The deep geothermal fluids in the Upper Rhine Graben exhibit rather similar chemical compositions. They have a high NaCl content of about 100 g/l. Circulation of geothermal fluid through the rock formations of the Upper Rhine Graben leads to chemical interactions at the liquid solid interfaces. Some of these processes result in the transfer of natural radionuclides from solid to fluid. Due to the variation of the fluid's temperature and pressure as it passes through the surface plant during operation, solid deposits containing trapped radionuclides can accumulate in heat exchangers and filters causing health hazard, disposal cost and adverse effects on the surface installation. Radiological investigations at Bruchsal and Soultz-sous-Forets were performed for the quantitative and qualitative determination of natural radionuclides that may occur in geothermal fluids and in the host rock with the aim to find a link between the specific activity of the source rock and the measured activity concentration in the fluid. The results of the fluid study show a distinct dominance of radium isotopes; however radium appears to be not supported by its parent nuclides. Thus, the geothermal fluid is in a state of radioactive imbalance. This phenomenon is observed for both geothermal sites at Bruchsal and Soultz-sous-Forets. It seems that there are interaction processes for radium release in the reservoir fluids that are working for both geothermal sites despite of different reservoir rocks (sandstone at Bruchsal vs. granite at Soultz). Furthermore, radium could be transported into the geothermal power plant and incorporate in barium/strontium sulphate precipitations. By regular monitoring of the local dose rate all over the surface installation, the plant operator meets its obligation concerning employee safety. In addition, the operator receives site-specific information about the formation of scalings and their growth characteristics, which can be used to determine strategies to avoid precipitations. (c) 2013 Elsevier Ltd. All rights reserved.