Pedogenic radon fluxes predictions from geochemical data and gamma ray: The Campania region experiment

The products of the natural decay of uranium-238, an unstable isotope of a radioactive element almost ubiquitous in geological materials, include radon-222 (Rn-222) and radium-226 (Ra-226). Rn-222 has a half-life of 3.8 days and its decay products (progeny) are represented by further radioactive isotopes of polonium (Po-218, Po-214, Po-210), bismuth (Bi-214, Bi-210) and lead (Pb-214, Pb-210). Radon (mostly Rn-222), abundant in igneous rock and volcanic soils, is the main source of natural radiation to which human beings can be exposed during their life and it is considered the second indirect leading cause of lung cancer after cigarette smoking. In fact, while radon is an inert gas, his progeny is electrically charged and can adhere to airborne particles. If inhaled, Rn-222 can decay in its progeny and particles can deposit in the lungs and irradiate brachial tissue leading to lung cancer in the long run. In Campania, a region located on the south-western sector of the Italian peninsula whose territory is mostly characterized by the presence of volcanic lithotypes and sediments, in 2003 an intensive gamma-ray spectrometry survey was performed, for a total of 2396 measurements stations. In addition, during several sampling campaigns concluded in 2015, a total of 3822 topsoil samples were collected over the whole territory. Radiometric and geochemical compositional data were elaborated to estimate the potential fluctuation of Rn-222 in the soil of Campania region. Terrestrial Gamma Dose Rate (TGDR), which represents the gamma-radiation emitted by the decay of naturally occurring radioactive nuclides present in the Earth’s crust, is considered a good proxy to determine the pedogenic Rn-222 flux. The K, Th and U soil concentrations and K-40, U-238 and Th-232 activity values, acquired during the two different environmental prospecting campaigns reported above, through a series of empirical relationship, were used to calculate the TGDR at each soil sampling and gamma radiation measurement location, with the final purpose of quantifying the amount of Rn-222 flux from the ground. Flux data generated were mapped for each individual dataset and the resulting distribution patterns and values were compared. A regression analysis was performed to quantitatively assess the functional relationship among the flux data determined from the gamma radiation and the ones determined from geochemical data. For the inner part of the region, some strong discrepancies among the flux maps roughly correspod with the presence of several quaternary active faults which could facilitate the rising of gases (probably including CO2 as a carrier) from the deepest geological layers. For the outer part of the region, where volcanic complexes are located (i.e. Mt. Somma-Vesuvius, Mt. Roccamonfina, Phlegrean Fields), discrepancies could be explained by the presence thick layers of volcanic rocks/sediments below the alluvial sandy soils.