Application of geochemistry and radioactivity in the ... › bbldoc › articulos › 16692299.pdf · Application of geochemistry and radioactivity in the hydrogeological investigation

Application of geochemistry and radioactivity in thehydrogeological investigation of carbonate aquifers

(Sierras Blanca and Mijas, southern Spain)

BartolomeÂ Andreo, Francisco Carrasco

Departament of Geology, Faculty of Sciences, University of Malaga, E-29071 Malaga, Spain

Received 26 May 1997; accepted 11 May 1998

Editorial handling by R. R. Raiswell

Abstract

The chemical characteristics, 3H contents and radioactivity of groundwaters from the Sierras Blanca and Mijas(Southern Spain) have been studied in relation to the chemical composition and radioactivity of the aquifer hostrocks, and the residence time of the water. The Sierras Blanca and Mijas are made up of calcitic and dolomiticmarbles of Triassic age. The groundwaters that drain the calcitic marbles (which outcrop principally in the westernSierra Blanca) have less mineralization, which descreases quickly with recharge (as does the 3H content), and thegross alpha and beta activities are below detection limit. This is due to the short residence time of water inside theaquifers which are conduit ¯ow systems. The waters of the dolomitic marbles (eastern Sierra Blanca and SierraMijas) have higher and less variable mineralization and contain greater concentrations of Mg2+, SiO2 and SO2ÿ

4

(ions normally associated with slow ¯ows). The 3H contents are more uniform with time (indicating an older age)and there is detectable natural radioactivity, because the waters have a longer residence time in the aquifers, whichare di�use ¯ow systems. # 1999 Elsevier Science Ltd. All rights reserved.

1. Introduction

The study area of 170 km2 lies in the Blanca and

Mijas mountains, situated 100 km E of the Strait of

Gibraltar and at an average of 5 km from the

Mediterranean coast. These Sierras mark the northern

limit of the Costa del Sol to the E of Marbella city,

southern Spain (Fig. 1).

Climatically the region has a mean annual tempera-

ture between 17±188C. The average rainfall during the

period 1963/64±1993/94 was approximately 700 mm,

although it varies from more than 800 mm in the wes-

tern sector of the Sierra Blanca down to 600 mm in

the eastern part of the Sierra Mijas. For this reason,

the mineralization of the rainwater in general, and the

Clÿ and Na+ contents in particular, increase from W

to E (Andreo, 1997).

In karst hydrogeology, variations in hydrochemical

parameters (both major and trace elements) are com-

monly used to establish the hydrodynamic function of

the studied systems (Shuster and White, 1971;

Atkinson, 1977; Hess and White, 1993; Blavoux and

Mudry, 1993; Bakalowicz, 1994). The 3H content of

waters is also useful (Malozewsky and Zuber, 1982;

Fontes, 1983; Bradbury, 1991, among others), but

natural radioactivity has rarely been considered

(Andrews and Wood, 1972; Smart, 1996).

The chemical composition of groundwater is a func-

tion of the aquifer rocks, ¯ow conditions and residence

time in the aquifer (Schoeller, 1962; Back and

Hanshaw, 1971). Groundwater ¯ow and residence time

depend on the degree to which karstic conduits are

developed. If karst development is limited, water ¯ow

is slow and the residence time in the aquifer is long. In

Applied Geochemistry 14 (1999) 283±299

0883-2927/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved.

PII: S0883-2927(98 )00049-3

PERGAMON

Fig.1.GeographicallocationandgeologicalmapoftheSierrasBlanca

andMijas(after

SanzDeGaldeanoandAndreo,1994).Thenumbersindicate

thesolidsamplesstudied.

B. Andreo, F. Carrasco / Applied Geochemistry 14 (1999) 283±299284

this case, mineralization is generally greater, the chemi-cal composition varies slightly with time and it is par-

ticularly conditioned by components such as Mg2+

and SiO2 (Drever, 1982; Appelo and Postma, 1993), ifthey are constituents of the aquifer rocks. A more

developed karst drainage implies shorter residencetimes, thus normally lower mineralization, and a tem-porally more variable water composition, which mainly

comprises Ca2+ and alkalinity derived from rapid dis-solution of carbonates (Dreybrodt, 1981) plus ad-ditional components introduced by the rain. The

residence time can be estimated if the 3H contents ofboth rainwater and groundwater are known over arepresentative period (Fontes, 1983).The objective of this work is to examine the appli-

cation of hydrochemistry, together with the radioac-tivity of the waters, to research on carbonate aquifers.The chemical composition, the 3H contents and the

natural radioactivity (gross alpha and beta activities)of the groundwater are studied as a function of thechemical and radioactive composition of the aquifer

marbles and the water residence time inside the aqui-fers. The integration of these di�erent researchmethods should permit a better understanding of aqui-

fer hydrogeology.

2. Geological setting

The Sierras Blanca and Mijas form part of the

Alpujarride Complex of the Betic Cordillera, and areformed by a lower metapelitic formation with migma-tites and gneisses of Paleozoic age overlaying carbon-

ates comprising lower white marbles and upper bluemarbles with metapelitic intercalations. The carbonateshave a total thickness of 600 m and are of middle andupper Triassic age. The white marbles have a fractured

aspect and outcrop mainly in the eastern sector of theSierra Blanca and Sierra Mijas, while the blue marblesinclude karstic cavities and are present mainly in the

western sector of the Sierra Blanca (Fig. 1). These cav-ities show a hierarchy in underground drainage, withpotholes (reaching depths of up to 80 m) at high alti-

tudes, larger subhorizontal cavities at lower altitudes(near the springs) and cavities mixed (vertical and hori-zontal development) at intermediate altitudes (Andreoet al., 1997a).

The geological structure is very complex but 3 sec-tors can be di�erentiated (Sanz De Galdeano andAndreo, 1994): the western Sierra Blanca is made up

of N±S and E±W folds, the eastern Sierra Blanca andSierra Mijas are principally formed by ESE±WNWfolds. The two Sierras are separated by an outcrop of

peridotites in the Puerto Pescadores, which is the con-tinuation of the Sierra Alpujata (Fig. 1). All the foldedstructures are truncated by NNE±SSW and NNW±

SSE fractures which have a�ected the surface drainageas well as the groundwater ¯ow (Andreo, 1997).

3. Hydrogeological characteristics of the Sierras Blanca

and Mijas

Marbles outcrop in the studied area constituting theBlanca±Mijas hydrogeological unit (Fig. 2), whose dis-

charge is produced from springs and from pumpingthe numerous wells which exist all over the SierraMijas. In this Sierra the springs have been dry sincethe seventies and only drained water during 1990 and

1991, in response to an important recharge at the endof 1989 and beginning of 1990.The elevation of the springs and the height of the

piezometric level in the wells are di�erent dependingon the sector considered (Linares and Trenado, 1981),due to the geological structure (Fig. 2). It is possible to

distinguish 8 aquifer systems in the unit (Andreo,1997): 3 in the western sector of the Sierra Blanca(Istan, Marbella and Ojen), one in the eastern sector(Coin) and 4 in the Sierra Mijas (Alhaurin el Grande,

Mijas, Benalmadena and Torremolinos).The springs of the western Sierra Blanca increase

rapidly in ¯ow, from 0 to several hundreds of l/s after

precipitation (Fig. 3A). In the same way, in the piezo-metric wells, rapid and signi®cant piezometric vari-ations are registered, in some cases above 100 m.

However, in the eastern Sierra Blanca and Sierra Mijasthe di�erence between maximum and minimum ¯owvalues (Fig. 4A) and/or piezometric levels are less

marked and extend for a longer time. In the Coinspring (MB-2), a greater ¯ow variability is registered,because of the pumping that is carried out during sum-mer and autumn.

The application of hydrodynamic methods, such asthe analysis of the recession curves, correlation andspectral analysis and analysis of classi®ed ¯ow, to the

out¯ow data (which is available for natural dischargefrom the Sierra Mijas, before 1970) shows that ¯ow bykarstic conduits takes place in the western sector of

the Sierra Blanca, while in the eastern sector and inthe Sierra Mijas di�use ¯ow exists (Andreo, 1997).The many boreholes in the area (200 aproximately)

corroborate the above observations. Thus, in the wes-

tern sector of the Sierra Blanca, the regulation ofresources by means of wells is complex and dependslargely on the number and size of the karstic conduits

penetrated (which is normally low). In this sector,during 1994 and 1995 (a drought period in SouthernSpain) 16 wells were drilled for water supply to

Marbella and Ojen but only one (near NaguÈ eles spring,MB-9 in Fig. 1) was successful. However, the exploita-ble water reserves were scarce because, after several

B. Andreo, F. Carrasco / Applied Geochemistry 14 (1999) 283±299 285

Fig.2.HydrogeologicalmapoftheSierrasBlanca

andMijas.(1)PlioceneandQuaternary

sedim

ents,(2)Triassic

marbles,(3)low

permeabilitymaterials,(4)peridotites,(5)stra-

tigraphic

contact,(6)unconform

ity,(7)tectonic

contact,(8)norm

alfault,(9)thrust,(10)strike-slip

fault,(11)anticline,

(12)reversedanticline,

(13)hydrographic

boundary,(14)

hydrogeologic

boundary,(15)possible

transfer

ofresources,(16)springswithreference

andelevationin

m.a.s.l.,(17)wellandreference.


months of discontinous pumping (15±20 l/s as averagevalue), the piezometric level dropped more than 130 m.

Conversely, in the eastern sector of the Sierra Blancaand in the Sierra Mijas, practically all the wells are

used to exploit or regulate water resources, because thegroundwater ¯ow occurs mainly through joints includ-

ing some of centimeter width, which have beenwidened by solution processes (Andreo, 1997).

Therefore the aquifers of the western Sierra Blanca areconduit ¯ow systems whilst the aquifers of the eastern

Sierra Blanca and Sierra Mijas are di�use ¯ow sys-tems.

Fig. 3. Temporal evolution of the ¯ows (A) and electrical conductivity of waters (B) in the western Sierra Blanca springs. The

period considered in both this ®gure and Fig. 4 is from July 1990 to October 1991 because the Torremolinos springs (in the Sierra

Mijas) are still draining water. For the following years the evolution is similar in the western Sierra Blanca springs. Thus, the con-

tinuous ¯ow record available in Ojen and NaguÈ eles springs indicates very quick response after several hours of rainfall.


4. Methodology

Samples of both the marbles (blue and white) and

the groundwaters from the main water points of

Sierras Blanca and Mijas have been collected.

Major (CaO, MgO, Na2O, K2O, SiO2, MnO, Fe2O3)

and trace element (Sr, Pb, Zn, Cu and Ni), compo-

sition were determined by X-Ray ¯uorescence at the

University of Granada (Spain).

The 226Ra, 232Th and 40K activities of both marbles

and peridotites were measured by gamma spectrometry

in the Laboratory of Environmental Radioactivity at

the University of Malaga. Samples were dried, sieved

to remove rock fragments and sealed in Marinelli type

Fig. 4. Temporal evolution of the ¯ows (A) and electrical conductivity of waters (B) in the eastern Sierra Blanca and Sierra Mijas

springs. The ¯ow of Coin spring (MB-2), in summer and autumn, is a�ected by pumping. The available ¯ow data for the natural

discharge of the Torremolinos springs (before the seventies), indicate that the maximum value is recorded one month after the high-

est rainfall.


containers for several weeks. They were then counteddirectly on a Germanium detector with an e�ciency of

25% and a resolution of 2 KeV. The 610 KeV gamma of214Bi was used as a measure of 226Ra activity and the 911KeV gamma of 228Ac for 232Th activity. The 40 K activity

was determined from the 40K peak at 1.461 KeV.The electrical conductivity of water was measured in

the ®eld, with a conductivity meter, and major dissolved

species (Ca2+, Mg2+, Na+ , K+ , Clÿ , alkalinity, SO2ÿ4

and SiO2) were analyzed in the Laboratory ofHydrogeology at the University of Malaga. The total

hardness (Ca2+ plus Mg2+) was determined by titrationwith 0.02 N EDTA using eriocrome-T black as indicator.The Ca2+ was titrated with 0.02 N EDTA using calconas indicator and the Mg2+ contents were obtained from

the di�erence between the total hardness and the Ca2+

content. Concentrations of Na+ and K+ were deter-mined using ¯ame photometry. The Clÿ content was

obtained by argentometric titration using 0.01 N AgNO3

and K2CrO4 as indicator, the SO2ÿ4 content by gravime-

try using BaCl2, and alkalinity by titration with 0.02 N

H2SO4 to pH 4.45. SiO2 was determined spectrophoto-metrically using the ammonium molybdate method.Trace dissolved species in the groundwater (Mn, Fe,

Sr, Pb, Zn, Cu, Ni and also Cr) were determined byatomic absorption spectroscopy with electrothermal ato-mization, except for Zn (by ¯ame atomization). These de-terminations were carried out in the laboratory of

EMASA, a municipal water supply enterprise inMalaga.The 3H content of the groundwater samples was

determined, in the Autonoma University of Barcelona

(Spain), by electrolitic decomposition of the waterbelow 08C for 6 days followed by counting beta par-ticles for 1.5 days approximately.

The gross alpha and beta activities of water sampleswere analysed in the Laboratory of EnvironmentalRadioactivity at the University of Malaga. The watersamples were acidi®ed with concentrated HNO3 to

pH=1 and stored at room temperature. According tothe Krieger (1975) method, an appropiate volume ofsamples was evaporated to dryness to determine the

gross activity. The residue was transferred quantitat-ively to a stainless-steel planchet and dried. After dry-ing under an IR lamp, the samples were allowed to

equilibrate with ambient temperature and were thenweighed. The samples were kept in a desiccator for 3±4 days and then counted in order to ensure complete

decay of 222Rn.

5. Chemical composition and radioactivity of the

marbles

The most abundant components are CaO and MgO,which make up more than 50% of the rock as wouldbe expected for carbonate aquifers. Silica was found to

vary from 0.37% to 30% (Table 1). The most abun-dant trace element is Sr, which varies from 55 to 1254

ppm. The elements Pb, Zn, Cu and Ni have variablecontents depending on local geology, in particular theexistence of mineralization which has been exploited in

the past (Orueta, 1917).The natural radioactivity of the marbles is, in general,

low in the few samples analyzed (Table 1). The highest ac-

tivities correspond to 40K, which can reach 1000 Bq/kg.232Th activity is below 1.6 Bq/kg in all the analyzedsamples and the activity of 226Ra is less than 32.5 Bq/kg.

Two principal groups of marbles can be distin-guished (Fig. 5). The blue calcitic marbles have thehighest concentrations of all the major elements, exceptMgO. They also have the highest contents of Sr and40K. This group includes the blue marbles of the upperpart of the sequence which outcrops in the westernSierra Blanca. These are formed principally of calcite,

although they are the most impure because theyinclude interbedded metapelites, with minerals otherthan calcite (such as quartz, potassic feldspar, andalu-

site, calcic amphibole and pyroxene). The very variableK2O content of these calcitic marbles (Table 1) is dueto varying potassic feldspar in the pelite component.

The white dolomitic marbles mainly comprise CaOand MgO (Table 1). Most of the white marble samplesin this group belong to the lower part of the sequenceand outcrop in the eastern sector of the Sierra Blanca

and the Sierra Mijas. Their composition is less variablethan the calcitic marbles because they are relativelypure dolomite.

Data from Frey et al. (1985), show that the majorcomponents of the peridotites are SiO2 (43%) andMgO (47%), while the most abundant trace elements

are Ni (2380 ppm) and Cr (2000 ppm). The activity ofthe radioisotopes is low (Table 1).

6. Chemical composition of the waters

6.1. Major components

Representative water samples from the hydrogeolo-

gical unit were collected (Table 2 and Fig. 2). In allcases, mineralization of water was low, as indicated bythe conductivity values (between 314 and 554 mS/cm).The majority of the waters analysed are Ca±HCO3 and

Ca±Mg±HCO3 types.Principal component analysis (PCA) was carried out

to summarize the relations between the measured vari-

ables. The 3 principal axes of PCA explain 80.9% ofthe sampling variance, although only the ®rst two areof interest. Axis I (35.6%) is determined by conduc-

tivity, Mg2+, alkalinity, K+ , SiO2 and SO2ÿ4 and is

the mineralization or residence time factor (Fig. 6A).Axis II (30.2%) is a geographic factor de®ned by the


Table

1.Chem

icalandradioactivecompositionsofthesolidsamplesanalysed(see

situationin

Fig.1),

whose

data

havebeenusedto

makeFig.5.Thechem

icalcomposition

data

oftheperidotitesare

averagevalues

from

Freyet

al.(1985)

Majorcomponents

(%weight)

Trace

elem

ents

(ppm)

Radioisotopes

(Bq/kg)

Sample

Litology

MnO

Fe 2O

3CaO

K2O

SiO

2MgO

Na2O

Sr

Pb

Zn

Cu

Ni

226Ra

232Th

40K

1Bluemarbles

32.0

<1.6

1000.0

20.15

2.52

33.44

3.50

29.99

6.00

0.39

354.67

13.55

12.64

12.09

16.72

29.5

<1.2

637.0

30.14

0.85

53.98

0.05

0.62

1.25

0.26

749.05

5.48

0.86

3.64

5.48

40.14

0.92

50.76

0.13

3.37

2.81

0.24

1041.00

5.22

2.69

1.99

4.64

50.14

0.80

53.07

0.10

1.63

1.83

0.27

60.14

0.80

51.77

0.12

1.74

3.56

0.26

1253.91

6.68

0.00

3.59

3.89

70.15

0.79

55.64

0.01

0.37

0.78

0.26

1140.18

6.65

1.02

1.56

3.02

4.9

<1.6

<12.7

80.15

0.90

51.73

0.03

0.98

4.27

0.25

951.36

0.00

0.67

1.17

3.28

9Whitemarbles

0.09

0.52

30.23

0.01

0.49

23.05

0.17

79.5

42.9

112.07

0.47

3.63

17.0

<1.2

<12.8

10

0.10

0.55

30.29

0.02

0.48

22.44

0.17

118.56

68.5

200.29

2.26

8.32

11

0.09

0.47

28.97

0.01

0.62

22.80

0.17

54.86

4.54

0.17

1.32

3.49

32.5

<0.8

<3.0

12

0.10

0.54

29.50

0.01

0.47

22.35

0.16

60.99

8.13

6.14

1.34

3.67

13

peridotites

0.12

7.91

0.77

<0.003

42.59

46.71

0.03

0.50

47.00

6.00

2380.00

2.3

<1.2

<13.7


spatial distribution of the Clÿ and Na+ content in

the rainwater.

In factor plan I±II of the statistical units (Fig. 6B) 3

groups of waters (which belong to 3 sectors inside the

hydrogeological unit) can be distinguished.

Waters in the western sector of the Sierra Blanca

have the lowest mineralization levels and Mg2+ con-

tents. This suggests that the residence times are short,

because the marbles have a well developed karst drai-

nage system.

Waters in the eastern sector of the Sierra Blanca

have the highest contents of SiO2 and SO2ÿ4 . These

data indicate that the residence time is longer because

water ¯ows more slowly through a ®ssured or poorly-karsti®ed aquifer.

Waters in the Sierra Mijas are as mineralized as thelatter, because the marbles here are also poorly-karsti-®ed, but they have the highest contents of Clÿ and

Na+ due to an increase of these rainwater com-ponents from W to E.

6.2. Temporal evolution in the electrical conductivity ofthe groundwater

The information obtained from the hydrochemicalresponse of the carbonate aquifers is greater if, besides

Fig. 5. Relations among di�erent components of the marbles. (A) MgO versus CaO, (B) MgO/CaO versus SiO2, (C) MgO/CaO

versus Na2O, (D) MgO/CaO versus K2O, (E) MgO/CaO versus Fe2O3 and (F) MgO/CaO versus Sr. See sample locations in Fig. 1

and data in Table 1. (WM) white dolomitic marbles, (BM) blue calcitic marbles. Sample 2 is not considered in this ®gure because it

is a very impure marble (see Table 1) and plots outside the grouping shown.


Table 2. Major chemical components of the groundwaters used for the PCA of Fig. 6. All the contents are given in mg/l (the alka-

linity ±ALK± in mg/l of CO3Hÿ ) and the electrical conductivity (EC) in mS/cm to 208C

EC Ca2+ Mg2+ Na+ K+ Clÿ ALK SO2ÿ4 SiO2

Western sector Sierra Blanca Moratan MB-6 360 71.2 7.3 1.5 0.8 10.6 230.6 7.0 12.8

Istan MB-7 335 57.6 10.7 1.3 1.0 9.9 189.1 38.7 9.9

Alfahuara MB-8 320 61.6 8.8 1.5 O.5 12.1 209.8 18.1 8.4

Nagueles MB-9 368 70.4 9.2 2.5 0.8 13.5 217.2 11.5 8.8

Camojan MB-10 314 60.0 8.3 1.8 0.8 13.1 187.1 9.9 7.3

Ojen MB-14 416 65.8 21.3 1.5 0.5 10.6 287.9 3.3 11.2

Eastern sector Sierra Blanca Mancha MB-4 409 58.4 27.4 1.5 0.8 11.0 239.1 28.4 16.5

M. Marmol MB-3 415 56.8 25.4 1.8 1.3 10.6 239.1 30.0 17.9

Coin MB-2 554 83.2 35.9 3.0 1.5 12.4 237.9 113.2 21.7

B. Blanco MB-1 403 52.7 43.1 3.0 1.0 12.4 301.0 37.0 39.0

Las Torres MP-3 481 62.4 32.8 2.3 0.8 12.4 291.6 34.2 17.7

Cortes SP-5 398 50.6 30.5 2.0 0.5 10.7 228.1 33.3 15.7

Sierra Mijas sector Urique MP-1 433 50.6 35.4 2.3 0.5 13.1 281.8 10.0 21.0

San Anton MM-5 432 49.6 29.9 2.0 0.5 14.2 230.0 14.8 6.1

H. Vilches SM-41 431 56.0 28.9 2.3 0.8 12.8 295.2 22.6 7.5

Pavitas MM-9 447 40.4 35.2 3.0 0.5 16.3 230.0 10.3 4.9

C. Grajas SM-56 345 46.4 30.1 3.0 0.5 14.5 264.7 8.2 5.7

F. Pena SM-60 535 69.9 33.8 4.5 1.8 22.7 307.4 26.3 16.1

F. Grande SM-12 407 53.6 26.2 2.5 1.3 14.5 251.3 11.1 11.5

F. Sanguina SM-10 436 62.4 23.3 3.0 1.0 15.3 258.6 51.0 6.7

A. Miel SM-81 375 43.5 24.4 4.5 1.0 18.5 223.3 9.5 8.0

Pellejera MM-16 398 30.8 33.3 7.0 0.8 16.3 210 0 10.3 7.7

Inca MM-19 405 44.2 27.8 6,0 0.8 19.2 239.8 11.1 5.9

Albercon MM-1 407 41.0 27.7 8.0 2.5 23.1 246.9 18.1 7.9

San Jose SM-8 459 47.2 20.1 4.5 0.8 17.8 231.8 12.8 8.7

Rojas SM-88 427 53.8 26.1 4.0 1.3 18.8 258.6 14.4 10.2

Fig. 6. Principal components analysis on hydrochemical data. (A) factor plan of the variables, (B) factor plan of the statistical

units, (WSB) western sector of Sierra Blanca, (ESB) eastern sector of Sierra Blanca, (SM) Sierra Mijas. The PCA includes data

from a sampling in October 1991 and completed with several samples collected in the Sierra Mijas springs before they dried.

Similar distribution have been found in other sampling periods.


the spatial distribution, the temporal evolution is con-sidered (Shuster and White, 1971; Atkinson, 1977;

Bouyer and Kubler, 1981; Hess and White, 1993;Blavoux and Mudry, 1993; Bakalowicz, 1994). Theelectrical conductivity is a valuable hydrochemical par-

ameter because it indicates the mineralization of thewater. For this reason this parameter was monitoredin the main springs of the study area.

The springs of the western Sierra Blanca increaserapidly in ¯ow in response to precipitation and, at thesame time, decreases in the electrical conductivity of

the water occur (Fig. 3B) with a magnitude which var-ies between 50 and more than 100 mS/cm. So, this dataindicate that the dilution of the groundwater by thein®ltration of rainwater can be approximately esti-

mated as 15 to 30%; higher percentage dilutions werenot recorded, because the sampling periodicity wastwice a month. Therefore, a karst drainage exists in

the western Sierra Blanca systems.Mineralization of the water drained by the springs

in the eastern Sierra Blanca and Sierra Mijas shows a

low variability (Fig. 4B), lower than in the springwaters of the western Sierra Blanca, which indicatesthat the karst drainage is less developed. In the Coin

spring (MB-2) the electrical conductivity of the watershowed a progressively ascending trend between July1990 and February 1991, which demonstrates the sub-stantial inertia of the system after the important 1989±

1990 rainfall.

6.3. Trace elements

The trace element contents of the waters do not

further clarify the hydrogeology because trace values,except for Sr, are controlled by local geology as in

other areas (Sarrot-Reynauld et al., 1981; Arad et al.,

1986). The Sr contents vary from 112 to 1190 mg/l(Table 3). In general, the waters of the western sector

of the Sierra Blanca, that drain calcitic marbles, havehigher contents of Ca2+ and Sr than the waters

sampled in the eastern sector of this Sierra and theSierra Mijas (Fig. 7A).

Elements such as Pb and Cu, traditionally associated

with the mineralization existing in the Alpujarriderocks, are present in concentrations of 3 and 6 mg/l, re-spectively (Table 3). Fe and Zn are present at concen-trations of less 20 mg/l, except in the well SM-60

(average Zn content of 117 mg/l).Fig. 7B indicates that the Fe content is correlated

with SO2ÿ4 presumably due to pyrite oxidation. Except

for water from the IstaÂ n spring (MB-7), the lowestcontents of SO2ÿ

4 and Fe are found in the waters from

the western sector of the Sierra Blanca, although theblue marbles that outcrop in this sector have a greater

content of Fe2O3. In the Coin spring (MB-2) high con-centrations of Fe, Zn, Cu and Pb have not been

detected (Table 3). However waters from this springhave high Sr and Ca2+ contents, although the aquifer

is not in the calcitic blue marbles which have higherconcentrations of these components. The SO2ÿ

4 , Ca2+

and Sr concentrations of the Coin spring waters mighthave been a�ected by the dissolution of evaporites,

which are locally intercalated with Alpujarride carbon-ates (Cardenal et al., 1994) and high grade meta-

morphic marbles (GoÂ mez-Pugnaire et al., 1994) in

other parts of the Betic Cordillera.The elements Cr and Ni are especially abundant in

peridotites (Frey et al., 1985), which suggests these el-ements could be used as tracers for waters originating

from these rocks that are in®ltrated in the marbles.

Table 3. Trace element contents (in (mg/l) in the groundwaters used to make Fig. 7. The data are average values of the monthly

content obtained between March and September 1992

Fe Mn Sr Pb Zn Cu Ni Cr

Western Sierra Blanca Moratan MB-6 6.7 2.1 523.7 0.9 9.0 1.8 0.8 1.3

Istan MB-7 19.9 0.9 653.6 1.3 9.0 1.2 0.8 0.8

Nagueles MB-9 3.7 1.6 660.0 1.5 9.0 1.7 0.5 0.5

Camojan MB-10 3.2 3.5 435.7 0.5 7.1 1.9 0.5 1.0

Ojen MB-14 2 1 1.1 328.1 2.6 10.3 0.3 0.6 0.4

Eastern Sierra Blanca Mancha MB-4 12.2 2.2 485.3 1.0 9.0 1.3 1.0 1.2

M. Marmot MB-3 3.9 5.1 457.3 0.4 8.4 0.8 1.3 0.4

Coin MB-2 8.5 1.8 1189.6 1.7 10.3 0.9 0.8 0.7

Las Torres MP-3 5.9 1.1 219.0 1.6 11.3 0.7 1.3 1.3

Cortes SP-5 4.1 1.7 126.7 2.2 11.7 0.9 0.7 1.5

Sierra Mijas F. PenÄ a SM-60 10.3 5,0 184.5 2.2 117.0 5.3 1.9 0.6

San Jose SM-83 6.6 5.7 310.7 2.3 20.1 2.2 0.5 0.6

Rojas SM-88 8.8 0.9 112.0 1.7 9.0 2.0 1.7 0.7


Fig. 7. Relations among di�erent trace elements in the waters. The sample from Coin spring (MB-2) is not plotted in this ®gure

because of the high content in Ca2+, Sr, SO2ÿ4 and SiO2 (see Tables 2 and 3). (WSB) western sector of Sierra Blanca, (ESB) eastern

sector of Sierra Blanca, (SM) Sierra Mijas.


Fig. 7C shows that the groundwaters of the easternSierra Blanca have more SiO2 but their content of Cr

and Ni (Table 3) are similar to others from the westernSierra Blanca (MB-6) or the Sierra Mijas (SM-60, SM-88). So, the peridotites may have a slight in¯uence on

the chemistry of waters, although these mainly orig-inate from the marbles as their concentrations ofmajor chemical components indicate. In any case the

in¯uence of the peridotites on the hydrogeology in theeastern Sierra Blanca is quite limited because thestream waters from the Sierra Alpujata (Alcazarin y

Pereilas rivers in Fig. 1) increase in ¯ow when theycross marble outcrops, indicating that groundwaterdischarge from the aquifer to the rivers takes place(Andreo, 1997).

7. The tritium contents

The nearest rainfall 3H data available for this study

are from the Gibraltar station which belongs toInternational Atomic Energy Agency (IAEA). In a pre-vious study (GarcõÂ a-LoÂ pez et al., 1994) data from

another station located more to the E of the SierrasBlanca and Mijas were used, but these values weresimilar to those from the station in Gibraltar. Natural

processes give an average 3H content of 10 TU in rain-water from the Gibraltar station, with lower values inwinter and higher values in spring and summer, asoccur in other stations of the IAEA (Eriksson, 1983).

The thermonuclear tests between 1953 and 1963yielded maximum concentrations of 1630 TU (May1963) in the rainwater. Since this time, the content of

radioactive 3H has fallen although this trend was inter-rupted in 1986 with values of up to 400 TU.The 3H contents of water samples from the Blanca±

Mijas unit (Table 4) vary between 0.8 (MB-9) and 17.8TU (SM-41).The spring waters of the western sector of the Sierra

Blanca have 3H contents which vary (as do the hydro-chemical parameters) depending on the date of

sampling. Thus data in September are clearly higher

(more than double) those in March (Fig. 8). In Marchthe 3H content is lower due to in®ltration of winter

rainwater with smaller 3H concentrations. InSeptember, higher concentrations are found in the

spring waters, which may represent reserves severalyears old, stored in the inter-karstic blocks (between

the conduits). However these reserves must be quitelimited as well data indicate that the blocks have a low

permeability. The higher 3H content in September maybe also related to recharge, principally from April,

May and even September rainfall (Fig. 8), when thereare regular occurrences of precipitation (up to several

tens of mm/day) with average 3H contents of 11±14TU (in Gibraltar station). This recharge is much less

important than in winter and is noted in the springwaters from the western Sierra Blanca systems because

their stored reserves are scarce, as indicated by theirkarstic behaviour.

The waters of the eastern Sierra Blanca and SierraMijas have 3H contents which are more uniform with

time (Table 4), as is the case with their hydrochemicalparameters. Tritium data from the wells SM-44 (in the

Alhaurin el Grande system) and SM-83 (in the

Torremolinos system), during the period September1992±September 1994, were of a similar magnitude as

the maximum contents found in the summer rainwater(14 to 18 TU as the Gibraltar station average).

However, summer rainfall did not exist or was of littleimportance in the Sierra Mijas (Fig. 9) and produced

practically no in®ltration. In any case, the in®ltratedwater would mix with greater stored reserves coming

from the important winter recharges.The annual average water resources in Sierra Mijas

are 20 hm3, of which 11.5 hm3 corresponds to theTorremolinos system (the most important), while their

annual discharge (principally by pumping) can reach30 hm3 in Sierra Mijas and 20 hm3 in the

Torremolinos aquifer (Andreo et al., 1997b). Thiscauses a general drop in the piezometric level, because

exploitation is greater than recharge, and this trend

Table 4. Tritium contents (in TU) of the groundwaters sampled in this work

9/09/92 2/09/93 17/03/94 15/09/94 20/03/96

Western Sierra Blanca Istan MB-7 2.99 8.9 3.9

Nagueles MB-9 4.77 0.82 5.7 3.1

Ojen MB-14 2.23 9.4 3.1

Eastern Sierra Blanca Coin MB-2 2.82 8.5 5.3

Las Torres MP-3 8.33 7.5

Sierra Mijas San Anton SM-44 17.83 13.6

San Jose SM-83 13.77 16.48 12.9 8.0

Rojas SM-88 6.65 9.2


only reverses with important recharges in very rainy

years. The most important recharge originated from

rainwaters at the end of 1989 and beginning of 1990,

when the 3H content was less than 10 TU (Fig. 9).

So water sampled in the well SM-83 between 1992

and 1994 was a mixture of rainwater from 1989 with

Fig. 8. Variation in tritium content in the waters from NaguÈ eles (MB-9) and OjeÂ n (MB-14) springs, between 1/4/93 and 31/3/96,

compared to the daily ¯ow of the Ojen spring (whose variations are similar to NaguÈ eles spring). The hydrological years begin in

October and ®nish in September.

Fig. 9. Variation in tritium content in the waters of the San JoseÂ well (SM-83) between September 1992 and March 1996, compared

to the piezometric evolution of the aquifer (in SM-81 well) and the tritium content of the rainwater at the Gibraltar station during

1985±1996 period.


in®ltration from 1986. The rains occurring at the endof 1995 and beginning of 1996 reduced the 3H con-

tent of the groundwater from 13 to 8 TU in the wellSM-83.The rest of the sampling points in the eastern Sierra

Blanca and Sierra Mijas, represent waters whichshould correspond to recharge at the end of 1989 and1995. In other Alpujarride aquifers, located to the E of

the studied area here, but with a hydrogeological beha-viour similar to the eastern sector of Sierra Blanca andSierra Mijas, residence times also exceed 5 a (GarcõÂ a-

LoÂ pez et al., 1994).

8. Natural radioactivity of the waters

The gross alpha and beta activities have been deter-mined in sampled waters, although the source isotopes

have not been identi®ed at present. The gross alpha ac-tivity in the groundwater samples originates from the238U and its daughter 226Ra (because 222Rn is notincluded in the measures obtained by the Krieger

method), while beta activity is principally from 232Thand 40K (Cothern and Rebers, 1990). All the availabledata (Table 5) are below legislative limits, as is the

case for most of the waters in the region (DuenÄ as etal., 1993, 1997). These data, together with the hydro-geological distinctions previously made, permit a pre-

liminary evaluation of the application of the alpha andbeta activities to carbonate aquifer research (Fig. 10).Waters in the western sector of the Sierra Blanca

have alpha and beta activities below minimum detec-

tion limits (DLalpha=0.009 Bq/l and DLbeta=0.051

Bq/l). This prevents observations of any variations intime that occur specially in karstic aquifers (Andrewsand Wood, 1972; Smart, 1996). Nevertheless, the fact

that activities have not been detected (although somemarble samples of this sector have the highest 40K con-

tents) indicates once again that the waters of this sec-tor do not remain in the aquifer long enough to

acquire radioactive elements by dissolution. This is dueto the karstic behaviour of the aquifers in this sector.

Waters in the eastern sector of the Sierra Blancahave the highest values of natural radioactivity(Table 5). It is unlikely that this radioactivity orig-

inates in the peridotites (Table 1), or in the streamwaters from the Sierra Alpujata (Alcazarin and

Pereilas rivers in Fig. 1) which cross marble outcrops,because the alpha and beta activities of these waters(before arriving at the marbles) are below the detection

limits (Table 5). The alpha activity of the groundwaterincreases with ¯ow (as also does conductivity and tem-

perature of the water) in passing from the Manchaspring (MB-4), in the W, to the Coin spring (MB-2) in

the E. The alpha activity decreases eastwards probablyby mixing with low radioactivity waters originatingfrom the eastern edge of the Sierra Blanca and/or the

peridotites of the Puerto Pescadores. The beta activityis above detection limit in the Coin (MB-2) and Torres

springs (MP-3).Waters in the Sierra Mijas have values of alpha ac-

tivity slightly above detection limit (Table 5).However, the beta activity is below detection limitexcept for water from the well SM-60, which drains

Table 5. Gross alpha and beta activities (in Bq/l) from the waters of the main points of the Sierras Blanca and Mijas. (<DL)

below detection limit

18/10/92 17/03/94 15/09/94

Alpha Beta Alpha Beta Alpha Beta

Western Sierra Blanca Moratan MB-6 <DL <DL <DL <DL

Istan MB-7 <DL <DL <DL <DL

Nagueles MB-9 <DL <DL <DL <DL

Camojan MB-10 <DL <DL

Ojen MB-14 <DL <DL <DL <DL <DL <DL

Eastern Sierra Blanca Alcazarin river <DL <DL

Pereilas river <DL <DL

Mancha MB-4 31+13 <DL

Molino Marmol MB-3 36+12 <DL

Coin MB-2 100+20 140+40 91+21 130+30 100+40 120+40

Las Torres MP-3 90+20 90+30 80+12 100+30 100+30 100+40

Cortes SP-5 62+15 <DL

Sierra Mijas Fuenta PenÄ a SM-60 20+10 60+30

Rojas SM-88 37+13 <DL 34+13 <DL 33+18 <DL

San Jose SM-83 22+10 <DL


through marbles with metapelite intercalations whichcontain potassic feldspar.

The waters from the eastern sector of the SierraBlanca and Sierra Mijas have the highest alpha andbeta activities and are relatively constant in time

(Table 5), although the white dolomitic marbles whichoutcrop in these sectors have only low radioactive iso-tope contents (Table 1). Slow long distance ¯ows mustoccur in the white dolomitic marbles allowing pro-

longed contact with the aquifer rocks and increaseddissolution of radioactive elements.

9. Conclusions

This study con®rms that the chemical composition

of water in this area is a function of the chemical com-position of the aquifer rocks. Thus in the calciticmarbles of the western Sierra Blanca Ca±HCO3 waterswith higher Sr content occur, while in eastern Sierra

Blanca and Sierra Mijas the waters are a Ca±Mg±HCO3 type, because of the dolomitic composition ofthe marbles that outcrop in this sector.

However, the chemical contents or the mineraliz-ation level of the water and its evolution in time alsodepend on residence time inside the aquifer which, in a

carbonate aquifer (such as studied here), is controlledby karsti®cation. The degree of karsti®cation is im-portant in water exploitation and/or management and

can be infered by applying both geochemical and iso-

tope (3H data) methods. The two methods give consist-

ent results, and their joint usage produces improved

con®dence. Thus, the western Sierra Blanca waters

have lower electrical conductivity and 3H contents,

both of which decrease quickly during recharge

periods, because the aquifers of this sector are conduit

¯ow systems. On the other hand, waters from the east-

ern Sierra Blanca and Sierra Mijas have a higher min-

eralization which, like the 3H content, is less variable,

because the aquifers of this sector are di�use ¯ow sys-

tems. In the Sierra Mijas, waters in®ltrated after 1986

have been detected due to the extent of exploitation,

whilst in the eastern Sierra Blanca the sampled waters

should have been in®ltrated during the 1989±1990 and

1995±1996 recharge periods.

This hydrogeologycal behaviour can be used to test

a research methodology based on the natural radioac-

tivity (gross alpha and beta activities from long-lived

radioisotopes) of the waters. The western Sierra Blanca

waters have the lowest activity values (below the detec-

tion limit), while in the eastern sector of the Sierra

Blanca and Sierra Mijas higher values were observed.

The latter must be related to the longer residence times

of waters in the aquifers of these sectors, as also indi-

cated by the hydrochemical and 3H data and hence

validates the use of natural radioactivity measurements

in hydrogeological investigations.

Fig. 10. Beta activity versus alpha activity plotted on log scales for samples collected on 18/10/92. When the activities are below

detection limits (in broken lines), the half part of above limits were considered to plot the diagram.


Acknowledgements

This work has been ®nanced by the InvestigationGroup no. 4021 of the Junta de Andalucia and by theProject PB94-1495 of the DGICYT and it is a contri-

bution to the IGCP 379 of UNESCO. The authorsthank Dr T. Atkinson for his interesting suggestionsand criticisms and Dr R. Raiswell (University of

Leeds) his helpful English corrections.

References

Andreo, B., 1997. HidrogeologõÂ a de acuõÂ feros carbonatados

en las Sierras Blanca y Mijas (Cordillera BeÂ tica, Sur de

EspanÄ a). SPUMA-CHS, MaÂ laga.

Andreo, B., Carrasco, F., Sanz De Galdeano, C., 1997a.

Types of carbonate aquifers according to the fracturation

and the karsti®cation in a southern Spanish area.

Environm. Geology 30, 163±173.

Andreo, B., Carrasco, F., Vadillo, I., 1997b. EvaluacioÂ n de

recursos hõÂ dricos de las Sierras Blanca y Mijas (MaÂ laga).

Estudios Geol. 53, 33±44.

Andrews, J. P., Wood, D. F., 1972. Mechanism of radon

release in rock matrices and entry into groundwaters.

Trans. Inst. Mining and Metallurgy B 81, 198±209.

Appelo, C. A. J., Postma, D., 1993. Geochemistry, ground-

water and pollution. A. A. Balkema, Rotterdam.

Arad, A., Kafri, L., Halicz, L., Brenner, I., 1986. Genetic

identi®cation of the saline origins of groundwaters in Israel

by means of minor elements. Chem. Geol. 54, 251±270.

Atkinson, T., 1977. Di�use ¯ow and conduit ¯ow in limestone

terrain in the Mendip Hills. J. Hydrol. 35, 111±123.

Back, W., Hanshaw, B. B., 1971. Geochemical interpretations

of groundwater ¯ow systems. W.C. Bull. 7, 1008±1016.

Bakalowicz, M., 1994. Water geochemistry: water quality and

dynamics. In Groundwater Ecology (eds J. Gibert, J.

Stanford and D. Danielopol), Chap. 4, 97±127. Academic

Press.

Blavoux, B. Mudry, J., 1993. Use of the physico-chemical

variations of karst waters to understand how an aquifer

works: example from the Fontaine de Vaucluse system

(Southeastern France). In Hydrogeological Processes in

Karst Terranes (Eds.) GuÈ nay, G., Johnson, A. I. and Back,

W.). Vol. 207, 327±333. IAH Sci.

Bouyer, Y., Kubler, B., 1981. HeÂ teÂ rogeÂ neiteÂ chimique et

hydrologique des eaux souterraines d'un karst du Haut-

Jura neuchaÃ telois. Suisse. J. Hydrol. 54, 315±339.

Bradbury, K. R., 1991. Tritium as an indicator of ground-

water age in central Wisconsin. Ground water 29, 398±404.

Cardenal, J., Benavente, J., Cruz-SanjuliaÂ n, J. J., 1994.

Chemical evolution of groundwater in triassic gypsum-bear-

ing carbonate aquifers (Las Alpujarras, South Spain). J.

Hydrol. 161, 3±30.

Cothern, C. R., Rebers, P. A., 1990. Radium, Radon and

Uranium in drinking water. Lewis Publishers, Michigan.

Drever, J. I., 1982. The geochemistry of natural waters.

Prentice Hall, New Jersey.

Dreybrodt, W., 1981. Kinetics of the dissolution of calcite

and its application to karsti®cation. Chem. Geol. 31, 245±

269.

DuenÄ as, C., FernaÂ ndez, M. C., GonzaÂ lez, J. A., Carretero, J.,

PeÂ rez, M., 1993. Ra-226 and Ra-224 in waters in Spain.

Tox. and Environm. Chem. 39, 71±79.

DuenÄ as, C., FernaÂ ndez, M. C., Liger, E., Carretero, J., 1997.

Natural radioactivity levels in bottled water in Spain.

Water Research 31, 1919±1924.

Eriksson, E., 1983. Stable isotopes and tritium in precipi-

tation. In Guidebook on nuclear techniques in hydrology,

Vol. 91, 19±27. Technical Reports Series I.A.E.A.

Frey, F. A., Suen, C. J., Stockman, H. W., 1985. The Ronda

high temperature peridotite: geochemistry and petrogenesis.

Geochim. Cosmochim. Acta 49, 2469±2491.

Fontes, J. Ch., 1983. Dating of groundwater. In Guidebook on

nuclear techniques in hydrology, Vol. 91, 285±317. Technical

Reports Series I.A.E.A.

GarcõÂ a-LoÂ pez, S., Cardenal, J., Cruz-SanjuliaÂ n, J. J.,

Benavente, J., 1994. Natural and anthropogenic sources of

tritium in carbonate aquifers to the south of Sierra Nevada

(Spain). J. Environm. Hydrology 2, 2±31.

GoÂ mez-Pugnaire, M. T., Franz, G., LoÂ pez SaÂ nchez-VizcaõÂ no,

V., 1994. Retrograde formation of NaCl-scapolite in high

pressure metaevaporites from the Cordilleras BeÂ ticas

(Spain). Contrib. Mineral. Petrol. 116, 448±461.

Hess, J. W., White, W. B., 1993. Groundwater geochemistry

of the carbonate karst aquifer, southcentral Kentucky.

U.S.A. Applied Geochemistry 8, 189±204.

Krieger, L. H., 1975. Interim radiochemical methodology for

drinking water. EPA-600/4-75-008, US Environmental

Protection Agency, Cincinnati, Ohio.

Linares, L., Trenado, L., 1981. CaracterõÂ sticas hidrogeoloÂ gicas

generales del macizo carbonatado de Sierra Blanca±Sierra

de Mijas (MaÂ laga). I Simposio sobre el Agua en AndalucõÂ a,

Granada 2, 699±705.

Malozewsky, P., Zuber, A., 1982. Determining the turnover

time of groundwater systems with the aid of environmental

tracers. J. Hydrol. 57, 207±231.

Orueta, D., 1917. Estudio geoloÂ gico y petrograÂ ®co de la

SerranõÂ a de Ronda. Mem. Instituto GeoloÂ gico de EspanÄ a,

Madrid.

Sanz De Galdeano, C., Andreo, B., 1994. Structure of the

Blanca unit (Alpujarride Complex, Betic Cordillera, Spain).

Regional implications. Bull. Geol. Soc. of Greece 30, 439±

447.

Sarrot-Reynauld, J., Rochat, J., Dazy, J., 1981. ReÂ partition

des eÂ leÂ ments traces dans les eaux thermomineÂ rales. J.

Hydrol. 54, 33±50.

Schoeller, H., 1962. Les eaux souterraines. Masson, ParõÂ s.

Shuster, E. T., White, W. B., 1971. Seasonal ¯uctuations in

the chemistry of limestone springs: a possible means for

characterizing carbonate aquifers. J. Hydrol. 14, 93±128.

Smart, P., 1996. Hydrological implications of Radon concen-

trations in karst groundwaters. Abstract In International

Meeting on Hydrochemistry (noble gases, stable isotopes

and Radioelements) in memory of J. N. Andrews.

University of Reading, Whiteknights, Reading, UK.


Documents

Application of geochemistry and radioactivity in the ... › bbldoc › articulos › 16692299.pdf · Application of geochemistry and radioactivity in the hydrogeological investigation