Geochemistry of thermal waters on the island of Ischia (Campania, Italy)

The stratigraphic and structural situation on the island of Ischia (southern Italy), the recent volcanic activity and the presence of hot springs and fumaroles, suggest the existence of a geothermal field. The chemical and isotopic compositions of the waters from several springs and wells were examined to obtain information on deep temperatures and to formulate a geothermal model of the island. δD values range from −33.60 to −12.50‰ and δ¹⁸O from −7.10 to −1.71‰, relative to SMOW. These variations have mainly been attributed to the presence of seawater, as confirmed by the general shift to more positive values with the increase of Cl content. Water-rock reactions, evaporation and subsurface boiling also contribute to the δ¹⁸O−δD trend. The chemical analyses reveal the presence of alkaline sulphate chloride water (seawater), bicarbonate waters and waters interpreted as the result of mixing. The chemical and isotopic composition of the latter are dependent on water-rock interactions, water circulation rates and eventual evaporation and condensation phenomena. The silica geothermometer, which seems to be the most suitable for determining the deep temperatures of these waters, gave values of about 200°C, even for mixing models. Our data suggest the following geothermal model: the heat flow heats up a deep reservoir, causing steam to rise through faults and fractures and transfer heat to a shallower aquifer. The temperatures of 200°C obtained by the geothermometers are not the maximum reservoir temperatures, but are probably water-rock equilibrium temperatures for the shallower aquifers. The high boron contents and the isotopic data confirm the presence of steam in the system.