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

Prague, Czech Republic

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.