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

• Published in: Sustainable water resources management 2019-12, Vol.5 (4), p.1511-1523
• Main Authors: Marques, J. M., Carreira, P. M.
• Format: Article
• Language: English
• Subjects: Ascent; Carbon 14; Carbon dioxide; Computational fluid dynamics; Development Economics; Earth and Environmental Science; Earth Sciences; Fluids; Geology; Geotemperature; Geothermometers; Groundwater; Groundwater dating; Hydrogeology; Hydrology/Water Resources; Income; Isotopes; Meteoric water; Mineral resources; Mountains; Original Article; Plagioclase; Ratios; Recharge; Regional analysis; Regional development; Regional planning; Rock; Rocks; Silica; Silicon dioxide; Strontium 87; Strontium isotopes; Sustainable Development; Tectonics; Thermal water; Water analysis; Water circulation; Water Policy/Water Governance/Water Management; Water resources; Water sampling
• ISSN: 2363-5037; 2363-5045
• DOI: 10.1007/s40899-017-0190-8

In the North of Portugal, thermal and mineral groundwater resources (e.g., Chaves CO 2 -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 ( 2 H, 18 O, 13 C, 87 Sr, 3 H, 14 C and 3 He/ 4 He) methodologies, was used to assess local/regional conceptual circulation models. In the case of the Chaves CO 2 -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 HCO 3 /Na/CO 2 -rich type (with pH ≈7), in that the result of meteoric waters–granitic rocks–gas (CO 2 ) 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 ( 87 Sr/ 86 Sr mean  = 0.722419) is similar to that of the Sr isotopic ratios of the plagioclases of the granitic rocks (e.g. 87 Sr/ 86 Sr = 0.72087; 87 Sr/ 86 Sr = 0.71261), indicating that hydrolysis of plagioclase is the main water–rock interaction process, favoured by the presence of deep-seated (mantle-derived) CO 2 (δ 13 C values in the range of −6 to −1‰ vs. PDB, and the CO 2 / 3 He values from 5.1 × 10 8 to 7.5 × 10 9 , are typical of MORB fluids); (4) the income of carbon-14 free to Chaves CO 2 -rich thermal waters system does not permit reliable 14 C groundwater dating; (5) the geothermometric results (e.g. K 2 /Mg and silica geothermometers) indicate equilibrium temperatures around 120 °C, and a maximum depth of about 3.5 km reached by the Chaves CO 2 -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.