Isotopic study of the system CO2-H2-CH4-H2O in the Pozzuoli Solfatara (southern Italy).

The study of carbon, hydrogen and oxygen isotopes in the system CO2-H2-CH4-H2O in the Pozzuoli Solfatara has led to some interesting considerations: - the steam originates from meteoric water and not from seawater contained in the volcanites, as hypothesized by some authors. This modifies the hydrogeological model; - the water in the reservoir undergoes strong reactions at high temperatures with surrounding rock; - the absence of tritium indicates that the water remains in the underground for at least 30 years; - the isotope geothermometers indicate similar temperature intervals to those measured in the geothermal boreholes in the region. The D geothermometer for the pair H2-H2O, in particular, gives a temperature value for the productive horizons feeding the surface manifestations that is comparable with that calculated by other authors with geochemical geothermometers applied in a study of the thermal waters of the region. The same temperature indications can also be obtained from a computation of the equilibrium constant of the equation: CO2 + 4 H2 = CH4 + 2 H2O by using a new criterium for choosing the equilibrium pressure.