Natural radioactivity of thermal springs and related precipitates in Gellért Hill area, Buda Thermal Karst, Hungary.

The elevated radioactivity of the thermal waters of Buda Thermal Karst (BTK), Hungary is known and studied since the beginning of the 20th century. In the recent studies, the anomalous Rn/Ra ratios have drawn the attention to the existence of local Rn source. Biogeochemical precipitates (i.e. biofilms) in spring caves were found to have high adsorption capacity, accumulating e.g. Ra. Biogeochemical precipitates are ubiquitous in the thermal springs of BTK, occurring in different amount and colours (dark grey, brown, red, white), and have different microbial communities and elemental composition. The detailed investigation of the radioactivity of spring waters highlighted the different Ra and Rn activity concentrations. The present study aimed to survey the radioactivity of the thermal springs of Gellért Hill area, together with the biogeochemical precipitates and air above the water level, and to assess the evolution of the radioactivity of known-aged precipitates, formed during in situ experiments. We found that the basic physicochemical parameters of the spring waters (field parameters, major ions) do not affect the adsorption capacity of biogeochemical precipitates. It was revealed by the conducted in situ experiments, that the flow conditions influence the evolution rate of precipitates, so their adsorption capacity. The Rn activity concentrations of spring waters are dependent on the area of the water surface, volume of air space above the water level, ventilation of the caves/channels and presence of calcite layer on the water surface. The latter has a blocking effect on degassing.