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Application of Principal Components Analysis to the study of CO2-richthermomineral waters in the aquifer system of Alto Guadalentn (Spain) /Application de l'analyse en composantes principales l'tude des eauxthermominrales riches en CO2 du systme aquifre d'Alto Guadalentin(Espagne)JUAN CARLOS CERNa; ANTONIO PULIDO-BOSCHb; MICHEL BAKALOWICZca

Department of Geology, University of Huelva, Haelva, Spainb

Department of Hydrogeology andAnalytical Chemistry, University of Almeria, La Caada, Almeria, Spain c Laboratoire Souterrain,Centre National de la Recherche Scientifique (CNRS), Moulis, Saint-Girons, France

Online publication date: 25 December 2009

To cite this Article CERN, JUAN CARLOS , PULIDO-BOSCH, ANTONIO and BAKALOWICZ, MICHEL(1999)'Application of Principal Components Analysis to the study of CO

2-rich thermomineral waters in the aquifer system of

Alto Guadalentn (Spain) / Application de l'analyse en composantes principales l'tude des eaux thermominralesriches en CO2 du systme aquifre d'Alto Guadalentin (Espagne)', Hydrological Sciences Journal, 44: 6, 929 942

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HydrologicalSciences-Journal-des Sciences Hydrologiques, 44(6) December 1999 929

Application of Princ ipal C om pone nts Analysis tothe study of C0 2 -r ich thermomineral waters in theaquifer system of Alto Guadalentin (Spain)JUAN CARLOS CERNDepartment of Geology, University of Huelva, E-2189 Haelva, Spaine-mail: ceron@uhu.es ANTONIO PULIDO-BOSCHDepa rtment of Hydrogeology and Analytical Chemistry, University ofAlmeria, La Canada,E-04I20 Almeria, Spaine-mail: apulido@ualm.es MICHEL BAKALOWICZLaboratoire Souterrain, Centre National de la Recherche Scientifique (CN RS), Moulis,F-09200 Saint-Girons, Francee-mail: baka@dstu.univ-montp2.frAbstract The southeast of the Betic Cordil leras has long been recognized as an areawith numerous geothermal anomalies of regional character . Many thermal springsappear related to currently tectonically active fault systems. Carbon dioxide and othergases in these waters have been mobilized through those fault systems. The greatdepth of these "sl ip-str ike zones" affects the entire thickness of the l i thosphre andleads to contrasting crustal domains of different natures and structures. In this area, thedetr i tal aquifer of the Alto Guadalentin has thermal waters with high salinity andunusually high contents of C0 2 gas. The uti l ization of Principal Components Analysis(PCA) in the hydrogeochemical study of this aquifer has revealed that the origin of thesalinity of i ts waters is due essentially to processes of dissolution of the Mioceneevaporite rocks, pr incipally sulphate, and to the contr ibution of deep hydrothermalwaters that show signs of endogenous C0 2 contamination. To a lesser extent,inf i l trat ion waters also form an input, with elevated sulphate, chloride and nitratecontent. Likewise, PCA has enabled the differentiation of dist inct groups of water towhich these processes have had a variable contr ibution.Application de l'analyse en composantes principales l'tude deseaux therm om inrales riches en CO2 du systme aquifre d'AltoGuadalentin (Espagne)R s u m Le sud-est des Cordil lres Btiques a t longtemps reconnu comme unezone possdan t de nombreuses anomal ies go thermales rg iona les . Beaucoup desources thermales apparaissent l ies des systmes de fail les tectoniquement actifs.Le dioxyde de carbone et d 'autres gaz sont mobiliss dans ces eaux travers dessystmes de fail les. La grande profondeur de ces zones de dcrochement affecte toutel 'paisseur de la l i thosphre, ce qui permet de dist inguer deux domaines corticaux denature et structure diffrentes. Dans cette zone, l 'aquifre dtr i t ique de l 'AltoGua dalentin a des eaux thermale s de haute salinit et des teneurs en C 0 2 sous formegazeuse anormalement leves . L 'u t i l i sa t ion de l 'Analyse en Composan tes Pr inc ipa les(ACP) dans l ' tude hydrogochimique de cet aquifre a rvl que l 'or igine de lasalinit de ses eaux est essentiellement due aux processus de dissolution des rochesvaporit iques miocnes, pr incipalement de sulfates, et la contr ibution d 'eauxhydrothermales profondes qui montrent des signes de contamination endogne par leC 0 2 . Dans une moindre mesure, les eaux d ' inf i l trat ion consti tuent un apport de teneurleve en sulfates, chlorures et nitrates. L 'ACP a de plus permis de diffrencierplusieurs groupes d 'eaux, selon la contr ibution de ces divers processus.

Open for discussion until I June 2000

mailto:ceron@uhu.esmailto:apulido@ualm.esmailto:baka@dstu.univ-montp2.frmailto:baka@dstu.univ-montp2.frmailto:apulido@ualm.esmailto:ceron@uhu.es

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930 Juan Carlos Cern et al.I N T R O D U C T I O NCarbon dioxide rich waters have been described in many parts of the world. The CO2discharges are concentrated in two major areas: one is a circum-Pacific belt and theother is a broad area in central and southern Europe and Asia Minor (Barnes et ah,1978; Maisoneuve & Risler, 1979; Blavoux et ah, 1982; Btard et ah, 1982;Bakalowicz et ah, 1987; Kharaka et ah, 1988; Ciezkowski et ah, 1992; Tiercelin et ah,1993; Hadzisehovic et ah, 1993; Arthaud et ah, 1994; Pauwels et ah, 1997). Theseareas have a great deal of seismicity. Similarity in distribution of CO2 discharges andareas of seismicity suggested that the production of carbon dioxide may be related tofundamental tectonic processes (Barnes et ah, 1978). In these zones, the endogenicCO2 originates from mantle degasification processes or from thermometamorphism ofcarbonate rocks.

On the Iberian Peninsula, above all on the southern half of the peninsula, the BeticCordilleras have many hydrothermal manifestations (Cern, 1995; Martin-Vallejo,1997). Both thermomineral springs and wells with CO2 emissions are always related togreat recently-active strike-slip fault systems.

HYDROGEOLOGICAL BAC KGR OUN D OF THE STUDY AREAThe area under study is situated in the southeast of Spain and covers a surface area ofapproximately 236 km 2 (Fig. 1). Over the last fifteen years there has been muchagricultural development in the region which has provoked a notable increase in water

Fig. 1 Location map and geological setting of the Alto Guadalentin aquifer. 1, 2 and3: Alborn domain (Internal Zones)1: Nevado-Filbride; 2: Alpujrride;3: Malguide; 4: Neogene volcanism; 5: Miocene sediments; 6: Pliocene-Quaternarysediments; 7: Sample point.

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Application of Principal Components Analysis to the study of' C02 rich thermomineral waters 931demand. This has given rise to huge exploitation of groundwaters, to the point that theaquifer was declared overexploited in 1987.At almost the same time as the overexploitation took place, the pumped water ofsome of the boreholes began to contain gasmainly CO2. This trend increased witht ime, so that many of the boreholes had to be abandoned. Recent data indicate that thenumber of boreholes affected by CO2 is still increasing due to overexploitation, andthis has caused the gas to be considered as a contaminant (Cern & Pulido-Bosch,1993; Cern, 1995). This situation continues to give rise to serious problems both inpumping systems, where i t causes the breakdown of pumps and pipes due to corrosionand "gas-lift" phenomenon, and in utilization of water, affecting soil texture andcultivation of certain crops (Cern & Pulido-Bosch, 1996).The Alto Guadalentin aquifer is situated in the Betic Cordilleras, in the easternsector of the Internal Zone or Domain of Alborn (Balany & Garcia-Duenas, 1987)and comprises con glomerates, sands, sil ts and clays from the Pliocene-Quaternary. Therocks that limit the aquifer (Fig. 1) are formed essentially by quartzites, micaschistsand marbles of the Nevado-Filabride Complex (of the Upper Permo-Triassic), byquartzites, phyllites, micaschists, slates, dolomites and limestones from the AlpujarrideComplex (of Palaeozoic-Triassic age), sandstones, quartzites, slates, conglomeratesand limestones from the Malaguide Complex (of the Permo-Triassic), and finally,situated above these, are marls, marls with gypsum, marls with sands, conglomerates,limestones and calcarenites (Miocene).The Alto Guadalentin aquifer is limited to the northwest by the Guadalentin faultsystem and to the southeast by the Palomares fault system. Geophysical and petro-logical data and cores from petroleum boreholes reveal that the crust has a differentnature and thickness on either side of the two systems. Thus, to the west the crust isformed by four layers, whilst to the east it has three, of lower density than the former(Banda et al, 1993; Garcia-Duenas tal, 1994). These fault systems give rise to amoderate and constant seismicity (Sanz de Galdeano & Lpez-Casado, 1988).In the central part of the aquifer is a depressed area of appro xim ately 15 km 2 called"El Saladar"; this constitutes a small endorreic basin where a lake existed about40 years ago. Clays and silts predominate here: the soils developed during the salinephase, due to the drying-up of the lake waters, and are rich in chlorides and sulphatesof sodium, calcium and magnesium. When there is intense rainfall, waters are retainedin this sector for a while, permitting the concentration of salts by evaporation. Erosionby piping is also evident in this sector.In agreement with the data obtained from electrical geophysical prospection, theplioquaternary sediments (Fig. 2) are at their thickest close to both the northwesternand southwestern borders, where they exceed 350 m; conversely, towards the centre ofthe aquifer they are of relatively small extent, ranging between 200 and 300 m.Towards the south and extreme southwest of the aquifer, the depth of these sedimentsranges between 50 and 200 m.With respect to the Miocene substratum, its thickness is greatest close to thenortheastern and southeastern limits of the aquifer, being over 600 m deep, and inneither case is the metamorphic substratum detectable. On the contrary, the thinnerparts are found in the centre of the aquifer, generally with depths of less than 300 m,there being some sectors where it may not exist at all. In relation to the metamorphicsubstratum, it forms a central horstwhich in some zones may be at least at 300 m

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932 Juan Carlos Cern et al.

Depth (m) WNW

' I001000

BDepth (m) WNW

0 500

10002 km

1Fig. 2 Schematic geological cross-sections of the Alto Guadalentin aquifer.1: Plioquaternary detrital aquifer; 2: Miocene substratum; 3: Metamorphic substratum.

depthand two grabens, situated on either side of the horst and close to thenorthwestern and southeastern limits, where this substratum is not even to be found.Studies by the Regional Government of Murcia (1988) indicate that theexploitation of the aquifer is know n to be betw een 24 Mm year" (1973) and69 M m year" (1987), and that it caused a continued drop of the piezom etric level of2.5 m year"1 (1973-1976), 4.5 m year"1 (1976-1983) and 9-10 m year"1 (1984-1987) .However, between 1989 and 1992, a general reduction in the rate of piezometricdrawdo wn was ob served, such that, in some boreholes, the piezom etric level stabilized,and even reversed its trend, due to decrease or absence of pumping. This change inpumping regime is a direct result of the deterioration in water quality which is due, insome cases, to an increase in its salinity, and in others, to salinization and a markedincrease in CO2 (Cern, 1995). At the present time, a high rate of exploitation

continued in the western sector of the aquifer, where the water level exceeds 210 m indepth (Fig. 3).

O B J E C T IV E S A N D M E T H O D SThis study attempts to establish the possible origin of the salinity and of the CO2present in the aquifer waters through the application of Principal Com pone nts Analysis(PCA) to both variables (physicochemical parameters) and observations. Thesevariables were obtained from samples taken in September 1989 (34 samples) and inApril 1992 (57 samples). Tables 1 and 2 show the main phys icochem ical characteristics of these samples. At the same time, a study was made of the correlationsbetween the different parameters, providing the coefficients presented in Tables 3and 4. Electrical condu ctivity (EC) was determined in the field by direct

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Application of Principal Components Analysis to the study ofC02 rich thermomineral waters 933

Fig. 3 Map of groundwater depth in the Alto Guadalentin aquifer (April 1992).1: Curve of equal groundwater depth. (For geological reference see legend in Fig. 1.)measurements; chloride and bicarbonate were determined by colorimetry; sulphate wasanalysed by turbidimetry using double-beam spectrophotometry; nitrate was obtainedby polarography; calcium, magnesium, sodium and potassium ions were analysed byplasma spectrometry (ICP).The variables dpH, DSI and PCO2 were obtained through application of theSOLUTEQ program (a modification of the WATSPEC program, Bakalowicz &D'Hults, CNRS, unpublished). The dpH variable was obtained based on the formuladpH = pH r - pH e, where pH r is the pH value obtained in the field and pH e is the valuecalculated at equilibrium. This variable allows one to determine the state of thesolution with respect to the equilibrium of the Ca~ +-~HC03-PC02 system (Bakalowicz,1983); thus, for -0.05 < dpH < +0.05, the solution is considered in equilibrium withrespect to calcite and this situation can change only if PCO2 and temperatureconditions vary. The PCO2 of the water at equilibrium corresponds to PCO2 of afictitious gaseous phase associated with the solution for which all the equilibria areestablished, in agreement with the measured values of pH and HCC>3~. It givesrelatively precise information about the partial pressure of CO2 in the aquifer(Bakalowicz, 1983). The dolomite saturation index (DSI) was used because this

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934 Juan Carlos Ceron et al.

T ab l e 1 Summary sta t ist ics (September 1989).Var iableDw (m )Tempera ture , 7*(C)p H ,C (nS c m - ' ; 25 0 C )CF (mg I"')S 0 4 2" (mg F1)H C 0 3 - (mg l"1)C a 2 + ( m g r ' )M g 2 + (mg F ' )N a + ( m g r ' )K + (mg F1)N 0 3~ (mg I"1)d pHD S IPCO, (% vol . )

Average122.426.36.773378.9567.4996.4655.7330.3201.5271.850.418.40.18

0.4823.0

Standard deviat ion47.53.20.51205.5289 .8625.8413 .6162.778.7161.912.415.00.26

0.5532.7

Mi n i m um24.019.75.99129010923428510 03088301- 0 . 3 0- 0 . 2 40.3

M a x i m u m199.031.87.98654011982518204361035775085600.97

2.24128.7Dw: depth of groundwater; EC: electrical conductivity.Table 2 Summary sta t ist ics (Apri l 1992).Var iableDw (m )Te m pe ra t u re , 7*(C)p HEC (\iScmA\25"C)c r (mg r")S 0 4 2 ' ( m g r ' )HCCV (mg I" ' )C a 2+ (mg r 1 )M g 2 + (mg I"1)N a + (mg l"1)K + ( m g r ' )NO3- (mg r 1 )d p HD S IP C 0 2 ( % v o l . )

Average138.726.56.993487 .8682.8997.9774.1401.3230.6316.570.325.00.601.2515.0

Standard deviat ion43.33.00.41332.8337.7618.7396.5182.491.6182.812.026.30.300.5023.9

M i n i m u m27.319.36.091182

17 615132612 34 8993620.080.130.2

M a x i m u m21131.98.066 8 1 0

1456256918247614 5387210117 11.413.01112.4Dw: depth of groundwater; EC: electrical conductivity.

mineral is less soluble than calcite, it is not affected so much by the common ion effectand does not precipitate under normal conditions of pressure and temperature(Bakalowicz, 1979).R E S U L T SThe waters were of predominantly sulphate and chloride type, and to a lesser degreebicarbonate type (moderately predominant in the central sector of the aquifer) . Theconductivity increased markedly at wells where the exploitation was more intense, dueto the decrease in the piezometric level. In these areas, wells draw water from thedeepest zones of the aquifer and thus they have greater salinity. In general, temperature

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Application of Principal Comp onents Analysis to the study of CO 2 rich thermomineral waters 935

Table 3 Correlation matr ix (September 1989).DwTpHECCI"SO-rHC03'C a2+M g 2+N a*NO3'dp HDS IPCO2

Dw1.00-0.28-0.03-0.18-0.05-0.090.15-0.06-0.06-0.090.020.110.200.14

T1.00

-0.28- 0 .51-0.64- 0 .350.48-0.01-0.14-0.71-0.420.120.120.50

pH

1.00-0.290.14- 0 .31- 0 .7 1-0.64-0.500.160.130.660.72-0.73

EC

1.000.680.770.040.550.790.760.36-0.36-0.30- 0 . 0 6

cr

1.000.24-0.300.020.360.880.43-0.13-0.01-0.37

S O J - '

1.00-0.020.780.640.440.20-0.27-0.36-0.05

HC03'

1.000.420.45-0.32-0.41-0.06-0.080.87

Ca 2+

1.000.550.05-0.15-0.18-0.390.32

M g2+

1.000.410.15-0.34-0.190.32

N a"

1.000.53-0.17-0.05-0.38

N O J -

1.00-0.37-0.24-0.33

dpH

1.000.91-0.28

DSI PCO2

1.00-0.27 1.00ncal conductivity.

Table 4 Correlation matr ix (April 1992).DwTpHECci-SO rHCO3-Ca 2*M g 2+Na*NO3-dp HDS IPCO2

Dw1.000.520.20-0.46-0.25-0.48-0.08-0.39-0.41-0.31-0.230.240.28-0.08

T1.00-0.24-0.67-0.71-0.530.26-0.34-0.43

-0.77-0.610.27-0.070.26

pH

1.00-0.190.25-0.31-0.74-0.53-0.39-0.200.230.690.73-0.77

EC

1.000.800.830.100.750.850.820.51-0.04-0.050.09

cr

1.000.46-0.240.400.540.920.640.280.31-0.27

SO i2-

1.000.070.840.730.590.40-0.21-0.290.13

HCO3-

1.000.410.42-0.24-0.35-0.15-0.190.87

Ca 2*

1.000.690.430.25-0.11-0.260.38

Mg2*

1.000.530.24-0.13-0.050.37

N a+

1.000.740.160.16-0.25

NO3-

1.000.080.07-0.32

dpH

1.000.92-0.39

DSI PCO;

1.00-0.41 1.00Dw: depth of gro3acoo

* .Lm r

a.c

EH

(m) J0JBM jo MJdsa (LU) J8JEM JO md8Q ( tuo /sr i ) A j iAnonpuoo

ojoME

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Application of Principal Components Analysis to the study ofC02 rich thermomineral waters 937same way, the plots of temperature as a function of sulphate, chloride, sodium andnitrates (Fig. 4) allow one to see that the waters with a greater content of these ions arethose having a higher temperature and, therefore, those that are more closely related toinfiltration waters. Sulphate, chloride, sodium, magnesium and calcium are theprincipal ions responsible for the increase in salinity of the waters. A close correlationis found between the sulphate and calcium (r = 0.84), sulphate and magnesium(r = 0.73), and between chloride and sodium (r = 0.92) (Fig. 4), indicating an originderiving from the dissolution of the sulphate and chloride evaporite rocks surroundingthe aquifer and its substratum. The dissolution of sodium chloride salts provokes anincrease in the ionic strength of the water, which favours a greater dissolution of thesulphate salts and the enrichment of the waters by magnesium and calcium. Graphicalrepresentation of nitrates (whose origin is linked to processes of agriculturalcontamination) against chloride and sodium (Fig. 4) highlights the relationship of theseions to infiltration wate rs.P R I N C I PA L C O M P O N E N T S A N A L Y S I SPrincipal Components Analysis (PCA) is a well known general statistical methodwhich can provide a useful complementary tool to other geochemical methods, used toidentify different water types and factors that produce a change in their salinity(Melloul & Collin, 1992). Its principal advantage is the ability to analyse a largenumber of variables and observations simultaneously. Principal Components Analysisis a method which summarizes the information contained in a data matrix with anumber of variables, n; the main stages include the preparation of the correlationmatrix, the extraction of the initial factors (exploration of the possible reduction ofdata) and its transformation (through processes of mathematical rotation) until a finalsolution is reached (Davis, 1984).The technique has been used in this study to visualize similarities betweenvariables in wells which have been sampled and to see if the results are consistent withthose obtained in previous studies. The main studies related to the origin and evolutionof salinity and CO2 in the groundwaters of the Alto Guadalentin aquifer have beendone by the Regional Government of Murcia (1987), Rodriguez-Estrella et al. (1989),Cern & Pulido-Bosch (1993, 1996) and Ceron (1995). Calculations were achievedusing the multivariate methods of the STAT-ITCF program (Service des EtudesStatistiques et Mthodologiques de 1TTCF, 1991). Starting from the initialstandardized data (variable value minus the mean, divided by the standard deviation),the variables were selected according to their possible relationship with salinity(S0 4 2", CF, Ca2+ , M g 2 +, N a+ and N0 3") and C0 2 contamination (HCO3"). Factoranalysis was used to transform the initial data matrix of both sampling periods(September 1989 and April 1992) into a new set of composite variables or principalcomponents. From the similari ty matrix, the principal components have beendetermined. Component I, representing the first axis, explains as much as possible ofthe total variance of the observations, and accounts between 43% (September 1989)and 55% (April 1992) of the variation in the data. The second component, representingthe second axis, explains as much as possible of the residual variance (30% inSeptem ber 1989 and 25 % in April 1992).

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938 Juan Carlos Cern et al.Spatial distribution according to the two first components (Fig. 5) shows that thefirst component discriminates in terms of salinity and differentiates ion groupingsrepresenting sulphate water (S0 4 2" , Mg2 + and Ca 2+) and infiltration water (CI", Na+ and

NO3"); the second component is defined by HCO3" ion and representing the C0 2contamination. Component I is defined as "dissolution of evaporite salts" andcomponent I I as "endogenous C 0 2 contamination".Distribution of observation points with respect to components I and II is shown inFig. 6. The first component appears to be fundamental in separating the different

I I (30 %)

(43 %)

-0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5S e p t e m b e r - 1 9 8 9

I I (25 %)0 . 7 -0 . 6 -0 . 5 -0.40.30.20.10.0

-0.1-0.2-0.3 H

H C O 3C0 1 contamination

, s o ; Sulphate waterV O /Salinity

T

(55 %)

-0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5April-1992

Fig. 5 Representation of the principal components of the variables measured in fieldsamples.

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Application of Principal Comp onents A nalysis to the study ofCOi rich thermominera l waters 939

I I ( 3 0 % )

-6 -5 - 4 - 3 - 2 - 1 0 1 2 3S e p t e m b e r - 1 9 8 9

II ( 2 5 % )

-7 -6 -5 - 4 - 3 - 2 - 1 0 1 2 3April -1992

Fig. 6 Representation of the principal components of the observations.

degrees of salinity. Component I distinguishes observations in relation to sulphatewater flow and infiltration water (related to processes of agricultural pollution). Thesecond component could discriminate the observations from the bicarbonate content,where the effect of CO2 flow seems to be more evident.

I (55 %)

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940 Juan Carlos Cern et al.D I S C U S S I O NThe principal components considered represent the existence of waters whose saltshave their origin in the processes of dissolution of sulphate evaporite salts (presentprincipally in the Miocene substratum) and waters with an origin distinct from theabove-mentioned, from greater depth and with evidence of endogenous C0 2contamination. If component I is defined as "dissolution of evaporite salts" andcomponent II as "endogenous CO2 contamination" one can see (Fig. 5) the influence ofthe variables sulphate, magnesium and calcium with respect to component I.Moreover, sulphate is most closely correlated with the variables calcium (0.78 and0.84) and magnesium (0.64 and 0.73). In relation to component II, the bicarbonate ionshow clear positive influence.The grouping together of nitrate, chloride and sodium (clearly separate from thegroup formed by sulphate, calcium and magnesium) indicates a source of infiltrationwaters related to processes of agricultural pollution and of dissolution of superficialsalts (visible in the zone known as El Saladar). If one examines the correlationcoefficients, one can see that, for the samples taken in September, nitrate is mostclosely correlated, even though the values are not very high, with sodium (0.53) andchloride (0.43). In the same way, for the August sample set, this variable is correlatedmost closely with the variables sodium (0.74) and chloride (0.64).When considering the "spatial" influence of the components defined (Fig. 6), thesamples are grouped according to their salinity (which increases along the negativepart of component I). In turn, one can distinguish between those which are mixed withmore superficial waters (with higher nitrate concentrations) and those where the effectof sulphate water flow seems to be more evident, and those affected by processes ofCO2 contamination (towards the positive part of component II).The first group corresponds to samples that have high salinities with ions such aschloride and sulphate predominating, to which is added the ion nitrate, in markedlyhigh concentrations, and this would indicate a significant contribution of infiltrationwaters in their origin. At the other extreme, the second group corresponds to watersprobably originated essentially at depth where dissolution of evaporite salts wouldhave predominated, principally sulphates of Miocene origin. In these waters thecontribution from infiltration waters would be slight and would produce a endogenouscontamination of CO2, consistent with the 13C data (Cern et al, 1998). In anintermediate position with respect to the two extreme groups detailed above, lie thesamples that have sulphate as their principal ion, with bicarbonate and chloride insecond, suggesting a predominance of the phenomenon of endogenous CO2contamination or the infiltration of surface waters.C O N C L U S I O N SThe application of Principal Components Analysis to samples taken from the AltoGuadalentin aquifer emerges as a suitable tool for highlighting the basic processes thatcontrol the physicochemical characteristics of the aquifer waters. The hydrogeo-chemical study of the Alto Guadalentin aquifer shows the existence of a complexmechanism of salinization in which sulphate flow linked to Miocene substratum is notthe only process affecting this aquifer; infiltration waters also contribute to increase the

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Application of Principal Compon ents A nalysis to the study ofC0 2 rich thermomineral waters 941

water salinity. The application of PCA using only major ions roughly discriminatesboth salinization types and the endogenous CO2 contamination. It proves that thechemical characteristics of the aquifer waters may be modified due to mixing withwaters of surface origin and those proceeding from its substratum. It also enables oneto see the existence of a process of endogenous CO2 contamination, which isresponsible for the exceptionally high values of bicarbonate and PC0 2 that are found inmany of the samples. The importance of the fault systems of Guadalentin andPalomares for the detrital aquifer of Alto Guadalentin is evident, as is the presence ofthermal waters with high contents of CO2 gas.Acknowl edgem ent This research received financial support from the Spanish CICYTwithin the framework of projects AMB92-0211 and AMB95-0493.R E F E R E N C E SAlbert, J. (1979) Heat flow and temperature gradient data from Spain. In.- Terrestrial Heat Flow in Europe (ed. byV. Cermk & L. Rybach), 261-266. Springer Verlag, New York.Arthaud, F., Dazy, J. & Grillot, J. C. (1994) Distribution of deep carbon dioxide in relation to the structure and tectonicevolution of southeast France. Geodinamica Acta 7(2) , 86-102 .Bakalowicz, M. (1979) Contribution de gochimie des eaux a la connaissance de l'aquifre karstique et de la karstification.Thse de D octorat d 'Etat es Sciences Naturelles, Univ. Pierre et Marie Curie, Paris, France.Bakalowicz, M. (1983) La gnese de l 'aquifre karstique vue par un geochimiste. Reun. Monogrfica sobre el Karst deLarra 82 (Navarra). 159-147.Bakalowicz, M., Ford. D., Miller, T., Palmer. A. & Palmer, V. (1987) Thermal genesis of dissolution caves in the BlackHills, South Dakota. Geol. Soc. Am. Bull. 99, 729-738.Balany, J. C. & Garcia-Duenas, V. (1987) Les directions structurales dans le Domaine d'Alborn de part et d'autre duDtroit de Gibraltar. C. R. Acad. ScL, Paris 304, 929-933 .Banda, E., Gallart, J., Garcia-Duenas, V., Danobeitia, J. & Makris, J. (1993) Lateral variation of the crust in the Iberianpeninsula: new evidence from the Betic Cordillera. Tectonophysics 2 2 1 , 5 3 -6 6 .Barnes, I., Irwin, W. & White, D. (1978) Global distribution of carbon dioxide discharges, and major zones of seismicity.Open File Report , US Geol. Survey, Washington DC, USA.Btard, F., Baubron, J., Bosch, B., Marc, A. & Risler, J. (1982) Isotopic identification of gases of a deep origin in Frenchthermomineral waters.,/ . Hydrol. 56, 1-21.Blavoux, B., Dazy, J. & Sarrot-Reynauld, J. (1982) Information about the origin of thermomineral waters and gas bymeans of environmental isotopes in eastern Azerbaijan, Iran, and southeast France. J. Hydrol. 5 6 , 2 3 -3 8 .Ceron, J. C. (1995) Estudio hidrogeoquimico del acuifero del Alto Guadalentin (Murcia) (Hydrogeochemical study of AltoGuadalentin aquifer, Murcia, in Spanish). Unpublished PhD Thesis, Univ. Granada, Spain.Ceron, J. C. & Pulido-Bosch, A. (1993) Considrations gochimiques sur la contamination par le C0 2 des eaux thermominrales de l 'aquifre surexploit de l 'Alto Guadalentin (Mu rcie, Espagne). C. R. Acad. ScL, Paris 317, 1121-1127.Cern, J. C. & Pulido-Bosch, A. (1996) Groundwater problems resulting from CO2 pollution and overexploitation in theAlto Guadalentin aquifer (Murcia, Spain). Environ. Geol. 28(4), 223-228 .Ceron , J. C , Pu lido-B osch, A . & Sanz de Galde ano, C. (1998) Isotopic identification of CO2 from a deep origin inthermomineral waters of southeastern Spain. Chem. Geol. 149(34), 251-258.Ceron, J. C. & Pulido-Bosch, A. (1999) Geochemistry of thermomineral waters in the overexploited Alto Guadalentinaquifer (southeast Spain). Wat. Res. 33(1), 295-300.Ciezkowski, W., Groning, M., Lesniak, P., Weise, S. & Zuber, A. (1992) Origin and age of thermal waters in Cieplice Spa,Sudeten, Poland, inferred from isotope, chemical and noble gas data. J. Hydrol. 140, 89-117.Davis, J. C. (1984) Statistics and Data Analysis in Geology. John Wiley, New York.Garcia-Duenas, V., Banda, E., Torn, M., Cordoba, D. & Esci-Bticas Working Group (1994) A deep seismic reflectionsurvey across the Betic Chain (southern Spain): first results. Tectonophysics 232, 77-89 .Hadzisehovic, M., Miljevic, N., Sipka, V., Gologocanin, D. & Popovic, R. (1993) Isotopic analysis of groundwater andcarbonate system in the Surdulica geothermal aquifer. Radiocarbon 35(2) , 277-286 .Kharaka, Y., Ambats. G., Evans, W. & White, A. (1988) Geochemistry of water at Cajon Pass, California: preliminaryresults. Geophys. Res. Lett. 15(9), 1037-1040.Martin-Vallejo, M. (1997) El sistema hidrotermal de la cuenca del rio Almanzora (N de la Provincia de Almeria) (Thehydrothermal system of the Almanzora River basin (N of Almeria province), in Spanish). Unpublished PhD Thesis,Univ. Granada, Spain.Melloul, A. & Collin, M. 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942 Juan Carlos Cern et al.Pauwels, H., Fouillac, C, Goff, F. & Vuataz, F. D. (1997) The isotopic and chemical composition of C0 2 rich thermalwaters in the M ont-Dore region (Massif-Central, France). Appl. Geochem. 12,411-427.Regional Government of Murcia (1987) Estudio para evaluar los problemas en relacin con las surgencias gaseosas encaptaciones de aguas subterrneas del Alto Guadalentin (M urcia) (Study to evaluate the problems in relation to the gasfrom the wells of the Alto Guadalentin aquifer (Murcia), in Spanish). Tech. Report. Direction de RecursosHidruiicos. Murcia, Spain.Regional Government of Murcia (1988) El sistema acuifero del Alto Guadalentin (The aquifer system of Alto Guadalentin,in Spanish). In: El Alto Guadalentin (H Seminario sobre gestion de acuiferos sobreexplotados y comunidades deusuarios) (The Alto Guadalentin (Second Seminar on management of the over-exploited aquifers and usercommunities). D irection de Recursos H idruiicos, Murcia, Spain.Rodriguez-Estrella, T., Albacete, M., Garcia, U. & Solis, L. (1989) Evolution espacial y temporal de los gases en elacuifero sobreexplotado del Alto Guadalentin (Murcia) (Spatial and temporal evolution of the gases in the over-exploited aquifer of Alto Guadalentin (Murcia), in Spanish). In: Temas Geolgico-Minero (Proc. Symp.: LaSobreexplotacion de Acuiferos), 613 -619 . ITGE Publ. no. 10.Sanz de Galdeano, C. & Lopez-Casado, C. (1988) Fuentes sismicas en el mbito Btico-Rifeno (Seismic sources in theBetic and Rifain areas, in Spanish). Rev. Geojis. 44, 175-198.Service des Etudes Statistiques et Mthodologiques de l'ITCF (1991) STAT-ITCF. Logiciel de Statistique (version 5) . Inst.Techniques des Crales et des Fourrages, Paris, France.Tiercelin, J. J., Pflumio, C , Castre, M., Boulgue, J., Gente, P., Rolet, J., Coussement, C , Stetter, K., Huber, R., Buku, S.& M ifundu, W. (1993) Hydrothermal vents in Lake Tanganyika, East African Rift system. Geology 21,499-502.Received 8 October 199S; accepted 31March 1999