A contribution to groundwater flow models in hydrothermal systems from North of Portugal

Abstract

Geothermal resources are considered a sustainable and environmentally friendly alternative to produce energy. The north of Portugal seems to be a suitable area for the exploitation of geothermal resources due to its geothermal gradient and the occurrence of several thermal areas. A study based on 18O and 2H water isotope and geochemical data of shallow and deep geothermal aquifers was performed in selected seven hydrothermal systems located in the Cávado and Ave River watersheds, NW Portugal. The main goal of the study is the proposal of conceptual groundwater flow models, including information on recharge areas, groundwater flow paths, water-rock interaction, and mixing processes between water from different sources. The studied hydrothermal systems are ascribed to groundwater circulation in fractured granitic host rocks. Calcium is dominant cation in the thermal water of Caldelas, while sodium is dominant in the other studied thermal waters. Hydrogen carbonate is the dominant anion in the thermal water of Caldelas, Moimenta, Taipas and Vizela, while Cavadinho is represented by Cl-type water. The hydrothermal systems of Gestal and Eirogo are characterized by a mixed water type between chloride and hydrogen carbonate. The intensity of water-rock interaction suggests rather short residence times in the reservoir for the geothermal waters of Caldelas and Taipas compared to the other thermal waters. The 18O and 2H water isotope data indicate a dominant meteoric origin of the thermal waters. Considering the isotope altitude effect, the preferential thermal water recharge areas are located at altitudes between 315 and 585 m asl, which is coincident with the fault networks and could be a flow water circulation paths. Several geothermometers were applied, considering the geothermal gradient of the region, to estimate the reservoir depth and temperature. Estimated reservoir temperatures range from 40 °C for the Caldelas thermal area to 130 °C for the Vizela thermal area, indicating that correspond to low-temperature geothermal systems. The estimated temperatures suggest depths of the geothermal reservoirs between 0.8 and 3.3 km. Indications for mixing processes between upwelling geothermal waters and shallow aquifers were found at Vizela and Taipas area