Effects of steam-heating processes on a stratified volcanic aquifer: Stable isotopes and dissolved gases in thermal waters of Vulcano Island (Aeolian archipelago)

We report on a comprehensive study of major-ion chemistry, dissolved gases, and stable isotopes measured in water wells at Vulcano Island since 1988. The work focuses on a quantitative model describing steam condensation and boiling phenomena in shallow water bodies. The model is based on the differences in partition coefficients between liquid water and vapor characterizing oxygen and hydrogen isotopes, as well as volcanic gases (CO2, S species, and HCl). Based on both physical conditions of aquifers identified during drilling campaigns and the composition of the volcanic vapor, mass and enthalpy balances are applied in a multistep process of steam separation and condensation in shallower aquifers. By comparing the model results with measured data, we infer that (i) strong isotope enrichment observed in some shallow thermal waters can result from an increasing mass rate of condensing deep vapor, even in water meteoric in origin; (ii) the high CO2 content measured in the fumarolic vapor during 1988–1993 affected the δ18O value of the steam-heated water due to CO2–H2O isotope exchange; (iii) the high pCO2 measured in the coldest and peripheral waters are explained by the progressive enrichment of this gas in the vapor phase during multistep boiling; and (iv) the high Cl− and SO42−contents in the hottest waters can be attributed to the direct condensation (single-step) of volcanic vapor. The model also takes into account both the mass fluxes and the compositions of the involved endmembers (steam and shallow groundwater), which provides important inferences on the modifications observed or expected during periods of increasing mass and heat input from depth.