Geosciences in the assessment of thermal and mineral groundwater systems in N-Portugal: a review

Abstract

In the North of Portugal, thermal and mineral groundwater resources (e.g., Chaves CO2-rich thermal—76 °C—waters) are being used in the local Spas, and are considered one of the main sources of local/regional development/income. A multidisciplinary approach, including geological, tectonic, geochemical and isotopic (2H, 18O, 13C, 87Sr, 3H, 14C and 3He/4He) methodologies, was used to assess local/regional conceptual circulation models. In the case of the Chaves CO2-rich thermal waters, this approach provided important data to answer the most commonly asked questions so as to enable the elaboration of a robust hydrogeological conceptual model, namely (1) Chaves thermal waters belong to the HCO3/Na/CO2-rich type (with pH ≈7), in that the result of meteoric waters–granitic rocks–gas (CO2) interaction; (2) Padrela Mountain (NE-Chaves), ascribed to high-fractured rocks, is the main recharge area; the stable isotopic composition of the groundwater samples indicate a mean recharge altitude higher than 1150 m a.s.l.; (3) the mean Sr isotopic ratio of the thermomineral waters (87Sr/86Srmean = 0.722419) is similar to that of the Sr isotopic ratios of the plagioclases of the granitic rocks (e.g. 87Sr/86Sr = 0.72087; 87Sr/86Sr = 0.71261), indicating that hydrolysis of plagioclase is the main water–rock interaction process, favoured by the presence of deep-seated (mantle-derived) CO2 (δ13C values in the range of −6 to −1‰ vs. PDB, and the CO2/3He values from 5.1 × 108 to 7.5 × 109, are typical of MORB fluids); (4) the income of carbon-14 free to Chaves CO2-rich thermal waters system does not permit reliable 14C groundwater dating; (5) the geothermometric results (e.g. K2/Mg and silica geothermometers) indicate equilibrium temperatures around 120 °C, and a maximum depth of about 3.5 km reached by the Chaves CO2-rich thermal waters system was estimated. The discharge zones are mainly related to the intersection of the main local/regional fault lineaments (and conjugate structures), responsible for promoting the mineral and thermal groundwater ascent.