Groundwaters of Mt. Vulture volcano, southern Italy - CiteSeerX

Geochemical Journal, Vol. 43, pp. 000 to 000, 2009

GJ050 galley proofs

Groundwaters of Mt. Vulture volcano, southern Italy: Chemistry and sulfur isotope composition of dissolved sulfate MICHELE PATERNOSTER,1* SERENA PARISI,2 ANTONIO CARACAUSI ,3 ROCCO FAVARA3 and GIOVANNI MONGELLI 1 2

1 Dipartimento di Chimica, Universitá degli Studi della Basilicata, Campus Macchia Romana, I-85100 Potenza, Italy Dipartimento di Scienze Geologiche, Universitá degli Studi della Basilicata, Campus Macchia Romana, I-85100 Potenza, Italy 3 Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Palermo, Via Ugo La Malfa 153, I-90146 Palermo, Italy

(Received April 9, 2009; Accepted July 13, 2009) We report the chemical composition of groundwaters—including the first data on the sulfur isotopic composition of dissolved sulfate—from the volcanic aquifers of Mt. Vulture, one of the most important hydrological basins of southern Italy. A total of 27 water samples taken at different altitudes among drilled wells and springs were collected. The majority of groundwaters have a bicarbonate alkaline and bicarbonate alkaline-earth composition. High-salinity waters are sulfatebicarbonate alkaline in composition. The water-rock interaction process is mainly affected from uprising of CO 2-rich gases which cause an increase of the water acidity promoting basalt weathering with an enrichment in certain chemical species (i.e., Na+ , Ca2+, SO 42–) and a high total carbon content. The δ 34S values of dissolved sulfate ranging from +4‰ to +8.6‰ can be explained by leaching of volcanites. Higher δ 34S values (from 9.6‰ to 10.4‰) detected in a few water springs can be ascribed either to the interaction with the pyroclastic layer rich in feldspathoids, such as haüyna, that have sulfur isotopic compositions up to +10.6‰ or animal manure contamination inducing localized bacterial sulfate reduction with an increase in the δ34S of sulfate. Taking into account that Upper Triassic evaporite deposits have higher δ34S values (from +13.5‰ to +17.4‰,) than those measured in all water samples the dissolution of these deposits could be excluded. Keywords: Mt. Vulture volcano, water-rock interaction,silicate weathering, hydrogeochemistry, sulfur isotope composition,

be either due i) to the dissolution of evaporite or ii) to the interaction of neutral Na–Cl waters with volcanic rocks affected by argillitic alteration (Gambardella et al., 2006). On the basis of isotopic studies (δD and δ18O), Paternoster et al. (2008) suggested that the Mt. Vulture groundwaters have a meteoric origin. Recently, Caracausi et al. (2009) have carried out a geochemical investigation of the two maar lakes suggesting that the dissolved CO2 is mainly of magmatic origin while the CH4 has biogenic source. Results from an extensive investigation of the chemistry (major and minor ions) and isotopic composition (sulfur in dissolved sulfate) of groundwaters sampled at 27 different sites in the Mt. Vulture area are here reported. The aim of this study was to better constrain the origin of these waters and water-rock interaction processes controlling their chemistry. We provide the first data for sulfur isotopic composition on dissolved SO42– in Mt. Vulture groundwaters. It is well known that sulfate in surface and ground waters can derive from a wide variety of both natural and anthropogenic sources: 1) oxidation of pyrite and other forms of reduced inorganic S during weathering; 2) mineralization of organically-bound S in soil; 3) leaching of sulfate minerals; 4) sulfate in rainwater; 5) geothermal waters and 6) sulfate of agricultural and in-

INTRODUCTION The Mt. Vulture hydrological basin is one of the more important in Southern Italy and the main Italian reservoir of effervescent-mineral waters that are pumped for drinking and irrigation purposes. It is characterized by the presence of numerous CO 2-rich mineral springs and wells, and the two volcanic maars—called “Laghi di Monticchio”—inside the caldera (Paternoster, 2005; Gambardella et al., 2006; Caracausi et al., 2009). The geochemistry of the Mt. Vulture groundwater has been the subject of study by Barbieri and Morotti (2003), Gambardella et al. (2006), Paternoster et al. (2008) and Caracausi et al. (2009). On the basis of Sr isotopic analyses, Barbieri and Morotti (2003) suggested that the chemical composition for most springs is likely to depend on their interaction with volcanic rocks. For a few waters with high salinity, their chemical composition might derive from the mixing of waters circulating in volcanic rocks with waters from Triassic evaporite deposits. Waters with sulfate-bicarbonate alkaline composition could *Corresponding author (e-mail: [email protected]) Copyright © 2009 by The Geochemical Society of Japan.

1

Rapolla

Mt. Vulture

Fig. 1. Geological sketch map of Mt. Vulture (modified from Giannandrea et al., 2004). Black squares indicate villages. Numbers indicate samples reported in Table 1.

dustrial origin such as fertilizers and industrial effluents (Yang et al., 1996; Robinson and Bottrell, 1997; Bottrell et al., 2000, 2008). To univocally constrain the sources of sulfate in groundwater, the isotopic composition of sulfate is the ideal solution, except when SO42– reduction is the dominant process. Sulfate from dissolution of marine evaporite, such as gypsum/anhydrite, has δ34S values ( δ34S = 34S/ 32 S sample / 34 S/ 32 S std ) ranging from +10‰ to +30‰ (Claypool et al., 1980), while sulfides from igneous rocks typically have δ34S values from –10‰ to 10‰ with an average of ~0‰ (Thode, 1991). Sulfate derived from oxidation of sedimentary sulfides shows usually negative δ 34 S values (Thode, 1991; Bottrell et al., 2008). Anthropogenically-contaminated rainwater has δ34S values ranging from +4‰ to +6‰ (Caron et al., 1986; 2

M. Paternoster et al.

McArdle and Liss, 1995; Wadleigh et al., 1996; Bottrell et al., 2008), while agricultural fertilizers show δ34S from +0.4‰ to +11.9‰ (Longinelli and Cortecci, 1970; Hoefs, 1997; Moncaster et al., 2000; Cortecci et al., 2002; Bol et al., 2005). GEOLOGICAL AND H YDROGEOLOGICAL SETTING The Mt. Vulture is a Pleistocene strato-volcano located in the most external part of the Apennine orogen (Southern Italy), close to the edge of the Apulian foreland (Fig. 1). Volcanic activity started at about 742 ± 11 ka and continued, interrupted by long-lasting quiescence periods, up 142 ± 11 ka (Laurenzi et al., 1993; Buettner et al., 2006). The volcanic products have a strongly undersaturated silica character with alkaline potassic to ultrapotassic af-

finities (De Fino et al., 1986) and range in composition from foidites (nephelinites, hauynites, leucitites) and melilitites (plus basanites and tephrites) to phonolitic tephrites, phonolites and trachytes (Melluso et al., 1996; Beccaluva et al., 2002; De Astis et al., 2006). The large Na and S contents of the Vulture magmas (Marini et al., 1994) result in the widespread presence of sodalite-group phases among the feldespathoids (De Fino et al., 1982; Di Muro et al., 2004). The main mineralogical phases occurring in the pyroclastic deposits and lava flows are feldspars and feldspathoids (De Fino et al., 1986; Beccaluva et al., 2002). The most recent eruptions produced multiple WNW-ESE aligned monogenic cones (Giannandrea et al., 2004) having a carbonatitic-melilititic composition (Stoppa and Principe, 1998). The primary origin of the carbonate fraction is currently being debated (see D’Orazio et al., 2007, 2008; Stoppa et al., 2008). The fluvio-lacustrine sediments are pre-, syn- and postvolcanic deposits and crop out in the southern and peripheral sectors of the volcano. These deposits are formed by clayely-sandy conglomerate with intercalations of pyroclastic products (Fiumara di Atella Super-synthem; Giannandrea et al., 2006). The basement consists principally of deep-sea sediments belonging to units ranging from the early Triassic to the lower-middle Miocene (Boenzi et al., 1987; Principe and Giannandrea, 2002). Beneath the Meso-Cenozoic substratum units, radiolarites and limestones of the Apulian platform reach a depth of about 5 km (La Volpe et al., 1984). Martinis and Pieri (1964) reported a Triassic evaporite horizon in the carbonates of the Mesozoic Apulian platform in a well (Foresta Umbra-1) drilled not far from the volcanic structure of Mt. Vulture. Structural analysis and morphological features have shown that the geomorphology of the studied area is widely due to the interplay between volcanogenic doming and regional tectonics. The fracture-fault system consists of two main orthogonal sets oriented N120° ± 10° and N40°–50° (Schiattarella et al., 2005). Recently, Spilotro et al. (2005), Celico and Summa (2004), and Paternoster et al. (2008) have investigated the hydrogeology of the Mt. Vulture. Spilotro et al. (2005) have suggested that the volcanic edifice represents a huge aquifer where the groundwater circulates from the highest altitudes following radial streamlines. The permeability and anisotropy features of the aquifer are conditioned by the magmatic and pyroclastic sequences and their rearrangements driven by tectonic and volcanogenic deformations. The bedrock is much less permeable than the overlying volcanites and the aquifer system is composed of alternation of horizons of high and low permeability. Alternatively, Celico and Summa (2004) have recognized two hydro-geologically independent basins. The first, confined between the most important fault systems of

Grigi Valley—Fosso del Corbo and the Southern Fault, has two main drainage axes oriented SE and NW, respectively. The second basin, located in the northern sector of the volcano, is characterized by radial drainage. The alternation of permeable and impermeable deposits locally determines the occurrence of inter-communicating basal layers. In the southern sector, in contact with clayelysandy conglomerate, the groundwater circulation is relatively low and the few wells reached the sedimentary substrate at shallow depths (Celico and Summa, 2004). Recently Paternoster et al. (2008) have shown that most of Mt. Vulture groundwaters are characterized by time-constant isotopic ratios (δD and δ18O) indicating that seasonal variations caused by the meteoric recharge are negligible. The mean altitude of the groundwater recharge area is about 1,000 meters a.s.l. METHODS Water samples were collected in March and April 2004 from 9 springs and 18 drilled wells located in the NW and NE of Mt. Vulture, within volcanites, and SE within the fluvio-lacustrine sediments (Fig. 1). Temperature, pH, Eh, conductivity were measured directly in situ and reported in Table 1 together with the geographical position of the sampled wells and springs and chemical and isotope data (δ34S). The samples were collected and stored in polyethylene bottles. An aliquot for analyses of dissolved cations was filtered in situ through micropore filters (0.45 µm) and acidified with Suprapure HNO3. Total alkalinity was determined in situ by acidimetric titration with 0.1N HCl. Major dissolved constituents were determined at the Istituto Nazionale di Geofisica e Vulcanologia (INGV - Sezione di Palermo) using a Dionex 2000i ion chromatograph. A Dionex CS-12 column was used for the determination of cations (Na+, K+, Mg2+, Ca2+) and a Dionex AS4A-SC column for anions (F–, Cl–, NO3–, SO42–). To verify the analytic error in ion concentration measurements, we estimated the electroneutrality by: Electroneutrality (e) ⇒

( CAT − AN ) × 100(%) ( CAT − AN )

where CAT and AN represent the cations and the anions sum (expressed in meq/l), respectively. Values in excess of 5% suggest an analytical error in the measurement (Matthess, 1982). In our case, such a boundary was never reached. Minor elements (Fe, Mn, Sr) analyses were performed by inductively coupled plasma optical emission spectroscopy (ICP-OES) at the Agenzia Regionale Protezione Ambiente Basilicata (ARPAB-Potenza). Boδ 34S (SO4) in Mt. Vulture groundwaters

3

4


Sample

Pozzo M2 Pozzo C3 Pozzo A7 Faggi Pozzo 4 Pozzo D Gaudio 21 Pozzo Madda Angelicchio Lilia 2 Fonte Itala 1 Savino S. Maria Luco 1 Atella 2 Crocco Gaudio 11V Eudria Fonte Toka Acetosella Rapolla Atella F3 Atella F1 Pozzo 22 Pozzo 5 Pozzo 2 Fonte Itala 2 Piloni

No.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

well well well well well well spring spring spring well spring well spring spring spring spring well well well well spring

well well well spring well well

Type

557800 527882 555727 556812 550803 554983 551362 557888 549448 553947 550060 555491 548394 550549 556944 555149 551337 548066 555579 556416 556416 556440 551154 554302 557800 556303 556203

(E°)

(N°) 4535826 4574930 4531832 4527553 4533677 4528064 4534020 4535856 4533483 4538480 4532963 4528962 4533815 4533213 4528665 4524642 4530875 4533875 4526680 4527365 4527365 4527304 4532938 4528490 4535826 4526817 4527672

Long.

Lat.

686 541 600 885 545 519 735 560 820 552 490 535 519 474 575 644 551 396 365 360 394 461 690 491 544 490 960

(m. asl)

Altitude

3.0 1.3 2.0 0.2 1.0 1.7 6.0 1.8 2.5 4.0 2.5 0.6 8.0 0.1 0.1 5.0 0.1 3.5 0.1 0.2 n.m. 0.0 2.5 2.0 2.9 4.3 0.2

l sec−1

Flow

7.5 6.2 7.2 7.9 6.5 6.8 5.7 7.3 7.4 6.1 5.7 7.1 6.1 5.7 5.8 5.9 6.4 6.3 6.4 6.8 6.5 6.7 5.9 7.0 5.8 5.9 7.4

pH

2,590 290 280 510 190

1,406 340 360 537 133

67 52 120

13.4 13.3 15.7 10.2 13.5 15.7 15.3 16.1 13.0 14.8 15.6 16.2 15.0 16.4 16.4 19.2 18.7 15.3 16.8 15.7 16.8 17.4 19.1 15.6 14.8 15.8 10.3

−47 29 180 130 12 −6 55 −36 180 32 59 92 31 117 51 225 130 160 220 156 95 −2 45 −13 203 410 288 198 325 310 538 480 611 525 638 780 2,100 960 1,375 1,350 2,200 2,600 3,260 7,600 11,980 17,760

°C

mV

µS cm−1

T

Eh

EC

232 465 305 276 351 338 586 483 692 491 692 907 2,380 969 1,582 1,662 3,548 3,257 3,862 8,449 12,908 21,286

mg L−1

TDS

Table 1. Chemical and sulphur isotopic composition of the investigated groundwaters

6.3 1.2 1.3 2.5 0.6

0.9 1.4 1.1 0.7 1.1 1.4 2.0 2.4 1.4 2.0 3.3 4.8 5.0 4.5 7.0 6.2 15.0 19.7 33 92 145 218

Na +

1.5 0.4 0.5 0.5 0.2

0.4 0.5 0.3 0.3 1.0 0.4 0.9 0.6 0.4 0.6 0.8 1.1 0.5 0.8 1.1 1.1 2.3 2.8 1.8 3.9 8.1 20.5

K+

4.9 0.5 0.4 0.8 0.4

0.3 1.1 0.5 0.6 0.4 0.6 1.0 0.6 1.7 0.9 1.1 1.0 1.4 1.6 4.3 4.6 10 4.9 1.4 3.3 8.2 12

Mg2+

Cl−

4.4 1.2 1.2 1.9 1.1

0.7 1.8 1.2 1.3 0.9 1.1 2.0 1.7 4.9 1.7 2.5 3.9 22.2 4.9 6.6 8.1 16 12.8 7.6 14.7 13.8 18.1

0.9 0.4 0.4 0.4 0.5

0.3 0.4 0.3 0.6 0.5 0.4 0.7 0.6 0.6 0.8 0.5 0.9 1.0 1.3 1.1 1.5 1.8 2.7 6.2 21.4 25 28.4

meq l−1

Ca 2+

0.1 0.1 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.5 0.2 0.1 0.5 0.2 0.1 0.2 0.0 0.6 0.5 1.6 13.2 1.0 0.0 0.4 0.2 0.6 0.1

NO 3−

1.5 0.2 0.2 0.4 0.3

0.1 0.1 0.2 0.2 0.2 0.3 0.3 0.5 0.5 0.5 0.7 1.0 1.4 2.0 2.6 2.6 6.1 7.4 14.2 41 61 95

SO 42−

13.8 2.2 2.4 4.2 1.4

1.8 4.1 2.4 2.0 2.6 2.8 5.0 4.2 6.4 3.3 6.0 8.5 26 7.8 15 15.5 36 30 23 49 69 153

HCO3−

δ 34S (SO4) in Mt. Vulture groundwaters

5

Pozzo M2 Pozzo C3 Pozzo A7 Faggi Pozzo 4 Pozzo D Gaudio 21 Pozzo Madda Angelicchio Lilia 2 Fonte Itala 1 Savino S. Maria Luco 1 Atella 2 Crocco Gaudio 11V Eudria

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 0.6 1.3 0.2 0.2 0.3 0.6 1.3 0.6 0.6 1.3 2.5 0.4 1.8 0.6 1.1 0.4 0.4 1.5 5.5 5.7 5.2 0.3 0.1 0.3 0.1 0.1

56 41 91 91 60 37

83 114 120 119

88 100 35 38 106 82 97 86 42

69

0.5

0.3 0.1 0.1 0.2 b.d.l.

3.0 3.2 26.5 30.3

0.4 0.4 0.4 0.5 1.3

0.1 0.1 0.1 0.2 0.4 0.2

0.1 0.1 0.1

0.1 0.1 b.d.l.

0.1

B+3 ppm

88 64 60 80 62 113

F−

48

SiO2

0.9 3.1 2.4 5.5 1.8 0.1 0.4 0.4 0.2

0.8 1.5 1.8 1.6 1.6

0.7 1.2 0.3 0.6 0.6 0.8

0.6 0.3 0.6 0.5 0.5 0.7

0.5

Sr+2 ppb

34 bdl bdl 18 bdl

40 1,150 6,700 11,900

16

44 10,350 45 11,600

34 57 bdl bdl 560 bdl

46 76 75

21 98 bdl

bdl

Fetot

bdl bdl bdl 20 bdl

bdl 684 776 903

27 180 bdl 130 450 25 699 261

39 bdl bdl

bdl bdl bdl bdl bdl 47

bdl

Mntot

8.6 7.7 8.5 8.6 7.1

4.0 8.2 7.8 9.3 nd 10.0 10.4 10.1 10.1

8.4 nd 9.8

8.3 6.6 7.3 8.0 7.4 6.3 nd 6.8 nd

nd

δ34S (SO4)

17.1 3.2 3.4 5.7 2.3

43.8 114 175 269

11.8 18.9 19.9 43.3 40.3

5.4 8.3 5.1 7.3 10.7 29.1

4.7 3.1 2.8 3.4 3.5 5.9

2.2

16.3 3.2 3.2 5.6 2.3

43.7 113 168 276

5.3 7.7 5.2 7.5 10.5 29.3 11.4 18.8 19.9 43.9 40.5

4.7 3.0 2.8 3.3 3.5 6.0

2.3

AS meq l−1

CS meq l−1

E

2.3 0.4 2.4 0.6 0.4

0.1 0.4 2.0 −1.4

1.7 0.3 0.0 −0.8 −0.2

0.7 1.3 −0.3

0.3 4.2 −0.3

0.4 0.4 −1.3

1.4 −0.7

−1.1 −0.1

%

Note: nd (not determined); m. asl (meter above sea level); bdl (below detection limit); E (electroneutrality, expressed in %); CS (cations sum); AS (anions sum).

Fonte Toka Acetosella Rapolla Atella F3 Atella F1 Pozzo 22 Pozzo 5 Pozzo 2 Fonte Itala 2 Piloni

Sample

No.

(a)

(b)

Fig. 2. (a) Ternary plot SO4–Cl–HCO3. It is possible to distinguish two water types. (b) Ternary plot Ca–K–Na. The point representing the composition of connate water is also plotted on this diagram (CW). Symbols: filled circles indicate investigated low to medium saline groundwaters; open circles indicate the high salinity samples. Numbers indicate samples reported in Table 1.

ron and SiO2 were determined by molecular spectrophotometry. Accuracy and precision of the measures were computed by analyzing certified reference materials (NIST standard, SLR 2) and by performing several replicates on samples, while the relative errors are less than 5% for all the analyzed elements. The sulfur isotope composition in aqueous sulfate was determined after precipitation as BaSO4 that was then thermally decomposed to yield SO2 for the mass-spectrometric analysis, following the procedure of Yanagisawa and Sakai (1983). Stable isotopic compositions are expressed in δ34S (‰) notation, relative to the Cañon Diablo Troilite (CDT) standard. The analyses were carried out at the Nevada Stable Isotope Laboratory (Dept. Geological Sciences - University of Nevada). Duplicate preparations and analyses agreed within ± 0.2‰. RESULTS AND DISCUSSION The chemical composition of groundwaters The temperature of the investigated groundwaters ranged from 10.2 to 19.1°C, while the pH values were from slightly acidic to near neutral (from 5.7 to 7.9). The electrical conductivity showed values between 203 and 17,760 µS/cm, while the total dissolved solids (TDS) between 232 and 21,286 mg/l. A wide range of redox conditions, from slightly reduced to oxidized (–47 mV < Eh < 225 mV) was observed. Using ternary diagrams (Fig. 2), two water types can be distinguished: (a) bicarbonate-alkaline-earth waters 6


characterized by Ca > Na > Mg > K and with TDS values ranging between 232 and 3,548 mg/L, which represent the most common type in the investigated area and (b) bicarbonate-sulfate-alkaline waters (only 4 samples) characterized by Na > Ca > K > Mg and with the highest TDS values up to about 21,300 mg/L. The sulfate-bicarbonatealkaline waters (hereafter briefly indicated as highsalinity waters) were enriched in sodium and sulfate respect to average groundwater composition and had the highest concentration of dissolved CO 2 (Paternoster, 2005). These samples were located in the north-eastern and south-eastern sector of Mt. Vulture. It is reasonable to correlate the high bicarbonate content to the dissolution of CO2 in groundwaters and related to the dissolution of volcanic bedrock during water circulation. The input of CO2-dominant gases increased the acidity of solution and consequently the CO2 was gradually converted into bicarbonate as cations were gradually brought into solution and pH increased to the typical values measured in the Vulture waters. This pattern indicates that the amount of metals released to groundwaters is determined by the intensity and extent of the acid attack of the host rocks by gas-charged water. The investigated Mt. Vulture groundwaters fall within the field of the so-called “immature water” as defined by Giggenbach (1988), showing that weathering processes took place in a cold environment, where thermodynamic equilibrium between rock and solution was never reached. This evidence prevented the use of classic aqueous chemical geothermometers (i.e., amorphous SiO2, Na/K, Na/

Fig. 3. Concentration of dissolved sodium versus chloride. The solid line represent a Na+/Cl– ratio of 1:1, CW (connate water). Symbols as well as in Fig. 2.

Li), which do provide unrealistic equilibrium temperatures for all samples. The dissolved SiO 2 concentrations in the analyzed waters range from 35 to 120 ppm. The higher concentrations of SiO2 have been observed in the water samples trending from bicarbonate alkaline-earth to bicarbonate alkaline following to water-volcanic rock interaction processes. The investigated waters are characterized by values of the concentrations of NO3– lower than 0.6 meq/l; only few samples (20, 21 and 22) have values higher than the maximum admissible concentration (0.81 meq/l) set by Drinking Water Directive (European Union—2000/60/ CEE). It is reasonable to assume for these samples an anthropogenic influence due to use of animal manure fertilizers in the surrounding areas. Minor elements (like F, B, Fe, Mn and Sr) show largely variable concentrations that could be explained by a variable extent of water-rock interaction. Fluorine concentrations are generally less than 2.5 ppm, except for the three samples, anthropogenically contaminated (20, 21 and 22) that show higher values, up to 5.7 ppm. Most of the investigated waters contain less than 0.5 ppm of B, few samples (18, 19 and 20) range between 1.3 and 3.2 ppm and only two waters (21 and 22) show values ranging from 26.5 to 30.3 ppm. The Sr concentration values are lower than 2 ppm, for highsalinity samples ranging from 3.1 to 5.5 ppm. The Fe and Mn concentration values display a wide range. The Fe/ Mn ratio in waters is often higher than unity and is similar to those measured in Vulture volcanics suggesting circulation in reducing environments and/or mixing with acidic solution. Only 4 samples (8, 14, 18, and 26) having lower Fe/Mn ratios, should be related to oxidizing and/or neutral to basic environments. In the Na–Cl graph (Fig. 3), the investigated waters show a good relation (r 2 = 0.95) with a Na/Cl ratio of about 6.21, higher than that of connate water (Na/Cl = 0.8), thus excluding a contribution from this source. Only

Fig. 4. Concentration of metals in the Mt. Vulture groundwater (Cw, ppb) vs. average concentration in local magmas (C s, ppm). Groundwater data are weighted averages and are computed basing on flow rates of springs and wells. Data for Vulture rocks derived from the literature (109 rock samples have been used from De Fino et al., 1982, 1986; Melluso et al., 1996; Bindi et al., 1999; Beccaluva et al., 2002, De Astis et al., 2006)

two springs (4 and 27) show a dominant meteoric component with a Na/Cl ratio similar to that of marine spray/ aerosol. In fact, these springs are located at a high altitude and they are very dilute. Here chloride is neither involved in mineral deposition nor in ionic exchange processes. Furthermore, considering the stoichiometry of the most common salts of evaporates, the expected Na/Cl ratio of the interacting waters should be close to unity. Thus, a Na/Cl ratio >1 may be explained in two different ways: (i) Ca–Na exchange (e.g., Plummer et al., 1990) involving a wide occurrence of Na-rich minerals, such as smectite, but which is only present in some pyroclastic deposits of Mt. Vulture (Fiore et al., 1992; Fiore, 1993) or; (ii) long-term interaction with Na–silicates, assuming Al and Si buffering and calcite deposition. Figure 4 reports a scatter plot (Aiuppa et al., 2000) of the average concentration of each metal in groundwater (Cw) and the volcanic-rocks (Cs) for Mt. Vulture area. Alkalis and alkaline earth elements show a good correlation between these two variables, while Fe and to a lesser extent Mn, appear to be depleted in the groundwater, which is consistent with their strong tendency to concentrate in the weathering minerals (oxides and clays). Hence, it may be suggested that rock composition plays a major role in controlling groundwater chemistry and that the basalt leaching is the dominant source of dissolved metals. The high-salinity samples enriched in Na+, Cl–, B3+ and SO 4 2– , probably reflect a more extended watervolcanic rock interaction. These springs emerge in areas where the aquifer overlies the fluvio-lacustrine sediments mainly constituted by clayely-sandy conglomerate with δ 34S (SO4) in Mt. Vulture groundwaters

7

Fig. 6. Plot of δ34S (SO 4) and SO4 concentrations (expressed in ppm) for the investigated groundwaters. The mass-balance theoretical curves between rainwater and Triassic evaporite deposits (solid curve), and rainwater and pyroclastic layers principally rich in feldspathoids (dashed curve) are shown. The rainwater data refer to November 2004 sample (δ34S = +3.65‰; SO 4 = 7 mg/l) measured in the Mt. Vulture area, while the data on Triassic evaporite deposits and haüyna minerals are literature values (δ34S = +15.5‰ by Cortecci et al., 1981; and δ34S = +10.6‰ by Marini et al., 1994; respectively). Numbers indicate samples in the Table 1 and symbols as for in Fig. 2. Fig. 5. Correlation between δ34S (SO4) and SO4 concentrations in groundwater. The reported boxes show the δ34S values for the volcanic rocks and haüyna minerals (Marini et al., 1994) and the Upper Triassic evaporite deposits (Burano Formation) of the Italian Adriatic area (Cortecci et al., 1981; Marini et al., 1994). Symbols as for Fig. 2.

intercalations of pyroclastic products (rich in alkali, feldspathoids, feldspars, pyroxenes, plagioclases etc.). These volcanic deposits are confined by impermeable layers, and could entrap saline groundwaters (Spilotro et al., 2005) derived from the leaching of weathering products of feldspathoids belonging to the sodalite group. The Naexcess compared to other samples may be due both to hydrolysis of Na–silicates and exchange for Ca2+ of Na+ ions in clay-mineral (Na–smectite) surfaces. The high amount of boron could be controlled by interaction with clay sediments, typically high in B3+ and K+ (Eriksson et al., 1996). Thus, the chemistry of the high-salinity water subset should be controlled by water circulation in the pyroclastic layer interbedded to clay deposits. Therefore, the water-evaporites interaction, proposed by Barbieri and Morotti (2003) for a few spring with high salinity, may be excluded. Sulfur isotope data of dissolved sulfate The volcanic rocks of the Vulture Volcano exhibit a range of δ34S values from +4‰ to +8.5‰ (Marini et al., 8


1994), much higher than its mantle source (about 0‰; e.g., Faure, 1986). Although the high δ34S values of primitive Vulture magmas may be explained in terms of the partial melting of mantle enriched in 34S, at present it is impossible to establish the cause (Marini et al., 1994). The δ 34 S data on Upper Triassic evaporite deposits (Burano Formation) in the Adriatic area of Italy range from +13.5‰ to +17.4‰ (Cortecci et al., 1981; Marini et al., 1994). Sulfate in the investigated groundwater samples is characterized by positive δ34S values ranging between +6.3‰ and +10.4‰. Only one sample (# 14) has a lower δ34S value of +4‰. In Fig. 5, no direct correlation between the δ 34 S (SO42–) values and the sulfate concentration is observed. Most samples show δ34S values of aqueous sulfate from +4‰ to +8.6‰ similar to sulfur isotope compositions measured by Marini et al. (1994) in the local magmas (4‰ to 8.5‰), thus supporting an origin mainly from leaching of volcanic rock. A few springs (13 and 17) show slightly higher δ 34S values (+9.6‰ and +9.8‰, respectively) than one measured in volcanics but lower than the Upper Triassic evaporite deposits (from +13.5‰ to +17.4‰, Cortecci et al., 1981 and Marini et al., 1994). The high-salinity waters have values from +10.0‰ to +10.4‰. The dissolution of sulfate minerals by groundwater does not modify the original δ34S signature, therefore either 1) an additional, 34S-enriched sulfate source

is involved, or 2) the pristine sulfate isotopic composition has been modified. The samples with 34S-enriched isotope values show evidence of significant pollution due to use of animal manure fertilizers (e.g., high nitrate and fluorine). However, animal manure fertilizers are characterized by low isotopic values ranging from +4.8‰ to +6.2‰ (Bol et al., 2005). Alternatively, animal manure contamination of groundwater might induce bacterial sulfate reduction and cause an increase in the δ 34S of sulfate (Goody et al., 2002; Stögbauer et al., 2004) since these organisms metabolize preferentially light 32SO42– leaving isotopically heavier sulfate in the residue (Thode, 1991; Bottrell et al., 2000). Additional sulfate may be related to the oxidation of volcanic gaseous sulfur species (SO2 and H2S). However these were not observed in the investigated waters (Paternoster, 2005) excluding this process. A reasonable explanation is that some of the isotopically heavy sulfate in the waters originated by interaction with pyroclastic layers rich in feldspathoids belonging to the sodalite group (haüyna). The S isotopic composition of haüyna samples of the Vulture volcanic rocks is systematically higher than volcanic rock samples (Marini et al., 1994), ranging from +8.7‰ to +10.6‰. This range well overlaps with the samples showing high δ34S values ranging from +9.6‰ to +10.4‰. In Fig. 6 mass-balance theoretical curves between rainwater and evaporitic deposits and rainwater-haüyna minerals have been calculated and plotted together with the measured isotopic data. Most of groundwaters show δ 34S values within the range of local volcanic rocks while the samples with higher δ34S values (from +9.6‰ to +10.4‰) fit the theoretical curve (dashed line) obtained by incrementally adding a solution with δ34S value of +10.6‰ (highest value of haüyna samples) to rainwater for which we obtained δ34S = +3.65‰ and SO42– = 7 mg/l. In short the sulfur isotope data show that there is no need to invoke an evaporite source. The groundwaters with the highest δ34S (SO 42–) values can be explained either: i) by weathering of pyroclastic layers rich in feldspathoids; or ii) slight modification of groundwater sulfate δ34S by sulfate reduction in agriculturally-contaminated parts of the aquifer. CONCLUSIONS The hydrogeochemical data and the new δ34S (SO 42–) isotope analysis, reported here for the first time, show the occurrence of two distinct water types. The first water is Na-rich and derived from low-temperature leaching of volcanic rocks of Mt. Vulture. The second water type, including the high-salinity waters, displays a bicarbonate-sulfate-alkaline composition and high values of

dissolved CO2 (Paternoster, 2005) possibly related to their circulation in fluvio-lacustrine sandy-clayey conglomerate deposits, with intercalations of feldspathoid-rich pyroclastic layers. These volcanic deposits are confined by impermeable layers, and may contain entrapped ancient saline waters. This could explain the very high content of sulfate, sodium, chlorine and other minor chemical species in these springs. A high concentration of F– and NO3–, due to anthropogenic sources, was detected in a few springs located in the north-eastern and southeastern sector of the studied area where animal manure fertilizers are used. The sulfate-δ 34S values from +4.0‰ to +8.6‰ are similar to those measured by Marini et al. (1994) in the local magmas (+4.0‰ to +8.5‰), supporting a dominant origin from leaching of the volcanic rocks. The highest sulfate-δ 34S values (+9.6‰ to +10.4‰) are due either to the interaction with pyroclastic layers principally rich in feldspathoids belonging to the sodalite group or slight modification of groundwater sulfate δ34S by sulfate reduction in parts of the aquifer contaminated by agriculture activity. Finally, this study shows that an evaporite source is not required to explain the high TDS of some Mt. Vulture groundwaters, as suggested by Barbieri and Morotti (2003). Acknowledgments—This paper, part of the Ph.D. thesis by M.P., was supported by the European Social Fund and the Istituto Nazionale di Geofisica e Vulcanologia (Sezione di Palermo, Italy). We wish to thank Prof. G. Cortecci and Prof. E. Dinelli for their constructive criticism during the drafting of this paper. We would like to thank Dr. S. Bottrell and an anonymous reviewer for their constructive comments, and viceeditor in chief Daniele Pinti for his editorial assistance and suggestions that have significantly improved this paper.

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