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 SO4 2−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.

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