The amount and distribution of radon (222Rn) and helium (4He) at the Earth’s surface are controlled not only by type and depth of the source, permeability of geological formations and hydrogeological setting, but also by disequilibria processes occurring during upward gas migration. In order to evaluate the latter processes, in
this work we discuss Rn and He data collected during surveys performed simultaneously in different types of natural occurrence (i.e. soil-air, groundwater, gas vent and soil-atmosphere exhalation flux) located along faults bearing pressurised CO2 (Siena basin, Central Italy). The obtained results emphasize the effects of gas-water partition and gas channelling processes induced by faults on Rn and He distribution in near-surface environment; accordingly they suggest that the analysis of a single
“sub-system” of the geologic environment (e.g., groundwater analysis only) may provide data not representative of gas occurrence or abundance in the investigated site
