurements were carried out in the adjacent area and interpreted in flow system context. A total of 53 samples were taken from surface water (springs and lakes) as well as from groundwater. Alpha spectrometry applied on Nucfilm discs was used to measure the uranium (234U+238U) and radium (226Ra) activity while radon (222Rn) activity was determined by TriCarb 1000 TR liquid scintillation detection. Pressure-elevation profiles, hydraulic cross sections, tomographic potential maps and potential difference maps were compiled to understand the groundwater flow directions and regime characteristics in the wider area. The areal distribution of the activity concentration values was interpreted regarding the groundwater flow system, physicochemical parameters measured onsite and in the laboratory. Those areas can be delineated where according to the flow conditions and the related geochemical environment the mobility of the uranium or radium and thus elevated activity concentration can be expected in groundwater. This novel approach may facilitate safe water management of drinking water supply systems. This study was supported by the ÚNKP-17-4 and ÚNKP-18-3 New National Excellence Program of the Ministry of Human Capacities.