Ischia Island (Italy) Geothermal System

Abstract

Ischia Island is located in the north-western part of the Gulf of Naples and is part of the Phlegrean Fields Volcanic District. Ischia is marked to be an example of resurgent caldera and is characterized by high heat flux comprised between 200 and 400 mW/m2. Hydrothermal activity is well know on the island since Roman Age and more than 200 hotels and SPA resources, located all over the island, use thermal waters (T ranges from 30°C to 99°C) for balneo-therapeutic medical cures. The deep wells, drilled in 1950s and subsequent investigations also revealed the occurrence of high temperature fluids in the subsoil of the island, hosted within a possible geothermal reservoir. The aim of this work is to characterize the main geochemical processes that explain the water geochemistry of the thermal fluids of Ischia Island, to classify the water composition data into genetic groups and to delineate a conceptual model to explain the composition of the discharges. The chemical composition of water sample allows distinguishing six chemical groups that reflect complex processes of water/rock interaction and admixtures in different proportions with surficial fluids and deep geothermal fluids. Geothermometric calculations (based either on solute geothermometers and Saturation Index of minerals vs temperature) suggest the existence of two different deep reservoirs in the subsoil of the Island, in which fluids attain equilibrium with host rock at about 160-180°C and 220-240°C. These results are in agreement with data from Ischia3 deep well (1053m depth) drilled in early 1950 where it was found T = 224°C and a ~180°C convective zone at shallower depths (between 400 and 890m depth). All these data allow proposing a conceptual model of the Ischia Island hydrothermal system.