Nature and Role of CO2 in Some Hot and Cold HCO3/Na/CO2-Rich Portuguese Mineral Waters: a Review and Reinterpretation José M
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Research article Nature and role of CO2 in some hot and cold HCO3/Na/CO2-rich Portuguese mineral waters: a review and reinterpretation José M. Marques 7 Paula M.M. Carreira 7 Luís Aires-Barros 7 Rui C. Graça Introduction Abstract At the northern part of the Portuguese mainland, the upflow zone of several hot and cold During the last decades, we have witnessed an increasing HCO3/Na/CO2-rich mineral waters is mainly asso- worldwide interest on the exploitation of low temperature ciated with important NNE–SSW faults. Several geothermal resources. Because low temperature geother- geochemical studies have been carried out on ther- mal systems and cold carbon dioxide waters are linked in mal and non-thermal hydromineral manifestations many parts of the world (e.g. Iceland, the Geyser in Cali- that occur along or near these long tectonic align- fornia and French Massif Central), special emphasis has ments. The slight chemical differences that exist be- been given to the study of hot and cold CO2-rich mineral tween these meteoric hot and cold HCO3/Na/CO2- water discharge in many parts of the world (Panichi and rich mineral waters seem to be mainly caused by Tongiorgi 1975; Barnes and Irwin 1980; Arnórsson and 13 CO2. d C(TIDC) values observed in these groundwat- Barnes 1983; Fouillac 1983; Wexteen and others 1988; Du- ers range between P6.00 and P1.00‰ versus V- chi and others 1991; Verma and others 1992; Arnórsson PDB (V denotes Vienna, the site of the Internation- and Gíslason 1994). al Atomic Energy Agency; PDB originates from the On the Portuguese mainland, most hot (Chaves) and cold CaCO3 of the rostrum of a Cretaceous belemnite, (Vilarelho da Raia, Vidago and Pedras Salgadas) HCO3/ Belemnitella americana, collected in the Peedee for- Na/CO2-rich mineral waters flow from natural springs mation of South Carolina, USA) indicating a deep- and drilled wells that are located either in granitic out- seated (mantle) origin for most of the CO2. Nev- crops or in the peribatholitic boundaries concordant to 13 ertheless, in the case of the heavier d C(TIDC) val- the main NNE–SSW fault trend (Fig. 1), which attains ues, the contribution of metamorphic CO2 or the 30 km depth in the study area (Baptista and others 1993). dissolution of carbonate rock levels at depth cannot Chaves thermal waters have been well known since Ro- be excluded. Concerning the hot waters, the lack of man times and today they are mainly used for heating a positive 18O-shift should be attributed to water- swimming pools and hydrotherapy. These thermal waters rock interaction in a low temperature environment, are also currently being utilised to heat an experimental rather than to the isotopic influence of CO2 on the greenhouse. d18O-value of the waters. Special emphasis has been given to the geohydrological characterisation of the Vilarelho da Raia-Pedras Salgadas Keywords d13C values 7 Northern Portugal 7 region to allow a more complete knowledge of ground- Origin of CO2 7 Stable isotopes 7 Thermal and cold water circulation paths and to determine correlations that mineral waters exist between the hot and cold CO2-rich mineral waters that discharge in the northern part of the Portuguese mainland (Aires-Barros and others 1991, 1994, 1995; Mar- ques 1993; Marques and others 1995, 1996, 1998). Geo- Received: 9 August 1999 7 Accepted: 8 March 2000 chemical and isotopic studies have enabled the formation of a conceptual geohydrological model of the low-temper- Y 7 7 José M. Marques ( ) Luís Aires-Barros Rui C. Graça ature geothermal system of Chaves (Aires-Barros and Instituto Superior Técnico. Laboratório de Mineralogia 18 2 3 e Petrologia (LAMPIST). Av. Rovisco Pais, 1049-001 Lisboa, others 1995). Isotope techniques ( O, H and H) have Portugal been used to identify groundwater origin, altitude of re- e-mail: pcd20456alfa.ist.utl.pt charge and underground flow paths. Recently, new re- Tel.: c351-21-8400806 search on isotope techniques in investigations on the na- Fax: c351-21-8400806 ture of hot and cold mineral waters was initiated. Special Paula M.M. Carreira attention has been given to the identification of the ori- Instituto Tecnológico e Nuclear (ITN). Estrada Nacional No. 10, gin of carbon dioxide in these hot and cold CO2-rich mi- 2686 Sacavém Codex, Portugal neral waters. Because of its peculiar geochemistry, the Environmental Geology 40 (1-2) December 2000 7 Q Springer-Verlag 53 Research article Fig. 1 Geological sketch map of the region, showing the position of main water samples. a Hercynian granites and granodiorites; b Silurian phyllites, quartzites, carbonaceous slates and migmatites; c Cenozoic cover sediments; d major faults; e hot saline waters; f cold saline waters; g dilute cold superficial waters (recharge waters). 1 Vilarelho da Raia (Facha, ACP1 and ACP2) area; 2 Chaves (AC2 and VI) area; 3 Campilho area; 4 Vidago (AC16 and AC18) area; 5 Sabroso area; 6 Pedras Salgadas (AC17) area role of carbonaceous slates, which are well displayed in 1982). The Nuclear and Technological Institute (ITN) in the study area, as a hydrochemical signature of the Portugal carried out the determinations of d18O, d2H and groundwaters, has been considered. The understanding of d13C values in waters by mass spectrometry. d13C ana- the origin of these waters is directly related to the origin lyses were performed on total inorganic dissolved carbon of carbon dioxide. (TIDC) precipitated in situ as BaCO3 at pH19.0. The gas 13 12 used in C/ C measurements was CO2. Carbonates were 3 reacted with 100% phosphoric acid to liberate CO2. H analyses were also performed at the ITN. d18O and d2H Field and laboratory methods were measured following the analytical methods of Ep- stein and Mayeda (1953) and Friedman (1953) respective- 18 13 Hot and cold CO2-rich mineral waters and cold superfi- ly. Analytical precision is B0.10‰ for d O and d C, cial low mineralised waters (spring waters) were sampled and B1.0‰ for d2H. Tritium determinations were per- and chemically and isotopically analysed. Physicochemi- formed using electrolytic enrichment and subsequent cal characteristics of the groundwater samples studied be- measurement of counting rates by liquid scintillation. For tween 1990 and 1992 are shown in Table 1. Table 2 in- 3H analyses, the standard deviation varied between B0.7 cludes the most relevant physicochemical and isotopic and B1.2 depending on tritium content. d13C values in variables analysed in the water samples taken during rocks (carbonaceous slates) have been measured by mass 1996 fieldwork campaigns. Some data from the literature spectrometry at Delta Isotopes Laboratory/The Nether- have been used for the Chaves hot waters (Almeida lands. The standard d notation per mil is used through- 54 Environmental Geology 40 (1-2) December 2000 7 Q Springer-Verlag Research article Table 1 and spring. SD Standard deviation. Number of samples: np6. Hydrogeochemical data of the water samples collected at Temp Issue temperature of sampled water; Cond conductivity at different times (during 1990, 1991, and 1992) in drilled wells the issue temperature. pH values are from field measurements Vidago AC16a Pedras Salgadas AC17a Vidago AC18a Chaves AC2a V. Raia Fachab Mean SD Mean SD Mean SD Mean SD Mean SD Temp. (7C) 18.3 0.5 17.0 0.4 17.8 0.3 75.1 1.5 16.3 1.5 pH 6.16 0.1 6.42 0.2 6.79 0.1 7.15 0.2 6.61 0.4 Cond. (mS/cm) 1618 100 2623 202 5648 337 2822 339 2543 489 Na (mg/l) 365 54 521 136 1246 255 574 66 724 126 K (mg/l) 32 5 28 2 105 3 65 6 28 11 Ca (mg/l) 62 13 182 13 203 12 22 2 33 5 Mg (mg/l) 10 3 28 5 37 4 5 1 5 2 Fetotal (mg/l) 3 1 4 2 2 2 0.2 0.1 0.9 0.5 Li (mg/l) 2 0.5 2 0.5 6 0.3 3 0.5 2 2 HCO3 (mg/l) 1262 88 2135 123 4716 169 1724 58 2059 311 SO4 (mg/l) 9 4 12 3 3 2 26 11 3 3 Cl (mg/l) 22 5 38 8 76 13 47 15 30 13 SiO2 (mg/l) 52 4 72 2 52 2 74 1 54 9 a Drilled wells b Spring Table 2 units. d13C is in ‰ vs V-PDB. Temp stands for the issue Chemical and isotopic analyses of water samples taken during temperature of sampled water (7C). pH values are from field 1996 fieldwork campaigns at Vilarelho da Raia, Chaves, Vidago measurements. Cond Conductivity (mS/cm) at the issue and Pedras Salgadas areas. Concentrations are in mg/l. DR Dry temperature; n.d. not detected (concentrations below detection d18 d2 3 residue. O and H are in ‰ vs V-SMOW. H is in tritium limit). PCO2 is in atm Vilarelho Vilarelho Chaves Chaves Vidago Vidago Pedras Boticasb,d Pedras da Raia da Raia No. VIb,c AC2a AC16a AC18a Salgadas Salgadasb,d ACP1a ACP2a AC17a Temp. 17.7 17.1 75.0 76.0 17.5 17.2 16.3 11.1 11.0 pH 5.78 – 6.80 7.30 6.10 6.75 6.25 5.26 5.26 Cond. 2100 3050 – 2890 1708 6060 2870 28 270 Na 706 832 610 551 379 1576 585 3 4 K 2841636434104270.70.2 Ca 32 34 21 24 66 207 171 0.8 0.7 Mg 7 9 6 6 12 39 35 0.8 0.2 Li 1.35 2.10 – 2.95 1.95 6.50 2.00 n.d.