Input of 87 Sr/ 86 Sr ratios and Sr geochemical signatures to update knowledge on thermal and mineral waters flow paths in fractured rocks (N-Portugal) J.M. Marques a,⇑ , P.M. Carreira b , F. Goff c , H.G.M. Eggenkamp a , M. Antunes da Silva d a Centro de Petrologia e Geoquímica, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal b Instituto Tecnológico e Nuclear / IST / UTL, Estrada Nacional No. 10, 2686-953 Sacavém, Portugal c Earth and Planetary Sciences Department, Univ. of New Mexico, Albuquerque, NM 87131, USA d Unicer Bebidas, S.A., Via Norte – Leça do Balio, Matosinhos, Apartado 1044, 4466-955 S. Mamede de Infesta, Portugal article info Article history: Received 21 November 2011 Accepted 20 March 2012 Available online 29 March 2012 Editorial handing by I. Cartwright abstract Strontium isotopes and other geochemical signatures are used to determine the relationships between CO 2 -rich thermal (Chaves: 76 °C) and mineral (Vilarelho da Raia, Vidago and Pedras Salgadas: 17 °C) waters discharging along one of the major NNE–SSW trending faults in the northern part of mainland Portugal. The regional geology consists of Hercynian granites (syn-tectonic-310 Ma and post-tectonic- 290 Ma) intruding Silurian metasediments (quartzites, phyllites and carbonaceous slates). Thermal and mineral waters have 87 Sr/ 86 Sr isotopic ratios between 0.716713 and 0.728035. 87 Sr/ 86 Sr vs. 1/Sr define three end-members (Vilarelho da Raia/Chaves, Vidago and Pedras Salgadas thermal and mineral waters) trending from rainfall composition towards that of the CO 2 -rich thermal and mineral waters, indicating different underground flow paths. Local granitic rocks have 87 Sr/ 86 Sr ratios of 0.735697–0.789683. There is no indication that equilibrium was reached between the CO 2 -rich thermal and mineral waters and the granitic rocks. The mean 87 Sr/ 86 Sr ratio of the thermal and mineral waters (0.722419) is similar to the Sr isotopic ratios of the plagioclases of the granitic rocks (0.71261–0.72087). The spatial distribution of Sr isotope and geochemical signatures of waters and the host rocks suggests that the thermal and mineral waters circulate in similar but not the same hydrogeological system. Results from this study could be used to evaluate the applicability of this isotope approach in other hydrogeologic investigations. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Recently, integrated advances for describing, modelling and pre- dicting physical, economic, social and political processes related to the hydrogeologic cycle, in particular with regard to Global Climate Change, have attracted worldwide attention both with governmen- tal authorities and the research community. An understanding of the processes related to the hydrogeologic cycle and the possible influ- ence of human activities upon this is crucial for developing methods for sustainable groundwater management. As simple scientific ap- proaches are unable to understand the complex interaction between nature, groundwaters and man, multidisciplinary studies are required to provide understanding for sustainable groundwater resources management. Increasing intensity of groundwater use and possible groundwater-related conflicts between stakeholders (e.g. society, governments, industry and nature) puts increasing pressure on the natural groundwater environment. For decision- making purposes (e.g. exploitation rates, avoiding overexploitation), indicators such as those presented in this paper should be accepted as driving forces to simplify complex information (such as the inter- relationship between several hydrogeological systems). The hydrogeology of the Vilarelho da Raia–Pedras Salgadas region (Fig. 1) has been extensively studied during the last 20 a (e.g. Aires-Barros et al., 1995, 1998; Marques et al., 1998, 2000, 2006, 2010b). In these papers, geochemical and isotopic (d 18 O, d 2 H, 3 H, d 13 C and 14 C) data have been presented and discussed. Aires-Barros et al. (1995, 1998) updated the hydrogeologic charac- terization of the Chaves geothermal area, based on coupled isoto- pic and geochemical studies carried out on CO 2 -rich thermal and mineral waters. Those studies concluded that the 3 H content (from 2.0 to 4.5 TU) in the Vidago and Pedras Salgadas mineral waters were mainly related to shallow underground flow paths, rather than mixing between deep mineralized waters and shallow cold dilute groundwaters of meteoric origin. Marques et al. (1998, 2000) studied the origin of CO 2 and the role of CO 2 in the mineral- ization of the CO 2 -rich thermal and mineral water systems. d 13 C values (ranging between 8‰ and 1‰) of CO 2 gas and Total Dis- solved Inorganic C (TDIC) indicated that the C in these CO 2 -rich thermal and mineral waters is mainly derived from a deep-seated (upper-mantle) source. Furthermore, the low 14 C content (from 7.9 to 9.9 pmC) of some of the CO 2 -rich mineral waters from the Vidago and Pedras Salgadas areas that contain 3 H indicates that 0883-2927/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apgeochem.2012.03.007 ⇑ Corresponding author. Tel./fax: +351 21 8400806. E-mail address: jose.marques@ist.utl.pt (J.M. Marques). Applied Geochemistry 27 (2012) 1471–1481 Contents lists available at SciVerse ScienceDirect Applied Geochemistry journal homepage: www.elsevier.com/locate/apgeochem