Status of the geogenic radon potential investigations in Bulgaria

Radon (222Rn) is a radioactive gas and formed as a result of the radioactive decay of radium. 222Rn relieved from the ground could accumulate in the building and contribute to human exposure. Exposure to indoor radon and its decay products contributes to half of the annual dose received by the public from all natural radioactive sources. Radon is recognized as a carcinogenic agent by the WHO and is the second leading cause of lung cancer after tobacco smoke. The radon concentration in buildings and the exposure to radon depend on many factors, but it can be assumed that geology is the main factor influencing the variation of indoor radon. In this regard, the geogenic radon potential (GRP) of the terrain is the probability of the presence of high radon concentration in a building, the genesis of which is directly related to the influence of the earth's surface, and not e.g. from building materials. In addition, there is a concept “radon index”, which is used to characterize GRP. One of the approaches for quantifying the radon index is based on a multivariate cross-tabulation, which includes two parameters – radon concentration in soil gas and gas permeability of the earth layer. Till 2019, complex studies concerning correlation between the measured radon concentrations and bedrock geology in Bulgaria are only scarce and quite general. In the last three years, there are attempts for detailed investigations for evaluation of the bedrock and superficial geology associating with radon potential or/and radon index determinations. Therefore, research works on geogenic radon so far in Bulgaria can be divided into two main groups: regional research works related to the "radon-rock" relationship and local research aimed at determining the radon index at specific sites. In the first group appertain investigations about defining the rock formations in Bulgaria, in their outcrop presence, with possible high radon potential based on geological and published more general “radon–rock correlations” data. Based on that, a GIS based map for the spatial distribution of the particular rock types in accordance with the expected radon potential is made. Also to this group belongs a study concerning spatial sampling design for the use of the polygons (Sliven Province) as the unit of sampling, data collection, and analysis and reduces the number of observations, as well as would optimize the gathering, analysis and systematization of the data for the preparation of a methodology for the preparation of geogenic radon potential map. In the second group appertain investigations of the radon index determination at the two pilot sites affected by fault systems in Sofia.