Geochemistry of groundwater from Graciosa Island (Azores): A contribution to the hydrothermal system conceptual model

Abstract

Graciosa island is located in the Azores Archipelago, along the so-called Terceira Rift, a major tectonic structure that makes the NE boundary of the Azores Plateau. In general terms, it includes a basaltic plataform on the NW and a silicic poligenetic volcano with caldera on the SE, the Graciosa Caldera Volcano. This volcano has produced significant tephra falls, pyroclastic flows, lahars, and lava flows, both of basaltic s.l. and trachitic s.l. composition. The hydrothermal system shows fumarolic emissions inside the volcano caldera and thermal springs located along the shoreline. This system is exploitated in a thermal building through shallow and deep (110 m) boreholes, near the coast. In Graciosa two types of Na-Cl groundwater systems can be identified: 1) a cold one emerging at springs and exploited by wells for public water supply, and 2) a hydrothermal system with temperatures around 40-44 °C. The cold groundwaters have pH higher than 7 and different degree of mineralization, according to the proximity to the sea. The thermal waters show mixing with seawater, pH varying between 6.20 and 6.94, 166 mg/L of SiO2, and significant concentration of metals, such as Mn, Fe, Co, Ni, Cu and Zn. The thermal water mineralization varies strongly, showing EC from 8.87 mS/cm (shallow water) to 47.4 mS/cm (deeper water). The higher mineralized water is rich in CO2(g), with 2130 mg/L of total dissolved CO2. Geothermometers application reveals aquifer temperature around 167 °C and immature/mixed waters, not reaching complete equilibrium with reservoir rock. The geochemistry of the thermal waters indicates the occurrence of seawater/host rock interaction processes at high temperature and slightly acid conditions, favored by CO2(g) input, and a different degrees of mixing with cold and shallow groundwaters.