An approach to relate uranium to indoor radon: a case study from the western Ligurian Alps (Italy)

Radiometric surveys are used to investigate different scientific and practical issues in Earth’s science studies, ranging from basic geophysics to mineral exploration and natural radiation monitoring. The latter is a topic of growing interest. ²³⁸U, ²³²Th and ⁴⁰K, which occur in variable quantities in the Earth’s crust rocks, are the primary cause of natural, potentially hazardous, gamma-ray exposure. Another important environmental aspect of natural radioactivity regards the effects of  ²²²Rn resulting from the ²³⁸U decay. This paper proposes an experimental approach to investigate the relationship between uranium and indoor radon concentration. We combined ground gamma-spectrometry with alpha-track detector measurements. We tested this methodology in the Alpine geological units of western Liguria (Italy). This area is densely populated and characterised by various lithotypes, spanning from sedimentary to metasedimentary and metavolcanic rocks. The latter are known for their abundance of natural radionuclides. Due to the width of the surveyed area (408 km²), we carried out about 300 gamma-ray determinations on the more extensive geological formations, with particular reference to those hosting the main residential areas, together with about 130 measurements of indoor radon. By considering the nineteenth percentile of the recorded specific activity, the largest value of ²³⁸U was 93 Bq/kg. It was found in the more acid metamorphic rocks (metarhyolites and porphyric schists) where we carried out also the largest number of gamma-ray spectrometry measurements. In the metasedimentary rocks, the largest activities of ²³⁸U were observed in quartzschists and micaschists (89 Bq/kg). In the sedimentary lithotypes, specific activities are generally lower than 40 Bq/kg. We found that the number of buildings with ²²²Rn exceeding 200 Bq/m³ increases where the specific activity of ²³⁸U is larger. About 40% of dwellings and public buildings with ²²²Rn>200 Bq/m³ occurs in the lithotypes (metarhyolites and micaschists) with ²³⁸U>60 Bq/kg. A comparison between the indoor ²²²Rn concentration and the ²³⁸U specific activity measured on the same geological formation showed a linear correlation. Using the records of ²³⁸U specific activity, we developed a map of the expected radon concentration on the different geological formations of the surveyed area. Despite the limitations and uncertainties, mainly related to the uneven data coverage and the complex interaction between the building and the bedrock, the proposed approach showed that gamma-ray spectrometry can be a valuable tool to identify areas of more significant potential risk of radon emanation.