Hydrogeology of the Nurra Region, Sardinia (Italy): basement-cover influences 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 lows are generated by synclines and normal faults. The (Italy) has a Palaeozoic basement and covers, consisting regional groundwater flow has been defined. The investi- of Mesozoic carbonates, Cenozoic pyroclastic rocks and gated groundwater shows relatively high TDS and Quaternary, mainly clastic, sediments. The faulting and chloride concentrations which, along with other hydro- folding affecting the covers predominantly control the geochemical evidence, rules out sea-water intrusion as the geomorphology. The morphology of the southern part is cause of high salinity. The high chloride and sulphate controlled by the Tertiary volcanic activity that generated concentrations can be related to deep hydrothermal a stack of pyroclastic flows. Geological structures and circuits and to Triassic evaporites, respectively. The lithology exert the main control on recharge and ground- source water chemistry has been modified by various water circulation, as well as its availability and quality. geochemical processes due to the groundwater–rock The watershed divides do not fit the groundwater divide; interaction, including ion exchange with hydrothermal the latter is conditioned by open folds and by faults. The minerals and clays, incongruent solution of dolomite, and Mesozoic folded carbonate sequences contain appreciable sulphate reduction. amounts of groundwater, particularly where structural Keywords Groundwater flow . Hydrogeochemistry . Salinization . Groundwater management . Italy

Received: 31 March 2006 /Accepted: 15 September 2008 Introduction © Springer-Verlag 2008 The Nurra district is located in the northwestern part of the island of Sardinia (Italy) in the Sassari Province, with G. Ghiglieri ()) 80 km of coastline with the Mediterranean Sea (Fig. 1a). Department of Territorial Engineering, Its geology records a long history from Paleozoic to Geopedology and Applied Geology Section, Desertification Research Group (NRD), University of Sassari, Quaternary, resulting in relative structural complexity and Viale Italia, 07100, Sassari, Italy in a wide variety of rocks. e-mail: [email protected] Due to intensive human activities and recent climatic Fax: +39-79-229261 changes, the area has become vulnerable to desertification. G. Oggiano As a result, the area is included in the national research Institute of Geological Sciences and Mineralogy, network under the RIADE project (Integrated research for University of Sassari, applying new technologies and processes for combating Corso Angioy 10, 07100, Sassari, Italy desertification (RIADE project 2002–2006), set up by the M. D. Fidelibus Italian Ministry of Research (Ghiglieri et al. 2006). Department of Civil and Environmental Engineering, The water demand in the study area is considerable, Technical University of Bari, water being required for industry, domestic use, tourism, Via Orabona 4, 70125, Bari, Italy agriculture, and animal rearing. Nurra relies on both T. Alemayehu surface and groundwaters. The seasonal and perennial School of Geosciences, rivers of the area are exploited using the Cuga and Wits University, Private bag 3, P. O. Box Wits 2050, Johannesburg, South Africa Surigheddu dams, built on the highlands. However, like : on other Mediterranean islands, surface-water resources G. Barbieri A. Vernier can periodically suffer from drastic shortage. Groundwater Department of Territorial Engineering, Applied Geology and Applied Geophysics Section, in different aquifers is exploited using deep boreholes University of Cagliari, which can attain discharges as high as 145 l/s. The water Piazza D’Armi, 09100, Cagliari, Italy demand of the city of Alghero, for example, is partially

Hydrogeology Journal DOI 10.1007/s10040-008-0369-z Hydrogeology Journal DOI 10.1007/s10040-008-0369-z ƒFig. 1 The Nurra district of Sardinia: a location map; b geological laboratory. The chemical analyses were performed at the map; c location of water sampling points in the Calich catchment University of Sassari (Italy), immediately after sample (delineated in b). In the legend gw stands for groundwater collection. The analysis of cations was undertaken using an Analyst 200 atomic absorption spectrometer. Anions were analyzed by an ion chromatograph with four satisfied by groundwater withdrawn through five wells components: a Waters pump (model 590), a Waters discharging a total of 96 l/s. electrical conductivity detector (model 431), an Alltech Notwithstanding the importance of local groundwater solid phase chemical suppressor (SPCS, model 335) and a as the main source of good quality water and its role of SRI PeakSimple data system (model 203). strategic reserve in such semiarid conditions, exploitation For stable oxygen and hydrogen isotopes, seven up to now has been uncontrolled (Barbieri et al. 2005a, b; groundwater samples were collected from one spring and Ghiglieri et al. 2006). An additional problem in the Nurra six boreholes. For tritium analysis only three waters district is that water users have a very scant knowledge of representative of the three main hydrogeologic units were the provenance and value of the fresh water they exploit, sampled. The analysis was carried out in the CNR isotope thus leading to a high rate of unofficial exploitation. hydrology laboratory, Pisa, Italy. The extensive exploitation of the Nurra aquifers and the consequent water-quality deterioration require a revision of current water management practices. This revision has to be Geological setting based on good knowledge of both the potential of aquifers in terms of geometry and storage and quality in terms of The Nurra district encompasses a structural high, which hydrogeochemical features, which, up to now, has been developed during the Tertiary and where older rock disregarded. This report presents the synthesis of lengthy sequences are progressively exposed westward (Fig. 1b). research, of which the main aims of have been: (1) to The northeastern limit of the area is marked by the upper reconstruct the hydrogeological setting and the regional Miocene deposits of a half-graben Porto Torres basin groundwater flow; (2) to ascertain the origin of salinity; (3) (Thomas and Gennessaux 1986; Funedda et al. 2000) that to recognise the boundary conditions of different hydro- cover the older rocks. The Variscan metamorphic basement geologic units by mean of processes that control the is well exposed in the westernmost sector near the coast concentration of major constituents in the different aquifers. (Fig. 1b). As regards the basement, grey Autunian arenites Achieving these aims will establish a basis for developing an and silts, and upper Permian and Triassic continental red appropriate monitoring programme and therefore improved beds with interlayered alkaline volcanics occur. The first management of the water resources of the region (Ghiglieri marine transgressive deposits consist of dolostones, lime- et al. 2006, 2007, 2008). stones and evaporites of Middle Triassic age showing the typical Germanic facies. Since this time, shallow marine sedimentation, in a Methodology carbonate platform environment, had been almost contin- uous until Aptian-Albian time. During the Albian-Aptian A geological and structural map of the area has been time, an important tectonic phase took place in Nurra, prepared on the basis of recent data and field surveys. The referred to as the Bedoulian movement (Oggiano et al. conceptualization of the hydrogeological setting led to the 1987). This tectonic event was responsible for the identification of the recharge and discharge areas and emersion that gave rise to widespread bauxite deposits major controlling structures. The field data have been and caused the partial erosion of the Jurassic succession. integrated with aerial photo interpretation and geophysical During the Coniacian stage, all the Nurra bauxitic prospecting (gravimetric profiles) (Ghiglieri et al. 2006). palaeosurface was submerged, due to a new transgression, For the purposes of the RIADE project, technical data which led to carbonate-terrigenous sedimentation lasting and relevant information from 424 boreholes were up to the Maastrichtian. The post-Maastrichtian emersion is collected together with data from 87 springs. A global supposedly related to a new tectonic phase (Laramic phase; positioning system (GPS) was used to locate each feature Oggiano et al. 1987). Since the Paleocene, the entire (Ghiglieri et al. 2006). region experienced weathering, erosion, widespread cal- From these locations, 99 wells and 21 springs were calkaline volcanism and two important deformation events selected for chemical monitoring purposes. In order to linked both to the Pirenaic and to the North Apennine investigate the behaviour of the aquifers at two different (Carmignani et al. 1995) orogenesis. These deformations times of the year, water was sampled for chemical analysis generated minor thrusts and mild NE trending folds which from 118 water points (97 wells and 21 springs) in dominate the present geometry of the Mesozoic cover and, December 2004, and from 55 water points (51 wells and 4 as a consequence, the geometry of the main aquifers. springs) in June 2005 (Fig. 1c). The Variscan basement outcrops in the westernmost part Water samples were collected from pumped wells and of the study area, consisting chieflyofblackphylliteswith directly from springs in 1,000-ml polyethylene bottles. minor quartzites, meta-basalts and oolitic ironstones. To- Electrical conductivity, pH, alkalinity and temperature wards the north, due to the increased metamorphic grade, it were measured in the field while Eh was determined in the consists of micaschists and paragneisses, while the phyllites

Hydrogeology Journal DOI 10.1007/s10040-008-0369-z inhibit vertical infiltration in this area. The only rock with Fig. 3 Hydrogeological map. K hydraulic conductivity „ very low secondary permeability (1×10−7 m/s) is the quartzite which is jointed due to its brittle nature. The Mesozoic succession overlies the basement (Fig. 2). The lowest unit is an arenite-conglomerate typical Urgonian facies. This limestone consists of a deposit, Permo-Triassic in age, which shows highly strongly karstified biosparite that, where not completely variable thickness and medium permeability. The remaining eroded, reaches 180 m in thickness (Figs. 1b and 2). part of the Triassic rocks consists of transgressive dolomitic, The upper Cretaceous sequence also consists of lime- calcareous and evaporitic deposits. The thickness of the stones with an important intercalation of glauconite-bearing, carbonate portion is about 80 m, whereas the stratigraphic more or less arenitic marls. Some boreholes penetrate 300 m thickness of the evaporitic deposits, mainly gypsum, is not of upper Cretaceous deposits in the area south of Olmedo known; due to their ductile behaviour, these deposits (Oggiano et al. 1987). are severely deformed. The permeability of the Triassic Above a karstic palaeosurface, developed on the carbonate and gypsum is high (Fig. 3) and the deposit Mesozoic carbonate rocks, several pyroclastic flows were plays an important role in conditioning the groundwater deposited during the lower Miocene. The pyroclastic salinity. deposits formed the volcanic plateau in the southeastern A carbonate sequence that encompasses the entire part of the area. The different flow units are separated by Jurassic system lies on the Triassic evaporites. Its base palaeosols; the thickness of each flow varies from a few to consists of alternations of marls and limestones with thin hundreds of metres. Bentonite deposits deriving from the dark pyrite-rich shale levels. Most of the sequence is made hydrothermal alteration of feldspar and glassy material of dolostones and limestones; green marls with typical generally seal the bottom of the pyroclastic stack. The Purbeckian facies also occur towards the top of this system welded tuffs that occur as a top cover are intensively grading into the Cretaceous limestone. The Jurassic fractured favouring vertical infiltration. deposits tend to increase their thickness to the southeast The pyroclastic flows (Figs. 1b and 2) are capped by and attain maximum thickness of 800 m to the south, close Burdigalian calcarenites that outcrop only at the eastern to the Su Zumbaru Fault (central part of the study area, boundary of the study area along with sands and Figs. 1–3). The permeability and the transmissivity of the conglomerates of Tortonian age derived from a reworking Jurassic sequence are high due to fractures and karstic of the basement. The latter clastic deposits are confined conduits. within structural lows interpreted as strike slip basins. The Cretaceous succession lies on “Purbeckian” marls; Quaternary aeolian sands, travertines and loose sedi- it consists of two sequences separated by an angular ments consisting of alluvial sands and gravels cover most unconformity, which is marked by bauxite deposits and of the plains in the west of the region, occurring as represents a hiatus corresponding to the mid-Cretaceous. pediment slope deposits and valley fill materials that range The lower sequence is represented by limestone with in thickness from 10 to 40 m. All these deposits have

Fig. 2 Representative geological cross sections

Hydrogeology Journal DOI 10.1007/s10040-008-0369-z Hydrogeology Journal DOI 10.1007/s10040-008-0369-z generally good permeability, allowing infiltration into the control on the following features of the geometry of the lower aquifers. aquifers:

– The Mid-Cretaceous erosive stage controls the thickness of Structural framework the Mesozoic carbonate rocks. In general, the thickness increases southwards and progressively diminishes north- ward in consequence of the presence of a palaeo-structural The basement tectonics reflect the polyphase evolution of high. The Mesozoic sequences, due to their huge the Variscan events in Sardinia. The only hydrogeologi- thickness, represent the main aquifer of the region, which cally relevant structure of this unit is linked to the latest is shallower and thinner moving northward (Figs. 2 and 3). Variscan folding phase that generated a wide synform with – Locally synformal geometries, due to Upper Creta- an east striking and dipping axis. This structure controls ceous deformation involving marly strata, can allow the superficial drainage, which is roughly directed the formation of perched aquifers; moreover, the eastwards, i.e. toward the Mesozoic limestones. The first shortening, accommodated by folds, causes the thick- tectonic instability affecting the Mesozoic cover started in ening of the cover and, consequently, of the aquifers. the Middle-Cretaceous time with transtensive Bedoulian movements followed by a transpressive regime (Oggiano et The volcanic succession thickens southward, where it al. 1987); these caused the angular unconformity between also crops out at high topographic levels (500 m a.s.l.). This the Lower Aptian and the Coniacian (Oggiano et al. 1987), unit hosts a multilayer aquifer due to alternance of weakly an interval which lead to the development of the bauxite welded, ignimbrites and deeply fractured high-grade ignim- mentioned above. These tectonic movements resulted in the brite. Each permeable layer is confined by clay-rich paleosols development of some uplifted blocks bounded by normal or by pumice and ash flows converted into bentonite. faults and mild folding within a sinistral wrench shear belt The strike-slip faults, due to their steep dip and deep running between the Olmedo area and Porto Conte bay penetration, allowed discharge of hypothermal fluid that (Fig. 2). Hence, during the peneplanation of the bauxitic resulted in the development of bentonite and zeolites surface, due to the erosive removal of at least 600 m of deposits in the volcanic rocks. Because of their high cation Mesozoic sequence towards the north, the actual thickness exchange capacities (CEC), these minerals exert a control of the Mesozoic cover increases to the south. on water chemistry. Hydrothermal alteration, which oc- The evidence for tectonic movements subsequent to the curred in Upper Miocene, is a widespread phenomena in development of the bauxite horizon and its Coniacian the study area and in the neighbouring localities. In the cover comprise syn-tectonic breccias and olistostromes study area, in particular, epigenetic kaolin and bentonite are within Upper Cretaceous sediments close to an important present (Mameli 2000). Zeolites with high CEC are fault. These tectonic movements caused the uplift of the described by Cerri et al. (2001) and Cerri and Mameli Mesozoic platform south of a line joining Uri and Alghero (2004). Bentonite occurs mostly in the calcium form, (Mamuntanas-Su Zumbaru Fault, Oggiano et al. 1987). while zeolite occurs in calcium-, sodium- and potassium- This tectonic activity was tentatively ascribed to the rich varieties (Cerri et al. 2001). With regards to chloride, Laramic phase. Other faults and folds, with NE axial concentrations up to 20,000 ppm have been detected in strike, involving the whole Mesozoic sequence except the some deeply kaolinized volcanic rocks (Mameli 2001). deposits younger than Lower Miocene; can be referred to In the southeastern and southern part of the area there are the Eocene Pyrenaic phase and the Oligocene Apenninic hypothermal manifestations in the form of thermal springs collision. During the Oligocene and early Miocene, new associated with deep running strike-slip faults. The field left lateral movements caused the reactivation of the ENE data also indicate that groundwaters in the volcanic rocks oriented, strike-slip fault (Carmignani et al. 1995) of mid- have high temperatures that range between 19 and 24°C, Cretaceous age. From the Burdigalian, at the same time as including the cold winter season. In order to identify the the opening of the Balearic basin, until the Pliocene, an recharge area, the groundwater divide has been recognized extensional regime was present, giving rise to normal (Fig. 3), allowing the mean annual recharge to be faults with various orientations. estimated (about 37×106 m3, Ghiglieri et al. 2006, 2008).

Hydrogeological features Growndwater regional flow

The thickness of the Mesozoic sequences is known only The mean annual rainfall in the Nurra district, calculated approximately because of the uncertainties associated over 30 years (1960–1990), averages 607 mm with a with the deep erosion during the Middle Cretaceous and bimodal pattern within a year, which is typical of the other erosion events, which occurred since the begin- Mediterranean region. The mean annual temperature of ning of the Cenozoic. However, in the main structural the area is 15.7°C. The recharge to the aquifers is also lows, the Mesozoic aquifer can easily reach thicknesses expected to take place during the rainy months of October of 1,000 m (Ghiglieri et al. 2006, 2007). The deforma- to December and February to April after soil moisture tion history of the Mesozoic rocks exerted a strong replenishment. The main aquifer is represented by the

Hydrogeology Journal DOI 10.1007/s10040-008-0369-z Mesozoic carbonate successions with a yield that varies water concentrated by permafrost salt exclusion, reverse between 20 and 145 l/s. The groundwater flow direction in osmosis, leakage of brines known to be present in a this aquifer is strictly controlled by structural deformation Carboniferous deltaic coal-containing sequence underly- and weathering processes (Ghiglieri et al. 2006, 2008). ing the sandstones, evaporite dissolution from an overly- Due to prevalent NE–SW aligned synclines and anticlines, ing Triassic mudstone/evaporite sequence, present day the direction of groundwater flow in the carbonate rocks is sea-water intrusion. Whatever the salt source, its involve- towards the SW. Anticlinal folds of the Paleozoic and ment is normally partly obscured by water–rock interac- Mesozoic sedimentary rocks gave rise to high rising tion processes triggered by ionic strength increase. As an ground in the western sector of the area; they play a very example, groundwater salinization in carbonate aquifers important role in recharging or dispersing surface water causes a renewal of karstification, with an enhancement of flows and by reducing water-rock contact time, thus secondary porosity (Hanshaw and Back 1979; Herman generating relatively fresh water (see the following). The and Back 1984; Tulipano et al. 2005; Whitaker and Smart structural highs of Monte Doglia, Monte Pedrosu, Monte 1997; Sanford and Konikow 1989a, b; Liu and Chen Zirra, Monte Timidone and Monte Cugiareddu act as 1996). In intergranular-flow aquifers, as well as in recharge areas for the confined Jurassic aquifer; the fractured or karstic aquifers with a few percent of clays culminations of the major anticlines also represent or other exchangers, salinization activates ion-exchange effective recharge areas. On the other hand, the synclines (Appelo and Geirnaert 1983). act as storage areas for groundwater. The most prominent The geological history of the Nurra Basin makes it synclines are those of Campu Calvagiu and Sabadiga- likely that among the several sources of salinity, sea water, Alghero (Figs. 2 and 3). In the huge Sabadiga-Alghero evaporites (mainly NaCl and CaSO4), and Tertiary syncline, groundwater converges from all directions and hydrothermal deposits (derived from hydrothermal fluids wells are high-yielding. Therefore, this synclinal zone with dissolved salts as KCl, CaCl2, NaCl, alunite) are the contains huge reserves of groundwater. Folded anticlines more reliable. Figure 4 shows the relationship between force groundwater flow through the synclinal axis and TDS and chloride concentration. The TDS-Cl plot and all influence the direction of groundwater flow, as confirmed the following plots distinguish all analyzed waters according by numerical modelling in similar geometries occurring in to their type—surface-, spring- or groundwater—and, in the Israel (Ben-Itzhak and Gvirtzman 2005). The structural case of groundwaters, their aquifer. Data from drillings, field frame also controls the boundary conditions: to the west surveys and the geological and structural studies, allowed the aquifers are encircled by the contact with very low initial attribution of the groundwater and spring samples to permeability Variscan basement, which is also the imper- the different aquifers; the hydrochemical study subsequently meable lower boundary; to the east the Mesozoic aquifer confirmed this attribution for the 95% of samples. is buried below the Miocene-hosted aquifers that feed it Moreover, no distinction is made as to the periods of laterally. To the south the main aquifers are in contact with sampling. The June 2005 sampling covered only 51% of the volcanic complex through important strike-slip faults. the wells and 20% of the springs sampled in December The productivity of the volcanic deposit is very low 2004, with the addition of other well points: the mean due to intensive weathering. The groundwater flow variation of TDS in repeated samples was only 10%, direction in the volcanic massifs is towards the NW, due indicating that mineralization does not vary much with to the dipping of the Pyroclastic units. season. Therefore, both data-sets are taken into account, with the main aim of describing the general characteristics of the different aquifers. Figure 4 shows that TDS ranges from minimum values Insights from analytical results of about 200 mg/l (springs from Oligo-Miocene volcanic complex) to maximum values of 5,000 mg/l (groundwaters Data reported in Table 1 relate to the superficial, spring from Triassic aquifer). Most of the springs, a large part of and groundwaters sampled in the Nurra district, sampled the Jurassic groundwaters and part of the Oligo-Miocene in the period September–December 2004. groundwaters show a TDS in the range 500–1,000 mg/l, while almost all Quaternary and Triassic groundwaters show TDS higher than 1,000 mg/l. Cretaceous ground- Total dissolved solids (TDS) waters cover the TDS range from 400 to 2,000 mg/l. The The principal feature emerging from the whole data set is value of about 4 meq/l has been chosen as an upper limit that a widespread salinization affects most of the analyzed for freshwater. The maximum TDS of waters having less groundwaters. Since many different salt sources, besides than 4 meq/l of chlorides is about 1,000 mg/l. present-day sea water, can be involved in salinization The TDS-Cl binary plot shows the lines representing processes (salt spray, evaporite dissolution, mixing with the chemically inert mixing between present-day sea water saline fluids and thermal fluids inflows) of coastal (represented by the mean of four samples of sea water aquifers, the identification of the sources can be difficult. collected offshore from the local coastal area) and, Tellam (1995) considered a number of potential salt respectively, the freshest waters (springs) sampled in the sources and salinization processes in a Triassic sandstone highland (corresponding to the Oligo-Miocene volcanic aquifer (Cheshire Basin, NE England, UK) such as sea aquifer) and in the plain (where all the other aquifers

Hydrogeology Journal DOI 10.1007/s10040-008-0369-z yrgooyJournal Hydrogeology Table 1 Hydrochemical parameters for the groundwaters sampled in December 2004

ID UTM E UTM N HU TDS T pH EC Ca Mg Na K HCO3 Cl SO4 DIC NO3 SiO2 Δca+ΔMg ΔNa+ΔK ΔSO4 SI SI SI PCO2 calcite dolomite gypsum 10C 443097 4493714 Q 1,539 19.2 6.7 2,380 8.68 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 7.24E-02 15C 441833 4494014 Q 2,453 17.5 6.5 4,190 9.48 9.05 28.71 0.52 8.48 33.37 6.26 10.13 49 30.5 5.23 1.35 2.30 0.0 0.0 −1.1 3.98E-02 20C 443007 4494522 Q 1,230 18.8 6.8 1,660 7.88 2.63 7.83 0.19 7.59 8.33 2.61 10.06 92 26.4 3.48 1.04 1.51 0.0 −0.5 −1.4 6.17E-02 21C 443428 4495249 Q 1,588 17.8 6.9 2,830 4.89 5.10 20.44 0.38 8.02 16.16 5.75 9.66 0 31.4 0.99 7.31 3.75 −0.1 −0.1 −1.3 3.89E-02 28C 442754 4494779 Q 1,140 19.5 6.6 1,540 8.28 2.30 6.18 0.15 7.23 7.77 2.19 10.94 67 22.3 3.69 −0.19 1.15 −0.2 −0.9 −1.4 9.33E-02 32C 443127 4496292 Q 1,346 17.8 7.1 1,690 7.68 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 2.19E-02 4C 441986 4493156 Q 2,040 19.0 7.2 2,980 15.22 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 1.62E-02 112C 450201 4491921 OM 997 19.0 6.2 1,900 2.00 2.72 14.14 0.46 2.20 15.87 1.54 3.66 5 93.3 −4.22 1.33 −0.42 −1.4 −2.7 −2.1 3.63E-02 145C 448770 4497340 OM 2,716 19.7 6.9 5,140 4.24 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 5.89E-02 152C 450451 4486999 OM 600 18.5 6.5 9,91 1.30 3.05 6.09 0.20 1.47 8.19 0.79 6.53 11 71.0 −2.67 −0.57 −0.30 −2.4 −4.4 −2.5 1.23E-01 157S 451199 4497403 OM 1,455 19.0 6.0 2,310 6.29 6.75 11.14 0.15 8.56 13.17 2.13 11.29 62 53.3 4.78 0.27 0.48 −0.1 −0.1 −1.6 6.76E-02 165S 454840 4495053 OM 1,985 18.7 6.8 4,080 6.74 7.41 27.40 0.61 2.73 35.63 2.98 3.26 64 48.7 0.28 −1.75 −1.23 −0.4 −0.8 −1.5 1.32E-02 167S 452777 4494629 OM 1,913 22.4 7.0 3,780 5.34 9.05 23.49 0.51 3.29 34.81 2.57 4.26 11 60.5 0.73 −5.08 −1.55 −0.6 −0.8 −1.7 2.63E-02 173S 461599 4492299 OM 822 20.2 6.8 1,322 2.99 1.15 8.87 0.30 6.53 6.01 1.19 8.25 0 35.5 −2.32 4.13 0.35 −0.3 −1.0 −2.0 4.47E-02 174S 462493 4493828 OM 713 18.8 7.6 1,085 1.90 1.15 7.31 0.33 5.12 4.99 0.78 6.23 2 88.3 −3.16 3.45 0.06 −0.5 −1.2 −2.3 2.75E-02 178S 458227 4492364 OM 1,980 17.7 7.1 4,420 7.14 11.52 25.66 0.41 0.30 35.93 4.03 0.53 99 47.8 4.72 −3.95 −0.22 −2.0 −3.7 −1.4 5.62E-03 180S 457964 4490889 OM 979 18.4 7.0 1,890 3.14 4.94 11.40 0.24 2.66 14.59 1.50 4.88 7 54.2 −0.53 −0.56 −0.32 −1.3 −2.3 −2.0 5.37E-02 181S 455244 4491162 OM 525 17.0 7.7 998 1.10 1.32 6.96 0.17 1.40 7.16 0.64 3.41 20 60.5 −4.34 1.13 −0.33 −2.1 −4.1 −2.6 4.68E-02 192S 459805 4490314 OM 540 16.9 5.8 980 1.62 1.93 5.39 0.13 1.86 6.38 0.58 4.52 25 57.4 −3.00 0.18 −0.30 −1.8 −3.6 −2.5 6.17E-02 197S 455328 4487011 OM 1,094 17.6 6.9 1,780 4.84 4.94 9.79 0.23 3.65 13.50 1.68 6.07 51 61.9 1.44 −1.28 −0.01 −0.9 −1.7 −1.8 5.75E-02 201S 452135 4487206 OM 692 21.6 6.6 1,312 1.90 1.32 9.35 0.32 2.37 10.70 0.55 2.68 0 42.8 −4.42 0.72 −0.82 −0.6 −1.3 −2.5 8.13E-03 205S 453047 4489723 OM 900 17.2 7.2 1,560 3.57 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 1.29E-01 206S 454045 4490894 OM 1,356 18.1 6.4 3,050 4.54 7.82 16.96 0.31 2.16 25.07 2.02 6.52 0 1.13 −3.67 −0.99 −1.7 −3.0 −1.8 1.05E-01 115C 444051 4493984 C 1,673 19.8 7.5 2,410 2.89 4.61 13.48 0.21 6.29 12.61 4.06 6.79 453 35.5 −0.61 3.15 2.47 0.0 0.0 −1.6 1.29E-02 118C 445604 4493897 C 2,072 21.0 6.8 3,500 6.09 6.09 26.53 0.36 9.19 24.62 5.81 9.38 62 37.3 1.06 6.32 2.85 0.1 0.1 −1.3 7.08E-03 125S 451597 4502826 C 1,119 18.2 6.7 1,363 8.28 2.30 5.31 0.10 7.59 7.04 2.07 10.75 59 21.8 3.87 −0.49 1.11 −0.1 −0.7 −1.4 7.76E-02 126C 446470 4496702 C 1,172 18.5 7.5 1,900 2.89 2.63 15.66 0.45 7.79 12.19 1.88 8.29 0 28.2 −2.48 5.91 0.34 0.0 0.0 −1.9 1.26E-02 135C 444754 4494778 C 1,599 20.7 7.0 2460 6.49 3.46 18.70 0.24 9.67 12.57 4.69 11.56 82 32.3 1.84 8.42 3.10 0.0 0.0 −1.3 4.90E-02 139S 448796 4500386 C 2,220 19.6 6.2 3600 8.98 11.11 24.36 0.52 7.74 30.26 3.50 11.41 61 62.8 7.57 −0.40 −0.10 −0.3 −0.4 −1.4 9.33E-02 142C 448568 4496603 C 1,465 19.9 6.4 2,580 2.30 2.47 23.49 0.98 7.56 18.45 1.77 8.78 27 76.9 −4.81 9.05 −0.49 −0.3 −0.4 −2.1 3.09E-02 18C 442349 4494343 C 1,758 19.4 6.7 2,600 9.68 5.10 10.87 0.31 8.38 17.87 4.64 11.67 63 25.0 5.36 −3.76 2.45 −0.1 −0.3 −1.1 8.32E-02 1C 440876 4494010 C 1,765 21.3 7.2 2,900 4.19 3.46 24.79 0.64 10.79 19.95 3.08 12.10 23 29.1 −2.30 8.76 0.65 0.0 0.0 −1.7 3.47E-02 29C 443463 4494493 C 446 17.5 6.6 673 2.82 1.03 3.00 0.10 2.76 3.00 0.80 3.26 22 12.3 −1.86 0.58 0.30 −0.5 −1.4 −2.1 1.17E-02 43C 442866 4498284 C 2,212 19.5 7.8 4,310 4.49 4.44 37.41 0.78 5.56 40.98 2.82 5.69 0 49.2 −6.27 3.95 −2.00 0.0 0.0 −1.7 4.27E-03 49C 438411 4494774 C 1,485 20.5 6.6 1,930 12.18 5.93 5.83 0.20 9.90 13.32 0.79 14.69 0 18.2 9.81 −5.12 −0.88 0.0 −0.1 −1.8 1.26E-01 79S 442799 4508227 C 876 17.7 7.0 1,110 5.49 1.15 4.35 0.07 6.88 4.21 1.21 8.37 86 8.6 0.63 0.88 0.58 0.0 −0.6 −1.7 3.55E-02

O 10.1007/s10040-008-0369-z DOI 98C 447038 4493027 C 1,252 20.9 5.7 2,070 2.89 3.46 18.27 0.53 7.74 12.41 2.43 8.01 19 41.9 −1.71 8.42 0.86 0.0 0.0 −1.8 7.94E-03 98S 443600 4506139 C 1,125 20.0 7.5 1,400 7.78 2.47 6.79 0.07 7.01 7.80 1.88 7.42 57 54.2 3.34 0.32 0.84 0.0 0.1 −1.5 1.15E-02 100S 445935 4506405 J 872 18.6 6.6 1,034 8.03 2.30 2.70 0.10 7.07 3.44 1.06 10.80 39 10.0 4.52 −0.10 0.51 −0.2 −0.9 −1.7 9.12E-02 101S 445695 4504022 J 1,030 15.2 6.7 1,178 8.68 2.30 3.48 0.45 8.21 5.02 1.32 11.83 31 10.0 4.77 −0.29 0.59 −0.1 −0.7 −1.6 7.94E-02 107C 447357 4493987 J 1,151 17.6 6.5 2,060 2.89 2.72 16.75 0.52 3.92 16.66 1.94 4.12 10 67.4 −3.52 3.33 −0.11 0.0 0.0 −1.9 4.90E-03 112S 445725 4501085 J 598 18.3 7.2 893 4.69 1.98 3.57 0.20 3.04 6.04 0.59 3.45 9 8.6 0.20 −1.30 −0.25 −0.2 −0.7 −2.1 1.00E-02 114S 444118 4504686 J 892 18.7 6.5 1,076 7.58 2.63 2.83 0.10 7.41 3.89 0.79 12.32 38 12.3 4.29 −0.34 0.19 −0.3 −1.0 −1.8 1.20E-01 120S 450400 4503746 J 1,021 21.6 6.6 1,314 6.49 3.62 4.70 0.15 8.15 6.75 0.87 12.22 20 15.0 3.46 −0.81 −0.05 −0.4 −1.0 −0.6 1.10E-01 121C 444373 4497045 J 842 16.4 7.5 1,052 5.69 1.81 5.92 0.08 6.86 3.36 2.28 7.32 12 14.6 1.70 3.17 1.74 0.0 0.0 −1.5 1.10E-02 122S 449270 4502595 J 673 18.1 7.3 802 4.79 3.46 1.83 0.07 5.50 2.41 1.17 6.08 23 7.3 2.68 −0.14 0.74 0.0 0.0 −1.8 1.41E-02 128S 448276 4503507 J 917 21.2 7.1 1,156 5.19 3.62 3.57 0.32 9.11 4.26 0.21 10.56 0 13.2 2.79 0.31 −0.43 0.0 0.0 −2.6 3.89E-02 130S 448813 4507119 J 1,097 19.1 6.5 1,474 8.18 3.62 5.87 0.12 7.79 7.91 1.06 12.82 34 14.6 4.87 −0.63 0.00 −0.3 −0.9 −1.7 1.26E-01 131C 444186 4501685 J 856 18.8 6.9 1,042 6.99 2.63 3.48 0.10 7.01 3.89 0.94 8.86 32 10.5 3.69 0.31 0.34 0.0 0.0 −1.3 4.57E-02 yrgooyJournal Hydrogeology 140S 448287 4501559 J 952 18.4 6.7 1,122 7.68 1.81 3.65 0.11 7.01 4.25 1.43 9.96 79 18.2 3.47 0.19 0.79 −0.1 −0.9 −1.6 7.24E-02 143S 445173 4502773 J 916 19.0 6.7 1,155 7.49 2.96 3.39 0.09 7.47 4.29 0.95 10.57 34 11.8 4.42 −0.12 0.31 −0.1 −0.6 −1.8 7.76E-02 16S 436002 4501003 J 1,367 18.7 6.9 1,930 8.18 4.12 11.09 0.36 7.03 11.35 4.41 8.82 49 12.7 4.50 1.95 2.96 0.0 −0.2 −1.2 4.47E-02 33C 442429 4496735 J 910 18.4 7.1 1,177 6.69 1.65 5.48 0.15 6.76 4.67 2.13 7.89 29 12.7 2.20 1.71 1.44 0.0 −0.2 −1.5 2.75E-02 36S 447026 4500702 J 847 19.2 6.7 1,160 7.29 2.14 3.31 0.09 5.98 5.03 1.05 8.47 34 17.3 3.21 −0.83 0.32 −0.2 −0.9 −1.7 6.17E-02 39S 438487 4499458 J 1,112 18.5 7.1 1,590 7.78 3.29 6.31 0.15 7.14 8.36 2.08 8.30 36 12.3 4.03 −0.55 0.97 0.0 0.0 −1.5 2.88E-02 41S 438108 4498022 J 1,079 18.2 7.0 1,510 8.68 2.96 5.65 0.15 7.05 7.50 1.85 8.51 37 11.8 4.81 −0.48 0.84 −1.7 −1.5 −1.5 3.55E-02 43S 438944 4497251 J 1,057 18.8 6.7 1,494 6.69 3.13 7.61 0.13 7.22 7.81 1.85 10.20 30 13.2 2.90 1.20 0.81 −0.2 −0.7 −1.6 7.41E-02 63C 440366 4499796 J 952 20.0 6.9 1,210 4.69 1.98 8.26 0.19 8.30 4.72 2.00 10.46 10 15.9 0.53 4.49 1.31 −0.1 −0.4 −1.6 5.50E-02 64S 437803 4510421 J 1,265 18.6 7.0 2,020 6.39 6.09 10.00 0.14 5.22 13.42 3.81 6.28 39 8.6 4.16 −1.08 2.13 −0.1 −0.2 −1.3 2.63E-02 69S 440106 4510126 J 1,147 18.6 6.5 1,366 9.98 3.13 4.35 0.10 6.97 5.81 4.23 11.50 33 10.5 6.69 −0.43 3.41 −0.3 −1.0 −1.1 1.12E-01 71C 440559 4503716 J 1,156 17.7 6.9 1,720 7.98 4.12 8.48 0.17 4.66 12.66 2.78 5.87 31 9.6 3.97 −1.94 1.18 −0.2 −0.6 −1.4 2.88E-02 72S 443776 4507878 J 1,005 18.3 6.5 1,368 6.79 1.65 6.74 0.10 5.08 6.72 1.88 8.46 162 14.1 1.79 1.21 0.96 −0.5 −1.7 −1.5 8.32E-02 73S 442081 4510021 J 711 18.1 6.6 889 5.56 2.22 2.91 0.09 5.35 3.45 1.27 8.25 24 8.6 1.96 0.11 0.72 −0.5 −1.3 −1.7 6.92E-02 74C 441064 4502626 J 676 16.3 6.8 867 5.29 0.99 3.48 0.08 5.62 3.15 0.85 7.62 23 9.6 0.53 0.91 0.34 −0.3 −1.3 −1.9 4.57E-02 76S 441420 4508244 J 1,319 18.0 6.6 1,860 9.38 5.76 7.18 0.16 6.06 10.83 4.99 9.17 23 8.2 7.47 −1.73 3.60 −0.3 −0.8 −1.1 7.59E-02 77C 443121 4501477 J 919 17.2 6.7 1,210 6.59 3.29 5.48 0.06 6.53 5.49 1.39 9.31 40 15.9 3.55 0.94 0.61 −0.3 −0.8 −1.7 6.61E-02 80S 444466 4508725 J 869 18.2 6.5 1,002 8.38 1.40 2.61 0.12 6.86 3.10 0.85 11.47 75 10.0 4.05 0.11 0.34 −0.3 −1.3 −1.8 1.12E-01 81C 440560 4498381 J 792 20.0 6.9 1,071 4.49 1.81 3.91 0.13 6.99 3.24 0.83 8.83 51 15.0 0.53 1.32 0.31 −0.1 −0.6 −2.0 4.68E-02 81S 445244 4507389 J 830 17.8 6.7 1,000 7.39 1.98 2.91 0.11 6.04 3.40 1.99 8.63 34 8.6 3.55 0.16 1.45 −0.2 −1.0 −1.4 6.17E-02 84S 442631 4506785 J 1,162 18.2 6.8 1,500 8.18 2.14 6.96 0.06 7.70 7.22 3.04 10.25 56 12.3 3.56 0.97 2.06 0.0 −0.6 −1.3 6.17E-02 87S 438374 4510924 J 943 19.6 6.6 1,206 7.49 2.63 3.91 0.14 7.93 5.07 0.74 12.03 19 10.9 3.89 −0.20 0.01 −0.2 −0.8 −1.9 1.05E-01 89S 440675 4510758 J 1,349 18.7 6.4 1,570 12.38 2.63 5.22 0.13 6.11 5.33 8.23 11.04 68 10.5 8.72 0.88 7.47 −0.4 −1.4 −0.8 1.23E-01 90S 445044 4505801 J 802 17.6 6.9 978 7.39 1.48 2.70 0.07 6.44 3.05 0.89 8.19 54 10.0 3.14 0.20 0.39 0.0 −0.6 −1.8 4.17E-02 11S 434404 4500459 T 1,428 19.7 6.9 2,140 8.98 3.95 11.53 0.26 6.78 11.20 6.06 8.47 41 12.3 5.17 2.42 4.63 0.0 −0.2 −1.0 4.27E-02 1S 434641 4498480 T 1,824 21.2 6.9 3,280 5.49 4.12 27.40 0.27 8.73 21.58 4.01 10.84 58 55.1 −0.75 9.64 1.40 −0.1 −0.2 −1.5 5.62E-02 26S 434811 4501819 T 885 21.1 8.9 1,870 2.40 2.80 16.31 0.49 3.37 14.62 0.40 3.09 0 1.0 −3.42 4.57 −1.42 0.1 0.1 −2.6 1.86E-04 29S 436491 4501864 T 1,589 16.2 6.8 2,800 8.38 5.43 16.53 0.35 5.43 21.87 3.07 7.22 34 14.1 3.39 −1.41 0.42 −0.2 −0.7 −1.4 4.07E-02 2S 435375 4500215 T 3,123 19.0 7.0 3,310 28.04 11.19 8.05 0.39 4.82 7.28 37.74 5.72 0 13.2 32.46 2.34 36.76 −0.1 −0.2 −1.5 2.24E-02 31S 436089 4502596 T 3,724 19.8 7.3 7,350 8.73 14.81 62.64 1.61 4.67 75.85 3.87 5.08 53 7.3 −0.38 0.90 −4.93 0.0 0.0 −1.5 1.05E-02 34S 433971 4502286 T 1,540 21.9 8.2 3,450 1.10 1.15 35.74 0.49 6.76 22.05 4.65 6.74 0 9.1 −8.22 17.79 1.98 0.0 0.1 −2.0 2.14E-03 48S 435563 4504909 T 3,693 18.8 6.6 7,140 8.23 17.28 61.77 1.42 6.09 71.84 5.18 8.84 0 10.5 2.60 3.19 −3.16 −0.5 −0.6 −1.4 6.92E-02 56S 439181 4508061 T 1,858 16.9 7.0 3,050 7.39 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 2.63E-02 57S 439164 4509234 T 3,480 18.8 6.7 4,610 23.45 18.93 23.49 0.47 6.35 30.94 25.27 8.63 0 16.8 29.69 −1.89 21.59 0.0 0.0 −0.3 5.62E-02 5S 434142 4499760 T 3,287 17.2 6.6 3,750 28.74 12.67 8.92 0.35 6.01 14.14 33.85 8.88 0 17.3 32.92 −2.56 32.08 −0.1 −0.4 −0.1 6.76E-02 66C 438858 4502128 T 1,620 20.0 6.3 2,070 13.67 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 1.95E-01 71S 440572 4510475 T 2,367 18.6 6.7 3,020 7.98 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 9.12E-02 75S 441866 4508696 T 1,971 16.5 7.1 2,750 14.87 6.75 10.22 0.17 6.52 13.92 11.68 7.57 82 16.8 13.18 −1.26 9.94 0.0 0.0 −0.6 2.40E-02 86S 440172 4507052 T 1,586 15.6 6.9 2,730 7.24 5.02 18.05 0.27 4.03 26.54 2.73 5.11 0 10.0 0.66 −3.86 −0.45 −0.3 −0.8 −1.5 2.40E-02 90C 437509 4501789 T 1,634 20.8 6.5 2,560 9.98 5.27 14.35 0.23 9.19 15.79 3.40 14.77 28 11.8 6.34 1.38 1.45 −0.2 −0.5 −1.3 1.48E-01 93C 437000 4505525 T 2,143 19.8 6.8 4,370 4.89 8.56 29.14 0.77 4.08 40.07 3.73 5.32 0 10.0 −1.53 −3.56 −0.99 −0.6 −0.9 −1.6 3.16E-02 O 10.1007/s10040-008-0369-z DOI 210S 449894 4505410 T 2,687 19.2 7.2 3,240 21.36 9.22 10.00 0.58 5.94 12.49 25.98 6.63 23 14.6 22.49 0.14 24.40 0.0 0.0 −0.3 1.78E-02

TDS, SiO2,NO3, major cations and anions are in mg/l; temperature (T) is in °C; electrical conductivity (EC) is at 18°C in mS/cm; dissolved inorganic carbon (DIC) is in mmole/l; ΔCa + ΔMg, ΔNa + ΔK and ΔSO4 are in meq/l. HU hydrogeologic unit (refer to legend in Fig. 3); SI saturation index; PCO2 partial pressure of CO2 (atm) Fig. 5 Piper diagram for groundwaters (gw) of the Nurra Basin Fig. 4 Variation of total dissolved solids (TDS) concentrations with chloride concentration. Curves fit data concerning sea water– freshwater conservative mixing, pure solution of calcium sulphate, groundwaters of Quaternary aquifers are mostly of a Ca– sodium chloride and a mole proportion (1:0.5) of both salts. In the Cl type, shifting to a Na–Cl type at the highest TDS. The legend, gw stands for groundwater 2 waters labelled in Fig. 6, being of the chloride-sulphate type, actually contain low percentages of sulphate; thus, locate). Moreover, to take into account other potential salt end-members besides sea water, the plot of Fig. 4 also shows the lines representing the chemically inert addition of soluble salts, as CaSO4, NaCl and a CaSO4 (NaCl proportion of 1:0.5, mole ratio). The comparison of groundwater sample distribution with the above lines may indicate that groundwater mineralization is due to multiple factors. As a whole, groundwaters from the plain aquifers show a higher TDS than groundwaters circulating in the highland.

Major ions and Hydrochemical water types The Piper plot of Fig. 5 shows, that in spring waters, the dominant anions are either bicarbonate or chloride, while groundwater samples can be described as bicarbonate, chloride or sulphate dominant.Figure 6 shows more clearly that groundwaters belonging to the Jurassic aquifer show a dominant calcium-bicarbonate type that, when TDS is over 1.1 g/l, convert into a Ca–SO4 or Ca–Cl2 type. Fresh groundwaters from Oligo-Miocene aquifer show a Na–Cl–HCO3 type, evolving to a Na–Cl type when TDS exceeds 2 g/l. Waters from the Triassic aquifer vary from Ca–SO4 to Ca–Cl2 and finally to a Na–Cl type according to TDS increase; fresh Cretaceous groundwaters Fig. 6 The relationship between the concentration of Na + K as a – percentage of total cations (meq/l per meq/l) and bicarbonate show a Ca or Na-HCO3 type, which evolve to Ca or Mg concentration as a percentage of total anions (meq/l per meq/l). In Cl2 and Na–Cl with increasing mineralization. Fresh the legend, gw stands for groundwater

Hydrogeology Journal DOI 10.1007/s10040-008-0369-z the increase in TDS is mainly related to the increase in silica contents in Cretaceous groundwater could originate chloride. Sulphate percentages greater than 20% are only as a result of silicate (probably the glassy fraction) found in groundwaters from the Triassic and Jurassic leaching from the overlying porous pyroclastic flows. aquifers. Figure 3 shows clearly that groundwater flow from the Plots of Fig. 7 show the major cation–chloride relations Oligo-Miocene aquifer is directed towards the NW and and include the lines representing fresh water and present- can feed the Cretaceous aquifer near Olmedo. Alterna- day sea-water mixing and, when appropriate, lines tively the high silica in the groundwater hosted in the representing the chemically inert solution of variable Cretaceous aquifer can derive from the thick (up to 200 m) amounts of soluble salts. As previously discussed, the glauconite rich calcarenites. chemical composition of waters derived from mixing different proportions of fresh and sea water, or from other sources, rarely matches the composition defined by a CO2 calculation that excludes the occurrence of reactions. Regarding bicarbonate concentrations (Fig. 7f), most of Indeed, Fig. 7a–d shows, above the limit of 4 meq/l of measured values for Jurassic and Cretaceous carbonate chloride (see the preceding), either excess or deficit of the formations are higher than typical for groundwaters from major cations with respect to the conservative mixing carbonate aquifers at normal temperatures and CO2 partial lines; excess prevails for calcium and magnesium, and pressures. Moreover, groundwater maintains such high deficit for sodium and potassium. The waters, which plot values under salinization as well. at concentrations below the limit, are mainly groundwaters The highest bicarbonate concentrations (8–11 meq/l) from the Jurassic aquifer and spring waters, though might be justified by high CO2 partial pressures. The members of both these groups also plot above the speciation of inorganic carbon (by PHREEQC, Parkhurst 4 meq/l Cl boundary. These waters are interpreted as 1995), for the data set of the winter 2004 survey (Table 1), −4 resulting from water–rock interaction with carbonate rocks shows that PCO2 varies between 1.9×10 and 1.95× −1 with limited addition of soluble salts. Figure 7e shows that 10 atm. For the data set of June 2005, PCO2 varies sulphate concentrations are mainly in excess with respect between 3.5×10−4 and 2.29×10−1 atm (Table 2). By to mixing lines; high excesses agree with the solution of considering both the surveys, the 13% of groundwaters CaSO4 with variable proportion of NaCl. Bicarbonates and the 10% of spring waters, however, show values −1 (Fig. 7f) span a range of 0.3–11 meq/l, maintaining high higher than 10 atm. The PCO2 frequency distribution of concentrations also at high TDS. Although there may be Nurra waters is lognormal and skewed to the left, which some component of mixing, the spread of data indicates confirms that the dominant population is characterized by that there is a range of other processes occurring. moderately high PCO2 values. On the whole, pH ranges from 5.7 to 8.9, but most groundwaters and spring waters (75%) are acidic, with a SiO2 pH between 6 and 7. For waters of both surveys, the Sulphate, bicarbonate (Fig. 7e and f) and silica contents decrease of pH with increasing PCO2 is shown in Fig. 9a, allow further characterization of the groundwaters into while Fig. 9b shows that groundwaters are progressively two main groups (highland and plain aquifers). Figure 8 more sub-saturated with respect to calcite as soon as PCO2 shows the relation between SiO2 and bicarbonate. One increases, thus indicating that they are not in equilibrium grouping of samples (A) can be defined by sulphate and with calcite and its dissolution takes place in an open sodium deficits relative to pure mixing with sea water, system. bicarbonates lower than 4 meq/l and silica concentrations There is no straightforward account for such high PCO2 greater than 35 mg/l. Group (A) includes samples of values. Influxes of carbon dioxide from depth through groundwaters from the Oligo-Miocene volcanic aquifer. tectonic discontinuities, CO2 production due to redox Group (B), including groundwaters sampled from the reactions or continuous dissolution of carbonates due to plain aquifers, shows mainly sulphate excess, bicarbonate ion-exchange could be responsible of anomalous values. ranging from 4 to 11 meq/l and silica concentrations less In any case, influxes of carbon dioxide from sub- than 35 mg/l (and mostly in the range 0–30 mg/l). A few crustal depth can be easily ruled out on a geological groundwater samples from the Oligo-Miocene volcanic basis. Even if Sardinia Island is characterized by aquifer and a few samples belonging to the Cretaceous numerous fault-controlled geothermal systems, their sequence (group C) have both bicarbonate contents higher origin is clearly independent from the recent alkaline to than 4 meq/l and silica contents higher than 35 mg/l; such transitional volcanic activity, which, in any case, ended waters (showing sulphate and sodium excess) come from around 80 ka before present (Beccaluva et al. 1977). wells reaching depths between 50 and 100 m below mean These systems are normally located, regardless the sea level (m.s.l.) or are located near or at the eastern crosscut formation, along inactive regional faults, mainly boundary of the volcanic sequence. Due to the complex strike–slip faults of Oligocene–Aquitanian age. Some structural setting, it is likely that aquifers come into thermal sources are located within the Variscan basement, contact both laterally and vertically. Thus, high bicarbon- others within the Oligocene-Miocene calc-alkaline volca- ate contents in volcanic groundwater might originate from nic complex or at the tectonic contact between the interconnection with carbonate aquifers. Conversely, high basement and the Tertiary or Mesozoic covers; no thermal

Hydrogeology Journal DOI 10.1007/s10040-008-0369-z Hydrogeology Journal DOI 10.1007/s10040-008-0369-z ƒFig. 7 Variation of major constituents with respect to chloride: a wintertime reflects the presence of recharge areas at high – Ca, b Mg, c Na, d K, e SO4, and f HCO3. Lines indicate sea water elevations in the same aquifer. freshwater conservative mixing and pure solution of gypsum and/or sodium chloride. In the legend, gw stands for groundwater In June 2005 the mean temperature of groundwaters in the region of the plains increases up to on average 17–25°C springs are linked to the Pliocene–Pleistocene basalt (average temperature 21.7°C); in the highland aquifer fl temperature varies between 19.5 and 24.4°C (mean oods. fl Figure 10 shows the trend of the above parameters for temperature 22°C). This range of temperature re ects the groundwater samples arranged according to DIC (dis- seasonal variation of the local isotherms. The only solved inorganic carbon) increase. The grey shadowed exception is represented by the groundwater coming from area in each single plot marks the samples with DIC a well close to the Su Zumbaru fault, which also in winter between 10 and 15.8 mmole/l (Fig. 10a), characterizing, shows a water temperature of 24°C. This important strike- as results from the analysis of the log probability plot of slip fault is deeply rooted in the Tertiary volcanic complex DIC show, the highest of almost six populations of Nurra and allows relatively quick upwelling of water heated by waters. thermal gradient which in western Sardinia is particularly fi high (Della Vedova et al. 1995) with HFD (heat flow As a rst approximation, water samples having DO > – −2 0.5 mg/l can be classified as aerobic; out of the 86 samples, density) in the range 50 70 mW/m . 83 are aerobic (with DO ranging from 1.1 to 9 mg/l), and three have DO less than 0.5 mg/l, whereas of the latter Sulphate solution and dedolomitization waters, which show appreciable contents of Fe, Mn and − Beside the large contribution of chloride, both parts a NH4, two also have nitrate exceeding 0.5 mg/l (NO3 - reducing samples) and the remaining (from the Triassic and e of Fig. 7 indicate that the solution of gypsum aquifer, with TDS of about 3.7 g/l) have nitrate less than contributes to the increase of salt content. In general, 0.1 mg/l, also indicating a sulphate deficit (Fe or sulphate calcium correlates with DIC (Fig. 11) except for waters reducing samples). However, some of the aerobic samples close to Triassic evaporites, while magnesium correlates with sulphate (Fig. 12), according to different trends with low DO contents have NH4 as the dominant N species; moreover, a few waters, with DO in the range 0.5– corresponding to different Mg/SO4 ratios. Mg/SO4 ratio 3 mg/l, show high Fe contents. These findings indicate varies between about 4 and 0.5 and waters from the anaerobic conditions; classification on the basis of DO Triassic aquifer belong to both extreme trends. seems to underestimate the reducing conditions of the The two highest values of magnesium (maximum aquifers (Tesoriero et al. 2004). 19 meq/l) correspond to sulphate concentrations of 8 and 25.3; the highest sulphate concentrations accompany Another source of CO2 could be the action of H2SO4 deriving from oxidization of both lower Jurassic pyrite- lower Mg concentrations and the highest calcium concen- rich coaly layers and pyrite occurring in the black phyllites of the exposed basement on carbonate rocks; the Eh-pH diagram indicates that moderately high-PCO2 water samples are located in the stability field of Fe2O3(s), which could justify the very low values of Fe in the grey shadowed area of Fig. 10c.

Temperature Groundwater temperature was measured at all water points during the two monitoring surveys. During winter 2004, the highland aquifer springs and groundwaters show temperatures ranging respectively from 13.4 to 17.5°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 the Triassic aquifer: plain springs show little variation between 16.6 and 17.9°C. In the studied area, the recharge waters have a mean temperature of about 10°C, corresponding to the average value of the atmospheric temperature during the recharge period (autumn-winter). Such a temperature is close to the temperature measured in winter 2004 at some of the

highland springs: the Oligo-Miocene aquifer has the highest Fig. 8 Relationship between bicarbonate concentration and SiO2 elevation of the area and the temperature of most springs in concentration. In the legend gw stands for groundwater

Hydrogeology Journal DOI 10.1007/s10040-008-0369-z yrgooyJournal Hydrogeology Table 2 Hydrochemical parameters for the groundwaters sampled in December 2004 and June 2005

ID UTM E UTM N Date HU TDS T pH EC DO Eh NO3 NO2 NH3 Fe Mn ΔSO4 ΔCa+ΔMg ΔNa+ΔKPCO2 DIC 4C 441986 4493156 Dec 2004 Q 2,040 19.0 7.2 2,980 3.6 194 309 0 40 0 0 46.9 10.2 −3.8 1.6E-02 6.10 10C 443097 4493714 Dec 2004 Q 1,539 19.2 6.7 2,380 4.0 264 154 0 50 20 0 28.2 3.1 4.3 7.2E-02 10.26 15C 441833 4494014 Dec 2004 Q 2,453 17.5 6.5 4,190 4.7 181 49 0 10 0 0 30.5 5.2 1.3 4.0E-02 10.13 20C 443007 4494522 Dec 2004 Q 1,230 18.8 6.8 1,660 6.9 249 92 0 60 20 0 26.4 3.5 1.0 6.2E-02 10.06 21C 443428 4495249 Dec 2004 Q 1,588 17.8 6.9 2,830 1.1 180 0 43 40 0 0 31.4 1.0 7.3 3.9E-02 9.66 28C 442754 4494779 Dec 2004 Q 1,140 19.5 6.6 1,540 2.9 257 67 0 30 0 0 22.3 3.7 −0.2 9.3E-02 10.94 32C 443127 4496292 Dec 2004 Q 1,346 17.8 7.1 1,690 3.8 216 278 0 70 0 0 22.8 4.8 3.7 2.2E-02 6.53 21C 443428 4495249 Jun 2005 Q 1,720 21.80 6.7 2,805 1.4 309 0 0 50 10 10 29.3 2.7 3.6 8.7E-02 11.70 32C 443127 4496292 Jun 2005 Q 1,343 22.00 7.3 1,971 2.4 269 236 0 40 20 20 18.3 7.7 3.4 1.1E-02 4.82 178S 458227 4492364 Dec 2004 OM 1,980 17.7 7.1 2,580 4.3 232 99 35 0 20 10 47.8 8.8 −4.4 5.6E-03 0.53 181S 455244 4491162 Dec 2004 OM 525 17.0 7.7 2,070 3.6 146 20 66 400 0 20 60.5 0.0 0.6 4.7E-02 3.41 145C 448770 4497340 Jun 2005 OM 2,613 22.70 6.8 4,653 2.7 323 181 0 160 0 0 56.9 6.0 5.2 7.4E-02 12.09 165S 454840 4495053 Jun 2005 OM 2,076 21.90 7.0 4,175 4.2 302 0 0 220 10 40 41.3 4.5 −7.2 1.4E-02 3.43 167S 452777 4494629 Jun 2005 OM 1,870 23.00 6.5 3,652 5.5 321 0 0 200 70 0 54.2 6.6 −4.7 5.9E-02 5.73 174S 462493 4493828 Jun 2005 OM 1,050 23.30 6.6 1,677 6.7 311 0 0 20 20 20 47.7 −1.0 6.0 1.1E-01 11.52 178S 458227 4492364 Jun 2005 OM 2,430 22.60 6.4 4,374 5.3 305 139 0 130 30 0 42.7 11.7 −9.5 2.1E-02 1.87 192S 459805 4490314 Jun 2005 OM 594 19.50 6.2 985 6.1 320 24 0 70 0 0 50.5 2.2 −0.6 1.3E-02 0.88 197S 455328 4487011 Jun 2005 OM 1,126 20.00 6.6 1,858 6.4 305 54 0 30 0 0 53.1 6.8 −1.8 5.0E-02 5.89 200S 455320 4488024 Jun 2005 OM 648 24.40 6.9 1,126 4.4 328 4 0 20 0 0 41.8 −2.9 −0.2 2.1E-02 3.63 205S 453047 4489723 Jun 2005 OM 1,070 20.40 6.0 1,686 5.6 385 152 0 200 0 0 77.3 4.8 −2.3 8.7E-02 5.04 29C 443463 4494493 Dec 2004 C 446 17.5 6.6 673 0.6 227 22 10 90 0 0 12.3 −1.9 0.6 1.2E-02 3.26 115C 444051 4493984 Dec 2004 C 1,673 19.8 7.5 2,410 4.9 219 453 0 20 0 0 35.5 −0.6 3.1 1.3E-02 6.79 118C 445604 4493897 Dec 2004 C 2,072 21.0 6.8 3,500 1.6 118 62 0 50 0 0 37.3 1.1 6.3 7.1E-03 9.38 125S 451597 4502826 Dec 2004 C 1,119 18.2 6.7 1,363 5.6 187 59 0 120 0 0 21.8 3.9 −0.5 7.8E-02 10.75 126C 446470 4496702 Dec 2004 C 1,172 18.5 7.5 1,900 1.9 224 0 0 400 0 0 28.2 −2.5 5.9 1.3E-02 8.29 135C 444754 4494778 Dec 2004 C 1,599 20.7 7.0 2,460 4.1 249 82 0 10 0 0 32.3 1.8 8.4 4.9E-02 11.56 115C 444051 4493984 Jun 2005 C 891 21.90 7.6 1,156 5.7 238 89 0 70 20 20 24.1 0.4 5.3 1.0E-02 7.65 125S 451597 4502826 Jun 2005 C 1,103 21.10 6.6 1,452 3.9 267 32 0 50 10 0 16.8 4.0 −0.8 1.0E-01 11.61 139S 448796 4500386 Jun 2005 C 2,115 22.50 6.8 3,589 2.2 254 17 0 60 20 0 48.1 12.5 −6.9 6.0E-02 9.89 18C 442349 4494343 Jun 2005 C 1,960 20.80 6.6 3,003 2.4 310 57 0 0 60 20 22.9 10.4 −1.8 1.0E-01 12.53 29C 443463 4494493 Jun 2005 C 345 24.00 6.8 441 5.7 280 5 0 40 1,120 80 12.8 −3.0 −0.3 1.8E-02 2.53 79S 442799 4508227 Jun 2005 C 1,046 24.40 6.9 1,434 2.3 319 66 0 30 30 20 8.3 4.2 −0.9 4.7E-02 8.37 98S 443600 4506139 Jun 2005 C 1,056 23.40 7.5 1,353 5.2 279 29 0 60 0 0 30.7 4.2 0.2 1.3E-02 8.02 120S 450400 4503746 Dec 2004 J 1,021 21.6 6.6 1,314 3.8 164 20 0 100 0 0 15.0 3.5 −0.8 1.1E-01 12.22 16S 436002 4501003 Jun 2005 J 1,342 21.20 7.1 1,984 3.5 301 29 0 110 240 50 12.6 6.5 0.6 2.8E-02 7.93 36S 447026 4500702 Jun 2005 J 973 21.30 6.6 1,188 7.3 315 29 0 260 20 10 15.8 3.6 0.3 1.1E-01 12.39 39S 438487 4499458 Jun 2005 J 1,192 18.90 7.1 1,641 5.4 303 32 0 50 60 0 11.0 4.5 −2.0 3.0E-02 8.64

O 10.1007/s10040-008-0369-z DOI 41S 438108 4498022 Jun 2005 J 1,117 21.50 6.7 1,551 5.4 381 31 0 0 10 0 9.3 4.9 0.1 7.8E-02 10.33 43S 438944 4497251 Jun 2005 J 1,107 17.80 6.6 1,506 4.0 329 25 0 60 30 0 10.5 4.6 −0.8 9.8E-02 11.71 47C 438711 4496925 Jun 2005 J 941 17.00 6.9 1,284 7.2 308 105 0 330 0 50 13.8 1.9 1.0 4.0E-02 7.99 58S 438503 4509770 Jun 2005 J 827 20.90 7.0 1,234 4.1 287 34 0 30 40 50 9.3 2.5 −0.6 2.2E-02 5.08 63C 440366 4499796 Jun 2005 J 535 21.20 7.3 703 3.7 285 4 0 90 180 90 6.9 −0.8 0.7 1.2E-02 4.87 69S 440106 4510126 Jun 2005 J 1,017 22.20 6.8 1,267 6.5 334 36 0 70 70 60 9.3 5.4 0.4 5.8E-02 8.98 72S 443776 4507878 Jun 2005 J 970 18.80 6.6 1,326 6.5 284 136 0 80 290 40 10.5 4.7 −0.4 6.8E-02 0.46 73S 442081 4510021 Jun 2005 J 647 22.30 6.9 810 4.7 294 13 0 80 20 60 3.2 1.4 0.2 3.7E-02 6.69 77C 443121 4501477 Jun 2005 J 979 20.80 6.6 1,300 1.8 323 31 0 40 50 10 14.5 4.2 −0.7 9.5E-02 10.88 81C 440560 4498381 Jun 2005 J 926 22.80 6.4 1,105 4.1 337 58 0 60 20 0 11.8 3.8 1.6 1.6E-01 13.47 81S 445244 4507389 Jun 2005 J 793 20.80 6.7 997 4.9 301 30 0 120 30 20 6.9 2.7 0.5 6.6E-02 8.71 89S 440675 4510758 Jun 2005 J 1,437 20.00 6.4 1,695 2.0 321 69 0 70 10 10 5.3 11.1 0.7 1.5E-01 13.25 90S 445044 4505801 Jun 2005 J 826 21.10 6.8 1,011 5.7 291 45 0 70 0 10 8.9 4.2 0.2 5.9E-02 9.17 yrgooyJournal Hydrogeology 101S 445695 4504022 Jun 2005 J 1,058 21.80 6.6 1,224 4.8 304 32 0 80 0 10 10.1 6.1 0.2 1.3E-01 13.98 107C 447357 4493987 Jun 2005 J 1,133 23.50 7.5 2,056 6.0 240 8 0 100 0 50 61.6 1.1 2.3 6.9E-03 4.31 120S 450400 4503746 Jun 2005 J 543 23.50 7.1 714 1.9 290 8 0 70 240 20 9.4 −0.1 −0.1 1.9E-02 4.92 121C 444373 4497045 Jun 2005 J 905 20.30 6.9 1,125 5.7 300 11 0 70 30 20 12.6 2.8 2.3 5.2E-02 9.87 128S 448276 4503507 Jun 2005 J 956 24.20 6.8 1,214 1.4 261 16 0 20 0 0 13.4 4.9 0.4 7.8E-02 11.56 131C 444186 4501685 Jun 2005 J 910 22.20 7.0 1,114 5.7 294 27 0 0 0 0 7.6 3.9 −0.7 4.1E-02 9.11 143S 445173 4502773 Jun 2005 J 963 18.60 6.7 1,235 6.5 303 33 0 40 20 20 11.8 4.8 −0.4 8.1E-02 11.36 1S 434641 4498480 Dec 2004 T 1,824 21.2 6.9 3,280 6.1 260 58 0 100 0 0 55.1 −0.8 9.6 5.6E-02 10.84 11S 434404 4500459 Dec 2004 T 1,428 19.7 6.9 2,140 5.9 242 41 0 40 0 10 12.3 5.2 2.4 4.3E-02 8.47 26S 434811 4501819 Dec 2004 T 885 21.1 8.9 1,870 3.3 115 0 54 200 20 10 1.0 −3.4 4.6 1.9E-04 3.09 29S 436491 4501864 Dec 2004 T 1,589 16.2 6.8 2,800 8.5 234 34 0 50 0 0 14.1 3.4 −1.4 4.1E-02 7.22 2S 435375 4500215 Dec 2004 T 3,123 19.0 7.0 3,310 3.0 234 0 0 50 10 60 13.2 32.5 2.3 2.2E-02 5.72 31S 436089 4502596 Dec 2004 T 3,724 19.8 7.3 7,350 1.4 136 53 54 1,500 30 140 7.3 −0.4 0.9 1.0E-02 5.08 34S 433971 4502286 Dec 2004 T 1,540 21.9 8.2 3,450 4.3 130 0 8 0 20 0 9.1 −8.2 17.8 2.1E-03 6.74 48S 435563 4504909 Dec 2004 T 3,693 18.8 6.6 7,140 0.4 159 0 0 5,800 140 500 10.5 2.6 3.2 6.9E-02 8.84 56S 439181 4508061 Dec 2004 T 1,858 16.9 7.0 3,050 6.6 234 223 0 50 0 0 24.1 2.8 2.3 2.6E-02 6.66 57S 439164 4509234 Dec 2004 T 3,480 18.8 6.7 4,610 9.1 192 0 0 700 10 10 16.8 29.7 −1.9 5.6E-02 8.63 5S 434142 4499760 Dec 2004 T 3,287 17.2 6.6 3,750 3.0 125 0 0 120 30 20 17.3 32.9 −2.6 6.8E-02 8.88 66C 438858 4502128 Dec 2004 T 1,620 20.0 6.3 2,070 5.6 246 340 0 70 0 0 10.5 8.7 −0.8 1.9E-01 15.16 71S 440572 4510475 Dec 2004 T 2,367 18.6 6.7 3,020 2.2 189 549 0 200 20 0 15.5 14.1 −2.3 9.1E-02 13.21 75S 441866 4508696 Dec 2004 T 1,971 16.5 7.1 2,750 7.0 268 82 0 1,600 10 0 16.8 13.2 −1.3 2.4E-02 7.57 86S 440172 4507052 Dec 2004 T 1,586 15.6 6.9 2,730 6.1 185 0 0 120 0 10 10.0 0.7 −3.9 2.4E-02 5.11 90C 437509 4501789 Dec 2004 T 1,634 20.8 6.5 2,560 4.8 203 28 0 40 0 20 11.8 6.3 1.4 1.5E-01 14.77 93C 437000 4505525 Dec 2004 T 2,143 19.8 6.8 4,370 2.6 45 0 0 40 120 160 10.0 −1.5 −3.6 3.2E-02 5.32 210S 449894 4505410 Dec 2004 T 2,687 19.2 7.2 3,240 3.1 256 23 0 130 0 20 14.6 22.5 0.1 1.8E-02 6.63 1S 434641 4498480 Jun 2005 T 2,096 24.60 7.2 2,769 3.8 282 0 0 140 0 30 9.0 16.3 3.1 2.1E-02 7.34 26S 434811 4501819 Jun 2005 T 978 23.40 8.7 2,011 1.3 166 0 0 1,600 370 30 1.0 −5.1 3.5 3.5E-04 3.34 31S 436089 4502596 Jun 2005 T 3,075 23.80 6.9 6,403 2.2 237 0 0 1,000 2,610 190 5.7 2.5 −2.5 3.9E-02 7.61 34S 433971 4502286 Jun 2005 T 1,237 23.60 8.2 2,354 1.6 197 3 0 100 790 40 5.3 −3.8 10.2 2.4E-03 7.20 48S 435563 4504909 Jun 2005 T 3,964 25.00 6.5 7,449 0.4 184 1 0 1,000 610 100 8.5 2.4 −9.6 9.8E-02 9.68 56S 439181 4508061 Jun 2005 T 1,577 22.70 6.7 2,715 2.3 262 89 0 220 730 40 14.6 4.9 −0.4 6.8E-02 8.99 71S 440572 4510475 Jun 2005 T 2,202 21.00 6.9 3,084 2.1 252 291 0 40 30 10 7.7 15.7 −1.8 6.3E-02 12.59 75S 441866 4508696 Jun 2005 T 2,306 19.30 7.1 3,084 5.8 300 70 0 60 150 70 11.4 17.1 −4.2 2.5E-02 7.61

HU is hydrogeological unit (refer to legend in Fig. 3); TDS, NO3,NO2 are in mg/l; Fe and Mn are in µg/l; Temperature (T) is in °C; electrical conductivity (EC) is at 18°C in mS/cm; redox potential (Eh) is in mV; dissolved inorganic carbon (DIC) is in mmole/l; ΔCa+ΔMg, ΔNa+ΔK and ΔSO4 are in meq/l O 10.1007/s10040-008-0369-z DOI Fig. 9 a Relationship between PCO2 and pH; b Variation of SI (calcite) with respect to PCO2. In the legend gw stands for groundwater

trations (maximum 29 meq/l): related groundwaters are of CaCl2. The chemistry of groundwaters in the volcanic area the Ca–Mg–SO4 water type. is therefore mainly controlled by leaching of volcanic Various processes compete in determining the calcium rocks, ion-exchange with kaolinite, zeolite and bentonite and magnesium concentrations, including calcite precipi- of calcium type and solution of salts, remnants of the tation and solution, incongruent solution of dolomite and hydrothermal activity (Mameli 2001), all of which dedolomitization driven by sulphate solution, which is compete in defining the mutual concentrations of Ca, Na particularly active at the contact between Giurassic dolo- and Mg. The efficacy of ion-exchange is suggested by the stone and Triassic evaporites. binary plots of Fig. 7a–d as well; after the limit of fresh In addition, within the Oligo-Miocene aquifer, these water (4 meq/l of chloride), as previously described, processes occur concurrently with increase of sodium and calcium and magnesium show an excess and sodium and chloride concentrations; due to the presence of potential potassium show a deficit relative to the sea water mixing ion exchangers in concerned formations, one other factor, line. ion-exchange, competes in determining the calcium and magnesium concentrations. In some cases, a deficit of sulphate with respect to conservative mixing is observed, Nitrate the leaching of welded or unwelded tuffs and of the Nitrate concentrations in the Nurra aquifers range from remnants of hydrothermal alteration might increase below detection limit (about 1 mg/l) to more than 500 mg/l. groundwater salt contents. The hydrothermal alteration Concentrations above 50 mg/l affect both fresh and can be traced back to hydrothermal fluid circulation; deep salinized waters: nitrate is likely to be derived from the running strike slip faults, present in the southern part of widespread use of fertilizers in the plain. High contents the Nurra area, functioned as conduits for the rising affect also the groundwaters of Jurassic sequence, which hydrothermal fluids that altered rocks by forming clay are destined for potable use. minerals and these potentially have played and play very important roles in affecting water quality due to ion exchange. Thus, deposits of kaolin, zeolite, bentonite, and travertine, which actually cover both the Miocene volcanic Stable isotopes and tritium tuffs and the Quaternary sediments, represent the memory of the widespread hydrothermal activity. Presently, the Seven water samples (Table 3), taken during November– area does not show important thermal waters, fumaroles December 2005, have been analyzed for stable isotope and hot grounds. The only memories of this hydrothermal (18O and 2H) and tritium contents.The stable isotope data activity, as already shown, are some warm waters, which are reported in Fig. 13 in comparison to the EMWL (East emerge close to the deep strike-slip faults. Hydrothermal Mediterranean water line) and the GMWL (global fluids which altered volcanic rocks could have been saline, meteoric water line). According to Longinelli and Selmo and contained dissolved salts as NaCl, KCl, CaSO4 and (2003), the local meteoric water line (LMWL) for central

Hydrogeology Journal DOI 10.1007/s10040-008-0369-z Fig. 10 Diagram of the variation of a Eh (left axis) and dissolved inorganic carbon (DIC, right axis); b pH; c Fe (left axis)andMn (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 and 15.2 mmole/l

Italy, where geographically the study area lies, is δ2H= could represent the effect of evaporation during infiltra- 7.05 δ18O + 5.6. The same study indicates that the tion. As an example, the groundwater sample no. 157S weighted mean 18O and 2H values for the rain at Sassari (located at the extreme right) lies on the Su Zumbaru (period February 1999–January 2000) is respectively - strike-slip fault; its position could correspond to enrich- 6.58‰ and -37.3‰. ment due to evaporation before infiltration. This hypoth- The clustering of stable isotope compositions of the esis could hold true due to the fact that the fault borders a groundwaters near the LMWL and the weighted mean structural depression towards which the surrounding precipitation indicates that the recharge to the volcanic surface flow converges and consequently evaporates due and carbonate aquifers is derived from local precipitation. to the low permeability enhanced by the occurrence of However, without a larger data set, the effective reference, tuffs whose clay content has been increased by weather- the weighted mean of “recharge waters” cannot be ing/hydrothermal fluids. However, other factors may be calculated and used for the comparison: the effective important, especially in the light of the previously value should be somewhat more negative, because of the discussed thermal anomalies and/or the possible addition exclusion of summer precipitation. of carbon dioxide; present data are not sufficient to solve However, even with respect to weighted mean rainfall the uncertainties, which would need to be clarified at least values, the analyzed groundwaters show a shift which by the analysis of 13C.

Hydrogeology Journal DOI 10.1007/s10040-008-0369-z ejective, folding-style strata related to the occurrence of evaporites and by later, generally normal, faults. Thus, in the quasi-flat area, where the Mesozoic rocks crop out, there is no relation between the topographic watershed and the hydrogeological basin. 3. The geometry of the folded and faulted Mesozoic cover allow for the identification of areas with prevailing Triassic rocks, mostly dolostones and evaporites, along with areas, in Jurassic and Cretaceous rocks, mostly made of limestones and marlstones. The latter gener- ally act as a barrier so that, in the Jurassic-Cretaceous sequence, several superposed aquifers can occur. 4. Within the Tertiary volcanic complex, infiltration is less effective with respect to the carbonate complex, the topographic watershed widely matches the hydrogeo- logical one, and the deepening of the piezometric contours follows the topography. This complex is affected by strike-slip faults along which deep circuits can develop giving rise to hypothermal waters that show the same temperature (24°C) regardless of the the seasonality. The residence time of waters in this circuit is very long as inferred by the close to zero tritium Fig. 11 Relationship between calcium concentration and DIC concentration. concentration. In the legend gw stands for groundwater The aquifers that develop in the different hydrogeo- logical units reflect the composition of the host rocks: The limited tritium values gave rise to meaningful sulphate (Triassic dolomites and gypsum aquifers), bicar- information on the timing of recharge and the nature of the bonate (Jurassic and Cretaceous limestone aquifer), and aquifer. Very low tritium levels (0.3±0.4 TU) in the silica (Oligo-Miocene volcanic aquifer). These ions can be volcanic aquifer of well 167S could indicate low used as markers for discriminating groundwater from permeability or deep circulation of groundwater, which different aquifers. Lithology and structural setting control can both result in large residence time. Samples coming the hydro-geochemical processes, which in turn determine from the Jurassic aquifer also show relatively low values (3.6±0.6, 3.1±0.6 TU). These values indicate only a limited degree of fracture development in the aquifer and slow groundwater circulation derived from rainwater. The three samples chosen for tritium analyses (43S, 143S and 167S) show that the decrease of tritium may be correlated with increasing TDS and temperature.

Conclusions

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

1. The basement that crops out to the west of the study area experiences poor infiltration; the surface water from this exposed sector of metamorphic rocks, due to its eastward tilt, when moving to the east, enters the carbonate aquifers. Possibly a basal detachment exists between the Mesozoic covers and the basement acting as a barrier. Fig. 12 Relationship between magnesium concentration and 2. Within the Mesozoic carbonate platform the ground- sulphate concentration. Lines indicate two different Mg/SO4 ratios. water flow path is controlled by wide, typically In the legend,gwstands for groundwaters

Hydrogeology Journal DOI 10.1007/s10040-008-0369-z 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

the enrichment in cations and anions; ultimately the Jurassic limestones represent the most productive chemical evolution leads to deterioration of water quality. (confined) aquifer in the Nurra district: groundwaters are The springs show hydrogeochemical characteristics mainly of calcium bicarbonate type and contain up to 20% that allow their clear separation into two main groups: of dissolved sulphate. The most saline groundwaters (1– bicarbonate-rich waters were found in springs emerging 1.5 g/l) are CaCl2 in character. Groundwaters from the from the limestones, whereas chloride type groundwaters Jurassic aquifer have the highest average PCO2 in the area, are dominantly found in the volcanic rocks despite the reflected in the high mean DIC. Green marls are the height above the sea level of the aquifers. Consideration substratum in which the most ion exchange takes place of the water compositions indicates that salinization in the (Fidelibus et al. 2004), directly and indirectly, affecting Nurra Basin derives from various sources, even though almost all waters. The Jurassic aquifer is the only reservoir most of the acquired salinity derives from sodium and of water with good natural quality in the district, but high chloride input. nitrate concentrations deriving from intensive agricultural The carbonate aquifer in contact with Triassic evaporite practices carried out in the area must be controlled. contains high sulphate groundwaters of poor quality. As Most of groundwaters from the Cretaceous aquifer for the other aquifers, this poor quality does not change have a TDS higher than 1 g/l. They show quite clear ion appreciably during the hydrologic year. TDS varies exchange, evidenced by the presence of NaHCO3, MgCl2, between 1.5 and 6.5 g/l. The solution of important and CaCl2 water types. Silica contents higher than the amounts of calcium sulphate in the gypsum levels reflects other aquifers of the plain are possibly linked with on the considerable excess of calcium and magnesium due glauconitic bearing materials and/or with a connection to dedolomitization. The presence of marls causes ion between this aquifer and the volcanic sequence; PCO2 is exchange; due to the masking effect of gypsum solution, low, with some isolated peaks. ion exchange between calcium and sodium is more The groundwater in the volcanic massif (Oligo-Mio- evident only in correspondence to high positive excess cene) has high hydraulic heads, thus excluding the of sodium (dilution). Typical water types are NaCl (having influence of sea water as the main salt source: the either excess or deficit of sodium), CaCl2 (salinization), chemical evolution of volcanic groundwaters develops and CaSO4. through water–rock interaction with welded and unwelded tuffs, addition of salts derived from past hydrothermal activity (Mark and Mauk 2001), and ion-exchange with zeolite, kaolinite, and bentonite, alteration products of the same activity. Dissolution of silicates and increase in alkalinity is facilitated by the acidity of groundwaters. Quaternary aquifers have groundwaters with TDS higher than 1.3 g/l all through the year; these groundwaters are NaCl and CaCl2 in character (with deficit and excess of both sodium and calcium). Sea water does not seem to play the principal role in the salinization of groundwaters of the area. Enlarging the spectrum of parameters to minor ions and isotopes will need to be part of future detailed studies of the complex Nurra basin. Finally this study represents a basic tool for sustainable water management in the framework of multidisciplinary research activities aiming to combat and/or mitigate desertification processes and to define appropriate policies for prospecting new ground- Fig. 13 Relationship between 2H and 18O. In the legend, gw water resources and assessment of their quality and stands for groundwater conservation.

Hydrogeology Journal DOI 10.1007/s10040-008-0369-z Acknowledgements The financial support from Ministry of Edu- di monitoraggio corpi idrici superficiali e sotterranei Nurra cation, University and Research (MIUR) for the development of the (Sardegna Nord-Occidentale): scala 1:50.000 [Hydrogeological research RIADE (Integrated Research for Applying new technolo- map of the Nurra area (NW Sardinia): surface water and gies and processes for combating Desertification: www.riade.net)is groundwater monitoring network: Scale 1:50.000]. Composita, acknowledged. Thanks are due to A. Carletti, N. Demurtas, R. Groningen, The Netherlands Pinna and A. Vigo for the sampling campaigns and sample Ghiglieri G, Barbieri, Vernier A (2008) Vulnerabilità all’inquina- preparation. The work of M. De Roma on chemical analysis is mento degli acquiferi della Nurra di Alghero (SS) per la particularly appreciated. We are also grateful to D. Pinna of the gestione integrata delle risorse idriche (Sardegna Nw) [Aquifer Silver & Barite S.p.A. for having allowed us to access the borehole vulnerability in the Nurra Region (Alghero) for integrated water data. The authors wish to thank G. M. Zuppi and B. Gambardella resources management: NW Sardinia]. IGEA Ing Geol Acquifer for the supporting discussion. Finally, the authors greatly appreciate 123:77–86 the critical review and the many helpful suggestions of J. Tellam. Hanshaw BB, Back W (1979) Major geochemical processes in the evolution of carbonates aquifer systems. J Hydrol 43:287–312 Herman JS, Back W (1984) Mass transfer simulation of diagenetic reactions in the groundwater mixing zone. 97th Annual Meeting References of the Geological Society of America Reno, Nevada, July 1984 Abstract book, p 16 Appelo CAJ, Geirnaert W (1983) Processes accompanying the Liu CW, Chen JF (1996) The simulation of geochemical reactions intrusion of salt water, Proc. of 8th SWIM, Bari, 1983. 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Hydrogeology Journal DOI 10.1007/s10040-008-0369-z