Radon fluxes estimate from geochemical data and gamma radiation in Campania region (Italy)

Radon is a ubiquitous radioactive gas proceeding from the decay of some naturally occurring radionuclides, mostly abundant in igneous rock and volcanic materials, including soils. It is the heaviest noble gas, thus chemically inert, and moderately soluble in water. The total amount of radon in nature derives from three unstable isotopes: 219Rn (235U decay chain), 220Rn (232Th decay chain) and 222Rn (238U decay chain). 222Rn, whose half-life is 3.8 days, represents the most important in terms of risks posed to human health since its progeny (e.g. 212Po, 214Po, 218Po, 210Bi, 214Bi, 210Pb, 214Pb) is chemically very reactive and decays emitting particles and large amounts of energy. Alpha particle radiation from Rn progeny is one of the main sources 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. An empirical method was applied to estimate the flux of 222Rn across Campania, a region of the southwestern sector of the Italian peninsula and whose territory is mostly characterized by the presence of volcanic lithotypes and sediments, by using radiometric and geochemical data recorded in two different prospecting campaigns completed in 2003 and 2015, respectively. The main objectives of the work were to compare the estimated 222Rn fluxes, resulting from the two sets of data, to analyse and interpret any differences and to identify areas with a high exposure to Rn for humans. As a first step, Terrestrial Gamma Dose Rates (TGDRs), which represent the gamma-radiation emitted by the decay of naturally occurring radioactive nuclides, were calculated from both datasets and, later, they were used as inputs to determine the 222Rn fluxes across the region. Data were processed in a GIS environment and were interpolated by means of the Multifractal Inverse Distance Weighted (MIDW) method. The resulting raster flux maps were compared by mean of a software that allowed to analyse the similarity among the values of the two maps, pixel by pixel. Further, a regression analysis was performed to quantitatively assess the functional relationship among the determined fluxes and residuals were also used to generate a raster (MIDW) map of their distribution, which was classified trough the C-A method. The results denote two areas with high flux in correspondence of the volcanic centers (Mt Roccamonfina, Mt Somma-Vesuvio, Phlegrean Field), an area with medium flux values roughly corresponding to the pyroclastic covers (overlapping the sedimentary formations) and areas with low values along the Apennines and south of the region. The analysis of the residuals and the similarities suggested the hypothesis of an underground contribution to the surficial 222Rn flux due to the presence of deep volcanic thick layers and the presence of tectonic structures.