Radon and helium as pathfinders of fault systems and groundwater evolution in different Italian areas

Groundwater surveys in some Italian areas with different geological and geodynamical features were performed in order to evaluate dissolved rare gases (222Rn, He and Ar) as potential tracers of fault systems and groundwater evolution (fluid origin, water-rock-gas interaction, fluid migration and mixing phenomena). The obtained results showed that the highest values of Rn and He were found along important fault systems cutting the investigated areas, where the fault-related permeability increases deep-seated fluid circulation. However, dissolved-radon anomalies may be due to: a) a rapid ascent of Rn-bearing fluids, equilibrated with lithologies richer in U than outcropping formation; b) a local enrichment of U due to the action of low enthalpy groundwater or strong carrier gas (CO2) discharges usually occurring along the fault systems.