In October 1995, we carried a geochemical study on the contents of major, minor and trace elements in the shallow and deep natural thermal waters (17 < T < 90°C) of Ischia island. The distribution of twenty-three water samples collected was chosen on the basis of their proximity of the main active tectonic structures present on the island. The chemical composition of major and trace elements is related to the main hydrological and lithological characteristics of the rock formations of the reservoir and allows us to obtain information about the origin of these waters and to evaluate both possible meteoric-sea water mixing and water-rock interaction processes. The analytical results showed that the main chemical characteristics of Ischia thermal waters derive mainly from mixing between meteoric water and sea-water modified by strong interaction processes with volcanic host rocks. In fact, this process is highlighted by the enrichment of water in K and B (derived from rock leaching) and the reduction in Mg content (due to the uptake in secondary minerals).
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