| Title | Geochemical Characterisation of Waters and Gases of Ischia Island (Italy) |
|---|---|
| Authors | Salvatore Inguaggiato and Giovannella Pecoraino |
| Year | 2000 |
| Conference | World Geothermal Congress |
| Keywords | gas and water geothermometers, fluid geochemistry, Ischia Island, ‰13CCO2, gas-water-deep fluids interactions |
| Abstract | Between 1994 and 1995 gas samples from some fumaroles and thermal waters were collected on Ischia Island. The chemical composition of the shallow and deep natural hydrothermal fluids discharged is related to the main hydrological and lithological characteristics of the rock formations present in the reservoir. The calculated mass vapor fraction (y) of the reservoir is always positive suggesting the presence of a biphase system where the dominant liquid has a temperature ranging from 240 to 280?C. The liquid phase is made up of modified seawater, heated fresh seawater and meteoric water. The mixing between these components together with interaction processes with the surrounding rocks is the origin of all of the islands groundwaters. The gaseous phase is mainly carbon dioxide with variable contents of helium that reflect different degrees of interaction with shallow groundwaters. The isotopic composition of the helium and the CO2 of these gases is characteristic of fluids that originate in a mantle that has undergone crustal contamination. The helium values are from 1 to 3 R/Ra and values of ‰13CO2 among -5 and ñ2 ‰â. The negative ‰13C value with respect to typical magmas in the Mediterranean (with ‰13CO2 ranging from 0 to - 2‰â) is the result of deep gases and groundwater interaction during their rise towards surface. The equilibrium temperature of the deep reservoir, found by using the chemical composition of the sampled gases, is uniform at 280?± 20?C. The homogeneity of the equilibrium temperatures of the sampled gases in different areas of the island probably indicates the existence of a common geothermal reservoir from which the gases originate. The differences in the equilibrium temperatures, estimated from the waterís chemical composition, are due to mixing processes between modified sea water that represents the deep recharge of the system, and shallow groundwaters recharged by meteoric and marine waters at lower temperatures. The amount of steam in the reservoir is below 2% of total water content. The calculated values of pCO2 in the reservoir are highly variable and range between 1 and 28 bars. The observed values do not show any correlation with other parameters and do not allow the evaluation of pCO2 in the geothermal reservoir. |