Interaction between fumarolic gases and thermal groundwaters at Vulcano Island (Italy): evidences from chemical composition of dissolved gases in waters

The chemical composition of some gaseous species dissolved in thermal groundwaters from Vulcano Island was determined directly in the field. The determinations were made using a methodology based on the equilibrium partitioning of gases between a liquid and a gaseous phase. The analyses were performed with a portable gas chromatograph using a simple analytical procedure lasting 70 s. He and CO2 partial pressures are notably higher than those of waters in equilibrium with the atmosphere confirming an interaction between volcanic gases and groundwaters. The highest CO2 values were recorded in wells located in areas of anomalous CO2 and He exhalations related to the active volcano-tectonic structures on the island. The temporal variations in CO2 concentrations both in the crater fumarolic gases and in the dissolved gases of some wells showed a very similar pattern. Lastly, the lowering of the He contents in the dissolved gases that occurred in 1996–1997 was probably due to a decreased interaction between the main degassing system (crater fumaroles) and the thermal groundwaters.