Mapping the geogenic radon potential and radon risk by using Empirical Bayesian Kriging regression: A case study from a volcanic area of central Italy.

A detailed geochemical study on radon related to local geology was carried out in the municipality of Celleno, a little settlement located in the eastern border of the Quaternary Vulsini volcanic district (central Italy). This study included soil-gas and terrestrial gamma dose rate survey, laboratory analyses of natural radionuclides (238U, 226Ra, 232Th, 40K) activity in rocks and soil samples, and indoor radon measurements carried out in selected private and public dwellings. Soil-gas radon and carbon dioxide concentrations range from 6 to 253 kBq/m3 and from 0.3 to11% v/v, respectively. Samples collected from outcropping volcanic and sedimentary rocks highlight: significant concentrations of 238U, 226Ra and 40K for lavas (151, 150 and 1587 Bq/kg, respectively), low concentrations for tuffs (126, 123 and 987 Bq/kg, respectively), and relatively low for sedimentary rocks (108, 109 and 662 Bq/kg, respectively). Terrestrial gamma dose rate values range between 0.130 and 0.417 μSv/h, being in good accordance with the different bedrock types. Indoor radon activity ranges from 162 to 1044 Bq/m3; the calculated values of the annual effective dose varied from 4.08 and 26.31 mSv/y. Empirical Bayesian Kriging Regression (EBKR) was used to develop the Geogenic Radon Potential (GRP) map. EBKR provided accurate predictions of data on a local scale developing a spatial regression model in which soil-gas radon concentrations were considered as the response variable; several proxy variables, derived from geological, topographic and geochemical data, were used as predictors. Risk prediction map for indoor radon was tentatively produced using the Gaussian Geostatistical Simulation and a soil-indoor transfer factor was defined for a 'standard' dwelling (i.e., a dwelling with well-defined construction properties). This approach could be successfully used in the case of homogeneous building characteristics and territory with uniform geological characteristics.