Variation in the total dissolved carbon isotope composition of thermal waters of the Island of Ischia (Italy) and its implications for volcanic surveillance

During surveys conducted in 1997, 30 thermal waters (for a total of 39 analysis) were collected from shallow wells utilised for resort spas on the Island of Ischia. Well temperatures ranged from 25° to 82°C and their depths from 30 to 150 m. Pools with temperatures of 26° to 67°C were also sampled on the northern flank of the Mt. Epomeo. The samples were analysed for bulk chemical composition, as well as H, O and C isotope composition. Isotopic analyses of water samples showed wide variations in both deuterium and oxygen-18, which ranged from −37.4 to +8.1‰ and −6.8 to +1.1‰, respectively. Overall δ13C in total dissolved carbon (TDC) ranged from −11.5 to +2.3‰, and computed δ13C of CO2(g) results ranged from −18.5 to −1.72‰. Water samples with CO2 content of up to about 1 bar partial pressure exhibited δ13C(CO2) ranging from −4 to −1‰. Water samples varied chemically from low-salinity HCO3 well waters (1–4 g/l) of essentially meteoric origin, to high-salinity Cl waters (up to 40 g/l). Mixing processes from local groundwater, seawater and evaporated waters have also been identified. Models for the chemical and isotopic data on the thermal manifestations have been developed for low- and high-pressure conditions in order to assess the isotopic composition of deep CO2, assuming that this component is continuously added to the solutions until degassing occurs at shallow depths or at the surface. Constraints due to the reaction of water with volcanic glass have been taken into account for in the computations in addition to those relative to the carbon components participating in the carbonate equilibria. Results from such simulations indicate that deep CO2 added to the shallow waters probably has a carbon isotope composition ≥−2‰, which most likely stems from a mixed magmatic–sedimentary origin.