Hydrogeochemistry of the thermal waters from the Sciacca Geothermal Field (Sicily, southern Italy)

Journal of Hydrology 396 (2011) 292–301

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Hydrogeochemistry of the thermal waters from the Sciacca Geothermal Field (Sicily, southern Italy) Bruno Capaccioni a,⇑, Orlando Vaselli b,c, Franco Tassi b,c, Alba P. Santo b,c, A. Delgado Huertas d a

Department of Earth and Geological-Environmental Sciences, University of Bologna, Piazza Porta San Donato 1, 40126 Bologna, Italy Department of Earth Sciences, University of Florence, Via G. La Pira 4, 50121 Florence, Italy c CNR-IGG Institute of Geosciences and Earth Resources, Via G. La Pira 4, 50121 Florence, Italy d Estacion Experimental de Zaidin (CSIC), Prof. Albareda 1, 18008 Granada, Spain b

a r t i c l e

i n f o

Article history: Received 12 February 2010 Received in revised form 6 October 2010 Accepted 23 November 2010 This manuscript was handled by L. Charlet, Editor-in-Chief, with the assistance of Philippe Négrel, Associate Editor Keywords: Sciacca Geothermal Field Fluid geochemistry Sicily Channel Isotope geochemistry

s u m m a r y The Sciacca basin (southern Sicily, Italy), well known since the Roman times for the importance of its thermo-mineral waters, is characterized by a large occurrence of thermal fluid discharges whose main thermal aquifer is located within the Triassic carbonate rocks. This reservoir represents the deepest formation cropping out in the area. Shallower and cooler thermal aquifers are located within the Tertiary limestone and Quaternary sand and gravel deposits. In this work new geochemical and isotopic data of thermal waters and dissolved gases from springs and wells were investigated to assess the origin of thermal fluids and the structure of the hydrothermal system. The main thermal aquifer, at about 60 °C and located at 43 m a.s.l., is fed by an 18O-enriched 18 (d O  +4‰; dD  0‰) thermal marine end-member, affected by cooling and dilution (1:2) caused by mixing with inland-originated meteoric waters (d18O  8‰; dD  58‰). This meteoric recharge is likely related to the area of Ficuzza and San Genuardo Mts., located 12 and 23 km N of Sciacca, respectively, at an altitude comprised between 900 and 1180 m a.s.l. After this primary dilution, the deep thermal aquifer, emerging at the ‘‘Antiche Terme Selinuntine’’ discharging site, is not apparently affected by further dilution processes in the Sciacca area, where the sedimentary cover does not allow infiltration of local meteoric waters down to the Triassic reservoir. Conversely, the shallower thermal aquifers hosted within the Tertiary–Quaternary sedimentary horizons are locally supplied by direct infiltration of meteoric water occurring at variable altitudes, up to about 400 m a.s.l. (San Calogero Mt.). With the only exception of the Molinelli thermal spring, direct contribution from the deep thermal end-member to thermal springs fed by the shallower aquifers can be considered negligible. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction The Sciacca Geothermal Field (hereafter SGF) is the southernmost of the three geothermal fields, including Montevago and Castellammare-Alcamo, which are hosted in Western Sicily, southern Italy (Fig. 1). The SGF is well known since the Roman times due to the relevance of its thermal fluids that ubiquitously seep out in the region, discharging also in karst-caves and as ‘‘stufe’’. The latter, located at San Calogero Mt. (396 m a.s.l.), consist of emissions of hot air and vapors and represent a unique attraction for locals and tourists. Presently, the thermal waters are a resort for wellness, fitness and therapeutic purposes. Nevertheless, the recent increase of global demand for alternative energy resources has stimulated the development of new research projects focused on the evaluation of the potential for low enthalpy energy production from the hydrothermal system of the Sciacca basin. ⇑ Corresponding author. Tel.: +39 051 2094947. E-mail address: [email protected] (B. Capaccioni). 0022-1694/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2010.11.015

Since the end of the 19th century, several authors have proposed interpretative models for the origin of thermal waters of the SGF. Gemmellaro (1831, 1833) and Hoffmann (1833) hypothesized that the thermal anomaly characterizing this area was related to the magmatic activity of the Sicily Channel (centralwestern Mediterranean sea). Alaimo et al. (1978) and Alaimo and Tonani (1984), on the basis of the chemical and isotopic compositions of deep thermal waters, supported the idea that the hydrothermalism at Sciacca was derived from chemically modified and diluted seawater. More recently, Favara et al. (2001) assessed the direct connection between the thermal springs and the main structural-tectonics in Western Sicily. These authors suggested for the SGF waters a feeding reservoir composed by 50% carbonate water and 50% sea water, able to interact with NaCl-rich evaporitic layers. Eventually, on the basis of helium isotopic ratios, Caracausi et al. (2005) suggested an accumulation of magma below the continental crust of western Sicily. This magma batch may be intruding and out-gassing along extensional N–S trending deep fault systems.

B. Capaccioni et al. / Journal of Hydrology 396 (2011) 292–301

293

Fig. 1. Sketch map of Sicily Island. The inset shows the position of the Sciacca Geothermal Field close to the Sciacca village; the outcropping Mesozoic–Cenozoic carbonates are represented by the gray areas.

By using new geochemical and isotopic data from several thermal waters of SGF, the present work is aimed to: (i) provide a conceptual geochemical model for the SGF and (ii) describe the origin and the chemical–physical processes that determine the compositional features of studied thermal fluid discharges in terms of a possible exploitation for geothermal purposes.

accretionary prism is located between two areas characterized by extensional tectonic activity: (a) the back-arc basin Tyrrhenian Sea to the north and (b) the Sicily Channel to the south. Here below, we summarize the main geological features of the Sicily Channel, characterized by intense Miocene to Present volcanic activity, and those of the Sciacca Geothermal Field for which no inland outcropping volcanic centre is recorded.

2. Geological, volcanological and hydrogeological setting 2.1. The Sicily Channel The Apennine-Maghrebide thrust system (e.g. Montone et al., 2004; Lavecchia et al., 2007 and references therein) in Sicily is represented by the southward convex Sciacca–Gela–Catania front. The latter represents the emergence of the arched shape Sicilian Basal Thrust (SBT) that dates back to Plio-Pleistocene times and with some evidences of activity on the Holocene (Lavecchia et al., 2007 and references therein). This part of the Apennine-Maghrebide

The Sicily Channel, related to the African–Europe subducting system, is a wide NW–SE elongated tectonic structure, belonging to the northern margin of the African foreland. This rift system is characterized by extensional NW–SE trending faults and by N–S strike-slip faults (e.g. Calanchi et al., 1989). Geophysical data suggest that tectonic deformation along the rift is still active (Cello,

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1987; Bonaccorso and Mattia, 2000). Three main rift zones are recognized in the Sicily Channel: Malta, Pantelleria and Linosa grabens (Boccaletti et al., 1984). The processes responsible of rifting are not fully understood, although this geodynamic system is crucial for the comprehension of the magmatism of the northern portion of the African plate. This area, floored by a variably thinned and faulted continental crust (e.g. Finetti, 1984; Boccaletti et al., 1987; Scarascia et al., 2000), is indeed characterized by intense magmatic activity. The Linosa (Rossi, 1996; Bindi et al., 2002) and Pantelleria island (Civetta et al., 1998), as well as several seamounts (e.g. Cimotoe, Tetide, Anfitrite, Graham, Senzanome, Foerstner) mainly located along extensional NW–SE trending faults (Calanchi et al., 1989; Rossi et al., 1990), show a large variety of volcanic rocks characterized by different magmatic affinities with a relatively wide compositional spectrum. They were produced by both sub-aerial and submarine magmatism (Miocene to Present). The Island of Linosa consists of mildly alkaline basalts and hawaiites, whereas that of Pantelleria also includes peralkaline riolites and trachites. The sub-aerial volcanic centres are relatively well known also due to the fact they had eruptive events in historical times. Conversely, volcanological investigations of the submarine activity in the Sicily Channel are rather poor. Part of them was detected by seismic profiles and magnetic survey, being covered by undisturbed Pliocene-Quaternary sediments (Allan and Morelli, 1971). For instance, the Senzanome Bank, dated 9.5 Ma (Beccaluva et al., 1981), is a pinnacle rising from a structural high, probably made of sedimentary rocks. Tetide, Anfitrite, Galatea e Cimotoe are NW–SE aligned volcanic centres on the eastern margin of the Avventura Bank, a large erosional sedimentary plateau located off the south-western Sicilian coast (Peccerillo, 2005 and references therein). The Graham Bank (Lyell, 1846), located about 50 km S of the Sicilian coast, shows a N–S elongated shape and rises from a sedimentary basement sited about 350 m b.s.l. It represents the feeding area of the Ferdinandea Island (1831) rapidly dismantled by marine erosion. The Bannock seamount is located between Malta and the Pantelleria graben. In spite of its conical shape, there are doubts about the volcanic nature of this seamount, since no magnetic anomalies were detected (Peccerillo, 2005). The volcanic nature is dubious also for three seamounts (Linosa 1, 2 and 3), located N of the Linosa Island. The most recent eruptive centre in the Sicily Channel is the Foerstner volcano, sited about 5 km NW of Pantelleria, probably erupting in October 1881 (Washington, 1909). The few petrological and geochemical data on magmatic rocks from the Sicily Channel, obtained on dredged samples from a few submarine cones (e.g. Colantoni et al., 1975; Carapezza et al., 1979), indicated that rock compositions range from tholeiitic basalt to alkali basalt, hawaiite and basanite (Calanchi et al., 1989). Tholeiites were found at the Tetide volcano and transitional basalts at Anfitrite. Alkali basalts to basanites were collected at the Senzanome Bank and hawaiites were dredged from the Graham Bank. Some petrological, geochemical and isotopic data, obtained on rock samples dredged from the Graham Bank, were recently published by Rotolo et al. (2006). However, it has to be pointed out that the available trace element abundances and isotopic ratios are not sufficient to produce a general picture of the magmatic activity for the Sicily Channel.

2.2. The Sciacca Geothermal Field The SGF is located in the Sciacca basin (10 km2) that roughly corresponds to that of the Sciacca town (Aureli, 1996). The occurrences of thick limestone sequences, abundant rainwater, and intense tectonics have given rise to significant karstification phenomena producing several large caves used in the past also for therapeutics purposes.

According to Mascle (1970), Catalano et al. (1978), D’Angelo (1980) and Ghisetti and Vezzani (1984), starting from the Upper Miocene, the Sciacca area was affected by intense tectonic activity related to the beginning of the collision process between African and European plates. Sedimentary deposition in the area started in the Upper Triassic in a subsiding carbonate platform likely active through 65 Ma. The basement underlying the Triassic carbonate rocks (limestone and dolostone formations) is unknown, since a 2800 m deep AGIP well did not reach their lower limit (Aureli, 1996). These carbonatic formations, hosting the main thermal aquifer, crop out at San Calogero Mt. and display a significant secondary permeability. Several sedimentary formations are superimposed on the Triassic carbonate rocks (Aureli, 1996): yellow–reddish limestone and white marlstone (Jurassic), shalelimestone and whitish calcareous marlstone (Cretaceous), whitish calcareous rudite and lutite (Eocene), limestone and calcareous sandstone (Oligocene), marl-sandstone, marlstone and shales (Miocene). During the Upper Miocene, regional tectonics, induced by the Africa–Europe collision, resulted into the compression, breakage and overlapping of the carbonate platform deposits and into the uplifting of a structural relief, bordered by inverse faults. During the Pliocene, a rapid increase of the sea level gave rise to the deposition of the Globigerinae marls formation (‘‘Trubi’’). During the Upper Pliocene and Lower Pleistocene, the area was affected by a vertical uplift and emersion of the San Calogero horst, followed by sedimentation of sandy–muddy shales in the adjoining subsiding areas (‘‘Argille azzurre’’ formation), presently observed W, S and E of Mt. San Calogero (Aureli, 1996). More recently, a still active extensional tectonics, driven by normal faulting, caused a general uplift of the Sciacca area. As described above, this extensional tectonic regime was accompanied by an intense volcanic activity, mostly located about 50 km offshore of the Sciacca shoreline, where in 1831 a submarine eruption took place, producing the ephemeral Ferdinandea Island. Sixteen different thermal springs and one hundred wells were documented by several authors (e.g. Alaimo and Tonani, 1984; Aureli, 1996). Some of them dried out or were modified (e.g. ‘‘Antiche Terme Selinuntine’’) in order to increase their discharge rate. Measured temperatures of thermal waters are generally in the range of 51–56 °C (Sommaruga, unpublished data; Aureli, 1996). Temperatures close to the boiling point were sometimes observed. In the past, several authors proposed different geological maps and interpretative models describing the water circulation pattern (e.g. Schoeller, 1948; Alaimo et al., 1978; Favara et al., 2001), but the origin and the structure of this hydrothermal system is still not completely understood. Gemmellaro (1831, 1833) and Hoffmann (1833) initially hypothesized a relationship between the thermalism of this area and the magmatic activity in the Sicily Channel. More than a century later, Sommaruga (unpublished data, in Aureli, 1996) described the Sciacca basin as a ‘‘closed cell’’ system. The basic idea was to have a basin characterized by impermeable rocks on its four sides and a convective circulation supposedly due to heating of the basement by local magmatic intrusions through paleo-karstic rocks. Alaimo et al. (1978), Alaimo and Tonani (1984) and Aureli (1996) supported a different geochemical model that was implying the presence of a deep marine-related reservoir within Triassic carbonate rocks, representing the deepest formation outcropping in this area, diluted by meteoric water at shallower levels. This conceptual model was partly revised by Favara et al. (2001) according to new geochemical data that also extended to the other geothermal field characterizing Western Sicily, e.g. Montevago and Castellammare-Alcamo. A schematic draft of the sedimentary sequence and related hydraulic more conductive layers (aquifers) of the Sciacca area proposed by Aureli (1996) is shown in Fig. 2. Other than a superficial alluvial aquifer (not reported in Fig. 2), three superimposed aquifers

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chromatograph equipped with a Flame Ionization Detector (FID) (Tassi et al., 2004), respectively. The complete composition of dissolved gas compounds was calculated on the basis of the Henry’s law constants, regulating the liquid–gas equilibrium for each volatile compound (Vaselli et al., 2006; Tassi et al., 2008). Dissolved CO2 concentrations (Table 1) were also calculated (CO2c) from the total carbonate concentrations, measured by automatic titration (Titroprocessor Methrom 636) with 0.5 M HCl, in water samples collected in 100 mL glass bottles containing 10 mL of a 1 M Na2CO3 solution, according to the following reaction:

H2 O þ CO2 þ CO2 $ 2HCO3 3 18

Fig. 2. Schematic stratigraphic section of the sedimentary sequence of the Sciacca basin with the main hydrogeological units.

can be recognized: (1) Triassic dolomitic limestone and dolostone and Jurassic limestone formations (a partially unconfined basal aquifer driven by secondary permeability, locally enhanced by karstification); (2) Eocene–Oligocene limestones (driven by secondary permeability); (3) Quaternary sands and gravels (characterized by primary permeability). As reported in Fig. 2, these aquifers are generally isolated each other by thick marl and shale formations, which act as aquicludes. 3. Sampling and analytical methods Nine thermal waters from hot springs and wells, two cold water discharges and one groundwater sample were collected in the area of the Sciacca town and analyzed for major and minor components, and d18OH2O, dDH2O and d13CDIC‰ (Table 1). Temperature and pH values were measured in situ. Chemical analyses were carried out by using Molecular Spectrophotometry (MS, Hach DR2010) for NHþ 4 and Acidimetric Titration (with 0.01 N HCl) for alkalinity, Ion Chromatography (IC, Dionex ICS 90) for F, Cl, Br, NO 3 and SO2 4 , Atomic Absorption Spectrophotometry (AAS, Perkin–Elmer AAnalyst 100) for Na+, K+, Ca2+, Mg2+ and Li+ and Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES, Thermo iCAP 6000) for H3BO3. The analytical error for IC, AAS, MS, ICP-AES and was 65%. Analytical data are reported in Table 1. As no free-bubbling gases are present in the thermal water discharges investigated in the present study, the chemical and isotopic analysis of the dissolved gas phase was carried out. The dissolved gases were collected in pre-evacuated 250 mL glass flasks tapped with Teflon valves after having filled the flask by 2/ 3 of its volume with water (Tassi et al., 2008). The composition of the main inorganic dissolved gas compounds (CO2, N2, O2 and Ar) and CH4 stored in the headspace of the sampling flasks was determined by a Shimadzu 15A gas chromatograph equipped with a Thermal Conductivity Detector (TCD) and a Shimadzu 14A gas

16

ð1Þ 18

The O/ O and D/H isotopic ratios (expressed as d O and dD‰ V-SMOW) in water samples were determined by using a Finnigan Delta Plus XL mass spectrometer according to standard protocols. Oxygen isotopes were analyzed using the CO2–H2O equilibration method proposed by Epstein and Mayeda (1953). Hydrogen isotopic ratios were measured on H2 after the reaction of 10 mL of water with metallic zinc at 500 °C (Coleman et al., 1982). The experimental error was ±0.1 and ±1.1‰ for d18O and dD‰ values, respectively, using EEZ-3 and EEZ-4 as internal standards that were previously calibrated versus V-SMOW and SLAP reference standards. Water samples were analyzed for d13CDIC‰ with a Finningan Delta Plus XL mass spectrometer after the reaction of 3 mL of water with 2 mL of anhydrous phosphoric acid in vacuum (Salata et al., 2000). The recovered CO2 was analyzed after a two-step extraction and purification procedures of the gas mixtures by using liquid N2 and a solid–liquid mixture of liquid N2 and trichloroethylene (e.g., Evans et al., 1998; Vaselli et al., 2006). The analytical error for d13CDIC‰ was ±0.05. The 13C/12C ratios of CO2 were determined on the basis of those measured in the separated gas phase stored in the headspace of the dissolved gas flasks (d13C–CO2-strip). The isotopic analyses were performed with a Finningan Delta S mass spectrometer, after a two-step extraction and purification procedures as described for the determination of the d13Cdic values. Internal (Carrara and San Vincenzo marbles) and international (NBS18 and NBS19) standards were used for estimating the external precision. The analytical error and the reproducibility are ±0.05% and ±0.1%, respectively. The d13C–CO2 values were corrected for the fractionation between dissolved and gas CO2, i.e. +0.83‰ (Luchetti, 2007; Tassi et al., 2008). The 3He/4He and He/Ne ratios were determined by mass spectrometry according to Poreda and Farley (1992). The gas samples were purified in a high vacuum line consisting of stainless steel and Corning-1724 glass to minimize He diffusion. Water vapor and CO2 were cryogenically trapped at 90 and 195 °C, respectively, whereas N2 and O2 were removed by the reaction with a Zr–Al alloy (SAES-ST 707). Argon and Ne were adsorbed on activated charcoal at 196 and at 233 °C, respectively. SAESST-101. Helium isotope ratios and concentrations were analyzed on a VG 5400 Rare Gas Mass Spectrometer fitted with a Faraday cup (resolution of 200) and a Johnston electron multiplier (resolution of 600) for sequential analyses of the 4He (F-cup) and 3He (multiplier) beams. The analytical error for the determination of 3 He/4He (hereafter expressed as R/Ra, where R is the measured 3 He/4He ratio and Ra is that of the air: 1.39  106 (Mamyrin and Tolstikhin, 1984) was ±0.3%. 4. Results The physical and chemical parameters, the oxygen and hydrogen isotopic composition in H2O and that of carbon in exsolved CO2 and Dissolved Inorganic Carbon (DIC) are reported in Table 1, whilst the chemical and isotopic (carbon and helium) data of two dissolved gases from 1 and 3 wells sampled at Hotel Terme are listed in Table 2.

296

Table 1 Geographical coordinates, chemical and isotopic features of the water samples considered in the present study. Location

Coordinates

Date of sampling

T (°C)

pH

HCO3

Cl

SO4

Na

K

Ca

Mg

NH4

H3BO3

F

SC1 SC1 SC7

Terme Selinuntine spring 1 Terme Selinuntine spring 1 Terme Selinuntine spring 2

37°300 28,990 0 N

13°050 33,190 0 E

10/20/05 07/11/06 10/20/05

54 55 50

6.35 6.38 6.03

506 477 497

12,600 11,422 14,500

1050 1554 62

6620 6350 6860

385 359 355

1181 1450 1144

404 350 316

7.8 5.7 7.9

140 132 139

1.2 4.6 3.1

SC3 SC3

Fontana Calda spring Fontana Calda spring

37°310 10,220 0 N

13°080 12,410 0 E

10/21/05 07/11/06

33 32

6.65 6.58

348 498

110 126

130 120

102 92

16 17

107 120

32 39

0.19 0.13

0.17 0.75

0.6 0.4

SC4 SC4 SC4 SC5 SC5 SC5

Hotel Hotel Hotel Hotel Hotel Hotel

37°300 17,340 0 N

13°050 24,570 0 E

10/22/05 07/11/06 06/10/09 10/22/05 07/11/06 06/10/09

55 55

5.32 5.95

55 55

5.50 6.01

543 482 500 537 339 506

12,750 12,745 13,426 12,125 12,618 12,711

550 1969 519 550 1987 524

6700 7225 6649 6500 7250 6638

372 367 345 388 367 350

627 1425 1297 1126 1420 1301

373 370 427 357 420 425

14 5.8 nd 15 5.7 nd

147 151 nd 147 125 nd

3.1 4.4 3.6 3.0 4.0 4.2

SC6 SC2

Vallone river Vallone river

21 22

7.08 7.11

464 395

245 135

300 280

183 148

12 13

136 120

64 52

0.39 4.4

1.1 1.1

2.0 1.0

SC8

Fontana di Campo

197sc 198sc 201sc

Terme Terme Terme Terme Terme Terme

well well well well well well

1 1 1 3 3 3

37°300 15,100 0 N

13°060 00,530 0 E

10/22/05 10/20/05

37°280 42,940 0 N

13°160 30,330 0 E

10/22/05

a

Molinelli spring Molinelli springa Molinelli springa

0.14

1.0

nd nd nd

nd nd nd

nd nd nd TDS g/L

13°050 33,190 0 E

10/20/05 07/11/06 10/20/05

10 0.51 12.5

155 93 119

1.9 1.6 1.9

4.34 4.85 5.23

nd nd nd

23.4 24.7 24.5

0.9 1.0 1.5

nd 0.2 nd

23.1 22.2 24.0

37°310 10,220 0 N

13°080 12,410 0 E

10/21/05 07/11/06

29 26

0.5 0.39

0.02 0.02

1.89 0.72

nd nd

29.9 32.6

2.8 4.7

nd 2.4

0.9 1.0

37°300 17,340 0 N

13°050 24,570 0 E

10/22/05

2.1

125

1.8

12.52

6.09

22.6

1.3

nd

22.1

07/11/06 06/10/09 10/22/05 07/11/06 06/10/09

0.16 bdl 0.25 0.32 bdl

103 103 137 98 101

1.6 1.7 1.9 1.6 1.7

10.78 nd 13.02 13.41 nd

6.99 nd nd 9.70 nd

24.6 nd 22.3 26.1 nd

1.7 nd 1.2 1.7 nd

0.5 nd nd 0.6 nd

24.9 23.3 21.9 24.6 22.6

10/22/05

3.4

0.94

0.06

nd

nd

nd

nd

nd

1.4

10/20/05

3.2

2.2

0.05

1.32

nd

nd

nd

nd

1.2

Hotel Terme well 1

SC4 SC4 SC5 SC5 SC5

Hotel Hotel Hotel Hotel Hotel

1 1 3 3 3

Fontana di Campo

0.26

d13C (V-PDB) DIC

SC4

Molinelli springa Molinelli springa Molinelli springa

26 255 295 312

d18O (V-SMOW)

Fontana Calda spring Fontana Calda spring

SC8

149 800 816 838

dD (V-SMOW)

SC3 SC3

197sc 198sc 201sc

7.2 204 232 264

d13C-CO2

37°300 28,990 0 N

Vallone river

102 3839 3986 4526

CO2c

Terme Selinuntine spring 1 Terme Selinuntine spring 1 Terme Selinuntine spring 2

Vallone river

65 317 288 331

Li

SC1 SC1 SC7

SC6

130 8242 8579 9536

Br

Coordinates

SC2

491 415 464 457

NO3

Location

well well well well well

7.15 7.30 6.20 6.10

Date of sampling

ID

Terme Terme Terme Terme Terme

18 32 34 33

37°300 15,100 0 N

0

00

37°28 42,94 N

13°060 00,5300 E

0

00

13°16 30,33 E

10/22/05

Concentration as mg/L; CO2c expressed as mmol/L; d notation as ‰; nd: not detected. a Compositions are taken from Aureli (1996).

5.7

0.62

0.05

3.33

nd

32.1

3.4

nd

1.0

nd nd nd

nd nd nd

nd nd nd

nd nd nd

nd nd nd

nd 22.9 nd

2.5 1.8 1.6

nd nd nd

14.1 14.7 16.3

B. Capaccioni et al. / Journal of Hydrology 396 (2011) 292–301

ID

297

B. Capaccioni et al. / Journal of Hydrology 396 (2011) 292–301 Table 2 Chemical and isotopic composition of the dissolved gases from the Hotel Terme wells.

Hotel Terme well 3 Hotel Terme well 1

10/22/2005 10/22/2005

CO2

N2

CH4

Ar

O2

Ne

He

d13C-CO2

d13C-DIC

He/Ne

R/Ra

10.34 11.42

8.12 0.93

0.004 0.015

0.197 0.023

1081.00 130.00

0.00011 0.00001

0.00120 0.00062

9.7 nd

0.64 nd

287 256

2.68 2.65

Concentration as mmol/L; d notation as ‰.

SO42- + Cl50

Selinuntine thermal springs

Na-Cl

Na-HCO3

Fontana Calda Hotel Terme wells Cold surficial waters Seawater

Ca2+ + Mg2+

Na+ + K+

Molinelli thermal spring

25

Ca-SO4

0 0

Ca-HCO3 -

HCO3 + CO32-

50

Fig. 3. Square diagram (Langelier and Ludwig, 1942) for the thermal and cold spring and well discharges collected in the Sciacca basin. The gray area represents the composition of the thermal and cold waters for the same area investigated by Alaimo and Tonani (1984).

Thermal water discharges have outlet temperatures in the range of 32 (Molinelli spring) and 55 °C (Terme Selinuntine and well 1 at Hotel Terme) and pH values close to the neutrality or slightly acidic. The lowest pH values were measured at wells 1 (5.32) and 3 (5.50) (Hotel Terme) in October 2005, whereas in November 2006 they had a pH of 5.95 and 6.01, respectively. No pH measurements were carried out in October 2009. In October 2005 the Vallone river and the cold spring of Fontana di Campo had temperatures around 20 °C and pH values close to 7. In the square classification diagram of Langelier and Ludwig (1942; Fig. 3), where the field of the thermal and cold waters investigated by Alaimo and Tonani (1984) is also reported, the Hotel Terme wells and Selinuntine springs have a Na–Cl composition and, according to their TDS values (Total Dissolved Solids) that range between 22 and 25 g/L (Table 1), they recall the chemical features of diluted seawater (SW). A similar composition is observed for the Molinelli thermal spring, although the lower TDS values (14–16 g/L) and temperatures suggest a more important dilution by cold and fresh aquifers. Conversely, Fontana Calda spring shows a Ca–HCO3(SO4) composition with relatively low TDS values (1 g/ L). The cold waters, located in proximity of the seashore have a Ca– HCO3(Cl) (Fontana di Campo) and Ca–SO4(Cl) (Vallone river) composition with TDS values P1 g/L and resemble most cold waters in the area (Alaimo and Tonani, 1984; Favara et al., 2001). As far as the minor and trace solutes, among the thermal waters NHþ 4 is between 0.19 (Fontana Calda) and 15 (well 3 at Hotel Terme) mg/L, while its contents is 0.26 and 4.4 mg/L for Fontana di Campo and Vallone river, respectively. Bromide is up to 155 mg/L (Terme Selinuntine spring 1) and the Cl/Br weight ratios are generally lower (<220) than those of seawater (296). Higher Cl/Br ratios are referred to Vallone river (260) and Fontana Calda November 2006 (323). Lithium concentrations of

the Hotel Terme wells and Selinuntine thermal springs are 1.6– 1.9 mg/L, one order of magnitude higher than that of seawater (0.18 mg/L). Lithium at Fontana Calda is 0.02 mg/L, lower than those measured for the cold waters (0.05 mg/L). A similar behavior is shown by H3BO3 whose values are up to 151 mg/L (well 1 at Hotel Terme), whereas Fontana Calda and the two cold waters have H3BO3 < 1.1 mg/L. It is noteworthy to point out that Terme Selinuntine and Hotel Terme wells show higher H3BO3 concentrations with respect to that of the seawater (26.3 mg/L). A similar consideration can be done for F, whose abundances are almost four times with that of seawater (4.6 and 4.4 mg/L for Terme Selinuntine and Hotel Terme, respectively). Interestingly, relatively high NO 3 contents are found at Fontana Calda (29 mg/L) and Terme Selinuntine (up to 12.5 mg/L), i.e. higher that those found in superficial waters (<6 mg/L). Geothermometric considerations based on the ionic solutes were not applied due to the presence of a large fraction of marine water. As a consequence, an estimation of the equilibrium temperatures can only be provided by SiO2 geothermometers. According to the SiO2 contents by Favara et al. (2001) for Terme Selinuntine springs (40 mg/L), and using the equations of Arnorsson et al. (1983) and Fournier (1991) for chalcedony solubility, the calculated equilibrium temperatures range between 61 and 63 °C. The d18O and dD‰ values (Table 1) are between 4.7 (Fontana Calda) and 0.9 (Terme Selinuntine spring 1) ‰ and 32.6 (Fontana Calda) and 22.5 (Hotel Terme well 3), respectively. The CO2c values in the Hotel Terme water samples (well 3) are up to 13.41 mmol/L, whereas the d13C–CO2 value is 9.7‰ V-PDB. Relatively high CO2c values (between 10.78 and 12.52 mmol/L) are shown by well 1 (Hotel Terme) that has a d13C–CO2 value of 7.0‰ V-PDB. Terme Selinuntine has CO2c values 65.23 mmol/L, whereas no d13C–CO2 data are available. The d13C–DIC values measured at Terme Selinuntine, Fontana Calda and Hotel Terme (wells 1 and 3) cluster around 0 or are slightly negative (Table 1). The whole composition of dissolved gases from wells 1 and 3 of the Hotel Terme are characterized by CO2 concentrations of 10.34 and 11.42 mmol/L (Table 2), respectively, which are consistent with the calculated values (CO2c; Table 1). Nitrogen (up to 8.12 mmol/L) and O2 (up to 1.08 mmol/L) are clearly atmospheric in origin as also supported by the N2/Ar ratios (41.2 and 40.3 at well 1 and well 3, respectively) that approach that of Air Saturated Water (ASW = 38.4 at 20 °C). It is to mention that well 1 is more air-contaminated and less representative of the deep source. Methane is present with concentrations up to 0.0015 (well 3) mmol/L, whereas no hydrocarbons and CO and H2 are detected. Neon is up to 0.00011 mmol/L (well 1) and the He/Ne ratios are 11.4 (well 1) and 48.3 (well 2), the latter indicating a deep-seated component carried to the surface with the thermal waters. This is well assessed by the R/Ra values that are up to 2.68 (Table 2) and similar to those found by Caracausi et al. (2005). 5. Discussion 5.1. Origin of dissolved ionic species The Hotel Terme, Selinuntine and Molinelli thermal waters are likely representing the expression the SGF reservoir at the surface.

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14000

500

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200

0 0

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Cl- mg/l Fig. 4. Cl vs. Na+ (in mg/L) binary diagram for the thermal and cold spring and well discharges collected in the Sciacca basin. Solid lines represent the Na+/Cl ratios of halite and seawater, respectively. Symbols as in Fig. 3.

These waters indicate they are related to a mixing of a seawater end-member and shallow cold and fresh aquifers fed by meteoric waters (Favara et al., 2001). Differently, the chemical composition of Fontana Calda spring better resembles that of steam-heated superficial waters without any significant contribution from the deep Na–Cl waters. On the other hand, the cold freshwaters from Sciacca (Fig. 3) are the typical composition of word wide superficial waters (Drever, 1997) at which a contribution of marine salts and/ or marine sprays is added. The water sample distribution in the Na+ vs. Cl (in mg/L) diagram of Fig. 4 clearly reflects a prevailing binary mixing, with variable dilution of a seawater end-member and negligible Cl contribution from other sources. As no 1:1 stoichiometric ratios for Na/Cl and Ca/SO4 are observed, significant effects of leaching of NaCl- and CaSO4- bearing evaporitic layers are to be excluded. Prolonged water-rock interaction result into a significant Li+ enrichment, although no Na+ enrichments are observed likely because they are masked by the relatively high concentrations due to the seawater contribution. Calculations based on the concentrations of Cl, characterized by a conservative behavior in hydrothermal environment, indicate that the Hotel Terme and Selinuntine thermal waters are produced by dilution of a seawater term with a ratio of 1:2. This ratio tends to decrease at 1:3 for the Molinelli thermal waters. The relatively high Br, H3BO3 and NHþ 4 contents (Table 1) at Hotel Terme wells and Selinuntine thermal springs cannot be 250

100 50 0 0

1000

2000

3000

[HCO3-] mg/L Fig. 6. [HCO3] vs. [Ca2+] (in mg/L) binary diagram for the thermal and cold spring and well discharges collected in the Sciacca basin. Solid line represents the [HCO 3 ]/ [Ca2+] ratio expected for dolomite dissolution according to the heterogeneous equilibrium: CaMg(CO3)2 + 2CO2 + 2H2O = Ca2+ + Mg2+ + 4HCO 3 . Solid curves represent equilibrium conditions at different PCO2. Symbols as in Fig. 3.

10

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"shifted" seawater by water-rock interactions

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condensed and meteoric waters mixing line

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isotopic compositions of meteoric waters

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δ18O -5

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Fig. 7. d18O vs. dD (V-SMOW) binary diagram for the thermal and cold spring and well discharges collected in the Sciacca basin. The solid line represents the ‘‘Mediterranean Meteoric Water Line’’ (MMWL, Gat and Carmi, 1970). Star is the inferred isotopic composition of the modified seawater. Closed (A) and Open (B) squares are the inferred isotopic compositions of the meteoric waters. Dashed lines are mixing and evaporation patterns, respectively. Symbols as in Fig. 3.

mg/L

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H3BO3

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100

50 Seawater

0 0

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Cl-

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mg/L

Fig. 5. Cl vs. H3BO3 (in mg/L) binary diagram for the thermal and cold spring and well discharges collected in the Sciacca basin. The solid line represents the Cl/ H3BO3 ratio of seawater. Symbols as in Fig. 3.

explained by a simple mixing between meteoric and seawater terms, as shown in the H3BO3 vs. Cl (mg/L) diagram of Fig. 5. They are probably related to the leaching of B-rich sedimentary rocks, although a deep contribution of vapor-bearing fluids for H3BO3 and NHþ 4 cannot be ruled out. The Ca2+/Mg2+ ratios in the thermal waters are >3 with the exception of Hotel Terme well 1 of October 2005 (1.6) and largely higher than that in seawater (0.3), likely due to dolomitization of Ca-carbonates. Calculated Ca2+ and HCO 3 ion activities (Pitzer, 1973) are plotted in Fig. 6. Solid curves represent equilibrium conditions in the MgCa(CO3)2/H2O/CO2 system at different PCO2 values and with a 1:1 stoichiometric Ca2+/Mg2+ ratio, whereas the solid line represents the expected Ca2+/HCO 3 values for dolomite dissolution. Calcium in excess with respect to water/rock equilibrium for thermal waters from Hotel Terme wells and Selinuntine thermal

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15000 10000 5000 0 -60

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Fig. 8. (a) dD vs. Cl and (b) d18O vs. Cl binary diagrams for the thermal and cold spring and well discharges collected in the Sciacca basin. Star is the inferred isotopic composition of the modified seawater. Closed (A) and Open (B) squares are the inferred isotopic compositions of the meteoric waters. Symbols as in Fig. 3.

springs can be interpreted as the results of CO2 loss, likely because of water uprising and degassing, followed by Ca-carbonates separation. Assuming the establishment of a water/rock equilibrium, the extrapolated equilibrium PCO2 provides a maximum value at 12 bars (Fig. 6). 5.2. Isotopic compositions of waters and dissolved gas species As shown in the dD vs. d18O diagram (Fig. 7), the Na–Cl waters of the Hotel Terme wells and Selinuntine thermal springs plot out of the Mediterranean Meteoric Water Line (MMWL; Gat and Carmi, 1970) and lie far from the theoretical alignment defining the mixing trend between seawater and local meteoric water. It can be envisaged that the isotopic composition of the Hotel Terme and Selinuntine thermal waters is related to a mixing between an isotopically light meteoric end-member ‘‘A’’ (Fig. 8a, b; d18O  8‰ V-SMOW; dD  58‰ V-SMOW) and an isotopically modified seawater term (d18O  +4‰ V-SMOW; dD  0‰ V-SMOW). The occurrence of isotopically modified connate or fossil thermal waters of marine origin has been documented in different geothermal systems, e.g. Biga Peninsula (northwestern Anatolia, Mützemberg et al., 1992) and El Ma’in in western Jordan (Capaccioni et al., 2003). The distribution of water samples in the Cl vs. dD and Cl vs. d18O binary diagrams (Fig. 8a and b) supports this hypothesis and suggests a negligible contribution of vapor separation processes. The Na–Cl waters from the Molinelli thermal spring

significantly diverge from the mixing lines depicted for the Hotel Terme wells and Selinuntine springs in Fig. 8a and b. This may be explained by admitting a contribution from different, isotopically heavier waters (‘‘B’’ term in Fig. 8a and b; d18O  3.5‰ V-SMOW; dD  23‰ V-SMOW), possibly related to direct infiltration of local rainwater. The isotopic composition of the meteoric end-member A is similar to that of the Ficuzza and San Genuardo springs reported by Aureli (1996), who indicated the homonymous reliefs, respectively located at 901 and 1180 m a.s.l., as the main meteoric recharge area. Conversely, the meteoric end-member B is isotopically consistent with rainwater in the urban area of Sciacca, close to the sea level. The isotopic composition of the Ca–HCO3(SO4) water of the Fontana Calda spring is very similar to local meteoric waters. It seems to be related to different degrees of mixing between condensed and meteoric waters recharged in the area of San Calogero Mt. (d18O = 5.5‰ V-SMOW; dD = 34‰ V-SMOW) at 369 m a.s.l. (Aureli, 1996). The composition of dissolved gases from thermal waters collected at the Hotel Terme (Hotel Terme well 1 and 3) are dominated by CO2, N2 and O2, with minor amounts of Ar, Ne, He and CH4 (Table 2). The absence of temperature and redox-dependent H2 and CO appears noteworthy. The N2/Ar ratios are close to that of the air saturated water (ASW), while O2 appears to be partially removed by oxidative reactions. The relatively high amounts of He are decoupled with respect to the helium isotopic ratios, this

Fig. 9. Simplified geological section through St. Calogero Mt. and conceptual model of fluid circulation in the Sciacca basin. The location of the investigated thermal wells and springs are also reported.

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gas being normally enriched in continental crust where radiogenic helium (4He) is produced. According to Caracausi et al. (2005), these high R/Ra values can be explained with the fact that helium is advectively transported to the SGF via deep fault systems. As a consequence, the presence of mantle melts can be invoked and likely related to volcanic activity that has recently affected the Ferdinandea Island, which is related to the rift of Pantelleria. It is probable that the whole western Sicily is suffering the presence of tectonic discontinuities that act as preferential pathways for the uprising of deep-seated fluids with a mantle-He contribution up to 50% (Caracausi et al., 2005). Thus, the helium content can be reconciled with the respective R/Ra values by considering an indirect enrichment of this gas by removal of more soluble gases (e.g. CO2) before reaching the surface. The d13C–CO2 values of Hotel Terme are in agreement with those of mantle CO2 (3‰ to 6‰, e.g. Hoefs, 1997; Ohmoto and Goldhaber, 1997), suggesting for this gas species a relatively deep source. However, by considering the value of 9.70 (‰ V-PDB), that is consistent with that of d13C – Dissolved Inorganic Carbon (DIC) (0.64‰ V-PDB), assuming a carbon isotopic fractionation factor between solid calcium carbonate and gaseous CO2 is 10.2‰ at 20 °C (Emrich et al., 1970), other contributions cannot be ruled out.

60–65 °C due to the meteoric waters infiltrating at an altitude of >900 m a.s.l. Thus, the main thermal aquifer consists of these two end-members and naturally emerges, slightly cooled, at Terme Selinuntine (at about 55 °C) and is intercepted by wells at Hotel Terme in the Sciacca town. The Molinelli spring has a similar origin, although affected by further mixing processes with local meteoric water, infiltrating at an altitude close to the sea level. Finally the Fontana Calda spring appears to be fed only by meteoric waters, heated by conductive heating and vapor condensation. The existence of a hotter and deeper hydrothermal system is clearly speculative, but it appears to be also supported by the presence of significant contents of possible deep-seated, vapor-bearing compounds, such as H3BO3 and NHþ 4 , and by the relatively high 3 He/4He values which can be related to an advective migration of the deep fluids to the SGF via deep fault systems. Moreover the measured d13C–CO2 values of Hotel Terme are in agreement with those of mantle CO2 also suggesting for this gas species a relatively deep contribution. The possible existence of a deep, medium to high enthalpy geothermal system could stimulate further investigations for the exploitation of the geothermal power as an alternative energy resource in the Sciacca area. Acknowledgements

6. The conceptual model of the SGF system According to the previously described interpretation of the hydrogeological and geochemical, a conceptual model of the SGF system can be proposed (Fig. 9). The SGF hosts a thermal and saline aquifer located in Triassic carbonates at an altitude of 43 m a.s.l. (Aureli, 1996). The measured temperatures of the wells and springs are not exceeding 55 °C. This deep thermal aquifer, directly feeding the Hotel Terme wells and Terme Selinuntine springs, appears to be related to 18O-enriched (d18O  4‰) seawater. This indicates a possible water/rock isotopic re-equilibration of a stagnant, rockdominated system (Giggenbach, 1991), at a temperature in excess of 150 °C (Truesdell and Hulston, 1980), cooled and diluted (1:2) by meteoric waters likely infiltrating in the areas of Ficuzza Mt. (901 m a.s.l.) and/or San Genuardo Mt. (1180 m a.s.l.) at about 12 and 23 km N of Sciacca, respectively. Due to this binary mixing, thermal waters cools down to 60–65 °C, which is the prevailing temperature of the main reservoir feeding the superficial thermal manifestations (Terme Selinuntine and Hotel Terme wells). The presence of the Cretaceous marls separates the main basal thermal aquifer from those hosted in the youngest limestone formations. The Molinelli thermal spring appears to be fed by a superficial aquifer hosted in the Eocene–Oligocene limestone deposits recharged by meteoric water locally infiltrating in the Sciacca area (Fig. 9). This aquifer is locally connected with the main reservoir and mix with it due to the upwelling of thermal waters throughout fracture and fault systems. The Fontana Calda spring can be considered as the emergence of the shallowest aquifer fed by meteoric waters infiltrating in the San Calogero Mt. area at an altitude of 378 m a.s.l. (Fig. 9). Both the chemical and isotopic data suggest that in this case interactions between the deep Na–Cl and the shallower Ca–HCO3(SO4) aquifers are negligible. Therefore, the thermalism of the Ca–HCO3(SO4) aquifer is possibly related to conductive heat transport and/or vapor condensation. On the basis of the relatively high R/Ra and the carbon isotopic ratios we may infer the presence of gas phase that enters the SGF and whose origin may be related to deep-seated mantle melts. 7. Conclusions The SGF waters are fed by isotopically modified seawater at temperatures around 150 °C that later mixes and cools down to

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