Preliminary Geochemical and Isotopic Results in Thermal and Cold Waters of Graciosa Volcanic Island (Azores)

Abstract

Graciosa Island is located in the Azores Archipelago, along the so-called Terceira Rift, NE boundary of the Azores Plateau. From the hydrochemical point of view, two types of Na-Cl groundwater systems were identified: a cold aquifer system emerging at springs and exploited through boreholes for public water supply with different degrees of mineralization, and a hydrothermal system with issuing temperatures around 45 ºC. Geothermometers applied to the thermal waters point to deep temperature around 167 ºC and to immature waters, not reaching complete equilibrium with the reservoir rock. The isotopic composition and geochemistry of the thermal waters indicate mixture groundwater - seawater in different percentages and ion-exchange mechanisms that will be able to: i) increase groundwater salinity, ii) strongly change the isotopic composition to more enriched values, with different degrees of mixing.