ABSTRACT

Gas samples for analysis of CO₂, Rn and He were collected simultaneously in different types of natural occurrence, i.e., soil-air, groundwater, gas vent and soil-atmosphere exhalation flux, in the Siena-Rapolano geothermal area (central Italy). The results showed differential enrichment and depletion of Rn and He within small areas as an effect of gas-water partitioning and gas channelling in CO₂-rich faulted subsoil. In the same site low concentrations of Rn or He in water co-exist with high concentrations in the ground; low Rn in soil-air occurs in correspondence with Rn-rich gas vents; soil-gas Rn may be higher than the level induced by U decay in the ground or outcropping rocks. These results suggest that the analysis of a single “sub-system” of the geological environment (e.g., groundwater analysis only) may not provide data representative of gas occurrence and abundance at the investigated site. This must be taken into account in natural resource exploration, earthquake precursor studies and radiation protection zoning based on gas geochemistry.