Rainstorm-induced soil ²²²Rn concentration spikes observed in Southern Italy

Abstract

Soil ²²²Rn concentrations have been monitored during the 2009 autumn and Spring 2010 rainy periods, in an alluvial soil, resting on a pyroclastic substrate (locally reworked), in the Pietramelara Plain, in Southern Italy. The dry soil diurnal oscillations detected at the site in August 2009 have been found to be present also during the rainy periods of the year, provided, however, that soil moisture conditions are not affected by rainfall infiltration or post storm gravity-driven percolation in the soil depth interval considered. With significant rainfall, the characteristic dry soil diurnal cycle is altered. This rainfall-induced perturbation produces a spike-like signal featuring a sharp rise, with commencement of precipitation, then followed, with persistent and significant rainfall, by an equally sharp drop to very low levels. Both the sharp rise, resulting from capping, and the sharp drop (wash-out), resulting from infiltration, have been found to follow not previously reported second-order trends tied to cumulative rainfall, with very high statistical significance (r ² > 0.97 for the sharp rise and r ² > 0.95 for the sharp drop). Furthermore, the sharp drop appears to be tied to wetting front migration of ²²²Rn-free, or ²²²Rn-poor, rainwater and to commence when it reaches the soil probe depth, effectively flushing and transferring radon to lower levels of the profile, thus validating previous studies. Low rainfall events (<1–2 mm), however, do not appear to alter significantly the dry soil diurnal cycle. Post rainstorm recovery of soil ²²²Rn concentrations, in the immediate aftermath of significant rainfall, appears to be very slow and limited. The spike-like pattern induced by rainstorm events poses significant questions as to the interpretation of soil ²²²Rn concentration time series data in geophysical precursor studies. Likewise the extreme variations resulting from rainfall, and likely affected by rainfall duration, intensity, frequency and spatial variability, pose significant implications in environmental monitoring studies aimed at characterising representative soil ²²²Rn concentration levels for the wet season.