Radon hazard in shallow groundwaters II: Dry season fracture drainage and alluvial fan upwelling

222Rn concentrations have been measured in a well located on the edge of a large Pleistocene-Holocene fan and belonging to the shallow pyroclastic aquifer of the Pietramelara Plain, southern Italy. The aim of this study has been both to characterise the hydrological inputs that determine the influx of 222Rn to the shallow aquifer and to understand the correlations between 222Rn, major ions, physical-chemical parameters and rainfall. Results obtained from the time series indicate that the studied well shows a 222Rn variability that is inconsistent with a mechanism of pure hydrological amplification, such as described in Radon hazard in shallow groundwaters: Amplification and long term variability induced by rainfall (De Francesco et al., 2010a). On the contrary, in this well hydrological amplification appears to be mainly tied to the upwelling of alluvial fan waters, rich in radon, in response to pistoning from recharge in the carbonate substrate. This upwelling of alluvial fan waters occurs during almost the whole period of the annual recharge and is also responsible of the constant increase in 222Rn levels during the autumn-spring period, when both the water table level and weekly rainfall totals drop. Furthermore, a rapid delivery mechanism for 222Rn likely operates through fracture drainage in concomitance with the very first late summer-early autumn rains, when rainfall totals appear largely insufficient to saturate the soil storage capacity. Results obtained from this study appear to be particularly significant in both radon hazard zoning in relation to the shallow aquifer and possibly also for indoor radon, owing to possible shallow aquifer-soil-building exchanges. Moreover, both the spike-like events and the long wave monthly scale background fluctuations detected can also have potential significance in interpreting 222Rn time series data as seismic and/or volcanic precursors. Finally, 222Rn has proved to be an excellent tracer for hydrological inputs to the shallow aquifer when combined with major ions, physical-chemical data and geological and geomorphological controls.

Research highlights► We describe the seasonal variability of radon in shallow groundwaters. ► We report the detection of two new mechanisms of hydrological amplification of radon in shallow groundwaters. ► These are significant for radon hazard evaluation and earthquake-volcanic precursor studies.