Radionuclides as natural tracers for the characterization of fluids in regional discharge areas, Buda Thermal Karst, Hungary

SummaryThe Buda Thermal Karst (Budapest, Hungary) developed in the regional discharge zone of a carbonate rock aquifer system. High radioactivity of the spring waters has already been reported in 1912, but there has been no detailed study and no consistent explanation for its origin. In this area mixing of cold and hot karst waters was hitherto assigned to be responsible for cave formation. However, the dissimilarity of the discharging waters within Budapest (in the North: Rózsadomb; in the South: Gellért Hill), may suggest also different cave forming processes. The application of radionuclides as natural tracers represents a novel approach to investigate these questions.For this study, we used uranium, radium and radon to identify mixing of fluids in the Buda Thermal Karst system and to infer the temperature and chemical composition of the end members. Chloride as a conservative component allowed the mixing ratios for the sampled waters to be calculated. Their fluid compositions were modeled and through the comparison of modeled and measured values, the end members were validated.As the result of this study, it was possible to characterize the mixing end members for the Rózsadomb area, whereas for the Gellért Hill discharge zone, mixing components could not be identified with the aid of radionuclides. Therefore, it is suggested that different processes are responsible for cave formation in these areas. In the Rózsadomb area, structurally-controlled mixing is the dominant cave forming process, whereas in the Gellért Hill area, due to the lack of mixing members, other processes have to be found, which are responsible for the formation of the caves, such as retrograde calcite solubility and/or geogenic acids, such as H2S. The application of radionuclides thus further supported the differences between the two study areas.This study identified moreover the source of elevated radon content of the waters in the Gellért Hill area in form of iron-hydroxide precipitates that accumulate in the spring caves. These precipitates are highly efficient in adsorbing radium, which generates radon by alpha decay, and hence act as local radon source for the waters.In this study we showed that uranium, radium and radon naturally occurring in groundwater can be used to characterize fluids of different flow systems in regional discharge areas owing to the contrasting geochemical behaviors of these elements.

► Radionuclides are natural tracers for mixing processes in regional discharge areas.
► Radionuclides help to identify and characterize mixing end members.
► We used radionuclides to differentiate the thermal waters of Budapest (Hungary).
► Iron-hydroxides are responsible for extremely high radon levels in spring water.