Hydrogeology of the Nurra Region, Sardinia (Italy): basement-cover influences on groundwater...

Hydrogeology of the Nurra Region, Sardinia (Italy): basement-coverinfluences on groundwater occurrence and hydrogeochemistry

Giorgio Ghiglieri & Giacomo Oggiano &

Maria Dolores Fidelibus & Tamiru Alemayehu &

Giulio Barbieri & Antonio Vernier

Abstract The Nurra district in the Island of Sardinia(Italy) has a Palaeozoic basement and covers, consistingof Mesozoic carbonates, Cenozoic pyroclastic rocks andQuaternary, mainly clastic, sediments. The faulting andfolding affecting the covers predominantly control thegeomorphology. The morphology of the southern part iscontrolled by the Tertiary volcanic activity that generateda stack of pyroclastic flows. Geological structures andlithology exert the main control on recharge and ground-water circulation, as well as its availability and quality.The watershed divides do not fit the groundwater divide;the latter is conditioned by open folds and by faults. TheMesozoic folded carbonate sequences contain appreciableamounts of groundwater, particularly where structural

lows are generated by synclines and normal faults. Theregional groundwater flow has been defined. The investi-gated groundwater shows relatively high TDS andchloride concentrations which, along with other hydro-geochemical evidence, rules out sea-water intrusion as thecause of high salinity. The high chloride and sulphateconcentrations can be related to deep hydrothermalcircuits and to Triassic evaporites, respectively. Thesource water chemistry has been modified by variousgeochemical processes due to the groundwater–rockinteraction, including ion exchange with hydrothermalminerals and clays, incongruent solution of dolomite, andsulphate reduction.

Keywords Groundwater flow . Hydrogeochemistry .Salinization . Groundwater management . Italy

Introduction

The Nurra district is located in the northwestern part of theisland of Sardinia (Italy) in the Sassari Province, with80 km of coastline with the Mediterranean Sea (Fig. 1a).Its geology records a long history from Paleozoic toQuaternary, resulting in relative structural complexity andin a wide variety of rocks.

Due to intensive human activities and recent climaticchanges, the area has become vulnerable to desertification.As a result, the area is included in the national researchnetwork under the RIADE project (Integrated research forapplying new technologies and processes for combatingdesertification (RIADE project 2002–2006), set up by theItalian Ministry of Research (Ghiglieri et al. 2006).

The water demand in the study area is considerable,water being required for industry, domestic use, tourism,agriculture, and animal rearing. Nurra relies on bothsurface and groundwaters. The seasonal and perennialrivers of the area are exploited using the Cuga andSurigheddu dams, built on the highlands. However, likeon other Mediterranean islands, surface-water resourcescan periodically suffer from drastic shortage. Groundwaterin different aquifers is exploited using deep boreholeswhich can attain discharges as high as 145 l/s. The waterdemand of the city of Alghero, for example, is partially

Received: 31 March 2006 /Accepted: 15 September 2008Published online: 15 October 2008

© Springer-Verlag 2008

G. Ghiglieri ())Department of Territorial Engineering,Geopedology and Applied Geology Section,Desertification Research Group (NRD), University of Sassari,Viale Italia, 07100, Sassari, Italye-mail: [email protected]: +39-79-229261

G. OggianoInstitute of Geological Sciences and Mineralogy,University of Sassari,Corso Angioy 10, 07100, Sassari, Italy

M. D. FidelibusDepartment of Civil and Environmental Engineering,Technical University of Bari,Via Orabona 4, 70125, Bari, Italy

T. AlemayehuSchool of Geosciences,Wits University,Private bag 3, P. O. Box Wits 2050, Johannesburg, South Africa

G. Barbieri :A. VernierDepartment of Territorial Engineering,Applied Geology and Applied Geophysics Section,University of Cagliari,Piazza D’Armi, 09100, Cagliari, Italy

Hydrogeology Journal (2009) 17: 447–466 DOI 10.1007/s10040-008-0369-z

448


satisfied by groundwater withdrawn through five wellsdischarging a total of 96 l/s.

Notwithstanding the importance of local groundwateras the main source of good quality water and its role ofstrategic reserve in such semiarid conditions, exploitationup to now has been uncontrolled (Barbieri et al. 2005a, b;Ghiglieri et al. 2006). An additional problem in the Nurradistrict is that water users have a very scant knowledge ofthe provenance and value of the fresh water they exploit,thus leading to a high rate of unofficial exploitation.

The extensive exploitation of the Nurra aquifers and theconsequent water-quality deterioration require a revision ofcurrent water management practices. This revision has to bebased on good knowledge of both the potential of aquifersin terms of geometry and storage and quality in terms ofhydrogeochemical features, which, up to now, has beendisregarded. This report presents the synthesis of lengthyresearch, of which the main aims of have been: (1) toreconstruct the hydrogeological setting and the regionalgroundwater flow; (2) to ascertain the origin of salinity; (3)to recognise the boundary conditions of different hydro-geologic units by mean of processes that control theconcentration of major constituents in the different aquifers.Achieving these aims will establish a basis for developing anappropriate monitoring programme and therefore improvedmanagement of the water resources of the region (Ghiglieriet al. 2006, 2007, 2008).

Methodology

A geological and structural map of the area has beenprepared on the basis of recent data and field surveys. Theconceptualization of the hydrogeological setting led to theidentification of the recharge and discharge areas andmajor controlling structures. The field data have beenintegrated with aerial photo interpretation and geophysicalprospecting (gravimetric profiles) (Ghiglieri et al. 2006).

For the purposes of the RIADE project, technical dataand relevant information from 424 boreholes werecollected together with data from 87 springs. A globalpositioning system (GPS) was used to locate each feature(Ghiglieri et al. 2006).

From these locations, 99 wells and 21 springs wereselected for chemical monitoring purposes. In order toinvestigate the behaviour of the aquifers at two differenttimes of the year, water was sampled for chemical analysisfrom 118 water points (97 wells and 21 springs) inDecember 2004, and from 55 water points (51 wells and 4springs) in June 2005 (Fig. 1c).

Water samples were collected from pumped wells anddirectly from springs in 1,000-ml polyethylene bottles.Electrical conductivity, pH, alkalinity and temperaturewere measured in the field while Eh was determined in the

laboratory. The chemical analyses were performed at theUniversity of Sassari (Italy), immediately after samplecollection. The analysis of cations was undertaken usingan Analyst 200 atomic absorption spectrometer. Anionswere analyzed by an ion chromatograph with fourcomponents: a Waters pump (model 590), a Waterselectrical conductivity detector (model 431), an Alltechsolid phase chemical suppressor (SPCS, model 335) and aSRI PeakSimple data system (model 203).

For stable oxygen and hydrogen isotopes, sevengroundwater samples were collected from one spring andsix boreholes. For tritium analysis only three watersrepresentative of the three main hydrogeologic units weresampled. The analysis was carried out in the CNR isotopehydrology laboratory, Pisa, Italy.

Geological setting

The Nurra district encompasses a structural high, whichdeveloped during the Tertiary and where older rocksequences are progressively exposed westward (Fig. 1b).The northeastern limit of the area is marked by the upperMiocene deposits of a half-graben Porto Torres basin(Thomas and Gennessaux 1986; Funedda et al. 2000) thatcover the older rocks. The Variscan metamorphic basementis well exposed in the westernmost sector near the coast(Fig. 1b). As regards the basement, grey Autunian arenitesand silts, and upper Permian and Triassic continental redbeds with interlayered alkaline volcanics occur. The firstmarine transgressive deposits consist of dolostones, lime-stones and evaporites of Middle Triassic age showing thetypical Germanic facies.

Since this time, shallow marine sedimentation, in acarbonate platform environment, had been almost contin-uous until Aptian-Albian time. During the Albian-Aptiantime, an important tectonic phase took place in Nurra,referred to as the Bedoulian movement (Oggiano et al.1987). This tectonic event was responsible for theemersion that gave rise to widespread bauxite depositsand caused the partial erosion of the Jurassic succession.

During the Coniacian stage, all the Nurra bauxiticpalaeosurface was submerged, due to a new transgression,which led to carbonate-terrigenous sedimentation lastingup to the Maastrichtian. The post-Maastrichtian emersion issupposedly related to a new tectonic phase (Laramic phase;Oggiano et al. 1987). Since the Paleocene, the entireregion experienced weathering, erosion, widespread cal-calkaline volcanism and two important deformation eventslinked both to the Pirenaic and to the North Apennine(Carmignani et al. 1995) orogenesis. These deformationsgenerated minor thrusts and mild NE trending folds whichdominate the present geometry of the Mesozoic cover and,as a consequence, the geometry of the main aquifers.

The Variscan basement outcrops in the westernmost partof the study area, consisting chiefly of black phyllites withminor quartzites, meta-basalts and oolitic ironstones. To-wards the north, due to the increased metamorphic grade, itconsists of micaschists and paragneisses, while the phyllites

�Fig. 1 The Nurra district of Sardinia: a location map; b geologicalmap; c location of water sampling points in the Calich catchment(delineated in b). In the legend gw stands for groundwater

449


inhibit vertical infiltration in this area. The only rock withvery low secondary permeability (1×10−7 m/s) is thequartzite which is jointed due to its brittle nature.

The Mesozoic succession overlies the basement(Fig. 2). The lowest unit is an arenite-conglomeratedeposit, Permo-Triassic in age, which shows highlyvariable thickness and medium permeability. The remainingpart of the Triassic rocks consists of transgressive dolomitic,calcareous and evaporitic deposits. The thickness of thecarbonate portion is about 80 m, whereas the stratigraphicthickness of the evaporitic deposits, mainly gypsum, is notknown; due to their ductile behaviour, these depositsare severely deformed. The permeability of the Triassiccarbonate and gypsum is high (Fig. 3) and the depositplays an important role in conditioning the groundwatersalinity.

A carbonate sequence that encompasses the entireJurassic system lies on the Triassic evaporites. Its baseconsists of alternations of marls and limestones with thindark pyrite-rich shale levels. Most of the sequence is madeof dolostones and limestones; green marls with typicalPurbeckian facies also occur towards the top of this systemgrading into the Cretaceous limestone. The Jurassicdeposits tend to increase their thickness to the southeastand attain maximum thickness of 800 m to the south, closeto the Su Zumbaru Fault (central part of the study area,Figs. 1–3). The permeability and the transmissivity of theJurassic sequence are high due to fractures and karsticconduits.

The Cretaceous succession lies on “Purbeckian” marls;it consists of two sequences separated by an angularunconformity, which is marked by bauxite deposits andrepresents a hiatus corresponding to the mid-Cretaceous.The lower sequence is represented by limestone with

typical Urgonian facies. This limestone consists of astrongly karstified biosparite that, where not completelyeroded, reaches 180 m in thickness (Figs. 1b and 2).

The upper Cretaceous sequence also consists of lime-stones with an important intercalation of glauconite-bearing,more or less arenitic marls. Some boreholes penetrate 300 mof upper Cretaceous deposits in the area south of Olmedo(Oggiano et al. 1987).

Above a karstic palaeosurface, developed on theMesozoic carbonate rocks, several pyroclastic flows weredeposited during the lower Miocene. The pyroclasticdeposits formed the volcanic plateau in the southeasternpart of the area. The different flow units are separated bypalaeosols; the thickness of each flow varies from a few tohundreds of metres. Bentonite deposits deriving from thehydrothermal alteration of feldspar and glassy materialgenerally seal the bottom of the pyroclastic stack. Thewelded tuffs that occur as a top cover are intensivelyfractured favouring vertical infiltration.

The pyroclastic flows (Figs. 1b and 2) are capped byBurdigalian calcarenites that outcrop only at the easternboundary of the study area along with sands andconglomerates of Tortonian age derived from a reworkingof the basement. The latter clastic deposits are confinedwithin structural lows interpreted as strike slip basins.

Quaternary aeolian sands, travertines and loose sedi-ments consisting of alluvial sands and gravels cover mostof the plains in the west of the region, occurring aspediment slope deposits and valley fill materials that rangein thickness from 10 to 40 m. All these deposits have

Fig. 2 Representative geological cross sections

Fig. 3 Hydrogeological map. K hydraulic conductivity �

450


451


generally good permeability, allowing infiltration into thelower aquifers.

Structural framework

The basement tectonics reflect the polyphase evolution ofthe Variscan events in Sardinia. The only hydrogeologi-cally relevant structure of this unit is linked to the latestVariscan folding phase that generated a wide synform withan east striking and dipping axis. This structure controlsthe superficial drainage, which is roughly directedeastwards, i.e. toward the Mesozoic limestones. The firsttectonic instability affecting the Mesozoic cover started inthe Middle-Cretaceous time with transtensive Bedoulianmovements followed by a transpressive regime (Oggiano etal. 1987); these caused the angular unconformity betweenthe Lower Aptian and the Coniacian (Oggiano et al. 1987),an interval which lead to the development of the bauxitementioned above. These tectonic movements resulted in thedevelopment of some uplifted blocks bounded by normalfaults and mild folding within a sinistral wrench shear beltrunning between the Olmedo area and Porto Conte bay(Fig. 2). Hence, during the peneplanation of the bauxiticsurface, due to the erosive removal of at least 600 m ofMesozoic sequence towards the north, the actual thicknessof the Mesozoic cover increases to the south.

The evidence for tectonic movements subsequent to thedevelopment of the bauxite horizon and its Coniaciancover comprise syn-tectonic breccias and olistostromeswithin Upper Cretaceous sediments close to an importantfault. These tectonic movements caused the uplift of theMesozoic platform south of a line joining Uri and Alghero(Mamuntanas-Su Zumbaru Fault, Oggiano et al. 1987).This tectonic activity was tentatively ascribed to theLaramic phase. Other faults and folds, with NE axialstrike, involving the whole Mesozoic sequence except thedeposits younger than Lower Miocene; can be referred tothe Eocene Pyrenaic phase and the Oligocene Apenniniccollision. During the Oligocene and early Miocene, newleft lateral movements caused the reactivation of the ENEoriented, strike-slip fault (Carmignani et al. 1995) of mid-Cretaceous age. From the Burdigalian, at the same time asthe opening of the Balearic basin, until the Pliocene, anextensional regime was present, giving rise to normalfaults with various orientations.

Hydrogeological features

The thickness of the Mesozoic sequences is known onlyapproximately because of the uncertainties associatedwith the deep erosion during the Middle Cretaceous andother erosion events, which occurred since the begin-ning of the Cenozoic. However, in the main structurallows, the Mesozoic aquifer can easily reach thicknessesof 1,000 m (Ghiglieri et al. 2006, 2007). The deforma-tion history of the Mesozoic rocks exerted a strong

control on the following features of the geometry of theaquifers:

– TheMid-Cretaceous erosive stage controls the thickness ofthe Mesozoic carbonate rocks. In general, the thicknessincreases southwards and progressively diminishes north-ward in consequence of the presence of a palaeo-structuralhigh. The Mesozoic sequences, due to their hugethickness, represent the main aquifer of the region, whichis shallower and thinner moving northward (Figs. 2 and 3).

– Locally synformal geometries, due to Upper Creta-ceous deformation involving marly strata, can allowthe formation of perched aquifers; moreover, theshortening, accommodated by folds, causes the thick-ening of the cover and, consequently, of the aquifers.

The volcanic succession thickens southward, where italso crops out at high topographic levels (500 m a.s.l.). Thisunit hosts a multilayer aquifer due to alternance of weaklywelded, ignimbrites and deeply fractured high-grade ignim-brite. Each permeable layer is confined by clay-rich paleosolsor by pumice and ash flows converted into bentonite.

The strike-slip faults, due to their steep dip and deeppenetration, allowed discharge of hypothermal fluid thatresulted in the development of bentonite and zeolitesdeposits in the volcanic rocks. Because of their high cationexchange capacities (CEC), these minerals exert a controlon water chemistry. Hydrothermal alteration, which oc-curred in Upper Miocene, is a widespread phenomena inthe study area and in the neighbouring localities. In thestudy area, in particular, epigenetic kaolin and bentonite arepresent (Mameli 2000). Zeolites with high CEC aredescribed by Cerri et al. (2001) and Cerri and Mameli(2004). Bentonite occurs mostly in the calcium form,while zeolite occurs in calcium-, sodium- and potassium-rich varieties (Cerri et al. 2001). With regards to chloride,concentrations up to 20,000 ppm have been detected insome deeply kaolinized volcanic rocks (Mameli 2001).

In the southeastern and southern part of the area there arehypothermal manifestations in the form of thermal springsassociated with deep running strike-slip faults. The fielddata also indicate that groundwaters in the volcanic rockshave high temperatures that range between 19 and 24°C,including the cold winter season. In order to identify therecharge area, the groundwater divide has been recognized(Fig. 3), allowing the mean annual recharge to beestimated (about 37×106 m3, Ghiglieri et al. 2006, 2008).

Growndwater regional flow

The mean annual rainfall in the Nurra district, calculatedover 30 years (1960–1990), averages 607 mm with abimodal pattern within a year, which is typical of theMediterranean region. The mean annual temperature ofthe area is 15.7°C. The recharge to the aquifers is alsoexpected to take place during the rainy months of Octoberto December and February to April after soil moisturereplenishment. The main aquifer is represented by the

452


Mesozoic carbonate successions with a yield that variesbetween 20 and 145 l/s. The groundwater flow direction inthis aquifer is strictly controlled by structural deformationand weathering processes (Ghiglieri et al. 2006, 2008).Due to prevalent NE–SW aligned synclines and anticlines,the direction of groundwater flow in the carbonate rocks istowards the SW. Anticlinal folds of the Paleozoic andMesozoic sedimentary rocks gave rise to high risingground in the western sector of the area; they play a veryimportant role in recharging or dispersing surface waterflows and by reducing water-rock contact time, thusgenerating relatively fresh water (see the following). Thestructural highs of Monte Doglia, Monte Pedrosu, MonteZirra, Monte Timidone and Monte Cugiareddu act asrecharge areas for the confined Jurassic aquifer; theculminations of the major anticlines also representeffective recharge areas. On the other hand, the synclinesact as storage areas for groundwater. The most prominentsynclines are those of Campu Calvagiu and Sabadiga-Alghero (Figs. 2 and 3). In the huge Sabadiga-Algherosyncline, groundwater converges from all directions andwells are high-yielding. Therefore, this synclinal zonecontains huge reserves of groundwater. Folded anticlinesforce groundwater flow through the synclinal axis andinfluence the direction of groundwater flow, as confirmedby numerical modelling in similar geometries occurring inIsrael (Ben-Itzhak and Gvirtzman 2005). The structuralframe also controls the boundary conditions: to the westthe aquifers are encircled by the contact with very lowpermeability Variscan basement, which is also the imper-meable lower boundary; to the east the Mesozoic aquiferis buried below the Miocene-hosted aquifers that feed itlaterally. To the south the main aquifers are in contact withthe volcanic complex through important strike-slip faults.

The productivity of the volcanic deposit is very lowdue to intensive weathering. The groundwater flowdirection in the volcanic massifs is towards the NW, dueto the dipping of the Pyroclastic units.

Insights from analytical results

Data reported in Table 1 relate to the superficial, springand groundwaters sampled in the Nurra district, sampledin the period September–December 2004.

Total dissolved solids (TDS)The principal feature emerging from the whole data set isthat a widespread salinization affects most of the analyzedgroundwaters. Since many different salt sources, besidespresent-day sea water, can be involved in salinizationprocesses (salt spray, evaporite dissolution, mixing withsaline fluids and thermal fluids inflows) of coastalaquifers, the identification of the sources can be difficult.

Tellam (1995) considered a number of potential saltsources and salinization processes in a Triassic sandstoneaquifer (Cheshire Basin, NE England, UK) such as sea

water concentrated by permafrost salt exclusion, reverseosmosis, leakage of brines known to be present in aCarboniferous deltaic coal-containing sequence underly-ing the sandstones, evaporite dissolution from an overly-ing Triassic mudstone/evaporite sequence, present daysea-water intrusion. Whatever the salt source, its involve-ment is normally partly obscured by water–rock interac-tion processes triggered by ionic strength increase. As anexample, groundwater salinization in carbonate aquiferscauses a renewal of karstification, with an enhancement ofsecondary porosity (Hanshaw and Back 1979; Hermanand Back 1984; Tulipano et al. 2005; Whitaker and Smart1997; Sanford and Konikow 1989a, b; Liu and Chen1996). In intergranular-flow aquifers, as well as infractured or karstic aquifers with a few percent of claysor other exchangers, salinization activates ion-exchange(Appelo and Geirnaert 1983).

The geological history of the Nurra Basin makes itlikely that among the several sources of salinity, sea water,evaporites (mainly NaCl and CaSO4), and Tertiaryhydrothermal deposits (derived from hydrothermal fluidswith dissolved salts as KCl, CaCl2, NaCl, alunite) are themore reliable. Figure 4 shows the relationship betweenTDS and chloride concentration. The TDS-Cl plot and allthe following plots distinguish all analyzed waters accordingto their type—surface-, spring- or groundwater—and, in thecase of groundwaters, their aquifer. Data from drillings, fieldsurveys and the geological and structural studies, allowedinitial attribution of the groundwater and spring samples tothe different aquifers; the hydrochemical study subsequentlyconfirmed this attribution for the 95% of samples.

Moreover, no distinction is made as to the periods ofsampling. The June 2005 sampling covered only 51% ofthe wells and 20% of the springs sampled in December2004, with the addition of other well points: the meanvariation of TDS in repeated samples was only 10%,indicating that mineralization does not vary much withseason. Therefore, both data-sets are taken into account,with the main aim of describing the general characteristicsof the different aquifers.

Figure 4 shows that TDS ranges from minimum valuesof about 200 mg/l (springs from Oligo-Miocene volcaniccomplex) to maximum values of 5,000 mg/l (groundwatersfrom Triassic aquifer). Most of the springs, a large part ofthe Jurassic groundwaters and part of the Oligo-Miocenegroundwaters show a TDS in the range 500–1,000 mg/l,while almost all Quaternary and Triassic groundwatersshow TDS higher than 1,000 mg/l. Cretaceous ground-waters cover the TDS range from 400 to 2,000 mg/l. Thevalue of about 4 meq/l has been chosen as an upper limitfor freshwater. The maximum TDS of waters having lessthan 4 meq/l of chlorides is about 1,000 mg/l.

The TDS-Cl binary plot shows the lines representingthe chemically inert mixing between present-day sea water(represented by the mean of four samples of sea watercollected offshore from the local coastal area) and,respectively, the freshest waters (springs) sampled in thehighland (corresponding to the Oligo-Miocene volcanicaquifer) and in the plain (where all the other aquifers

453


Tab

le1

Hyd

rochem

ical

parametersforthegrou

ndwaterssampled

inDecem

ber20

04

IDUTM

EUTM

NHU

TDS

TpH

EC

Ca

Mg

Na

KHCO3

Cl

SO4

DIC

NO3

SiO

2Δca+ΔMg

ΔNa+

ΔK

ΔSO4

SI

calcite

SI

dolomite

SI

gypsum

PCO2

10C

4430

9744

9371

4Q

1,53

919

.26.7

2,38

08.68

2.80

15.66

0.13

7.34

13.79

3.50

10.26

154

28.2

3.07

4.25

1.77

−0.1

−0.7

−1.3

7.24

E-02

15C

4418

3344

9401

4Q

2,45

317

.56.5

4,19

09.48

9.05

28.71

0.52

8.48

33.37

6.26

10.13

4930

.55.23

1.35

2.30

0.0

0.0

−1.1

3.98

E-02

20C

4430

0744

9452

2Q

1,23

018

.86.8

1,66

07.88

2.63

7.83

0.19

7.59

8.33

2.61

10.06

9226

.43.48

1.04

1.51

0.0

−0.5

−1.4

6.17

E-02

21C

4434

2844

9524

9Q

1,58

817

.86.9

2,83

04.89

5.10

20.44

0.38

8.02

16.16

5.75

9.66

031

.40.99

7.31

3.75

−0.1

−0.1

−1.3

3.89

E-02

28C

4427

5444

9477

9Q

1,14

019

.56.6

1,54

08.28

2.30

6.18

0.15

7.23

7.77

2.19

10.94

6722

.33.69

−0.19

1.15

−0.2

−0.9

−1.4

9.33

E-02

32C

4431

2744

9629

2Q

1,34

617

.87.1

1,69

07.68

3.79

9.13

0.15

5.61

6.65

4.77

6.53

278

22.8

4.85

3.71

3.86

0.0

−0.1

−1.2

2.19

E-02

4C44

1986

4493

156

Q2,04

019

.07.2

2,98

015

.22

5.43

14.35

0.29

5.46

22.11

3.06

6.10

309

46.9

10.16

−3.83

0.39

0.0

0.0

−1.2

1.62

E-02

112C

4502

0144

9192

1OM

997

19.0

6.2

1,90

02.00

2.72

14.14

0.46

2.20

15.87

1.54

3.66

593

.3−4

.22

1.33

−0.42

−1.4

−2.7

−2.1

3.63

E-02

145C

4487

7044

9734

0OM

2,71

619

.76.9

5,14

04.24

10.37

35.67

1.76

9.78

35.88

4.40

12.11

235

62.4

0.68

7.46

0.16

−0.2

0.0

−1.6

5.89

E-02

152C

4504

5144

8699

9OM

600

18.5

6.5

9,91

1.30

3.05

6.09

0.20

1.47

8.19

0.79

6.53

1171

.0−2

.67

−0.57

−0.30

−2.4

−4.4

−2.5

1.23

E-01

157S

451199

4497

403

OM

1,45

519

.06.0

2,31

06.29

6.75

11.14

0.15

8.56

13.17

2.13

11.29

6253

.34.78

0.27

0.48

−0.1

−0.1

−1.6

6.76

E-02

165S

4548

4044

9505

3OM

1,98

518

.76.8

4,08

06.74

7.41

27.40

0.61

2.73

35.63

2.98

3.26

6448

.70.28

−1.75

−1.23

−0.4

−0.8

−1.5

1.32

E-02

167S

4527

7744

9462

9OM

1,91

322

.47.0

3,78

05.34

9.05

23.49

0.51

3.29

34.81

2.57

4.26

1160

.50.73

−5.08

−1.55

−0.6

−0.8

−1.7

2.63

E-02

173S

4615

9944

9229

9OM

822

20.2

6.8

1,32

22.99

1.15

8.87

0.30

6.53

6.01

1.19

8.25

035

.5− 2

.32

4.13

0.35

−0.3

−1.0

−2.0

4.47

E-02

174S

4624

9344

9382

8OM

713

18.8

7.6

1,08

51.90

1.15

7.31

0.33

5.12

4.99

0.78

6.23

288

.3−3

.16

3.45

0.06

−0.5

−1.2

−2.3

2.75

E-02

178S

4582

2744

9236

4OM

1,98

017

.77.1

4,42

07.14

11.52

25.66

0.41

0.30

35.93

4.03

0.53

9947

.84.72

−3.95

−0.22

−2.0

−3.7

−1.4

5.62

E-03

180S

4579

6444

9088

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979

18.4

7.0

1,89

03.14

4.94

11.40

0.24

2.66

14.59

1.50

4.88

754

.2−0

.53

−0.56

−0.32

−1.3

−2.3

−2.0

5.37

E-02

181S

4552

4444

9116

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525

17.0

7.7

998

1.10

1.32

6.96

0.17

1.40

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0.64

3.41

2060

.5−4

.34

1.13

−0.33

−2.1

−4.1

−2.6

4.68

E-02

192S

4598

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9031

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540

16.9

5.8

980

1.62

1.93

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0.13

1.86

6.38

0.58

4.52

2557

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.00

0.18

−0.30

−1.8

−3.6

−2.5

6.17

E-02

197S

4553

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OM

1,09

417

.66.9

1,78

04.84

4.94

9.79

0.23

3.65

13.50

1.68

6.07

5161

.91.44

−1.28

−0.01

−0.9

−1.7

−1.8

5.75

E-02

201S

4521

3544

8720

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692

21.6

6.6

1,31

21.90

1.32

9.35

0.32

2.37

10.70

0.55

2.68

042

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.42

0.72

−0.82

−0.6

−1.3

−2.5

8.13

E-03

205S

4530

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8972

3OM

900

17.2

7.2

1,56

03.57

4.12

8.26

0.33

1.26

10.30

1.48

6.67

133

88.3

0.15

−0.03

0.15

−2.2

−4.4

−1.9

1.29

E-01

206S

4540

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1,35

618

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3,05

04.54

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16.96

0.31

2.16

25.07

2.02

6.52

01.13

−3.67

−0.99

−1.7

−3.0

−1.8

1.05

E-01

115C

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1,67

319

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2,41

02.89

4.61

13.48

0.21

6.29

12.61

4.06

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35.5

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3.15

2.47

0.0

0.0

−1.6

1.29

E-02

118C

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2,07

221

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3,50

06.09

6.09

26.53

0.36

9.19

24.62

5.81

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6237

.31.06

6.32

2.85

0.1

0.1

−1.3

7.08

E-03

125S

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9745

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1,119

18.2

6.7

1,36

38.28

2.30

5.31

0.10

7.59

7.04

2.07

10.75

5921

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−0.49

1.11

−0.1

−0.7

−1.4

7.76

E-02

126C

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1,17

218

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1,90

02.89

2.63

15.66

0.45

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12.19

1.88

8.29

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.48

5.91

0.34

0.0

0.0

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1.26

E-02

135C

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2460

6.49

3.46

18.70

0.24

9.67

12.57

4.69

11.56

8232

.31.84

8.42

3.10

0.0

0.0

−1.3

4.90

E-02

139S

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8.98

11.11

24.36

0.52

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30.26

3.50

11.41

6162

.87.57

−0.40

−0.10

−0.3

−0.4

−1.4

9.33

E-02

142C

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9660

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1,46

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2,58

02.30

2.47

23.49

0.98

7.56

18.45

1.77

8.78

2776

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.81

9.05

−0.49

−0.3

−0.4

−2.1

3.09

E-02

18C

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1,75

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2,60

09.68

5.10

10.87

0.31

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17.87

4.64

11.67

6325

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2.45

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8.32

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2,90

04.19

3.46

24.79

0.64

10.79

19.95

3.08

12.10

2329

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8.76

0.65

0.0

0.0

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3.47

E-02

29C

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17.5

6.6

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2.82

1.03

3.00

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2.76

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0.80

3.26

2212

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0.58

0.30

−0.5

−1.4

−2.1

1.17

E-02

43C

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2,21

219

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4,31

04.49

4.44

37.41

0.78

5.56

40.98

2.82

5.69

049

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.27

3.95

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0.0

0.0

−1.7

4.27

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49C

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1,48

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1,93

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5.93

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0.20

9.90

13.32

0.79

14.69

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.29.81

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−0.88

0.0

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−1.8

1.26

E-01

79S

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17.7

7.0

1,110

5.49

1.15

4.35

0.07

6.88

4.21

1.21

8.37

868.6

0.63

0.88

0.58

0.0

−0.6

−1.7

3.55

E-02

98C

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1,25

220

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2,07

02.89

3.46

18.27

0.53

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12.41

2.43

8.01

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8.42

0.86

0.0

0.0

−1.8

7.94

E-03

98S

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1,40

07.78

2.47

6.79

0.07

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7.80

1.88

7.42

5754

.23.34

0.32

0.84

0.0

0.1

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1.15

E-02

100S

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18.6

6.6

1,03

48.03

2.30

2.70

0.10

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3.44

1.06

10.80

3910

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−0.10

0.51

−0.2

−0.9

−1.7

9.12

E-02

101S

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1,03

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1,17

88.68

2.30

3.48

0.45

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5.02

1.32

11.83

3110

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0.59

−0.1

−0.7

−1.6

7.94

E-02

107C

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1,15

117

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2,06

02.89

2.72

16.75

0.52

3.92

16.66

1.94

4.12

1067

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.52

3.33

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0.0

0.0

−1.9

4.90

E-03

112S

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18.3

7.2

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4.69

1.98

3.57

0.20

3.04

6.04

0.59

3.45

98.6

0.20

−1.30

−0.25

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−0.7

−2.1

1.00

E-02

114S

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218

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1,07

67.58

2.63

2.83

0.10

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3.89

0.79

12.32

3812

.34.29

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0.19

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−1.0

−1.8

1.20

E-01

120S

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121

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1,31

46.49

3.62

4.70

0.15

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6.75

0.87

12.22

2015

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−0.81

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−0.4

−1.0

−0.6

1.10

E-01

121C

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16.4

7.5

1,05

25.69

1.81

5.92

0.08

6.86

3.36

2.28

7.32

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3.17

1.74

0.0

0.0

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1.10

E-02

122S

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18.1

7.3

802

4.79

3.46

1.83

0.07

5.50

2.41

1.17

6.08

237.3

2.68

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0.74

0.0

0.0

−1.8

1.41

E-02

128S

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7.1

1,15

65.19

3.62

3.57

0.32

9.11

4.26

0.21

10.56

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0.31

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0.0

0.0

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3.89

E-02

130S

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1,47

48.18

3.62

5.87

0.12

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7.91

1.06

12.82

3414

.64.87

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−0.3

−0.9

−1.7

1.26

E-01

131C

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18.8

6.9

1,04

26.99

2.63

3.48

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7.01

3.89

0.94

8.86

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0.31

0.34

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4.57

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27.68

1.81

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1.43

9.96

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0.19

0.79

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7.24

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143S

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6.7

1,15

57.49

2.96

3.39

0.09

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4.29

0.95

10.57

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4.42

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0.31

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7.76

E-02

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1,93

08.18

4.12

11.09

0.36

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11.35

4.41

8.82

4912

.74.50

1.95

2.96

0.0

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−1.2

4.47

E-02

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7.1

1,17

76.69

1.65

5.48

0.15

6.76

4.67

2.13

7.89

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1.71

1.44

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2.75

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5.98

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0.32

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6.17

E-02

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0.97

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2.88

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1,51

08.68

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5.65

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7.05

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1.85

8.51

3711.8

4.81

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0.84

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−1.5

−1.5

3.55

E-02

43S

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1,49

46.69

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7.22

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1.85

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0.81

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7.41

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4.72

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5.50

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2.63

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11.50

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1,72

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4.66

12.66

2.78

5.87

319.6

3.97

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2.88

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6.72

1.88

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162

14.1

1.79

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0.96

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8.32

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5.35

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8.25

248.6

1.96

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0.72

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6.92

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5.62

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0.85

7.62

239.6

0.53

0.91

0.34

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−1.9

4.57

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1,86

09.38

5.76

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6.06

10.83

4.99

9.17

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7.47

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3.60

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−1.1

7.59

E-02

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06.59

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0.61

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6.61

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1.12

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4.68

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6.7

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1.99

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3.55

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6.17

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6.17

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6.11

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1.23

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4.17

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2,14

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11.53

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6.78

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6.06

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2.42

4.63

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4.27

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1.40

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5.62

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1,87

02.40

2.80

16.31

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3.37

14.62

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4.57

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0.1

−2.6

1.86

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2,80

08.38

5.43

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0.35

5.43

21.87

3.07

7.22

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0.42

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4.07

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11.19

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4.82

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37.74

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2.34

36.76

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2.24

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31S

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3,72

419

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7,35

08.73

14.81

62.64

1.61

4.67

75.85

3.87

5.08

537.3

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0.90

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0.0

−1.5

1.05

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3,45

01.10

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0.49

6.76

22.05

4.65

6.74

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17.79

1.98

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0.1

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2.14

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7,14

08.23

17.28

61.77

1.42

6.09

71.84

5.18

8.84

010

.52.60

3.19

−3.16

−0.5

−0.6

−1.4

6.92

E-02

56S

4391

8145

0806

1T

1,85

816

.97.0

3,05

07.39

5.76

20.01

0.36

5.53

21.63

4.81

6.66

223

24.1

2.78

2.29

2.19

−0.1

−0.3

−1.2

2.63

E-02

57S

4391

6445

0923

4T

3,48

018

.86.7

4,61

023

.45

18.93

23.49

0.47

6.35

30.94

25.27

8.63

016

.829

.69

−1.89

21.59

0.0

0.0

−0.3

5.62

E-02

5S43

4142

4499

760

T3,28

717

.26.6

3,75

028

.74

12.67

8.92

0.35

6.01

14.14

33.85

8.88

017

.332

.92

−2.56

32.08

−0.1

−0.4

−0.1

6.76

E-02

66C

4388

5845

0212

8T

1,62

020

.06.3

2,07

013

.67

2.14

5.31

1.15

7.64

8.62

2.60

15.16

340

10.5

8.70

−0.76

1.46

−0.3

−1.3

−1.2

1.95

E-01

71S

4405

7245

1047

5T

2,36

718

.66.7

3,02

07.98

15.23

11.31

0.20

9.55

16.50

5.17

13.21

549

15.5

14.12

−2.29

3.14

−0.1

0.0

−1.3

9.12

E-02

75S

4418

6645

0869

6T

1,97

116

.57.1

2,75

014

.87

6.75

10.22

0.17

6.52

13.92

11.68

7.57

8216

.813

.18

−1.26

9.94

0.0

0.0

−0.6

2.40

E-02

86S

4401

7245

0705

2T

1,58

615

.66.9

2,73

07.24

5.02

18.05

0.27

4.03

26.54

2.73

5.11

010

.00.66

−3.86

−0.45

−0.3

−0.8

−1.5

2.40

E-02

90C

4375

0945

0178

9T

1,63

420

.86.5

2,56

09.98

5.27

14.35

0.23

9.19

15.79

3.40

14.77

2811.8

6.34

1.38

1.45

−0.2

−0.5

−1.3

1.48

E-01

93C

4370

0045

0552

5T

2,14

319

.86.8

4,37

04.89

8.56

29.14

0.77

4.08

40.07

3.73

5.32

010

.0−1

.53

−3.56

−0.99

−0.6

−0.9

−1.6

3.16

E-02

210S

4498

9445

0541

0T

2,68

719

.27.2

3,24

021

.36

9.22

10.00

0.58

5.94

12.49

25.98

6.63

2314

.622

.49

0.14

24.40

0.0

0.0

−0.3

1.78

E-02

TDS,S

iO2,N

O3,m

ajor

catio

nsandanions

arein

mg/l;temperature

(T)isin

°C;electricalcond

uctiv

ity(EC)isat18°C

inmS/cm;dissolvedinorganiccarbon

(DIC)isin

mmole/l;ΔCa+

ΔMg,

ΔNa+ΔK

andΔSO4arein

meq/l.

HU

hydrog

eologicun

it(refer

tolegend

inFig.3);SI

saturatio

nindex;

PCO2partialpressure

ofCO2(atm

)

455


locate). Moreover, to take into account other potential saltend-members besides sea water, the plot of Fig. 4 alsoshows the lines representing the chemically inert additionof soluble salts, as CaSO4, NaCl and a CaSO4 (NaClproportion of 1:0.5, mole ratio). The comparison ofgroundwater sample distribution with the above linesmay indicate that groundwater mineralization is due tomultiple factors. As a whole, groundwaters from the plainaquifers show a higher TDS than groundwaters circulatingin the highland.

Major ions and Hydrochemical water typesThe Piper plot of Fig. 5 shows, that in spring waters, thedominant anions are either bicarbonate or chloride, whilegroundwater samples can be described as bicarbonate,chloride or sulphate dominant.Figure 6 shows moreclearly that groundwaters belonging to the Jurassic aquifershow a dominant calcium-bicarbonate type that, whenTDS is over 1.1 g/l, convert into a Ca–SO4 or Ca–Cl2type. Fresh groundwaters from Oligo-Miocene aquifershow a Na–Cl–HCO3 type, evolving to a Na–Cl typewhen TDS exceeds 2 g/l. Waters from the Triassic aquifervary from Ca–SO4 to Ca–Cl2 and finally to a Na–Cl typeaccording to TDS increase; fresh Cretaceous groundwatersshow a Ca or Na-HCO3 type, which evolve to Ca or Mg–Cl2 and Na–Cl with increasing mineralization. Fresh

groundwaters of Quaternary aquifers are mostly of a Ca–Cl2 type, shifting to a Na–Cl type at the highest TDS. Thewaters labelled in Fig. 6, being of the chloride-sulphatetype, actually contain low percentages of sulphate; thus,

Fig. 4 Variation of total dissolved solids (TDS) concentrationswith chloride concentration. Curves fit data concerning sea water–freshwater conservative mixing, pure solution of calcium sulphate,sodium chloride and a mole proportion (1:0.5) of both salts. In thelegend, gw stands for groundwater

Fig. 5 Piper diagram for groundwaters (gw) of the Nurra Basin

Fig. 6 The relationship between the concentration of Na + K as apercentage of total cations (meq/l per meq/l) and bicarbonateconcentration as a percentage of total anions (meq/l per meq/l). Inthe legend, gw stands for groundwater

456


the increase in TDS is mainly related to the increase inchloride. Sulphate percentages greater than 20% are onlyfound in groundwaters from the Triassic and Jurassicaquifers.

Plots of Fig. 7 show the major cation–chloride relationsand include the lines representing fresh water and present-day sea-water mixing and, when appropriate, linesrepresenting the chemically inert solution of variableamounts of soluble salts. As previously discussed, thechemical composition of waters derived from mixingdifferent proportions of fresh and sea water, or from othersources, rarely matches the composition defined by acalculation that excludes the occurrence of reactions.

Indeed, Fig. 7a–d shows, above the limit of 4 meq/l ofchloride (see the preceding), either excess or deficit of themajor cations with respect to the conservative mixinglines; excess prevails for calcium and magnesium, anddeficit for sodium and potassium. The waters, which plotat concentrations below the limit, are mainly groundwatersfrom the Jurassic aquifer and spring waters, thoughmembers of both these groups also plot above the4 meq/l Cl boundary. These waters are interpreted asresulting from water–rock interaction with carbonate rockswith limited addition of soluble salts. Figure 7e shows thatsulphate concentrations are mainly in excess with respectto mixing lines; high excesses agree with the solution ofCaSO4 with variable proportion of NaCl. Bicarbonates(Fig. 7f) span a range of 0.3–11 meq/l, maintaining highconcentrations also at high TDS. Although there may besome component of mixing, the spread of data indicatesthat there is a range of other processes occurring.

SiO2Sulphate, bicarbonate (Fig. 7e and f) and silica contentsallow further characterization of the groundwaters intotwo main groups (highland and plain aquifers). Figure 8shows the relation between SiO2 and bicarbonate. Onegrouping of samples (A) can be defined by sulphate andsodium deficits relative to pure mixing with sea water,bicarbonates lower than 4 meq/l and silica concentrationsgreater than 35 mg/l. Group (A) includes samples ofgroundwaters from the Oligo-Miocene volcanic aquifer.Group (B), including groundwaters sampled from theplain aquifers, shows mainly sulphate excess, bicarbonateranging from 4 to 11 meq/l and silica concentrations lessthan 35 mg/l (and mostly in the range 0–30 mg/l). A fewgroundwater samples from the Oligo-Miocene volcanicaquifer and a few samples belonging to the Cretaceoussequence (group C) have both bicarbonate contents higherthan 4 meq/l and silica contents higher than 35 mg/l; suchwaters (showing sulphate and sodium excess) come fromwells reaching depths between 50 and 100 m below meansea level (m.s.l.) or are located near or at the easternboundary of the volcanic sequence. Due to the complexstructural setting, it is likely that aquifers come intocontact both laterally and vertically. Thus, high bicarbon-ate contents in volcanic groundwater might originate frominterconnection with carbonate aquifers. Conversely, high

silica contents in Cretaceous groundwater could originateas a result of silicate (probably the glassy fraction)leaching from the overlying porous pyroclastic flows.Figure 3 shows clearly that groundwater flow from theOligo-Miocene aquifer is directed towards the NW andcan feed the Cretaceous aquifer near Olmedo. Alterna-tively the high silica in the groundwater hosted in theCretaceous aquifer can derive from the thick (up to 200 m)glauconite rich calcarenites.

CO2Regarding bicarbonate concentrations (Fig. 7f), most ofmeasured values for Jurassic and Cretaceous carbonateformations are higher than typical for groundwaters fromcarbonate aquifers at normal temperatures and CO2 partialpressures. Moreover, groundwater maintains such highvalues under salinization as well.

The highest bicarbonate concentrations (8–11 meq/l)might be justified by high CO2 partial pressures. Thespeciation of inorganic carbon (by PHREEQC, Parkhurst1995), for the data set of the winter 2004 survey (Table 1),shows that PCO2 varies between 1.9×10−4 and 1.95×10−1 atm. For the data set of June 2005, PCO2 variesbetween 3.5×10−4 and 2.29×10−1 atm (Table 2). Byconsidering both the surveys, the 13% of groundwatersand the 10% of spring waters, however, show valueshigher than 10−1 atm. The PCO2 frequency distribution ofNurra waters is lognormal and skewed to the left, whichconfirms that the dominant population is characterized bymoderately high PCO2 values.

On the whole, pH ranges from 5.7 to 8.9, but mostgroundwaters and spring waters (75%) are acidic, with apH between 6 and 7. For waters of both surveys, thedecrease of pH with increasing PCO2 is shown in Fig. 9a,while Fig. 9b shows that groundwaters are progressivelymore sub-saturated with respect to calcite as soon as PCO2increases, thus indicating that they are not in equilibriumwith calcite and its dissolution takes place in an opensystem.

There is no straightforward account for such high PCO2values. Influxes of carbon dioxide from depth throughtectonic discontinuities, CO2 production due to redoxreactions or continuous dissolution of carbonates due toion-exchange could be responsible of anomalous values.

In any case, influxes of carbon dioxide from sub-crustal depth can be easily ruled out on a geologicalbasis. Even if Sardinia Island is characterized bynumerous fault-controlled geothermal systems, theirorigin is clearly independent from the recent alkaline totransitional volcanic activity, which, in any case, endedaround 80 ka before present (Beccaluva et al. 1977).These systems are normally located, regardless thecrosscut formation, along inactive regional faults, mainlystrike–slip faults of Oligocene–Aquitanian age. Somethermal sources are located within the Variscan basement,others within the Oligocene-Miocene calc-alkaline volca-nic complex or at the tectonic contact between thebasement and the Tertiary or Mesozoic covers; no thermal

457


458


springs are linked to the Pliocene–Pleistocene basaltfloods.

Figure 10 shows the trend of the above parameters forgroundwater samples arranged according to DIC (dis-solved inorganic carbon) increase. The grey shadowedarea in each single plot marks the samples with DICbetween 10 and 15.8 mmole/l (Fig. 10a), characterizing,as results from the analysis of the log probability plot ofDIC show, the highest of almost six populations of Nurrawaters.

As a first approximation, water samples having DO >0.5 mg/l can be classified as aerobic; out of the 86 samples,83 are aerobic (with DO ranging from 1.1 to 9 mg/l), andthree have DO less than 0.5 mg/l, whereas of the latterwaters, which show appreciable contents of Fe, Mn andNH4, two also have nitrate exceeding 0.5 mg/l (NO3

− -reducing samples) and the remaining (from the Triassicaquifer, with TDS of about 3.7 g/l) have nitrate less than0.1 mg/l, also indicating a sulphate deficit (Fe or sulphatereducing samples). However, some of the aerobic sampleswith low DO contents have NH4 as the dominant Nspecies; moreover, a few waters, with DO in the range 0.5–3 mg/l, show high Fe contents. These findings indicateanaerobic conditions; classification on the basis of DOseems to underestimate the reducing conditions of theaquifers (Tesoriero et al. 2004).

Another source of CO2 could be the action of H2SO4

deriving from oxidization of both lower Jurassic pyrite-rich coaly layers and pyrite occurring in the blackphyllites of the exposed basement on carbonate rocks;the Eh-pH diagram indicates that moderately high-PCO2water samples are located in the stability field of Fe2O3(s),which could justify the very low values of Fe in the greyshadowed area of Fig. 10c.

TemperatureGroundwater temperature was measured at all waterpoints during the two monitoring surveys. During winter2004, the highland aquifer springs and groundwatersshow temperatures ranging respectively from 13.4 to17.5°C and from 17.7 to 22.4°C (mean T of ground-waters=19.3°C). In the same period, in the plain aquifers,groundwater temperature is in the range 15.2–21.9°C(mean T=18.7), the maximum value corresponding to theTriassic aquifer: plain springs show little variationbetween 16.6 and 17.9°C.

In the studied area, the recharge waters have a meantemperature of about 10°C, corresponding to the averagevalue of the atmospheric temperature during the rechargeperiod (autumn-winter). Such a temperature is close to thetemperature measured in winter 2004 at some of thehighland springs: the Oligo-Miocene aquifer has the highestelevation of the area and the temperature of most springs in

wintertime reflects the presence of recharge areas at highelevations in the same aquifer.

In June 2005 the mean temperature of groundwaters inthe region of the plains increases up to on average 17–25°C(average temperature 21.7°C); in the highland aquifertemperature varies between 19.5 and 24.4°C (meantemperature 22°C). This range of temperature reflects theseasonal variation of the local isotherms. The onlyexception is represented by the groundwater coming froma well close to the Su Zumbaru fault, which also in wintershows a water temperature of 24°C. This important strike-slip fault is deeply rooted in the Tertiary volcanic complexand allows relatively quick upwelling of water heated bythermal gradient which in western Sardinia is particularlyhigh (Della Vedova et al. 1995) with HFD (heat flowdensity) in the range 50–70 mW/m−2.

Sulphate solution and dedolomitizationBeside the large contribution of chloride, both parts aand e of Fig. 7 indicate that the solution of gypsumcontributes to the increase of salt content. In general,calcium correlates with DIC (Fig. 11) except for watersclose to Triassic evaporites, while magnesium correlateswith sulphate (Fig. 12), according to different trendscorresponding to different Mg/SO4 ratios. Mg/SO4 ratiovaries between about 4 and 0.5 and waters from theTriassic aquifer belong to both extreme trends.

The two highest values of magnesium (maximum19 meq/l) correspond to sulphate concentrations of 8 and25.3; the highest sulphate concentrations accompanylower Mg concentrations and the highest calcium concen-

Fig. 8 Relationship between bicarbonate concentration and SiO2concentration. In the legend gw stands for groundwater

�Fig. 7 Variation of major constituents with respect to chloride: aCa, b Mg, c Na, d K, e SO4, and f HCO3. Lines indicate sea water–freshwater conservative mixing and pure solution of gypsum and/orsodium chloride. In the legend, gw stands for groundwater

459


Tab

le2

Hyd

rochem

ical

parametersforthegrou

ndwaterssampled

inDecem

ber20

04andJune

2005

IDUTM

EUTM

NDate

HU

TDS

TpH

EC

DO

Eh

NO3

NO2

NH3

Fe

Mn

ΔSO4

ΔCa+ΔMg

ΔNa+

ΔK

PCO2

DIC

4C44

1986

4493

156

Dec

2004

Q2,04

019

.07.2

2,98

03.6

194

309

040

00

46.9

10.2

−3.8

1.6E

-02

6.10

10C

4430

9744

9371

4Dec

2004

Q1,53

919

.26.7

2,38

04.0

264

154

050

200

28.2

3.1

4.3

7.2E

-02

10.26

15C

4418

3344

9401

4Dec

2004

Q2,45

317

.56.5

4,19

04.7

181

490

100

030

.55.2

1.3

4.0E

-02

10.13

20C

4430

0744

9452

2Dec

2004

Q1,23

018

.86.8

1,66

06.9

249

920

6020

026

.43.5

1.0

6.2E

-02

10.06

21C

4434

2844

9524

9Dec

2004

Q1,58

817

.86.9

2,83

01.1

180

043

400

031

.41.0

7.3

3.9E

-02

9.66

28C

4427

5444

9477

9Dec

2004

Q1,14

019

.56.6

1,54

02.9

257

670

300

022

.33.7

−0.2

9.3E

-02

10.94

32C

4431

2744

9629

2Dec

2004

Q1,34

617

.87.1

1,69

03.8

216

278

070

00

22.8

4.8

3.7

2.2E

-02

6.53

21C

4434

2844

9524

9Jun20

05Q

1,72

021

.80

6.7

2,80

51.4

309

00

5010

1029

.32.7

3.6

8.7E

-02

11.70

32C

4431

2744

9629

2Jun20

05Q

1,34

322

.00

7.3

1,97

12.4

269

236

040

2020

18.3

7.7

3.4

1.1E

-02

4.82

178S

4582

2744

9236

4Dec

2004

OM

1,98

017

.77.1

2,58

04.3

232

9935

020

1047

.88.8

−4.4

5.6E

-03

0.53

181S

4552

4444

9116

2Dec

2004

OM

525

17.0

7.7

2,07

03.6

146

2066

400

020

60.5

0.0

0.6

4.7E

-02

3.41

145C

4487

7044

9734

0Jun20

05OM

2,61

322

.70

6.8

4,65

32.7

323

181

016

00

056

.96.0

5.2

7.4E

-02

12.09

165S

4548

4044

9505

3Jun20

05OM

2,07

621

.90

7.0

4,17

54.2

302

00

220

1040

41.3

4.5

−7.2

1.4E

-02

3.43

167S

4527

7744

9462

9Jun20

05OM

1,87

023

.00

6.5

3,65

25.5

321

00

200

700

54.2

6.6

−4.7

5.9E

-02

5.73

174S

4624

9344

9382

8Jun20

05OM

1,05

023

.30

6.6

1,67

76.7

311

00

2020

2047

.7−1

.06.0

1.1E

-01

11.52

178S

4582

2744

9236

4Jun20

05OM

2,43

022

.60

6.4

4,37

45.3

305

139

013

030

042

.711.7

−9.5

2.1E

-02

1.87

192S

4598

0544

9031

4Jun20

05OM

594

19.50

6.2

985

6.1

320

240

700

050

.52.2

−0.6

1.3E

-02

0.88

197S

4553

2844

87011

Jun20

05OM

1,12

620

.00

6.6

1,85

86.4

305

540

300

053

.16.8

−1.8

5.0E

-02

5.89

200S

4553

2044

8802

4Jun20

05OM

648

24.40

6.9

1,12

64.4

328

40

200

041

.8−2

.9−0

.22.1E

-02

3.63

205S

4530

4744

8972

3Jun20

05OM

1,07

020

.40

6.0

1,68

65.6

385

152

020

00

077

.34.8

−2.3

8.7E

-02

5.04

29C

4434

6344

9449

3Dec

2004

C44

617

.56.6

673

0.6

227

2210

900

012

.3−1

.90.6

1.2E

-02

3.26

115C

4440

5144

9398

4Dec

2004

C1,67

319

.87.5

2,41

04.9

219

453

020

00

35.5

−0.6

3.1

1.3E

-02

6.79

118C

4456

0444

9389

7Dec

2004

C2,07

221

.06.8

3,50

01.6

118

620

500

037

.31.1

6.3

7.1E

-03

9.38

125S

4515

9745

0282

6Dec

2004

C1,119

18.2

6.7

1,36

35.6

187

590

120

00

21.8

3.9

−0.5

7.8E

-02

10.75

126C

4464

7044

9670

2Dec

2004

C1,17

218

.57.5

1,90

01.9

224

00

400

00

28.2

−2.5

5.9

1.3E

-02

8.29

135C

4447

5444

9477

8Dec

2004

C1,59

920

.77.0

2,46

04.1

249

820

100

032

.31.8

8.4

4.9E

-02

11.56

115C

4440

5144

9398

4Jun20

05C

891

21.90

7.6

1,15

65.7

238

890

7020

2024

.10.4

5.3

1.0E

-02

7.65

125S

4515

9745

0282

6Jun20

05C

1,10

321

.10

6.6

1,45

23.9

267

320

5010

016

.84.0

−0.8

1.0E

-01

11.61

139S

4487

9645

0038

6Jun20

05C

2,115

22.50

6.8

3,58

92.2

254

170

6020

048

.112

.5−6

.96.0E

-02

9.89

18C

4423

4944

9434

3Jun20

05C

1,96

020

.80

6.6

3,00

32.4

310

570

060

2022

.910

.4−1

.81.0E

-01

12.53

29C

4434

6344

9449

3Jun20

05C

345

24.00

6.8

441

5.7

280

50

401,12

080

12.8

−3.0

−0.3

1.8E

-02

2.53

79S

4427

9945

0822

7Jun20

05C

1,04

624

.40

6.9

1,43

42.3

319

660

3030

208.3

4.2

−0.9

4.7E

-02

8.37

98S

4436

0045

0613

9Jun20

05C

1,05

623

.40

7.5

1,35

35.2

279

290

600

030

.74.2

0.2

1.3E

-02

8.02

120S

4504

0045

0374

6Dec

2004

J1,02

121

.66.6

1,31

43.8

164

200

100

00

15.0

3.5

−0.8

1.1E

-01

12.22

16S

4360

0245

0100

3Jun20

05J

1,34

221

.20

7.1

1,98

43.5

301

290

110

240

5012

.66.5

0.6

2.8E

-02

7.93

36S

4470

2645

0070

2Jun20

05J

973

21.30

6.6

1,18

87.3

315

290

260

2010

15.8

3.6

0.3

1.1E

-01

12.39

39S

4384

8744

9945

8Jun20

05J

1,19

218

.90

7.1

1,64

15.4

303

320

5060

011.0

4.5

−2.0

3.0E

-02

8.64

41S

4381

0844

9802

2Jun20

05J

1,117

21.50

6.7

1,55

15.4

381

310

010

09.3

4.9

0.1

7.8E

-02

10.33

43S

4389

4444

9725

1Jun20

05J

1,10

717

.80

6.6

1,50

64.0

329

250

6030

010

.54.6

−0.8

9.8E

-02

11.71

47C

4387

1144

9692

5Jun20

05J

941

17.00

6.9

1,28

47.2

308

105

033

00

5013

.81.9

1.0

4.0E

-02

7.99

58S

4385

0345

0977

0Jun20

05J

827

20.90

7.0

1,23

44.1

287

340

3040

509.3

2.5

−0.6

2.2E

-02

5.08

63C

4403

6644

9979

6Jun20

05J

535

21.20

7.3

703

3.7

285

40

9018

090

6.9

−0.8

0.7

1.2E

-02

4.87

69S

4401

0645

1012

6Jun20

05J

1,01

722

.20

6.8

1,26

76.5

334

360

7070

609.3

5.4

0.4

5.8E

-02

8.98

72S

4437

7645

0787

8Jun20

05J

970

18.80

6.6

1,32

66.5

284

136

080

290

4010

.54.7

−0.4

6.8E

-02

0.46

73S

4420

8145

1002

1Jun20

05J

647

22.30

6.9

810

4.7

294

130

8020

603.2

1.4

0.2

3.7E

-02

6.69

77C

4431

2145

0147

7Jun20

05J

979

20.80

6.6

1,30

01.8

323

310

4050

1014

.54.2

−0.7

9.5E

-02

10.88

81C

4405

6044

9838

1Jun20

05J

926

22.80

6.4

1,10

54.1

337

580

6020

011.8

3.8

1.6

1.6E

-01

13.47

81S

4452

4445

0738

9Jun20

05J

793

20.80

6.7

997

4.9

301

300

120

3020

6.9

2.7

0.5

6.6E

-02

8.71

89S

4406

7545

1075

8Jun20

05J

1,43

720

.00

6.4

1,69

52.0

321

690

7010

105.3

11.1

0.7

1.5E

-01

13.25

90S

4450

4445

0580

1Jun20

05J

826

21.10

6.8

1,011

5.7

291

450

700

108.9

4.2

0.2

5.9E

-02

9.17

460


101S

4456

9545

0402

2Jun20

05J

1,05

821

.80

6.6

1,22

44.8

304

320

800

1010

.16.1

0.2

1.3E

-01

13.98

107C

4473

5744

9398

7Jun20

05J

1,13

323

.50

7.5

2,05

66.0

240

80

100

050

61.6

1.1

2.3

6.9E

-03

4.31

120S

4504

0045

0374

6Jun20

05J

543

23.50

7.1

714

1.9

290

80

7024

020

9.4

−0.1

−0.1

1.9E

-02

4.92

121C

4443

7344

9704

5Jun20

05J

905

20.30

6.9

1,12

55.7

300

110

7030

2012

.62.8

2.3

5.2E

-02

9.87

128S

4482

7645

0350

7Jun20

05J

956

24.20

6.8

1,21

41.4

261

160

200

013

.44.9

0.4

7.8E

-02

11.56

131C

4441

8645

0168

5Jun20

05J

910

22.20

7.0

1,114

5.7

294

270

00

07.6

3.9

−0.7

4.1E

-02

9.11

143S

4451

7345

0277

3Jun20

05J

963

18.60

6.7

1,23

56.5

303

330

4020

2011.8

4.8

−0.4

8.1E

-02

11.36

1S43

4641

4498

480

Dec

2004

T1,82

421

.26.9

3,28

06.1

260

580

100

00

55.1

−0.8

9.6

5.6E

-02

10.84

11S

4344

0445

0045

9Dec

2004

T1,42

819

.76.9

2,14

05.9

242

410

400

1012

.35.2

2.4

4.3E

-02

8.47

26S

4348

1145

0181

9Dec

2004

T88

521

.18.9

1,87

03.3

115

054

200

2010

1.0

−3.4

4.6

1.9E

-04

3.09

29S

4364

9145

0186

4Dec

2004

T1,58

916

.26.8

2,80

08.5

234

340

500

014

.13.4

−1.4

4.1E

-02

7.22

2S43

5375

4500

215

Dec

2004

T3,12

319

.07.0

3,31

03.0

234

00

5010

6013

.232

.52.3

2.2E

-02

5.72

31S

4360

8945

0259

6Dec

2004

T3,72

419

.87.3

7,35

01.4

136

5354

1,50

030

140

7.3

−0.4

0.9

1.0E

-02

5.08

34S

4339

7145

0228

6Dec

2004

T1,54

021

.98.2

3,45

04.3

130

08

020

09.1

−8.2

17.8

2.1E

-03

6.74

48S

4355

6345

0490

9Dec

2004

T3,69

318

.86.6

7,14

00.4

159

00

5,80

014

050

010

.52.6

3.2

6.9E

-02

8.84

56S

4391

8145

0806

1Dec

2004

T1,85

816

.97.0

3,05

06.6

234

223

050

00

24.1

2.8

2.3

2.6E

-02

6.66

57S

4391

6445

0923

4Dec

2004

T3,48

018

.86.7

4,61

09.1

192

00

700

1010

16.8

29.7

−1.9

5.6E

-02

8.63

5S43

4142

4499

760

Dec

2004

T3,28

717

.26.6

3,75

03.0

125

00

120

3020

17.3

32.9

−2.6

6.8E

-02

8.88

66C

4388

5845

0212

8Dec

2004

T1,62

020

.06.3

2,07

05.6

246

340

070

00

10.5

8.7

−0.8

1.9E

-01

15.16

71S

4405

7245

1047

5Dec

2004

T2,36

718

.66.7

3,02

02.2

189

549

020

020

015

.514

.1−2

.39.1E

-02

13.21

75S

4418

6645

0869

6Dec

2004

T1,97

116

.57.1

2,75

07.0

268

820

1,60

010

016

.813

.2−1

.32.4E

-02

7.57

86S

4401

7245

0705

2Dec

2004

T1,58

615

.66.9

2,73

06.1

185

00

120

010

10.0

0.7

−3.9

2.4E

-02

5.11

90C

4375

0945

0178

9Dec

2004

T1,63

420

.86.5

2,56

04.8

203

280

400

2011.8

6.3

1.4

1.5E

-01

14.77

93C

4370

0045

0552

5Dec

2004

T2,14

319

.86.8

4,37

02.6

450

040

120

160

10.0

−1.5

−3.6

3.2E

-02

5.32

210S

4498

9445

0541

0Dec

2004

T2,68

719

.27.2

3,24

03.1

256

230

130

020

14.6

22.5

0.1

1.8E

-02

6.63

1S43

4641

4498

480

Jun20

05T

2,09

624

.60

7.2

2,76

93.8

282

00

140

030

9.0

16.3

3.1

2.1E

-02

7.34

26S

4348

1145

0181

9Jun20

05T

978

23.40

8.7

2,011

1.3

166

00

1,60

037

030

1.0

−5.1

3.5

3.5E

-04

3.34

31S

4360

8945

0259

6Jun20

05T

3,07

523

.80

6.9

6,40

32.2

237

00

1,00

02,61

019

05.7

2.5

−2.5

3.9E

-02

7.61

34S

4339

7145

0228

6Jun20

05T

1,23

723

.60

8.2

2,35

41.6

197

30

100

790

405.3

−3.8

10.2

2.4E

-03

7.20

48S

4355

6345

0490

9Jun20

05T

3,96

425

.00

6.5

7,44

90.4

184

10

1,00

061

010

08.5

2.4

−9.6

9.8E

-02

9.68

56S

4391

8145

0806

1Jun20

05T

1,57

722

.70

6.7

2,71

52.3

262

890

220

730

4014

.64.9

−0.4

6.8E

-02

8.99

71S

4405

7245

1047

5Jun20

05T

2,20

221

.00

6.9

3,08

42.1

252

291

040

3010

7.7

15.7

−1.8

6.3E

-02

12.59

75S

4418

6645

0869

6Jun20

05T

2,30

619

.30

7.1

3,08

45.8

300

700

6015

070

11.4

17.1

−4.2

2.5E

-02

7.61

HUishy

drog

eologicalu

nit(referto

legend

inFig.3);TDS,N

O3,N

O2arein

mg/l;FeandMnarein

µg/l;Tem

perature

(T)isin

°C;electricalcon

ductivity

(EC)isat18

°Cin

mS/cm;redox

potential(Eh)

isin

mV;dissolvedinorganiccarbon

(DIC)isin

mmole/l;ΔCa+ΔMg,

ΔNa+

ΔK

andΔSO4arein

meq/l

461


trations (maximum 29 meq/l): related groundwaters are ofthe Ca–Mg–SO4 water type.

Various processes compete in determining the calciumand magnesium concentrations, including calcite precipi-tation and solution, incongruent solution of dolomite anddedolomitization driven by sulphate solution, which isparticularly active at the contact between Giurassic dolo-stone and Triassic evaporites.

In addition, within the Oligo-Miocene aquifer, theseprocesses occur concurrently with increase of sodium andchloride concentrations; due to the presence of potentialion exchangers in concerned formations, one other factor,ion-exchange, competes in determining the calcium andmagnesium concentrations. In some cases, a deficit ofsulphate with respect to conservative mixing is observed,the leaching of welded or unwelded tuffs and of theremnants of hydrothermal alteration might increasegroundwater salt contents. The hydrothermal alterationcan be traced back to hydrothermal fluid circulation; deeprunning strike slip faults, present in the southern part ofthe Nurra area, functioned as conduits for the risinghydrothermal fluids that altered rocks by forming clayminerals and these potentially have played and play veryimportant roles in affecting water quality due to ionexchange. Thus, deposits of kaolin, zeolite, bentonite, andtravertine, which actually cover both the Miocene volcanictuffs and the Quaternary sediments, represent the memoryof the widespread hydrothermal activity. Presently, thearea does not show important thermal waters, fumarolesand hot grounds. The only memories of this hydrothermalactivity, as already shown, are some warm waters, whichemerge close to the deep strike-slip faults. Hydrothermalfluids which altered volcanic rocks could have been saline,and contained dissolved salts as NaCl, KCl, CaSO4 and

CaCl2. The chemistry of groundwaters in the volcanic areais therefore mainly controlled by leaching of volcanicrocks, ion-exchange with kaolinite, zeolite and bentoniteof calcium type and solution of salts, remnants of thehydrothermal activity (Mameli 2001), all of whichcompete in defining the mutual concentrations of Ca, Naand Mg. The efficacy of ion-exchange is suggested by thebinary plots of Fig. 7a–d as well; after the limit of freshwater (4 meq/l of chloride), as previously described,calcium and magnesium show an excess and sodium andpotassium show a deficit relative to the sea water mixingline.

NitrateNitrate concentrations in the Nurra aquifers range frombelow detection limit (about 1 mg/l) to more than 500 mg/l.Concentrations above 50 mg/l affect both fresh andsalinized waters: nitrate is likely to be derived from thewidespread use of fertilizers in the plain. High contentsaffect also the groundwaters of Jurassic sequence, whichare destined for potable use.

Stable isotopes and tritium

Seven water samples (Table 3), taken during November–December 2005, have been analyzed for stable isotope(18O and 2H) and tritium contents.The stable isotope dataare reported in Fig. 13 in comparison to the EMWL (EastMediterranean water line) and the GMWL (globalmeteoric water line). According to Longinelli and Selmo(2003), the local meteoric water line (LMWL) for central

Fig. 9 a Relationship between PCO2 and pH; b Variation of SI (calcite) with respect to PCO2. In the legend gw stands for groundwater

462


Italy, where geographically the study area lies, is δ2H=7.05 δ18O + 5.6. The same study indicates that theweighted mean 18O and 2H values for the rain at Sassari(period February 1999–January 2000) is respectively -6.58‰ and -37.3‰.

The clustering of stable isotope compositions of thegroundwaters near the LMWL and the weighted meanprecipitation indicates that the recharge to the volcanicand carbonate aquifers is derived from local precipitation.However, without a larger data set, the effective reference,the weighted mean of “recharge waters” cannot becalculated and used for the comparison: the effectivevalue should be somewhat more negative, because of theexclusion of summer precipitation.

However, even with respect to weighted mean rainfallvalues, the analyzed groundwaters show a shift which

could represent the effect of evaporation during infiltra-tion. As an example, the groundwater sample no. 157S(located at the extreme right) lies on the Su Zumbarustrike-slip fault; its position could correspond to enrich-ment due to evaporation before infiltration. This hypoth-esis could hold true due to the fact that the fault borders astructural depression towards which the surroundingsurface flow converges and consequently evaporates dueto the low permeability enhanced by the occurrence oftuffs whose clay content has been increased by weather-ing/hydrothermal fluids. However, other factors may beimportant, especially in the light of the previouslydiscussed thermal anomalies and/or the possible additionof carbon dioxide; present data are not sufficient to solvethe uncertainties, which would need to be clarified at leastby the analysis of 13C.

Fig. 10 Diagram of the variation of a Eh (left axis) and dissolved inorganic carbon (DIC, right axis); b pH; c Fe (left axis) and Mn(right axis); d ΔSO4 (where Δ indicates excess or deficit with respect to sea water-freshwater conservative mixing); e dissolved oxygen;f nitrate; g ammonium (as N); h nitrite. Data refer to 51 samples the June 2005 survey with 33 samples of the December 2004 survey:they are arranged according to increasing DIC (mmole/l). Grey shadowed areas mark the 24 samples with DIC ranging between 10 and15.2 mmole/l

463


The limited tritium values gave rise to meaningfulinformation on the timing of recharge and the nature of theaquifer. Very low tritium levels (0.3±0.4 TU) in thevolcanic aquifer of well 167S could indicate lowpermeability or deep circulation of groundwater, whichcan both result in large residence time. Samples comingfrom the Jurassic aquifer also show relatively low values(3.6±0.6, 3.1±0.6 TU). These values indicate only alimited degree of fracture development in the aquifer andslow groundwater circulation derived from rainwater. Thethree samples chosen for tritium analyses (43S, 143S and167S) show that the decrease of tritium may be correlatedwith increasing TDS and temperature.

Conclusions

The geological and hydrogeological survey on the Nurraregion allow a good reconstruction of stratigraphy,structures and related aquifers in an area where base-ment-cover relationships exert the prominent role incontrolling the path and the storage of groundwaters.

In detail:

1. The basement that crops out to the west of the studyarea experiences poor infiltration; the surface waterfrom this exposed sector of metamorphic rocks, due toits eastward tilt, when moving to the east, enters thecarbonate aquifers. Possibly a basal detachment existsbetween the Mesozoic covers and the basement actingas a barrier.

2. Within the Mesozoic carbonate platform the ground-water flow path is controlled by wide, typically

ejective, folding-style strata related to the occurrenceof evaporites and by later, generally normal, faults.Thus, in the quasi-flat area, where the Mesozoic rockscrop out, there is no relation between the topographicwatershed and the hydrogeological basin.

3. The geometry of the folded and faulted Mesozoic coverallow for the identification of areas with prevailingTriassic rocks, mostly dolostones and evaporites, alongwith areas, in Jurassic and Cretaceous rocks, mostlymade of limestones and marlstones. The latter gener-ally act as a barrier so that, in the Jurassic-Cretaceoussequence, several superposed aquifers can occur.

4. Within the Tertiary volcanic complex, infiltration is lesseffective with respect to the carbonate complex, thetopographic watershed widely matches the hydrogeo-logical one, and the deepening of the piezometriccontours follows the topography. This complex isaffected by strike-slip faults along which deep circuitscan develop giving rise to hypothermal waters thatshow the same temperature (24°C) regardless of the theseasonality. The residence time of waters in this circuitis very long as inferred by the close to zero tritiumconcentration.

The aquifers that develop in the different hydrogeo-logical units reflect the composition of the host rocks:sulphate (Triassic dolomites and gypsum aquifers), bicar-bonate (Jurassic and Cretaceous limestone aquifer), andsilica (Oligo-Miocene volcanic aquifer). These ions can beused as markers for discriminating groundwater fromdifferent aquifers. Lithology and structural setting controlthe hydro-geochemical processes, which in turn determine

Fig. 12 Relationship between magnesium concentration andsulphate concentration. Lines indicate two different Mg/SO4 ratios.In the legend, gw stands for groundwaters

Fig. 11 Relationship between calcium concentration and DICconcentration. In the legend gw stands for groundwater

464


the enrichment in cations and anions; ultimately thechemical evolution leads to deterioration of water quality.

The springs show hydrogeochemical characteristicsthat allow their clear separation into two main groups:bicarbonate-rich waters were found in springs emergingfrom the limestones, whereas chloride type groundwatersare dominantly found in the volcanic rocks despite theheight above the sea level of the aquifers. Considerationof the water compositions indicates that salinization in theNurra Basin derives from various sources, even thoughmost of the acquired salinity derives from sodium andchloride input.

The carbonate aquifer in contact with Triassic evaporitecontains high sulphate groundwaters of poor quality. Asfor the other aquifers, this poor quality does not changeappreciably during the hydrologic year. TDS variesbetween 1.5 and 6.5 g/l. The solution of importantamounts of calcium sulphate in the gypsum levels reflectson the considerable excess of calcium and magnesium dueto dedolomitization. The presence of marls causes ionexchange; due to the masking effect of gypsum solution,ion exchange between calcium and sodium is moreevident only in correspondence to high positive excessof sodium (dilution). Typical water types are NaCl (havingeither excess or deficit of sodium), CaCl2 (salinization),and CaSO4.

Jurassic limestones represent the most productive(confined) aquifer in the Nurra district: groundwaters aremainly of calcium bicarbonate type and contain up to 20%of dissolved sulphate. The most saline groundwaters (1–1.5 g/l) are CaCl2 in character. Groundwaters from theJurassic aquifer have the highest average PCO2 in the area,reflected in the high mean DIC. Green marls are thesubstratum in which the most ion exchange takes place(Fidelibus et al. 2004), directly and indirectly, affectingalmost all waters. The Jurassic aquifer is the only reservoirof water with good natural quality in the district, but highnitrate concentrations deriving from intensive agriculturalpractices carried out in the area must be controlled.

Most of groundwaters from the Cretaceous aquiferhave a TDS higher than 1 g/l. They show quite clear ionexchange, evidenced by the presence of NaHCO3, MgCl2,and CaCl2 water types. Silica contents higher than theother aquifers of the plain are possibly linked withglauconitic bearing materials and/or with a connectionbetween this aquifer and the volcanic sequence; PCO2 islow, with some isolated peaks.

The groundwater in the volcanic massif (Oligo-Mio-cene) has high hydraulic heads, thus excluding theinfluence of sea water as the main salt source: thechemical evolution of volcanic groundwaters developsthrough water–rock interaction with welded and unweldedtuffs, addition of salts derived from past hydrothermalactivity (Mark and Mauk 2001), and ion-exchange withzeolite, kaolinite, and bentonite, alteration products of thesame activity. Dissolution of silicates and increase inalkalinity is facilitated by the acidity of groundwaters.Quaternary aquifers have groundwaters with TDS higherthan 1.3 g/l all through the year; these groundwaters areNaCl and CaCl2 in character (with deficit and excess ofboth sodium and calcium).

Sea water does not seem to play the principal role inthe salinization of groundwaters of the area. Enlargingthe spectrum of parameters to minor ions and isotopeswill need to be part of future detailed studies of thecomplex Nurra basin. Finally this study represents abasic tool for sustainable water management in theframework of multidisciplinary research activities aimingto combat and/or mitigate desertification processes and todefine appropriate policies for prospecting new ground-water resources and assessment of their quality andconservation.

Fig. 13 Relationship between 2H and 18O. In the legend, gwstands for groundwater

Table 3 Stable isotopes and tritium in groundwaters

ID Type Date δ18O‰ VSMOW δ2H‰ VSMOW 3H TU Aquifer

S6 Spring 30/11/2005 −6.44 −35.6 – Tuff (Oligo-Miocene)157S Borehole 30/11/2005 −5.03 −34.4 – Tuff (Oligo-Miocene)167S Borehole 06/12/2005 −5.95 −34.6 0.3±0.4 Tuff (Oligo-Miocene)43S Borehole 06/12/2005 −5.81 −36.8 3.6±0.6 Limestone (Jurassic)98C Borehole 30/11/2005 −5.72 −34.7 – Tuff and limestone (Cretaceus)43C Borehole 30/11/2005 −5.53 −33.4 – Limestone (Jurassic)143S Borehole 06/12/2005 −5.36 −33.8 3.1±0.6 Limestone (Jurassic)

VSMOW Vienna standard mean ocean water; TU tritium units

465


Acknowledgements The financial support from Ministry of Edu-cation, University and Research (MIUR) for the development of theresearch RIADE (Integrated Research for Applying new technolo-gies and processes for combating Desertification: www.riade.net) isacknowledged. Thanks are due to A. Carletti, N. Demurtas, R.Pinna and A. Vigo for the sampling campaigns and samplepreparation. The work of M. De Roma on chemical analysis isparticularly appreciated. We are also grateful to D. Pinna of theSilver & Barite S.p.A. for having allowed us to access the boreholedata. The authors wish to thank G. M. Zuppi and B. Gambardellafor the supporting discussion. Finally, the authors greatly appreciatethe critical review and the many helpful suggestions of J. Tellam.

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