Caldelas and Geres hydrothermal systems (NW Portugal): a comparative study based on geochemical and isotopic signatures

A study based on geochemical and environmental isotope data was performed in two low-temperature hydrothermal systems in NW Portugal (Caldelas and Geres hydrothermal systems). This study aims to demonstrate the role of integrated hydrogeological tools for developing conceptual models of groundwater circulation. The studied hydrothermal systems are ascribed to groundwater circulation in fractured calc-alkaline/alkaline granitic contexts, responsible for different groundwater geochemical types. Caldelas hydrothermal system is dominated by Ca/Na-HCO3 waters, while at Geres, the hydrothermal system is characterized by Na-HCO3-type waters. The isotopic signatures indicate that the preferential recharge areas are located at very different altitudes (Caldelas around 171 m a.s.l. and Geres between 912 and 1118 m a.s.l.). The thermomineral waters issue with a mean temperature of 27 degrees C (Caldelas) and 43 degrees C (Geres). Several geothermometers were used to estimate the reservoir temperature and the corresponding depth reached by the hydrothermal systems. At Caldelas, the mean estimated reservoir temperature was 42 +/- 6 degrees C, using only the chalcedony and K-2/Mg geothermometers, which suggests depths around 0.93 km. In the Geres thermal area, the chalcedony, K-2/Mg, and Na/K/Ca (beta =4/3) geothermometers gave a mean estimated reservoir temperature of 96 +/- 5 degrees C, suggesting depths close to 2.8 km. In both case studies, conceptual circulation models are proposed, based on: geological heterogeneities, geochemical and isotopic signatures, mean preferential recharge altitudes, groundwater circulation paths, and mean residence time.