Natural radioactivity due to radon in northeastern Sicily (Italy): application, measurement and radiological hazard

A radiological study was carried out over the Tyrrhenian margin of the Peloritani Mountains (northeastern Sicily, southern Italy) to investigate the natural radioactivity with the aim of defining if radiation levels, particularly those produced by radon, are higher than the global averages exposing inhabitants to dangerous radiation doses. The collected results demonstrate that groundwater radon levels and gamma radiation derived from soils and rocks do not constitute a health threat. On the other hand, anomalous radon concentrations in soil gases were detected along a ~NW-SE oriented zone located in the Barcellona-Milazzo alluvial plain. The diffuse degassing appears to be generated by the uprising of deep-originated fluids along faults and fractures, and it is favoured by the high permeability of the clastic Quaternary sediments outcropping in the area. Structural and geophysical indications allow considering the tectonic lineament promoting the radon degassing as a potential “silent” on-land prolongation of the Vulcano-Milazzo fault zone (VMFZ), a transtentional tectonic element located in the Gulf of Patti and belonging to the Aeolian-Tindari-Letojanni System. In this framework, as already suggested by other authors, a regional degassing of CO2-dominated fluids is triggered by the activity of lithospheric faults. Mantle volatiles are produced by the depressurization of the upper mantle and they are driven toward the surface due to the enhanced vertical permeability occurring along tectonic structures. During their ascent, mantle volatiles mix with radon-rich crustal-derived fluids likely produced by thermo-mechanical processes. Therefore, radon atoms, carried by CO2, migrate from the crust to the atmosphere generating local radiological hazards for inhabitants. In detail, radiological measurements were carried out by means of gamma and alpha spectroscopy on different types of natural samples including crystalline rocks, soil gases and groundwaters. Crystalline rocks show 226Ra, 232Th and 40K activity ranging from (17 ± 4) to (56 ± 8) Bq kg−1, from (14 ± 3) to (77 ± 14) Bq kg−1 and from (167 ± 84) to (1760 ± 242) Bq kg−1, respectively. The effective dose outdoor derived from gamma radiation is slightly above the global average. Despite this evidence, it does not constitute a menace to human health. Since some of the crystalline rocks are extensively used as building materials, the potential hazard for population derived from the radon released from the rock matrix was assessed. The results show that the 222Rn emanation coefficient and the 222Rn surface exhalation rate range from (0.63 ± 0.3) to (8.27 ± 1.6)% and from (0.12 ± 0.03) to (2.75 ± 0.17) Bq m−2 h−1, respectively. These values do not generate dangerous indoor radon levels, and therefore the crystalline rocks can be used in construction without restrictions. Concerning soil gases, a total of 172 samples were analysed for the 222Rn and 220Rn activity concentrations. Analyses were carried out by using a hollow probe to pump out soil gas from depths of 0.5-0.6 m and the RAD7 Durridge to determine the radon concentration. 222Rn and 220Rn concentrations range from (0.69 ± 0.2) to (81.3 ± 2.5) kBq m-3 and from (2.63 ± 0.6) to (123.48 ± 5) kBq m-3, respectively. Radon index (RI) estimations point out that the radiological hazard for the population living in the studied area is low to moderate, except along the ~NW-SE oriented zone already described, where widespread radon degassing from soils takes place. Moreover, 70 groundwater samples were collected from wells and natural springs, and the activity of dissolved 222Rn was measured by using the alpha counter RAD7 Durridge coupled with the RAD H2O setup. Analyses show that the 222Rn activity concentration ranges between (1.6 ± 1.1) and (57.5 ± 5.3) Bq L-1. Groundwater coming from natural springs and wells is generally used as tap water for human consumption and other domestic purposes. All the measured radon activities are lower than the reference limit set by the Italian legislation, and the annual effective dose received by the population due to ingestion is in the range of 4-146.8 µSv y-1, then below the worldwide average value. As a result, groundwaters extracted in northeastern Sicily can be used for domestic purposes and human consumption without taking remedial actions. It is important to understand that all the data were collected within a restricted amount of time, providing only an instantaneous photograph that is valid only for a very short time span. Future events, such as increased seismicity or powerful earthquakes, may strongly increase the degassing rate by further enhancing the vertical permeability along the crust. Therefore, the concept of “temporal variation” of radiological parameters becomes a key factor in the field of human health protection, since harmful injections of deep-originated radioactive fluids into the atmosphere could suddenly occur in those areas affected by intense tectonic and seismic activity. Moreover, performing physical-radiological analyses in other areas of northeastern Sicily together with the deployment of monitoring stations with continuous recording of radiological data should be the future goal in order to improve the health hazard assessment due to natural sources.