Geogenic Radon Potential map of the Celleno municipality (Lazio, central Italy)

In the international literature new methodological approaches and modelling, based on spatially continuous geological and environmental radon proxy parameters (e.g., lithology, permeability and airborne total gamma radiation), were recently proposed to construct maps of the Geogenic Radon Potential (GRP) in order to identify radon-prone areas. In this work are presented data collected in the municipality of Celleno (VT) that was selected for a detailed study within the framework of the research project INAIL/CNR- IGAG P19L06 "Studio dei processi di migrazione e accumulo dei gas endogeni e del radon dai suoli verso gli ambienti di vita e di lavoro in funzione delle caratteristiche geologiche presenti al contorno". The aims of the project were: 1) to investigate the main factors affecting the GRP at local scale through the development of a spatial regression model considering soil-gas radon concentrations as the response variable and developing proxy variables as predictors; 2) to construct a map of the GRP, thus providing the local administration of a useful tool for land use planning and strategies aimed at radon health risk reduction; 3) to use a soil-indoor transfer factor to discriminate the amount of the geogenic radon vs the rate due to building and habit characteristics. The study area is located along the western margin of central Italy characterized by extensional tectonics, high heat flow and widespread CO2 gas emissions. The municipality of Celleno extends on a surface of about 25 km2 and is located 80 km NW of Rome, at the eastern border of the Quaternary Vulsini volcanic district, whose activity produced mainly pyroclastic products and minor lava flows with potassic to ultrapotassic affinity. The outcropping volcanic rocks were traditionally used as building materials in the old center of the Celleno village. Soil-gas surveys and laboratory analyses were carried out to measure the concentrations of radon and other endogenic gases in the shallow environment, as well as the activity concentrations of natural radionuclides (238U, 226Ra, 232Th, 40K) and the radon emanation coefficient of several soil and rock samples, representative of the different lithotypes outcropping in the area. Indoor radon measurements were also carried out in selected private and public dwellings and cellars. Soil gas 222Rn concentrations range from 6 to 253 kBq/m3; CO2 concentrations range between 0.3% and 11%. Samples collected from outcropping volcanic and sedimentary rocks highlight: significant concentrations of 238U, 226Ra and 40K for lavas (183, 181 and 1671 Bq/kg, respectively), lower concentrations for tuffs (110, 92 and 756 Bq/kg, respectively) and very low for sedimentary rocks (42, 47 and 572 Bq/kg, respectively). Indoor radon activity ranges from 162 to 1044 Bq/m3, the highest value (4256 Bq/m3) was collected in a cellar directly carved into a tuff deposit. About 50% of the measured dwellings show radon indoor values above 300 Bq/m3 (threshold value recommended in the 2013/59/Euratom Directive), whereas all investigated sites show radon indoor values above 100 Bq/m3. Classical (Ordinary Least Square, OLS) and spatial (Geographically Weighted Regression, GWR) regression models were applied to investigate the relationships between soil-gas radon concentrations and some proxy explanatory variables, and to construct the GRP map. The OLS regression highlights that CO2 concentrations in soil gas, 226Ra content, radon emanation coefficient and Digital Terrain Model are statistical significant in the model. These variables were used to improve the model performances within GWR. The final map shows areas with high GRP on the western sector of the municipality of Celleno; as this area is sparsely populated, the GPR map may facilitate the future urban planning and should allow the improvement of public health management strategies. A radon soil-indoor transfer factor was calculated to discriminate the geogenic vs indoor contribution. Preliminary results highlight that ventilation habits and building materials (mainly tuff) appear to be important parameters affecting the radon accumulation. According to these promising results, the Celleno municipality has been selected and further investigated within the new LIFE-Respire project.