Conceptualizing a mountain hydrogeologic system by using an integrated groundwater assessment (Serra da Estrela, Central Portugal): a review

Geosciences Journal DOI 10.1007/s12303-013-0019-x ⓒ The Association of Korean Geoscience Societies and Springer 2013

REVIEW ARTICLE

Conceptualizing a mountain hydrogeologic system by using an integrated groundwater assessment (Serra da Estrela, Central Portugal): a review Jorge Espinha Marques*

Centro de Geologia da Universidade do Porto, Departamento de Geociências, Ambiente e Ordenamento do Território, Faculdade de Ciências da Universidade do Porto, Portugal CEPGIST/CERENA, Instituto Superior Técnico, Universidade Tecnica de Lisboa, Portugal José M. Marques Laboratório de Cartografia e Geologia Aplicada (LABCARGA), Departamento de Engenharia Helder I. Chaminé Geotécnica, Instituto Superior de Engenharia do Porto (ISEP), Politécnicodo Porto; and Centro GeoBioTec|UA, Portugal Instituto Superior Técnico/Campus Tecnológico e Nuclear, Universidade Técnica de Lisboa, Paula M. Carreira Sacavém, Portugal Centro de Geologia da Universidade de Lisboa, Departamento de Geologia, Faculdade de CiênPaulo E. Fonseca cias da Universidade de Lisboa, Portugal Fernando A. Monteiro Santos Centro de Geofísica do Instituto D. Luís and Departamento de Engenharia Geográfica, Geofísica e Energia, Faculdade de Ciências da Universidade de Lisboa, Portugal Centro de Geologia da Universidade do Porto, Departamento de Geociências, Ambiente e OrdeRui Moura namento do Território, Faculdade de Ciências da Universidade do Porto, Portugal Escuela Técnica Superior de Ingenieros de Caminos, Canales y Puertos, Universidad de A Javier Samper Coruña, Spain Bruno Pisani Laboratório de Cartografia e Geologia Aplicada (LABCARGA), DEG, Instituto Superior de José Teixeira Engenharia do Porto (ISEP), Politécnico do Porto; and Centro GeoBioTec|UA, Portugal José Martins Carvalho Centro GeoBioTec|UA, Departamento de Geociências, Universidade de Aveiro, Portugal Fernando Rocha Centro de Geologia da Universidade do Porto, Faculdade de Ciências da Universidade do Porto, Portugal Frederico S. Borges

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ABSTRACT: Mountains are often considered as the world’s water towers. This paper presents a critical review on the research concerning the integrated assessment of groundwater resources of the mountain hydrogeologic system of Serra da Estrela Natural Park (central Portugal). The study area is the Zêzere river basin upstream of Manteigas village located at the Serra da Estrela Mountain in Central Portugal. It provides the source of strategic water resources for the Portuguese mainland, including normal groundwaters, thermomineral waters and surface waters. An integrated approach has been used to formulate a conceptual model for this complex mountain hydrogeological system by integrating the geological, morphotectonic, hydroclimatic, unsaturated soil zone, hydrogeological, hydrogeophysical, hydrogeochemical and isotopic data. This model has been useful to: i) evaluate the water resources; ii) provide the basis for a sustainable management of water resources, iii) design measures for groundwater exploitation and contamination control; and iv) set up land-use policies. Key words: mountain hydrogeology, groundwater resources, recharge, conceptual model, Serra da Estrela (Portugal)

1. INTRODUCTION There is an increasing awareness regarding the importance of mountain hydrological systems as the source of a significant fraction of the World’s water resources. This has *Corresponding author: [email protected]

been recognized by UNESCO in several editions of the International Hydrological Programme (Aureli, 2002; Hassan et al., 2005) and the UN International Year of Mountains (Hofer, 2005). In many parts of the world, mountains are crucial for supplying freshwater to populations (e.g., Lang and Musy, 1990; Bandyopadhyay et al., 1997; Wilson and Guan, 2004; Grunewald et al., 2007; Espinha Marques et al., 2007, 2011). Mountain water resources usually have great quality and strategic socioeconomic importance. Therefore, it is crucial to assure the sustainable management of mountain ecosystems in order to maintain the adequate quantity and quality of water supplies to the populations living downstream (Viviroli and Weingartner, 2008). Yet, in many parts of the world, mountain sustainability is at risk due to several factors, such as a growing water demand for urban, industrial and agricultural uses, changes in land use for urbanization or agriculture, building of dams for water supply or hydropower, recreational activities and climate change (Schreier, 2003). Mountain hydrological systems are difficult to study due to their complex geology, geomorphology and climate. Although they are in isolated areas, they may show anthropic and climate change influences. All these factors produce several interactions that affect the local water cycle. Despite its importance to human populations, mountain hydrological

Jorge Espinha Marques, et al.

studies are uncommon. They are rarely considered a priority by the authorities, resulting in data shortage and poor understanding of such systems (Domenico and Schwartz, 1998; Schreier, 2003; Zektser and Everett, 2004). Hydrological modeling of mountain areas is a challenging task, even when relevant data are available. As a result, the effect of changes in land use in the regional scale water balance and in water quality is normally hard to predict. Nevertheless, mountain basins provide an exceptional opportunity to raise the knowledge on the connection between those complex variables as well as their impacts on the water quality at different elevation zones, under different cultural settings (Chalise, 1994; Beniston, 2003; Neves, 2012). Serra da Estrela is the highest mountain in the Portuguese mainland (1993 m a.s.l.). Its geographical boundary corresponds to the limit of a protected area (Serra da Estrela Natural Park, SENP). This mountain has specific climatic, geologic and geomorphologic features that contribute to control the local water cycle and generate a complex hydrogeological system. The region is the origin of strategic water resources including high quality normal groundwater for agricultural and domestic uses as well as for the bottling industry. Several industrial units are installed in the area; thermomineral

waters utilized for therapeutic purposes at spas; surface waters stored in dams, especially the one that supplies the city of Lisbon (Castelo do Bode dam). Given this scenario, it was fundamental to provide both the Natural Park’s and the national water authorities a groundwater framework focused in a hydrogeologic conceptual model with the ability to support the decision-making process concerning water management (e.g., groundwater recharge rates), water contamination/pollution control (e.g., use of road deicing chemicals), land use (e.g., protection of aquifer recharge areas) and nature conservation (e.g., identification of the ecosystem’s most sensitive areas). This conceptual model could also help private water companies to improve groundwater exploration and exploitation, with clear technical, economic and environmental advantages. Previous hydrogeological knowledge of this region was very scarce. Therefore, this research was a challenging task and the best way to overcome the difficulties was to adopt a multidisciplinary approach encompassing geology, geomorphology, climatology, hydrogeology, hydrogeochemistry, isotope hydrology, hydropedology and geophysics (Fig. 1). Between 2003 and 2008, in order to understand the hydrogeological mountain system of the river Zêzere Basin

Fig. 1. Scheme of the multidisciplinary integrated approach to study the hydrogeological mountain system.

Conceptualizing a mountain hydrogeologic system (Serra da Estrela, Portugal)

Fig. 2. Some views from the river Zêzere Basin Upstream of Manteigas area: (a) river Zêzere valley; (b) alluvia and glacial deposits in Nave de Santo António; (c) Umbrisol in a glacial deposit; (d) river Zêzere; (e) Nave de Santo António spring; (f) Cântaro Magro granitic peak.

Upstream of Manteigas village (ZBUM, Fig. 2), several fieldwork campaigns were performed in the region, under the scope of the HIMOCATCH R&D Project “Role of high mountain areas in catchment water resources, Central Portugal” (e.g., Espinha Marques et al., 2005, 2006a, b, 2007; Espinha Marques, 2007; Marques et al., 2007, 2008a, b, 2011; Carreira et al., 2011). This paper presents a critical review illustrating how such a multidisciplinary approach may be very useful to overcome the specific research difficulties of a complex mountain hydrogeological system in a context of data shortage. The main results are presented in an integrative manner, culminating on the hydrogeologic conceptual model. 2. HYDROGEOLOGICAL FRAMEWORK: STUDY SITE Serra da Estrela (40°15–38N; 7°18–47W) is part of the Cordilheira Central, an ENE-WSW mountain range that crosses the Iberian Peninsula (see Figs. 3 and 4).

The Serra da Estrela climate has Mediterranean influence, with mean annual precipitation around 2500 mm in the most elevated areas. Precipitation (in the type of rain and snow) seems to be mainly controlled by the slope orientation regarding the North Atlantic air circulation patterns and the altitude (Daveau et al., 1997; Mora, 2006). Mean annual air temperatures are below 7 °C in most of the plateau area. They may be as low as 4 °C in the vicinity of the summit (Torre site). According to the Köppen-Geiger climate classification, the study region has a Csb climate (warm temperate, with dry and warm summers), together with Northwestern Iberia (Kottek et al., 2006; Peel et al., 2007). To the South of Serra da Estrela the climate subtype is Csa (warm temperate, with dry and hot summers). Consequently, the climate is another complex issue to deal with because Serra da Estrela climate is not typically Mediterranean since this region is located in a transition zone between the influences of the Atlantic Ocean and the Mediterranean Sea. Serra da Estrela is located in the Central-Iberian Zone of the Iberian Massif (Ribeiro et al., 2007). The main crustal deep-structure is the NNE-SSW Bragança-Vilariça-Manteigas fault zone (Fig. 3), which controls the thermomineral water occurrences (Carvalho, 1996; Espinha Marques et al., 2006a). This structure corresponds to a major seismological fault zone with a locally expressed fault breccia gouge, where the sinistral strike-slip structure is evident (Cabral, 1989; Rockwell et al., 2009; Vicente and Vegas, 2009). The regional hydrogeological units in the ZBUM sector (Fig. 3) are: i) sedimentary cover; ii) metasedimentary rocks; and iii) granitic rocks. The geological and tectonic conditions determine some of the major hydrogeologic features and processes, such as infiltration, aquifer recharge, type of flow medium (porous vs. fractured), type of groundwater flow paths, or hydrogeochemistry. Fractured media occur in poorly weathered granitic and metasedimentary rocks. Porous media are dominant in the alluvium and quaternary glacial deposits as well as in the most weathered granites and metasedimentary rocks. The ZBUM has an area of about 28 km2 and mean altitude of 1505 m a.s.l., ranging from 875 m a.s.l., at the streamflow gauge measurement weir of Manteigas, to 1993 m a.s.l., at the Torre summit (Fig. 4). The mean slope is 20°; the perimeter is 24 km; the shape index (basin area/length of the basin’s longest axis) is 0.39; the Gravelius compactness index (0.28 × basin perimeter/basin area1/2) is 1.31. The landforms of the ZBUM region consist of two major plateaus, separated by the NNE-SSW U-shape glacial valley of the Zêzere River (Daveau et al., 1997). Late Pleistocene glacial landforms and deposits are a distinctive feature of the upper Zêzere basin, since the majority of the plateau area was glaciated during the Last Glacial Maximum (Daveau et al., 1997; Vieira, 2004).

Jorge Espinha Marques, et al.

Fig. 3. Hydrogeological framework of the Serra da Estrela region (geological background revised from Oliveira et al., 1992; and hydrogeological mapping adapted from Carvalho et al., 2007).

Conceptualizing a mountain hydrogeologic system (Serra da Estrela, Portugal)

Fig. 4. Topographic map of the river Zêzere basin upstream Manteigas. Hydromorphological units: (a) Eastern plateau; (b) Zêzere valley eastern slopes; (c) Lower Zêzere valley floor; (d) Nave de Santo António col; (e) Upper Zêzere valley floor; (f) Zêzere valley western slopes; (g) Cântaros slopes; (h) Lower western plateau; and (i) Upper western plateau. For water sampling sites refer to Table 1; for unsaturated zone study sites (I to X) refer to Table 2; for geophysical profiles (GP-a and GP-d) refer to Figure 6.

3. AN INTEGRATED GROUNDWATER ASSESSMENT OF SERRA DA ESTRELA 3.1. Hydrogeochemistry and Isotope Hydrology 3.1.1. Research methods During the fieldwork surveys, three water-types were sampled in the research region: surface waters, shallow cold groundwaters (spring waters – “normal” waters) and ther-

momineral waters. Temperature (°C), pH and electrical conductivity (EC: in µS/cm) were measured in situ using a portable equipment for T °C, pH and EC determinations (WTW pH 330i SET/ SENTIX ORP and COND330i SET, respectively – accuracy T °C: ±0.1 °C, pH: ±0.1 and EC: ±0.1 µS/cm). Water samples were specifically treated by ultrafiltration. Total alkalinity was measured a few hours after collection. The chemical analyses of the waters were performed at the Laboratory of Mineralogy and Petrology of Instituto Superior Técnico, Por-

Jorge Espinha Marques, et al.

Table 1. Representative physico-chemical data of waters from the studied region (average values from April 2004 and September 2005 campaigns) Reference Bisaa Paulo L. Martinsa Covão do Boia Jonjaa Nave de Santo Antónioa Espinhaço de Cãoa AC2 boreholeb AC3 boreholeb Barroqueira Streamc Zêzere river (Spa)c

ID 1 2 3 4 5 6 7 8 9 10

T 10.4 7.5 13.0 13.0 8.7 11.2 41.9 44.4 10.1 13.4

pH 6.1 5.9 6.1 5.7 5.6 6.4 9.1 9.1 6.3 6.6

EC 70 19 12 11 38 137 296 309 16 29

Na 9.5 6.2 2.7 3.0 10.7 12.3 42.7 47.6 4.7 8.0

K 1.02 0.3 0.21 0.16 0.23 0.51 1.12 1.04 0.21 0.21

Ca 3.0 3.7 1.0 0.6 1.0 7.8 3.4 3.5 0.7 1.7

Mg 0.22 0.58 0.01 0.01 0.01 0.55 n.d. n.d. 0.01 0.09

HCO3 27.8 7.4 1.7 1.2 3.3 9.2 77.8 77.2 1.5 8.0

SO4 1.0 1.0 1.1 0.2 0.2 0.5 15.6 15.6 0.3 1.1

NO3 0.11 0.35 0.03 n.d. 0.88 1.95 0.21 0.45 0.52 0.56

Cl 3.5 2.9 1.2 1.1 7.2 17.0 6.1 6.2 3.6 3.2

SiO2 24.1 10.4 13.8 11.9 9.8 23.1 56.7 56.3 11.1 9.9

F n.d. 0.22 n.d. n.d. 0.02 n.d. 8.77 8.76 n.d. n.d.

Notes: Concentrations in mg/L; Temperature (T) in °C; Electrical Conductivity (EC) in µS/cm; n.d. = not detected. a cold spring water; bthermomineral water; csurface water; for sampling locations (ID 1 to 10), refer to Figure 4.

tugal, by the following methods: atomic absorption spectrometry for Ca2+ and Mg2+ and emission spectrometry for Na+ and K+ (Atomic Absorption Spectrometry (AAS) Varian AA 280FS – accuracy: ±0.1 mg/L); colorimetric methods for SiO2 and F– and potentiometry for alkalinity, here referred to as HCO3– (Titroprocessor Metrohm 682 + 665 Dosimat – accuracy: ±0.1 mg/L); ion chromatography for SO42–, NO3– and Cl– (Ion Chromatograph DIONEX ICS-900 – accuracy: ±0.1 mg/L); The main physico-chemical signatures of the sampled waters are summarized in Table 1. The δ2H and δ18O were determined three times for each sample to increase the analytical precision. The measurements were conducted on a mass spectrometer SIRA 10 VG-ISOGAS using the methods proposed by Friedman (1953) and Epstein and Mayeda (1953) for 2H and 18O, respectively (the associated measurement errors are 0.1‰ for 18O and 1‰ for 2H). The tritium content was determined using the electrolytic enrichment and liquid scintillation counting method (IAEA, 1976; Lucas and Unterweger, 2000) using a Packard TRI-CARB 2000 CA/LL. The error associated to the 3H measurements (usually around 0.6 TU) varies with the 3H concentration in the sample. All isotopic determinations were performed in the Environmental Analytical Chemistry Group of Instituto Tecnológico e Nuclear (ITN/IST), Portugal. 14 C determination in thermomineral water was performed at the Geochron Laboratories (Billerica, USA) by accelerator mass spectrometry (AMS). 3.1.2. Hydrogeochemistry Concerning local “normal” groundwaters (shallow cold dilute groundwaters) and surface waters, the geochemical signatures can be defined as: i) Na-HCO3-type waters displaying low total dissolved solids (TDS) values (which are representative of local recharge waters), and ii) Na-Cl-type waters (also low mineralized waters). The Na-Cl geochemical facies found within some of these waters (clearly detected

in the field through the higher electrical conductivity values, up to 150 µS/cm – in April 2004) could be related to the local use of NaCl to promote snowmelt in the roads during the winter season (Table 1 and Fig. 5 – see Espinhaço de Cão and Nave de Sto. António springs). The Caldas de Manteigas thermomineral waters (with output temperatures around 44 °C), used in the local Spa are characterized by: i) pH values ≈ 9; ii) TDS values usually in the range of 160 to 170 mg/L; iii) HCO3 is the dominant anion; iv) Na is the dominant cation; v) the presence of reduced species of sulphur (HS– ≈ 1.7 mg/L); vi) silica values representing a considerable percentage of total mineralization and vii) high fluoride concentrations (greater than 10 mg/L). The geochemical signatures (Table 1 and Fig. 5) could be explained by the fact that at greater depths the groundwaters tend to evolve, with increasing salinity and pH, and a gradual preponderance of Na over Ca. Sulphate probably derive from the oxidation of sulphide minerals (e.g., pyrite), present in the granitic basement. Most reaction rates increase with increasing temperature, and the mineralization of the Caldas de Manteigas thermomineral waters should be strongly dominated by the hydrolysis of Na-plagioclases from the granitic rocks. The strong Na-HCO3 signatures of Caldas de Manteigas thermomineral waters can be clearly observed in the Stiff diagram (Fig. 5). 3.1.3. Reservoir temperature The main objective of geothermometric interpretation is to use the chemistry of the thermomineral waters to estimate the depth of the geothermal reservoir. This methodology relies on the temperature dependence of the concentrations of certain species, the chemical equilibrium between minerals and geofluids. Using reservoir temperatures (between 98 and 103 °C) given by the quartz geothermometer applied to Caldas de Manteigas AC2 and AC3 borehole waters (Espinha Marques, 2007), and considering a mean geother-

Conceptualizing a mountain hydrogeologic system (Serra da Estrela, Portugal)

Fig. 5. Stiff diagrams of the studied waters (average values from April 2004 and September 2005 campaigns). Examples of water sampling in studied area: (a) “normal” groundwater from Jonja spring; (b) thermomineral water from borehole AC3; (c) surface water from Zêzere river, near the Spa; (d) representative isotopic data for the study area.

mal gradient of 25 °C/km (IGM, 1998), we can estimate a maximum depth of about 3.7 km reached by the Caldas de Manteigas thermal water system. This value was obtained considering that: depth = (Tr – Ta) / Gg = (100.5 – 7) / 25 = 3.7 km (1) where Tr is the reservoir temperature (°C), Ta the mean annual air temperature (°C) of the recharge area and Gg the geothermal gradient (°C/km). 3.1.4. Isotope hydrology At Serra da Estrela, from November to March, winter precipitation is usually accumulated in the upper zone of the catchment as snow. Snowmelt often becomes more intense at the end of March. The isotopic composition of the surface water, shallow groundwater and thermomineral water samples range from –8.0 to –7.2‰ for δ18O and from –52 to –42‰ for δ2H (Espinha Marques, 2007). The long term weighted mean value of precipitation obtained at Penhas Douradas meteorological station (located at an altitude of 1380 m a.s.l.) is δ2H = –43‰; δ18O = –7.2‰

(Carreira et al., 2005). The δ18O and δ2H values measured in normal groundwaters at the end of the winter season (April) do not show a significant depletion in heavy isotopes when compared with the data from the end of the summer season (September). This trend seems to suggest the existence of mixing between different water bodies, with the tendency of homogenization of the isotopic composition of the groundwaters (Espinha Marques, 2007; Carreira et al., 2011). In the case of the Caldas de Manteigas thermomineral waters, no shift towards heavier δ18O values, due to exchange with oxygen-18 from silicates, is observed. The methodology used to study the recharge of thermomineral waters was based on the definition of a relationship between δ18O and the altitude of recharge by assuming a conservative behavior of the oxygen isotopic composition from recharge to discharge areas. Samples of normal groundwaters were taken at several altitudes (Fig. 4). The δ18O isotopic gradient obtained for the study area was –0.14‰/100 m of altitude. The isotopic signatures of Caldas de Manteigas thermomineral waters (δ18Omean = –7.8‰ vs. V-SMOW; see Fig. 5) indicate that the recharge of the thermomineral aqui-

Jorge Espinha Marques, et al.

to an expressive glacial basin deposits with granitic boulder accumulations and sandy sediments –, as well as its fill and basement. The Nave de Santo António basin is usually interpreted as corresponding to an infilled lake surrounded by morainic accumulations resulting from two different glaciers during the last glacial maximum (Daveau, 1971; Vieira, 2008). Two moraine ridges from both glaciers and a boulder field are present in the borders of the basin. Alluvial deposits occupy the central area (Figs. 2 and 6). The geophysical survey included (Fig. 6): (1) A NW-SE 600 m dipole-dipole profile (dipole distance of 10 m) placed in the south part of the basin; (2) Three vertical electric soundings (VES) with an AB/2 spacing varying between 1 m and 200 m (VES 2 lies over the dipole-dipole profile; VES 1 is located ca. 150 m to NE of the profile; VES 3 is located ca. 150 m to SE of the profile); and (3) Three NE-SW 188 m electric resistivity profiles using the Wenner array which are located in the NE border of the basin. The dipole-dipole profile and the vertical electric soundings were carried out by means of a SYSCAL Pro device from Iris Instruments. In the Wenner profiles a SYSCAL R1 Plus Switch-48 device, also from Iris Instruments, was applied. In both cases the mean measurement error is less (2) t = 8267 ln(Co/C) than 2%. Co = [100(δDIC – δR)(1 + 2.3ε13/1000)]/(δS – δR + ε13) (3) The results were useful to delineate a model of the depression where C is the measured 14C activity and Co is the “initial” consisting of an irregular paleorelief corresponding to gran14 C activity in the total dissolved inorganic carbon (DIC) in ite with different degrees of weathering overlaid by sedithe measured groundwater (expressed in pmC); δDIC is the ments. According to the VES and the dipole-dipole profile, measured 13C content of carbonate species dissolved in the the sedimentary cover reaches at least 60 m of depth in the sample; δR is the 13C content of CaCO3 in the soil and in central zone of the basin. These sediments consist of layers the rock matrix; δS represents the 13C content of soil CO2; of coarse materials (consisting of alluvial and fluvioglacial ε13 is the 13C enrichment factor during dissolution of soil sand with resistivity values bellow 7000 ohm.m) to very CO2 in the infiltrating water. The value of δS = 21 ± 2‰ was coarse materials (which include a large percentage of glaadopted for calculating the initial 14C activity using Equa- cial and fluvioglacial blocks and boulders, with resistivity tion (3). As representative for the soil and rock carbonates values higher than 7000 ohm.m). In the deepest part of the the value of 0 ± 1‰ was adopted for δR. To account for depression, higher resistivity values could correspond to the fractionation during dissolution of soil CO2 in the infiltrat- granitic basement (Fig. 6). ing water we adopted ε13 = 9.98 ± 0.1‰ was adopted. The The resolution of the Wenner profile is much higher than ε13 value was estimated applying the relation εb(g) = 9.483 the one of the dipole-dipole profile and therefore allowed a ×103/T + 23.89‰ described by Mook et al. (1974), by more detailed interpretation. Resistivity values below 2000 assuming a mean weighted temperature of the area of 7 °C. ohm.m were interpreted as corresponding to finer sandy The apparent groundwater age obtained is 10540 ± 80 years sedimentary layers or to highly weathered granite (W4-5), posBP (pmC = 26.93 ± 0.27 and δ13C = –17.6‰) – Espinha sibly related to a fault zone. Values higher than 2000 ohm.m Marques (2007). The isotopic age points to a long residence point to very coarse glacial and fluvioglacial sediments or time, and is consistent with the geothermometer and 3H data. to poorly weathered granite (W1-2). Some large glacial boulders could explain several high resistivity points observed in Figure 6d. 3.2. Geophysics The resistivity values associated to the sediments are genA geophysical survey was carried out by means of geo- erally high, due to the coarse particle size and to the circulation electric techniques in the Nave de Santo António col (Figs. of poorly mineralized groundwater (as confirmed by the 4 and 6) due to its importance as a potential recharge for the low electrical conductivity of the water from Barroqueira thermomineral aquifer subsystem. This survey intended to stream, which drains the Nave de Santo António basin — investigate the depression’s morphology – which corresponds Fig. 4 and Table 1). fer takes place between 1400 and 1600 m a.s.l. (Espinha Marques, 2007). Tritium content, close to the detection limit, was determined on Caldas de Manteigas thermomineral waters (boreholes AC2 and AC3) indicating long residence times ascribed to groundwater circulation reaching considerable depth. These results support the geothermometric signatures obtained which indicate a maximum circulation depth of the thermomineral waters around 3.7 km. On the other hand, the high 3H content (between 2 and 6 TU is associated to the local shallow cold dilute groundwaters (“normal” groundwaters), indicating that they are young and have relatively short underground flow paths. The normal groundwaters collected during the summer field work campaigns present higher 3H values (Espinha Marques, 2007). This trend can be related to the so-called “spring leak effect” – caused by the temporary mixing between the stratosphere and troposphere at high latitude regions in early spring (Gat et al., 2000). Carbon-14 determination has been used to estimate the “apparent groundwater age” of Caldas de Manteigas thermomineral water. This radiocarbon age was obtained using the following expressions (Salem et al., 1980; Gonfiantini, 1988):

Conceptualizing a mountain hydrogeologic system (Serra da Estrela, Portugal)

Fig. 6. Geophysical study of Nave de Santo António: (a) dipole-dipole electric resistivity profile with the approximate location of the VES; (b) interpretation of the dipole-dipole profile; (c) VES results; (d) Wenner electric resistivity profile; (e) interpretation of the Wenner profile.

Jorge Espinha Marques, et al.

3.3. Unsaturated Zone The unsaturated zone study was carried out at a regional scale and focused on soils due to its influence both on the volume (via aquifer recharge rate) and chemistry of groundwater in the aquifer system. First, soil physical, chemical and mineralogical features were described at 9 sites (Fig. 4, Table 2); finally, the unsaturated zone characteristics that promote interflow were investigated. The following soil units, according to the FAO (2006) classification, occur in the ZBUM: (i) Humic, Leptic and Skeletic Umbrisols; (ii) Lithic and Umbric Leptosols; (iii) Umbric Fluvisols; (iv) Rock outcrops. According to Espinha Marques et al. (2007), the dominant soil profile is ACR type, with an umbric A horizon. These soils are coarse textured (mainly sand and loamy sand), acid (pH ≈ 4.5), with high A horizon organic matter content, mostly controlled by altitude (soils from the mountain summit have organic-matter contents from 180 g.kg–1 to 240 g.kg–1, whereas soils from lower sites present ranging from 49 g.kg–1 to 84 g.kg–1); its total porosity values (≈ 50%) are greater than the reference ones indicated by Carsel and Parrish (1988) for coarse soils. The fine fraction soil mineralogy has detrital origin (with absolute predominance of quartz, mica/illite and feldspars). The assessment of permeability is fundamental to understand water movement in soils, as well as the balance between infiltration and overland flow (Fitts, 2002). Therefore, soil hydraulic conductivity was evaluated in the ZBUM area through 40 field tests (Figs. 4 and 7) – measurement of the field-saturated hydraulic conductivity by means of the constant head Guelph permeameter (Reynolds, 2007). The average value obtained was 7.855 cm/h, which is high according to the SSDS (1993) classification. Further permeability studies, currently in course by means of laboratory tests in undisturbed soil samples (e.g., Klute and Dirksen, 1986), reveal that laboratory results could be

greater than field results by one order of magnitude. According to Bouwer (1978) and Reynolds and Elrick (1987) an important explanatory reason is that complete soil saturation is not achieved in field tests (and therefore is more representative of natural conditions), whereas it is practically complete in laboratory tests. Another factor that may contribute to such results is the water repellency caused by soil organic matter (e.g., Eynard et al., 2003; Arye et al., 2007; Bens et al., 2007) which is more effective in field conditions. In the ZBUM area, four types of unsaturated zone structures were identified (Espinha Marques et al., 2007; Table 2) and classified in terms of the Hydrologic Soil Groups system (from low overland flow potential soils, group A, to high overland flow potential soils, group D) – (e.g., USSCS, 1972; Langan and Lammers, 1991; Boulding, 1993). Type I structure consists of a single layer of poorly weathered granite with very thin or absent soil cover and is related to the hydrologic soil group D. Type II structure has a soil layer usually less than 0.5 m thick overlying a continuous and hard granitic layer. It includes Lithic and Umbric Leptosols from plateaus and slopes and corresponds to hydrologic soil group D. In Type III structure a soil layer between 0.5 and 1.0 m thick overlies intensely weathered granite layer and/or a slope deposit. It corresponds to Leptic Umbrisols from lower altitude areas of slopes and plateaus which are included in hydrologic group C. Type IV structure consists of a soil layer frequently over 1 m thick that overlies a glacial deposit. It is related to soils from the base of slopes as well as from cols and valley floors: Skeletic and Humic Umbrisols (A, B or C hydrologic groups) and Umbric Fluvisols (C or D hydrologic groups). The high hydraulic conductivity of soils promotes infiltration. Yet, like in other high-mountain basins (e.g., Gurtz et al., 2001; Gupta and Wang, 2002; Wu and Xu 2005), several factors tend to simultaneously favor interflow and

Table 2. Features of the unsaturated zone study sites Study site FAO soil class I Humic Umbrisol II Humic Umbrisol III IV V VI VII VIII IX X Average

Parent material Glacial deposit Glacial deposit

Humic Umbrisol Glacial deposit Leptic Umbrisol Granite/Glacial deposit Humic Umbrisol Glacial deposit Umbric Leptosol Granite Skeletic Umbrisol Glacial deposit Leptic Umbrisol Granite Umbric Leptosol Granite Humic Umbrisol Granite – –

Landform Land cover Soil profile Kfs (cm/h) Base of slope Maritime pine woodland A-C 7.786 Base of slope Genista florida and Cytisus sp.pl. A-C 4.740 scrubland Valley-bottom Meso-hygrophilous grassland A-C 4.195 Slope Meso-xerophilous grassland A-C or A-B-C – Col Nardus stricta grassland A-C 3.314 Base of slope Heathland A-C or A-C-R 12.717 Base of slope Heathland A-C – Plateau Nardus stricta grassland A-R 4.674 Plateau Common juniper shrubland A-R 12.821 Slope Quercus pyrenaica forest A-B-C-R 12.589 – – – 7.855

Data compiled from Espinha Marques (2007) and Espinha Marques et al. (2007). Kfs: field saturated hydraulic conductivity; for unsaturated study site location (I to X) refer to Figure 4.

Conceptualizing a mountain hydrogeologic system (Serra da Estrela, Portugal)

Fig. 7. (a) Field tests for the evaluation of soil hydraulic conductivity; (b) shallow granitic layer with sub-horizontal fractures.

therefore decrease aquifer recharge. In the studied basin, the first three types of unsaturated zone structures are affected by one or more of the following factors: (i) The steep relief, that shortens the water underground paths; (ii) The combination of highly permeable topsoil and fractured and much less permeable shallow granitic bedrock; (iii) The wide presence of a shallow layer (up to five meters of depth) of granitic rock showing intense sub-horizontal fracturing that increase horizontal permeability (Fig. 7). These structures – which play an important role in the water circulation in the unsaturated zone – are known as “sheeting fractures” or “pseudostratification”, and could be originated by exogenous (e.g., offloading related to deglaciation) or endogenous (e.g., lateral compressive stress) factors (CFCFF, 1996; Twidale and Vidal Romaní, 2005). 4. HYDROGEOLOGICAL CONCEPTUAL MODEL The elaboration of a hydrogeologic conceptual model must bear in mind that it is a formal and simplified representation of a given reality and it should be subject of a close interaction with mathematical modeling (NAP, 2001). Therefore it is fundamental to understand the aquifer nature, its broad characteristics and the physical and chemical processes involved. The VISUAL BALAN v2.0 code (e.g., Samper et al., 1999, 2007) was applied to evaluate its water resources and especially the aquifer recharge, resulting in a better comprehension of the local water cycle dynamics (Espinha Marques et al., 2011). This code is based on a semi-distributed mathematical model that performs daily water balances in the soil, in the underlying unsaturated zone and in the aquifer. Table 3 presents the most relevant modeling results, particularly the estimate of a groundwater recharge rate of 15%,

Table 3. Mean annual results of the water balance of ZBUM

Precipitation Snow precipitation Actual evapotranspiration Overland flow Potential recharge Interflow Groundwater recharge Total stream flow

Balance component (%) of mean annual (mm) precipitation 2336 100 373 16 727 31 310 13 1308 56 962 41 349 15 1621 69

which are close to the ones achieved with other techniques by previous studies of hard-rock aquifer systems located in mountains from the Central-Iberian Zone (e.g., Carvalho et al., 2000; Oliveira, 2006). Another important result regards interflow which reaches 41% of mean annual precipitation and is in agreement with the unsaturated zone features described in the previous section. The monthly distribution of the stream flow components (surface runoff, interflow and groundwater flow) is illustrated in Figure 8. From October to May (the wet season) the weight of interflow is very clear and emphasizes the importance of this water balance component in the assessment of groundwater resources in mountain areas. The hydrogeological conceptual model for the ZBUM considers three main types of aquifers: (i) unconfined aquifers, hydraulically connected to the unsaturated zone; (ii) shallow semi-confined aquifers; iii) a deep thermomineral aquifer. Waters from aquifer types i) and ii) have TDS values 40 mg/L, pH 6, temperature 10 °C and relatively short

Jorge Espinha Marques, et al.

Fig. 8. Stacked area chart displaying monthly distribution of stream flow components.

flowpaths and residence times (usually, up to some decades). The thermomineral waters have TDS values 160 mg/L, pH 9, temperature 44 °C and much longer flowpaths and residence times (around 10000 years). The unsaturated zone types of structure identified in the ZBUM area tend to facilitate infiltration from rain and snowmelt. Yet, as discussed earlier, the unconfined aquifer recharge is greatly reduced due to the influence of the factors that promote interflow. Most of this recharge occurs where the unsaturated zone structures of type II, III and IV prevail: the eastern and western plateaus and the slopes of the Zêzere valley and its tributaries. The main groundwater discharge areas from aquifer types i) and ii) are the Zêzere and the Candeeira valley-bottoms and the Nave de Santo António col (see Fig. 4). The recharge of the thermomineral aquifer takes place on more permeable zones of the granitic massif, which correspond to the main tectonic structures in the basin (Figs. 3 and 4) and act, at the same time, as discharge areas of the shallow aquifer subsystem. These recharge areas consist of small sedimentary basins which include a sedimentary cover (alluvium and quaternary glacial deposits) overlying tectonized granite. Water enters the basins due to vertical infiltration (from rain and snowmelt) as well as to lateral flow from the bordering shallow aquifers. The resulting water table lies very close to the surface throughout the year. Part of the groundwater flow reaches the surface and outflows in springs or along the river bottoms, while another part circulates downward through the tectonic structures, eventually reaching the deep thermomineral reservoir (Fig. 9). The morphostructural and isotopic results point out three main recharge areas of the thermomineral aquifer located between 1400 m a.s.l. and 1600 m a.s.l. (Fig. 4): (i) the

Nave de Santo António col, lying over the main BVMFZ lineament; (ii) the Covão de Ametade valley and (iii) the Candeeira valley, both corresponding to conjugate faults of the main structure. The thermomineral waters flow along the BVMFZ towards the discharge zone at the Caldas de Manteigas Spa (800 m a.s.l.) where it ascends from the deep reservoir probably due to the intersection of the main NNESSW structure by WNW-ESE conjugate structures and the presence of a geologic limit separating different granitic facies. 5. CONCLUSIONS Mountains are more recognized as the World’s water towers due to the quantity and high quality of its water resources. The Serra da Estrela massif is regarded as a fundamental source of freshwater for the Portuguese mainland and therefore should be carefully managed to assure the system’s sustainability in the long term. Bearing in mind this framework, the assessment of the hydrogeologic system of the river Zêzere Basin Upstream of Manteigas village was carried out through an integrated approach meant to face the great research difficulties resulting from the shortage of previous data and from the complexity of the system itself. The study received contributions from several domains of Earth Sciences such as geology, hydrogeology, isotope hydrology, hydropedology and geophysics. The research dedicated special attention to the regional hydrogeologic framework, to the unsaturated zone and to the geochemical and isotopic features from groundwater and, also, surface water. The study results provided the basis of a conceptual model

Conceptualizing a mountain hydrogeologic system (Serra da Estrela, Portugal)

Fig. 9. ZBUM hydrogeological conceptual model (updated from Espinha Marques, 2007) as resulted from the multidisciplinary integrative approach.

which reflects the complexity of the ZBUM mountain hydrogeologic system, encompassing the unsaturated zone (emphasizing the interflow and recharge conditions), the shallow and intermediate aquifers (with circulation of normal groundwater) and the deep aquifer (with circulation of thermomineral water). The integrative effort made possible to develop a conceptual model in close relationship to the mathematical model carried out through the VISUAL BALAN v2.0 code. Based on the isotope hydrology as well as on morphotectonical and geophysical results, the conceptual

model was also used to determine the location of recharge areas, the flow paths and the residence times of the thermomineral waters. The integrated research methodology has proven to be very suitable to investigate a complex mountain hydrogeologic system with limited previous data. The results may be used by the authorities to promote the sustainable management of water resources, especially in what concerns groundwater exploitation and contamination control, as well as land-use policies.

Jorge Espinha Marques, et al.

ACKNOWLEDGMENTS: This study was performed within the scope of the HIMOCATCH R&D Project granted by the Portuguese Foundation for Science and Technology (FCT) and FEDER EU funds (POCTI/CTA/44235/02). The research was conducted within the framework of the PEst-OE/CTE/UI0039/2011/2012 project (CGUP), the PEst-C/CTE/UI4035/2011/2012 project (GeoBioTec|UA) and the re-equipment program (LABCARGA-IPP-ISEP|PAD'2007/08). JT holds a doctoral scholarship from the FCT (SFRH/BD/29762/2006). The authors thank the anonymous reviewer and the editor Prof. Kang-Kun Lee for their critical comments and suggestions that helped to improve the manuscript.

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