Radon hazard in shallow groundwaters: amplification and long term variability induced by rainfall.

(222)Rn concentrations have been determined with a RAD7 radon detector in shallow groundwaters of the Pietramelara Plain, north-western Campania, southern Italy, where pyroclastic deposits, along with recent stream alluvial sediments, come in contact with Mesozoic carbonate reservoirs. The aim of this study has been to study the annual variation of (222)Rn concentration in the shallow groundwaters, scarcely considered in the literature and of obvious relevance for radon hazard evaluation. Our results definitely show that (222)Rn levels are characterized by a clear annual periodicity, strictly related to rainfall and water table levels, with a pronounced difference between the dry and the wet season. In this last case with concentrations increasing up to two orders of magnitude (up to two times the lower threshold given in the Recommendation 2001/928/EURATOM for public waters). In relation to this, experimental field data will be presented to demonstrate that this variability is due to purely hydrological mechanisms, mainly rinse out and discharge that control leaching efficiency. The detected cycle (Radon Hydrological Amplification Cycle, RHAC) has been generalized for the Mediterranean Tyrrhenian climate. The marked and seasonally persistent amplification in (222)Rn levels poses the problem of evaluating the epidemiological risk brought up by this previously not yet reported mechanism. This mechanism, occurring in shallow groundwaters, very likely should strongly influence indoor radon levels via groundwater-soil-building exchange.