Radionuclides as natural tracers of the groundwater flow systems in the thermal karst system of Budapest, Hungary

Global changes are increasingly pushing for more sustainable resources, such as geothermal energy. The thermal waters of the karst system in Budapest (Hungary) have so far been used mainly for balneological purposes. It is thus expected that in the near future they will more intensely be used for geothermal purposes as well. To this end, a comprehensive research project has been launched by the Geological Survey of the Supervisory Authority for Regulatory Affairs to gain a better understanding of the system and to assess the impact of future projects via numerical simulations. One task of this research was to perform a geochemical water sampling campaign to gain simultaneous geochemical results and characterize a baseline. In this project, also the most abundant natural radioactive isotopes in groundwater, uranium, radium and radon were measured, which were previously successfully used in the natural discharge areas to characterize fluids of different flow paths. Now, these natural tracers were applied in the entire regional study area. 

Liquid scintillation technique was used to measure the activity concentration of radon. For radium and uranium, an innovative method, selectively adsorbing Nucfilm discs measured by alpha spectrometry was applied.  

Based on the first results, the radon content in water samples was either below the detection limit or less than 40 Bq/L. However, 284 Bq/L activity concentration was measured in one location, which is high compared to the 100 Bq/L value for drinking water. Fluid mixing was hypothesized here. 

Uranium activity concentrations were also low (8-17 mBq/l), which is associated with the mostly reductive conditions of the sampled groundwater. The reducing environment of higher order flow paths is confirmed by our sampling results by the measured higher radium contents. The radium content of the samples ranged from 10 to 1823 mBq/L. The highest radium content was found in sample BPGT-08, which with its high dissolved solid content suggests the presence of waters with a long groundwater residence time and a possible connection with organic matter (hydrocarbons). The other radium activities show similar values to those measured by previous studies in the region. For two water wells, we contributed to the interpretation of the total alpha measurements by measuring the activity concentrations of radium and uranium as alpha decay radionuclides. Our measurements allowed for the characterisation of groundwater flow systems, the identification of different geochemical environments and possible fluid mixing.