Centro de Recursos Naturais e Ambiente (CERENA), Instituto Superior Técnico, University of Lisbon, Av. Rovisco Pais , Lisbon, Portugal.

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1 Use of environmental isotopes ( 2 H, 13 C, 18 O, 3 H and 14 C) in the characterization of hydromineral and geothermal systems in north of Portugal (Monção and Gerês spas) Guerra, A. 1, Marques,.M. 1 and Carreira, P.M. 2 1 Centro de Recursos Naturais e Ambiente (CERENA), Instituto Superior Técnico, University of Lisbon, Av. Rovisco Pais , Lisbon, Portugal. 2 Centro de Ciências e Tecnologias Nucleares (C2TN), Instituto Superior Técnico, University of Lisbon, Estrada Nacional 10, ao km 139, obadela LRS, Portugal. Abstract The studied thermomineral waters are situated in the north of Portuguese mainland. In order to update the conceptual circulation models of Monção and Gerês thermomineral water, isotope hydrology approach was used in addition to traditional hydrogeological and hydrogeochemical techniques. The environmental isotope techniques are based on the variability of stable and radioactive, in nature and this variability can be used as natural tracers of groundwater flow. Chemical geothermometers were also used to estimate the reservoir temperature and the maximum depth reached by the thermomineral water systems. oth thermomineral waters (Monção and Gerês) are meteoric waters with ages over years (based on the absence of tritium). The chemical composition is strongly dependent on water-rock interaction processes. The Monção thermomineral water infiltrates at an elevation between 350 and 475 m a.s.l., flowing at a maximum depth of 2.9 km to 3.2 km, and emerging with a temperature of 47 ºC. The Gerês thermomineral waters infiltrates at a recharge altitude between 900 to 1115 m a.s.l., with a maximum circulation depth of 2.6 km to 3.4 km, and rising to the surface with a temperature of 43 ºC. oth groundwater flow paths systems of Gerês and Monção, are associated with fault and fracture structures. Key-words: Hydromineral and Geothermal Systems; Isotope Hydrology; N Portugal; Monção; Gerês 1. Introduction The geothermal resources are classified into high and low enthalpy resources. In Portuguese mainland only low enthalpy resources can be observed (reservoir temperature below 150 ºC). However, according to Lourenço & Cruz (2005) the occurrences of hydromineral and geothermal resources in Portuguese mainland are distributed asymmetrically, due to the different geological and structural features of the country. The geothermal potential in Portuguese mainland is related to tectonic accidents that benefit the rapid upward fluids movement. Nevertheless, climatology and geomorphology also have an important 1

2 contribution to that asymmetry (Lourenço, 1998; IGM, 1998; Aires-arros & Marques, 2000). Most of the hydromineral and geothermal resources are meteoric waters. When the precipitation waters infiltrate at depth they acquire specific physical and chemical characteristics, depending on the mineralogical composition of the percolated geological formations. The water issue temperature is a function of the depth of flow paths. The geochemical signatures are associated with water-rock interaction. Usually, higher water mineralization is promoted by higher temperatures (Marques et al., 2012). For a better understanding, characterization and development of conceptual circulation models of the hydromineral and/or geothermal systems, in addition of using conventional hydrogeological, geological, geochemical and geophysical techniques, the use of isotope hydrology, through the study of the variability of environmental stable ( 2 H, 13 C and 18 O) and radioactive ( 3 H and 14 C) isotopes, is usually regarded as a valuable tool. The application of such analytical nuclear techniques has been extremely useful in many case studies, enabling the definition of recharge areas, of groundwater dating and estimation of residence time, direction of groundwater flow system, identification of palaeowaters, evaluation of water salinization mechanisms and pollution sources, as well as the quantification of mixing between different aquifer systems (e.g., Clark & Fritz, 1997; Mook, 2000b; Edmunds, 2005; Kharaka & Mariner, 2005). The aim of this paper is to use environmental isotopes (stable and radioactive) together with geochemical data to update the conceptual circulation models of the thermomineral waters systems from Monção and Gerês Spas. Chemical geothermometry was used to estimate the reservoir temperature and the maximum depth reached by these thermomineral water systems. 2. Methods 2.1. Physical and chemical analysis The physico-chemical parameters measured in situ were the temperature, ph, electrical conductivity and redox potential. The chemical parameters analysed at laboratory were: Na, K, Ca, Mg, Li, HCO3, SO4, Cl, NO3, F and SiO2 (taken from Carreira et al., 2004) Environmental isotopes The most frequently used environmental isotopes correspond to the heavy isotopes of the elements founding in the water molecule, such as hydrogen ( 2 H and 3 H) and oxygen ( 18 O), and also carbon isotopes ( 13 C and 14 C) present in the aqueous systems as dissolved inorganic and organic carbon. The use of stable isotopes is based on the variation of the isotopic composition comparative to an international standard; these deviations are defined by the following equations: δ( ) = ( R sample R standard 1) 1000 Where: R sample ratio 2 H/ 1 H, 18 O/ 16 O or 13 C/ 12 C; R standard the same ratio determined in the standard. In isotope hydrology studies for 2 H and 18 O the reference standard is V-SMOW (Vienna Standard Mean Ocean Water). For 13 C is the V-PD (Vienna Pee Dee elemnite) (e.g., Clark & Fritz, 1997; Mook, 2000a). 2

3 The use of radioactive isotopes is based on the radioactive decay law. The content in tritium is expressed in Tritium Units (TU). The 14 C concentration is expressed in terms of pmc (percent modern Carbon), given by the following equation: A sample pmc = ( A oxalic acid eλ(y 1950)) 100 (%) Where: A sample specific activity of 14 C measured in the sample; A oxalic acid specific activity of 14 C determined in the oxalic acid (modern reference standard); λ - 1/8267 years; y counting year of oxalic acid (Mook, 2000a) Chemical geothermometry Chemical geothermometers are hydrogeological tools that allow estimating the reservoir temperature of hydrothermal systems from the chemical composition of the up flowing groundwaters. According to Davraz (2014), for the application of these geothermometers it must be assumed that: i) the chemical equilibrium is reached, at depth, in the reservoir, being a function of the temperature; ii) the rise of the fluid from the reservoir to the surface is fast, preventing a significant re-equilibrium; iii) there is no mixing between different water systems during the rise of the geothermal fluids from the reservoir to the surface. The chemical geothermometers used in this study have been proposed by Truesdell (1975) and Giggenbach (1988) due to the acceptable and consistent estimation of reservoir temperature that usually provide in case of low enthalpy systems, which is the case of Monção and Gerês systems. The Truedell (1975) is a silica geothermometer given by T(ºC) = 1315 ( ) 273,15, where SiO2 is in mg/l. 5,205 log SiO 2 The Giggenbach (1988) is a K Mg geothermometer given by T(ºC) = ( log( K2 Mg ) ) 273,15, where K and Mg is in mg/l. 3. Results and discussion 3.1. Monção thermomineral groundwater system Geological and hydrogeologic setting The Monção region is located in the NW of Portugal. This region is characterized by granitic rocks. It s also possible to find small veins in mafic rocks, pegmatitic and aplitic veins and quartz veins. The metasedimentary rocks are also present in this region, sometimes strictly related with migmatites. Recent sediments, that constitutes the alluvial plain and terraces of the Minho River, can also be found. The main fracture network systems are represented by tectonic lineaments (strike-slip faults), with directions ENE-WSW, WNW-ESE, NNE-SSW and NNW-SSE (Carreira et al., 2005). The thermal springs of the Monção Spa are located on the left shore of the Minho River in the alluvial terraces, and are aligned according to the ENE-WSW direction, indicating a possible relation between the rise of these fluids and the fault system (Carreira et al., 2005) Hydrogeochemical approach The thermomineral waters have a temperature (T) of about 47 ºC, ph 7, electrical conductivity (EC) from 633 to 793 μs/cm, and Total Dissolved Solids (TDS) from 420 mg/l to 470 mg/l. The local shallow 3

4 HCO 3 (mg/l) Na (mg/l) cold groundwaters have T around 14 ºC, ph between 4 and 6, EC ranging from 39 to 162 μs/cm, and TDS between 24 and 120 mg/l. In general, the both water types of show a HCO3-Na faceis, as can be observed in a Piper Diagram (Figure 1). Mg K K Ca Figure 1 Piper diagram. ( ) thermomineral waters; ( ) shallow cold groundwaters; ( ) Minho River. In Piper Diagram of Figure 1, we can observe the existence of "two clusters of waters", one formed by the thermomineral waters and another one by the shallow cold dilute groundwaters. This trend can also be observed in the projection of the dominant ion composition (Na-HCO3) with TDS, supporting the inexistence of mixing between these two water groups (Figure 2a,b). SO 4 + Cl The dominant presence of HCO3 - anion and Na +, K +, Ca 2+ and Mg 2+ cations can be explained by water-rock interaction (WRI) dominated by a granitic environment. HCO3 -, Na + and (in less extent) Ca 2+ derive from the hydrolysis of Naplagioclase (e.g., Freeze & Cherry, 1979). The K + is ascribed to WRI with K-feldspar, and Mg 2+ cation is associated with mafic minerals, such as biotite. The presence of silica is mainly the result of the hydrolysis of silicates (that origin clay minerals). The clay minerals are responsible for the K + ion absorption (Custódio & Lhamas, 1983 in: Marques, 2012). Na + K K HCO 3 Ca + Mg K Cl SO (b) Figure 2 (a) Na vs. TDS; (b) HCO 3 vs. TDS. ( ) thermomineral waters; ( ) shallow cold groundwaters; ( ) Minho River. (a) Concerning the increase of chlorine in thermomineral waters relative to shallow cold groundwaters (e.g. 25<Cl - <45 mg/l for a small range of Na + values) some hypotheses were formulated: i) water-rock interaction (due to apatite weathering), however this hypotheses seems to be unreliable due to the poor correlation of chlorine with Ca and F (-0.22 and 0.01, respectively); ii) in addition chlorine could have a deep origin (in the upper mantle), ascending to the surface through the major tectonic structures (faults) of the region, being corroborated by the δ 13 C values (see item ); iii) chlorine concentration resulting from evapotranspiration process due to the conservative character of this ion (Skrzypek et al., 2013) TDS (mg/l) TDS (mg/l) Isotopic approach Projecting the available isotopic data ( 18 O and 2 H) of the thermomineral waters and of the shallow cold groundwaters of Monção region in the diagram of Figure 3, we can observed that 4

5 δ 2 H ( ) the groundwater samples are distributed over or near the Global Meteoric Water Line (G-MWL), indicating that these waters didn t suffer any significant evaporation prior to infiltration (the slope of Local Meteoric Water Line L-MWL is similar to the slope of G-MWL), as well no isotopic deviation as result of water-rock interaction at very high temperatures. The gap found between one thermomineral water sample and the remains can be explained due the different period of sampling with different climatic conditions (Oct 99 end of the dry season and middle of wet season Feb 02 and Feb 03). However, other samples taken in this period didn t show the seasonal and amount rainfall effects in their composition, which suggests a historical value. Figure 3 2 H vs. 18 O. ( ) thermomineral waters; ( ) shallow cold groundwaters; ( ) Minho River. In the Monção Spa region the isotopic gradient obtained for 18 O ( altitude effect ) was per 100 m of altitude. This value was estimated using the discharge altitude of the springs (shallow cold groundwater systems). The recharge altitude of Monção thermomineral waters was calculated considering the regional isotopic gradient obtained. Considering the average δ 18 O values of AC1 and AC2 boreholes and S. Saúde spring), we have obtained recharge altitudes between 350 m and 475 m a.s.l Global Meteoric Water Line δ 2 H = 8 δ 18 O + 10 Local Meteoric -40 Water Line δ 2 H = 9.02 δ 18 O R² = δ 18 O ( ) The absence of tritium in thermomineral waters indicates a relatively residence time, at least more than years. Although the AC1 sample of Oct 99 has a content of 4.1 TU, the absent of 3 H in the other thermomineral water samples can be explained by the decrease of tritium content in the atmosphere in the last years and due to the mean residence time, considering the short half-life of this isotope (Carreira et al., 2007). The shallow cold groundwaters have 3 H values around 2.1 TU to 5.2 TU, reflecting a short residence time and local recharge when compared with the tritium record measured at Porto meteorological station (4.5 TU weighted arithmetic mean) (Carreira et al., 2007). The δ 13 C signatures are and in AC1 and AC2 borehole waters, respectively, suggesting a mixture between different carbon origins, namely atmospheric CO2, CO2 derived from the decay of organic matter and plant roots respiration and deep-seated (upper mantle) CO2. The hypothesis of dissolution of limestones isn t feasible since that type of rocks is absence of the geological environment of the region (Carreira et al., 2004). The low 14 C content in the thermomineral waters, 4.82 ± 1.00 pmc in AC1 and 7.43 ± 0.34 pmc in AC2 borehole waters, together with the absence of tritium, support the hypothesis of a long residence time Reservoir temperature In order to obtain the reservoir temperature (Table 1) the quartz and K 2 /Mg geothermometers were used. The maximum depth reached by Monção thermomineral waters (Table 1) was estimated considering that: depth = (Tr - Ta)/gg, where Tr is the reservoir temperature (ºC), Ta the mean annual 5

6 temperature (14 ºC) and gg the regional geothermal gradient. Table 1 Reservoir temperature and maximum depth reached by Monção thermomineral waters. (a) Truesdell (1975); (b) Giggenbach (1988). Reference Date quartz T(ºC) (a) K 2 /Mg T(ºC) (b) Maximum depth reached (km) AC1* Out/ AC1* Fev/ AC1* Fev/ AC2* Out/ AC2* Fev/ AC2* Fev/ S. Saúde + Out/ Notes: * orehole waters; + Spring water Conceptual circulation model ased on the multidisciplinary approach, the conceptual circulation model of Figure 4 was proposed. The δ 18 O signatures indicate that the main recharge altitude of thermomineral waters system is around m a.s.l. As local meteoric waters infiltrate at depth, through major rock faults and fractures, water-rock interaction in enhanced increasing water mineralization, reaching a depth around 2.9 km to 3.2 km, and reservoir temperatures between 80 ºC to 130 ºC. Afterwards, the thermomineral waters will rise to the surface through major faults aligned according to the ENE-WSW direction Gerês thermomineral groundwater system Geological and hydrogeologic setting The Gerês Spa region is situated in NW of Portuguese mainland. This region is characterized by granitic rocks and some modern alluvial deposits. The geomorphology is dominated by high mountains with steep reliefs and tectonic valleys, such as the Gerês valley, with NNE-SSW direction (Medeiros et al., 1975). The thermomineral springs occur through diaclases in Gerês Valley (Medeiros et al., 1975) Hydrogeochemical approach The thermomineral waters have a T of 43 ºC, ph 8, EC from 314 to 353 μs/cm, and TDS around 420 mg/l to 470 mg/l. The shallow cold dilute groundwater samples shown T around 14 ºC, ph 6, EC from 12 to 136 μs/cm, TDS between 7 and 38 mg/l. The superficial waters (rivers and streams) have TDS between 7 to 15 mg/l. SO 4 + Cl Ca + Mg Figure 4 Representative scheme of the conceptual circulation model of Monção thermomineral system (a). The shallow cold waters system for comparison (b). Mg Na + K K K K K HCO 3 K KK SO 4 Ca Figure 5 Piper diagram. ( ) thermomineral waters; ( ) shallow cold groundwaters; ( ) river and streams waters. Cl 6

7 δ 2 H ( ) HCO 3 (mg/l) Na (mg/l) As observed in Piper Diagram of Figure 5, the groundwaters samples of this region have mostly a HCO3-Na faceis. In that diagram is possible to identify two groups of waters, one composed by the thermomineral waters and another by the shallow cold groundwaters. These two groups can also be observed in the projection of the dominant ion composition (Na- HCO3) vs. TDS, supporting the inexistence of mixing between the deeper water system with shallow or superficial waters (Figure 6) TSD (mg/l) TSD (mg/l) (b) Figure 6 (a) Na vs. TDS; (b) HCO 3 vs. TDS. ( ) thermomineral waters; ( ) shallow cold groundwaters; ( ) river and streams waters. (a) The main presence of HCO3 - anion and Na +, K +, Ca 2+ and Mg 2+ cations can be explained by water-rock interaction dominated by a granitic environment. HCO3 -, Na + and (in less extent) Ca 2+ derive from the hydrolysis of Naplagioclase (e.g., Freeze & Cherry, 1979). The K + is ascribed to WRI with K-feldspar, and Mg 2+ cation is associated with mafic minerals, such as biotite. The presence of silica is mainly the result of the hydrolysis of silicates (that origin clay minerals). The clay minerals are responsible for the K + ion absorption (Custódio & Lhamas, 1983 in: Marques, 2012). Concerning the higher chlorine concentration in the thermomineral waters relatively to the shallow cold groundwaters, the following hypotheses have been considered: i) water-rock (apatite) interaction, which seems to be plausible since the correlation values of Cl with Ca and F (-0.69 and 0.61, respectively) are reasonably high; ii) chlorine concentration resulting from evapotranspiration process due to the conservative character of this ion (Skrzypek et al., 2013) Isotopic approach Projecting the available isotopic data ( 18 O and 2 H) of the thermomineral waters and the shallow cold groundwaters of Gerês region in the diagram of Figure 7, we can observed that the groundwater samples are distributed over or near the Global Meteoric Water Line (G-MWL), indicating that these waters are meteoric waters. The slope of the Local Meteoric Water Line (L-MWL: δ 2 H = 6.06 δ 18 O ) is lower when compared to the G-MWL, indicating that the water samples have suffered isotopic fractionation due to evaporation before infiltration, inducing an enrichment in heavy isotopes in the water Local Meteoric Water Line δ 2 H = 6.06 δ 18 O R² = Global Meteoric Water Line δ 2 H = 8 δ 18 O δ 18 O ( ) Figure 7 2 H vs. 18 O. ( ) thermomineral waters; ( ) shallow cold groundwaters; ( ) river and streams waters. 7

8 The groundwater samples enriched in heavy isotopes correspond to shallow cold spring waters of Fontanário do Tanquinho. It should be noted that their sampling was carried out in Feb. 02 and Mar. 03, while the remaining shallow cold groundwaters, depleted in heavy isotopes, correspond to the May 10 field work campaign. This unusual trend in the isotopic water composition may be ascribed to sampling performed during different hydrological years (i.e. Fontanário do Tanquinho sampling was performed during a hydrological year characterized by less precipitation and/or higher mean annual air temperature, causing the enrichment on the isotopic composition. The recharge area was estimated from the following equation: altitude (m) = -588 δ 18 O 2734, representing the mean isotopic gradient with altitude, estimated by Lima (2011). The altitude of the recharge area is around 900 m to 1115 m a.s.l., based on the mean δ 18 O values for each sample (Forte, Contra Forte and ica springs). The thermomineral waters are characterized by the absence of tritium, suggesting a residence time at least more than years (considering the average content in atmospheric 3 H). The presence of 3 H in Fontanário do Tanquinho shallow cold spring water samples, indicates that these waters have a short residence time. (Table 2) was obtained considering that: depth = (Tr-Ta)/gg, where Tr is the reservoir temperature (ºC), Ta the mean annual temperature (13 ºC) and gg the regional geothermal gradient. Table 2 Reservoir temperature and maximum depth reached by Gerês thermomineral waters. (a) Truesdell (1975); (b) Giggenbach (1988). Reference Date quartz T (ºC) (a) K 2 /Mg T (ºC) (b) Maximum depth reached (km) Spring Forte Mar/ Spring Forte Fev/ Spring CForte Mar/ Spring CForte Fev/ Spring ica Ma/ Spring ica Fev/ Conceptual circulation model In the elaboration of the conceptual circulation model (Figure 8) of Gerês thermomineral waters, we have taken into account the different approaches undertaken in this study Reservoir Temperature ased on the quartz and K 2 /Mg geothermometers (Truesdell, 1975 and Giggenbach, 1988, respectively) the reservoir temperature was estimated. The results obtained are showed in Table 2. The maximum depth reached by the thermomineral waters Figure 8 Representative scheme of the conceptual circulation model of Gerês thermomineral system. The water that emerges as thermomineral springs comes from the rainfall, which infiltration occurs in places with an altitude of about 900 m to 1115 m a.s.l.. This water infiltrates at depth through major rock discontinuities (faults and 8

9 fractures) and water-rock interaction in enhanced increasing water mineralization, reaching a depth around 2.6 km to 3.4 km, and reservoir temperatures between 93ºC and 123ºC. The water will rise through fault system. The similar chemical and isotopic composition suggests that the thermomineral spring waters belong to the same hydrothermal system, with similar flow paths. This is support by the proximity of thermal springs (5 to 10 m between them). Another explanation for this similarity in chemical composition is the fact that the granitic rocks in this region are quite uniform in mineralogical composition. 4. Concluding remarks The stable isotopes of oxygen and hydrogen were used as natural tracers to estimate the origin and altitude of the recharge areas of the Monção and Gerês thermomineral aquifer systems, and characterization of groundwater flow. The tritium ( 3 H) content was used also as a natural tracer in issues related to the dynamics of the groundwater flow systems and existence (or not) of mixing processes between different aquifer units. Carbon isotopes ( 13 C and 14 C) allowed the determination of the main source carbon dissolved in the aqueous system and estimate an apparent age, respectively. oth case studies have similar conceptual circulation models. The recharge areas are located at higher elevations, the meteoric waters infiltrates at depth through major fractures and faults in the massive, water-rock interaction is enhanced, and therefore the TDS in water increases, being temperature dependent. The thermomineral waters rise to the surface through preferential fault systems. References Aires-arros, L., & Marques,. M. (2000). Portugal Country Update. World Geothermal Congress, (pp ). Kyushu-Tohoku, apan. Carreira, P. M., Marques,. M., Andrade, M., Matias, H., Luzio, R., Monteiro Santos, F., & Nunes, D. (2004). Isotopic, geochemical and geophysical studies to improve Caldas de Monção thermomineral waters conceptual circulation model (NW-Portugal). Revista de Xeologia Galega e do Hercínico Peninsular, 29, pp Carreira, P. M., Marques,. M., Carvalho, M. R., Monteiro Santos, F., Matias, H., Luzio, R., & Nunes, D. (2007). Fluid/mineral equilibrium calculations, isotopes and geophysics as a multidisciplinary approach to the characterization of Monção hydrothermal system (NW-Portugal). Em L. Chery, & G. Marsilly (Edits.), Aquifers Systems Managment: Darcy's Legacy in a World of Impeding Water Shortage, Nº 10 - IAH (Vol. Chapter 27, pp ). London: Taylor & Francis Group. Carreira, P. M., Marques,. M., Monteiro Santos, F. A., Andrade, M., Matias, H., Luzio, R., & Nunes, D. (2005). Role of geophysics, geochemistry and environmental isotopes in the assessment of Caldas de Monção lowtemperature geothermal system (Portugal). Geothermal Resources Council Transactions, 29, pp Clark, I. D., & Fritz, P. (1997). Environmental Isotopes in Hydrogeology. United States of America: CRC Press. Davraz, A. (2014). Application of hydro- -geochemical techniques in geothermal systems; exemples from the eastern Mediterranean region. Em A. aba,. 9

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