A combined approach for the correlation between indoor radon and geological background: application in the western Ligurian Alps (Italy)

Radon (222Rn) is one of the most common naturally occurring radioactive elements and is particularly interesting to environmental issues, for it is considered a carcinogenic gas. It is a decay product of 238U, contained in most rocks and soils, and can easily escape from the ground to accumulate in closed spaces where it may become dangerous. The knowledge of its potential is vital to urban development plans and to protect people from potential hazards. We recently conducted monitoring campaigns in Liguria (NW Italy) to investigate the relations between the observed indoor radon concentrations and the geo-lithological background. We focused on the geological units of the Western Alps, characterized by various lithotypes, ranging from sedimentary to metasedimentary and metavolcanic rocks. The natural gamma radiation was measured on outcrops. Spectrometric measurements indicated that metamorphic acid rocks have the highest specific activity values of 238U (75-85 Bq/kg). In metasedimentary rocks, quartz and mica schists show the highest concentration of 238U, with an average specific activity of 56 Bq/kg. Sedimentary rock types are characterized by average specific activities < 40 Bq/kg., The dosimetric indoor surveys highlighted that about 40% of the investigated public and private buildings show indoor radon values above 200 Bq/m3. These preliminary campaigns revealed a relationship between the uranium content of the bedrock and the indoor radon. The correlation can be used to predict the geogenic radon potential based on a geological background when dosimetric data are few or scattered. In this paper, we refined our early analysis by integrating the dataset with further spectrometric and indoor dosimetric records, which were also coupled with soil radon measurements. The radon concentration in soil was investigated focusing on the sites where the previous monitoring campaigns showed high indoor radon concentrations. Soil radon was recorded at depths between 50 and 80 cm, where radon diffusion from the ground to the buildings very likely occurs. Soil radon concentrations substantially agree with spectrometric measurements. The largest concentration of 222Rn was found in the soils on more acid metamorphic rocks (porphyroid and porphyric shists) with values of about 100 kBq/m3. The lowest values about (20 kBq/m3) were recorded in soils occurring in sedimentary rocks. Despite the limitations and uncertainties, mainly related to the uneven data coverage and the complex interaction between the building and the bedrock, the combined techniques can identify areas of potentially high indoor radon concentrations.