Dissolved gases in groundwater as tracer for gas-water interaction and volcanic system evolution: a case study at island of Vulcano (Italy)

Water-gas interaction is an ordinary process occurring in volcanic areas because of gases released from magma reservoir at depth interact and dissolve in groundwater and/or are discharged from the soils or fumaroles. At the island of Vulcano (Aeolian Islands), both thermal and geochemical anomalies in groundwater were detected along lines of structural weakness in the volcanic edifice behaving as preferential pathways for up-flows of heat and fluids discharged by the deep magmatic system.

The interaction between deep volcanic/hydrothermal gases and groundwater can develop at various extent due to both local hydrogeological conditions and volcano-tectonic setting, resulting in different dissolved gas concentrations. Herein, we report a comprehensive study of chemical and stable isotope composition of dissolved gases in thermal groundwater at island of Vulcano.

The data were acquired with systematic sampling in four selected well since 2010, and include data on dissolved helium isotopes and carbon isotope composition of dissolved CO₂. The chemistry and isotopic data (C and He) of dissolved gases reveal the magmatic origin of the gas interacting with the aquifer and point out as the pristine magmatic composition varies upon gas ascent because of either dilution by a soil-atmospheric component or fractionation processes during interaction with groundwater. Further we discussed dissolved gases variations recorded during the period of unrest which onset at Vulcano on September 2021 and is still ongoing. The period of unrest was characterized by huge increase, orders of magnitude over the background, of degassing activity both from main crater and in pericrateric area. The variations detected in the chemical and isotopic composition of the dissolved gases occurred at different times and intensities in relation to the location of the wells.