Geochemical evolution of groundwater in the carbonate aquifers of Sierra de Segura (Betic Cordillera, southern Spain)

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  • Geochemical ein the carbona(Betic Cordiller

    F a, Cruz

    a s Fsic41b sidadc

    Received 17 July 2007; received in revised form 30 June 2008; accepted 1 July 2008

    Mg/Ca ratio;Temperature;

    stituted of Mesozoic and Cenozoic sedimentary rocks, among which there are thicklimestonedolomitic formations which have given rise to extensive outcrops of permeable

    of which the 50 most important add up to a mean spring flow of about 13,500 l/s. The

    solution of dolomite. Finally, the distribution of the temperatures in the vadose zone,determined by atmospheric thermal gradient, implies an apparent stratification of thepredominant hydrochemical processes and of the groundwater physical and chemicalcharacteristics. 2008 Elsevier B.V. All rights reserved.

    0022-1694/$ - see front matter 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.jhydrol.2008.07.012

    * Corresponding author. Tel.: +34 954349829; fax: +34 954349814.E-mail addresses: fmormar@upo.es (F. Moral), jjcruz@ugr.es (J.J. Cruz-Sanjulian), manuel.olias@dgyp.uhu.es (M. Olas).

    Journal of Hydrology (2008) 360, 281296

    ava i lab le a t www.sc iencedi rec t . com

    joactive geochemical processes in aquifers of Sierra de Segura, with their correspondingtime sequence, are: dissolution of CO2, dissolution of calcite, incongruent dissolution ofdolomite, dedolomitization, exsolution of CO2, and precipitation of calcite. More evolvedwater has higher temperature, magnesium content and Mg/Ca ratio; therefore, theseparameters can be utilised as indicators of the degree of hydrochemical evolution. In addi-tion, a good correlation between water temperature and magnesium concentration (orMg/Ca ratio) indicates that an increase in temperature accelerates the kinetics of the dis-Evolutive model materials. In geomorphological terms, there is a large plateau intensively karstified thatconstitutes the main recharge area. Discharge takes place via a large number of springs,KEYWORDSCarbonate aquifer;Hydrochemicalprocesses;

    Summary Sierra de Segura (Betic Cordillera), with a total area of over 3000 km2, is thesource of the two principal rivers in southern Spain, the Guadalquivir and the Segura. Dueto the orographic effect of these mountains, precipitations are considerably more abun-dant than in nearby lowland areas, where the climate is semi-arid. Sierra de Segura is con-. Moral *, J.J.

    Departamento de Sistema013 Seville, SpainInstituto del Agua, UniverDepartamento de Geodinamica yvolution of groundwaterte aquifers of Sierra de Seguraa, southern Spain)

    -Sanjulian b, M. Olas c

    os, Qumicos y Naturales, Universidad Pablo de Olavide, Carretera de Utrera, Km 1,

    de Granada, C/ Ramon y Cajal No. 4, 18071 Granada, SpainPaleontologa, Universidad de Huelva, Campus El Carmen s/n, 21071 Huelva, Spain

    urnal homepage: www.elsevier .com/ locate / jhydrol

  • existence of perched water tables related to Segura pla-

    282 F. Moral et al.Introduction

    The physical and chemical characteristics of groundwater inkarstic aquifers are determined by the chemical and mineralcomposition of the aquifer rocks, by the predominant geo-chemical processes, by the residence time and by other fac-tors inherent to the medium and the groundwater flow, suchas hydraulic conductivity, the specific surface area ofwaterrock interface and the flow regime.

    Traditionally, studies of karstic systems have addressedthe above question by observing the physical, chemical andhydrodynamic characteristics of the springs flowing fromthe system, which in the case of unexploited aquifers aregenerally the only points at which access for observationof the groundwater is possible. This approach (Kiraly,2002) considers the behaviour of a karstic spring to repre-sent the global response of the system to the input sig-nal (infiltration episode). This response, which is relativelysimple to measure, enables us to hypothesize (sometimesin various, even contradictory, ways) about processes ofinfiltration and groundwater flow, and about the degreeof karstification.

    In this respect, the temporal variability of the hydrody-namic and physico-chemical characteristics of the springwater allows to classify hydrogeological systems on the ba-sis of the degree of karstification and the predominant typeof flow. Thus, Mangin (1975) proposed a classification basedmainly on the analysis of the recession curve; Shuster andWhite (1971, 1972) and Bakalowicz (1976) classified karsticsystems in accordance with the chemical variability ofgroundwater, while Bonacci (1987) presented a similar pro-posal, based on changes in water temperature.

    Nevertheless, present-day knowledge of the spatial vari-ability of the characteristics of the karstic medium leavesmany questions still unanswered. The conceptual modelsof karstic aquifers that are most commonly accepted todaycomprise a pattern of triple porosity and permeability,resulting from the existence of a matrix with primary poros-ity, a system of fractures and a network of conduits (Whiteand White, 2001; White, 2002), and the presence of an epik-arst, a superficial layer of high porosity and permeability,created by preferential fissuring and karstification (Gunn,1983; Williams, 1983; Klimchouk, 2000; White, 2002). Theacceptance of these concepts implies to assume the exis-tence of a high degree of heterogeneity on the hydraulicconductivity field.

    It is apparent, therefore, that karstic systems are nothomogeneous: the water temperature varies, as do the pre-dominant geochemical processes, the chemical compositionof water, the degree of karstification, the predominant typeof groundwater flow and the residence time of water.

    In Sierra de Segura, there are numerous karstic sys-tems, which are characteristic in that their principal re-charge zone is constituted of the remains of a formerkarstic plateau, at an altitude ranging from 1500 to1900 m. Scarce piezometric data and other hydrogeologi-cal, topographic, geomorphological and speleological dataallow us to assert that unsaturated zone thickness is gen-erally in the range of several hundreds of metres. So, dis-charge takes place at altitudes comprised between 800and 1400 m. However, in a few places, it is known theteau epikarst.These mountains provide an essential water supply of

    high quality for domestic and agricultural purposes in thelowlands of Guadalquivir and Segura basins, two regionscharacterized by semi-arid climate with severe droughtperiods and a structural water deficit.

    The goals of the present study are to characterize, inphysical and chemical terms, the water of the springs inSierra de Segura, to propose a model of groundwater geo-chemical evolution and to analyze the factors influencingthe spatial variability, and particularly the vertical variabil-ity, of the physical and chemical characteristics of ground-water, together with its possible relation to otherhydrogeological and environmental factors.

    Geological and hydrogeological context

    Situated in the central part of Betic Cordillera (southernSpain), the Sierra de Segura (surface area of over3000 km2) has an elongated shape (over 100 km) and is ori-ented approximately N30E. In these mountains, with sum-mits over 2000 m high, is located the origin of the twomain rivers of southern Spain, the Guadalquivir, which flowsinto the Atlantic Ocean, and the Segura, which flows to-wards the Mediterranean Sea.

    From a geologic standpoint, the area is part of the Prebet-ic Zone (Fig. 1), characterized by the presence of asequence, some 2000 m thick, of Mesozoic and Tertiary sed-imentary rocks that were mainly deposited in a marine med-ium close to the former Iberian margin (Azema et al., 1979;Foucault, 1971; Dabrio, 1972; Rodrguez-Estrella, 1979; Lo-pez-Garrido, 1971). The synthetic stratigraphic sequenceof the Prebetic Zone units includes two main aquifer materi-als, comprised of Lias-Dogger dolomites and Upper Creta-ceous dolomites and limestones, which may be superposedby Palaeogene and Miocene limestones (Pendas, 1971; Mor-al, 2005). Jurassic materials only outcrop, to a small extent,at the western edge of Sierra de Segura. The most importantpermeable outcrops are constituted of the Upper Cretaceousand Tertiary carbonate rocks.

    The geological structure is of alpine age and consists of asuccession of folds and fault-folds, oriented approximatelyN30E and with vergence towards NW, affected by dextralstrike-slip faults that are perpendicular to the fold axes.In a later distensive phase, various tectonic grabens werecreated, these being bounded by normal faults, that werefilled in by post-orogenic sediments (Azema et al., 1979;Dabrio, 1972; Rodrguez-Estrella, 1979). After the foldingand emergence of the region, an erosive phase during theupper Miocene occurred, which created a smooth erosionsurface. Since then, the region has been uplifted consider-ably, such that the former plateau is currently at an altitudeof 15001900 m. Due to this uplifting, there has been an in-crease in the erosive power of the river network, which hasprogressively worn the Cretaceous and Tertiary rocks,breaking up the ancient plateau, although remains of thelatter can still be observed, occupying a surface area of over800 km2.

    The orography of the region has led to Sierra de Seguracomprising a substantial mountain barrier to humid winds

  • Geochemical evolution of groundwater in the carbonate aquifers of Sierra de Segura 283from the west, which means that precipitations are abun-dant (over 1000 mm annually) on the western slopes andon the summits of the mountain range, while the climateis semi-arid to the east of the mountains, with precipita-tions lesser than 500 mm/year.

    The climate is of a Mediterranean type, with very drysummers and a rainy season extended from November toApril, during which over 70% of annual precipitation occurs.Approximately 90% of surface runoff takes place fromOctober to May.

    The abundance of outcropping carbonate rocks, the rel-atively shallow slopes and the large volume of precipitationshave favoured an extraordinary development of exokarsticforms in the remains of the plateau (Lopez-Limia, 1987;Moral, 2005); in consequence, this is the main groundwaterrecharge area of Sierra de Segura.

    Given the relation between the isotopic composition ofthe precipitation waters and the altitude, Cruz-Sanjulianet al. (1990) concluded that groundwaters infiltrate at anaverage altitude of around 1600 m, which is coherent withother geological and geomorphological observations.

    Figure 1 Regional geologicalSubterranean discharge from studied aquifers occurs innatural regime through more than 50 main springs, with amean flow of between 40 and 1500 l/s. These springs are sit-uated at the foot of the mountains, mainly at altitudes be-tween 900 and 1300 m (Fig. 2 and Table 1).

    In the northern sector of Sierra de Segura, fluvial erosionhas individualized some twenty hydrogeological units,mainly of small surface area and with limited water re-sources (100 hm3/year for the whole sector). In the centraland southern parts, extensive carbonate outcrops and theconsiderable development of the karstic morphology onthe plateau created conditions enabling the infiltration oflarge amount of water, estimated at 330 hm3/year (Moral,2005).

    Methods

    The experimental data, corresponding to the period 19882004, were obtained from field trips and from chemicalanalyses performed in the laboratory. Portable equipmentwas used to obtain in situ readings of temperature, pH

    setting of Sierra de Segura.

  • and electrical conductivity. At the same time, samples ofnon-acidified water in 500 ml polyethylene bottles were ta-ken. The measurement of HCO3 and Ca was carried out inthe shortest time practicable, although the field campaignsnormally took 3 or 4 days; during this period, the sampleswere kept at a low temperature in a portable refrigerator.

    The HCO3 content was determined as the total alkalinity,by titration with HCl 0.05 N and methyl orange as indicator.The cations were analyzed by atomic absorption spectrom-etry (Ca and Mg) and by emission spectrometry (Na and K). Avisible light spectrophotometer was used to analyse SO4 byturbidimetry and the SiO2, by colorimetry. The concentra-tions of Cl ions were determined by argentometric titration,using AgNO3 0.01 N and 5% K2CrO4 as indicator. Since theyear 2001, all the ions, except the bicarbonates, were ana-lysed by ionic chromatography.

    The hydrochemical calculations were performed usingthe AQUACHEM program (Calmbach, 1997), which makes itpossible, in a straightforward way, to use of PHREEQC (Park-

    hurst and Appelo, 1999). The computer processing of thesehydrochemical data had two main objectives: (1) to deter-mine the saturation indexes of calcite, dolomite and gyp-sum, and the partial equilibrium pressure of CO2, and (2)to calculate mass transfer along the flow path.

    Physical and chemical characteristics ofgroundwater

    Table 1 shows the mean water temperature in the mainsprings; the values recorded ranged from 8.5 C in FuenteSegura (ref. 22) to 21 C at Los Tubos (ref. 36). The springswith the highest flows tended to drain colder waters, withtemperatures ranging from 9 to 11 C. In addition, it shouldbe noted that there are a few weakly thermal springs (watertemperature close to 20 C), of which the most significantare Los Tubos (ref. 36), La Toba de Jartos (ref. 9) and ElNacimiento (ref. 10) springs.

    udy

    284 F. Moral et al.Figure 2 Carbonate outcrops in the st area and location of the main springs.

  • Table 1 Physicochemical characteristics of spring waters of Sierra de Segura

    Ref. Spring Height(m.a.s.l.)

    Ca(mg/l)

    Mg(mg/l)

    Na(mg/l)

    HCO3(mg/l)

    SO4(mg/l)

    Cl(mg/l)

    T(C)

    EC(lS/cm)

    pH log PCO2 SIcalcite SIdolomite Mg/Ca(molar ratio)

    1 El Molino 1000 36.7 11.7 0.9 184.1 3.2 2.0 10.6 253 7.93 2.754 0.133 0.099 0.3192 Arroyo Fro (II) 1000 40.5 13.2 0.9 206.8 2.3 1.9 11.8 263 7.74 2.507 0.049 0.236 0.3263 San Blas 980 43.9 15.5 1.1 233.8 3.7 2.1 13.0 305 8.42 3.146 0.800 1.327 0.5814 Era del Concejo 940 54.5 17.4 1.3 277.2 3.3 2.6 14.0 347 8.08 2.720 0.643 0.983 0.5265 El Ojuelo 820 63.4 16.5 2.2 272.2 16.9 4.2 13.4 414 7.36 2.005 0.027 0.457 0.2606 La Maleza 880 57.0 19.5 6.0 337.0 22.5 6.0 13.1 522 6.92 1.474 0.433 1.157 0.3427 Las Pegueras 920 37.0 10.0 3.0 191.0 4.0 4.0 11.0 2728 El Tejo 1320 53.4 15.1 2.0 243.6 6.2 5.7 10.0 328 7.66 2.369 0.086 0.182 0.2829 La Toba de Jartos 780 69.3 30.8 54.9 347.6 65.0 89.5 20.7 762 7.27 1.799 0.071 0.084 0.444

    10 El Nacimiento 810 71.0 37.0 30.0 353.0 85.5 46.0 17.8 692 7.27 1.789 0.044 0.057 0.52111 Alcantarilla 880 48.0 9.4 0.9 205.4 3.6 1.6 13.4 279 8.33 3.105 0.714 0.905 0.32212 Santa Ana 1330 47.2 18.1 1.1 259.1 5.4 1.6 335 8.2413 El Gorgocn 1250 42.2 17.4 1.7 210.8 4.0 3.4 11.7 282 7.84 2.603 0.163 0.094 0.41214 Cuatro Canos 820 41.7 18.7 2.6 226.2 5.1 4.3 13.7 292 8.07 2.797 0.438 0.717 0.73815 El Arroyo 950 53.8 22.2 3.2 271.6 3.8 5.3 15.3 376 7.52 2.157 0.094 0.017 0.41316 Sege 950 53.8 21.9 3.2 271.0 3.2 5.3 15.2 372 7.60 2.239 0.171 0.164 0.40717 Torcal 1020 49.9 20.3 2.5 252.5 2.9 4.8 13.8 360 7.79 2.467 0.281...

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