Impact of the geological and hydrogeological settings on the radon potential in Bulgaria

Natural radon (222Rn) is a radioactive noble gas that occurs in every rock or soil due to the content of radium (226Ra), part of the 238U family, in the lithosphere. One of the indicators for evaluating the impact of radon on a certain area/terrain, including on the biota, is the so-called geogenic radon potential. Different types of rocks and soils possess different 226Ra content and different permeability. Radon gas has high mobility and is driven by diffusion and convection with the soil gas throughout connected and water-unsaturated pores and/or cracks in permeable rocks and soils. Therefore the geogenic radon potential at the surface could be affected by the geological settings as a content of uranium and also by the hydrogeological settings as a degree of saturation near the surface. The present study mainly deals with the different by genesis shallow groundwater in Bulgaria based on the published since 1960’s sources. After analysis of the collected data several distinct regions have been determined regarding lithological and geomorphological conditions, and the 0–3 m depth of the groundwater table. In addition, a GIS-based map was elaborated relating the rocks with expected geogenic radon potential and shallow groundwater areas.