Chemical and isotopic compositions of the shallow groundwater system of Vulcano Island, Aeolian Archipelago, Italy: an update

8/9/2019 Chemical and isotopic compositions of the shallow groundwater system of Vulcano Island, Aeolian Archipelago, Italy

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Boschetti T, Cortecci G, Bolognesi L (2003) - Chemical and isotopic compositions of the shallow groundwater system of Vulcano Island, AeolianArchipelago, Italy: an update. GeoActa2, 1-34

erratum/corrigendum version (2014-12-22 update) for info: Tiziano Boschetti ([email protected]) 1

Chemical and isotopic compositions of the shallow groundwater system of

Vulcano Island, Aeolian Archipelago, Italy: an update

by

Tiziano Boschetti1, Gianni Cortecci2* and Luca Bolognesi3

1Dipartimento di Scienze della Terra, University of Parma, Parco Area delle Scienze 157a, I-

43100 Parma, Italy

2Dipartimento di Scienze della Terra e Geologico-Ambientali, University of Bologna, Piazza

San Donato 1, I-40126 Bologna, Italy.

3CNR- Istituto per i Processi Chimico-Fisici, Via G. Moruzzi 1, I-56124 Pisa, Italy.

*Corresponding author: tel. +39-051-2094944; fax: +39-051-2094904

E-mail address:[email protected]
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Abstract

Cold and thermal waters were sampled on Vulcano Island from shallow wells in June 1995,

June 1996 and January-February 1997; additional samples refer to a thermal spring, meteoric

water from domestic cisterns and local seawater.

The chemical and isotopic (hydrogen, oxygen, tritium and strontium in water, and sulfur

and oxygen in sulfate ions) compositions measured in the samples showed that: 1) the shallow

hydrothermal system is formed by the mixing of waters of meteoric origin and various ages

with crater-type fumarolic steam and gases (phreato-volcanic waters), with water rising from

the geothermal reservoir expected to exist at depth (sulfate-chloride waters), and with steam

and gases released from the geothermal reservoir itself (steam-heated waters); 2) a major

seawater contribution occurs only in water from a well exploited in the Togo-Togo Camping,

that is located very close to the shoreline; 3) dissolved sulfate mostly derives from oxidation

of fumarolic/magmatic sulfur species, with minor sporadic contributions from seawater or

from dissolution of secondary anhydrite; and 4) major changes in the chemical and isotopic

compositions of the bulk groundwater system were not observed during the study period, and

the appreciable variations in waters from few wells may be interpreted as due to changes in

the mixing proportions of the endmembers or to direct rainwater dilution.

Based on the present study and previous ones from literature, an updated geochemical

model is proposed for the groundwaters system at Vulcano and its relations with the crater

and beach fumarolic fluids.

Keywords: Vulcano Island; groundwater system; oxygen and hydrogen isotopes; tritium;

sulfur isotopes; strontium isotopes.
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1. Introduction

Vulcano Island belongs to the Aeolian Islands, a volcanic archipelago located in the

southern Tyrrhenian Sea, off the northern coast of Sicily (Fig. 1). Tectonic, structural

evolution and caldera formation of Vulcano are described in Gabbianelli et al. (1991) and

Ventura (1994). The magmatic activity of Vulcano began during the Upper Pleistocene and

generated a variety of deposits from leucitic tephrites to high-potassic trachytes; alkali-

rhyolitic obsidian erupted in historic times (Keller, 1980). The last eruption took place during

1888-1890; after that event, the island was and is the site of fumarolic and seismic activities

only.

High-temperature fluids are discharging from fumaroles on the rim of the active crater of

Vulcano, named Gran Cratere or La Fossa (392 m a.s.l.), and mainly consist of H2O;

major components of the dry fraction are CO2, SO2, H2S, HCl, HF and N2. Low-temperature

fumaroles at Porto di Levante beach release CO2, H2S and N2as major components of the

dry fraction. The origin of fumarolic fluids was debated in many papers (e.g. Cortecci et al.,

2001 and references therein).

Geothermal fluids at Vulcano were encountered only by the VU2/bis borehole (236 m deep;

Fig. 1) during 1952-1953. Aquifers were found at 7-14 m (T = 101C), 90-110 (T = 136C)

and 185-236 m (194-198C). Other boreholes (VP-1, 1000 m deep; IV-1, 2050 m deep)

drilled in the 1980s close to the volcanic cone were unproductive (Silvano, 1985; Gioncada

and Sbrana, 1991). Hydrothermal minerals found in these wells, including anhydrite and

pyrite, were described by Sommaruga (1984) and Fulignati et al., (1996 and 1998).

Several models were proposed to explain the genesis of the thermal waters in the shallow

wells located in the Vulcano Porto plain to the northwest of Gran Cratere (see Cortecci et al.,

2001 for a review). The piezometric level in the southern part of the plain is about 2 m a.s.l.

and decreases to sea level toward the north, with a radial flow of meteoric groundwater from

the volcanic cone to the sea (Bolognesi, 1997). Some information on the bottom depth of the

studied wells are reported in Cortecci et al. (2001).

Almost all the authors agree that meteoric water is the main component of the shallow

groundwater system of Vulcano, with local contributions from condensates of magmatic

origin. Steam, gases and heat from the geothermal reservoir produce the steam-heated waters

located at the foot of the volcanic cone (Bolognesi and DAmore, 1993; Cortecci et al., 2001).

Bolognesi and DAmore (1993) proposed that the geothermal reservoir mainly consists of

magmatic condensate evolved via water-rock reaction; the composition of the geothermalwater varies with time (Bolognesi, 2000) depending on new earthquake-induced contributions


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from the magmatic source to the reservoir and subsequent water-rock interactions. The only

known major upflow of geothermal water from the reservoir occurs in well W2 (Fig. 1), that

supplies water to the camp-site Camping Sicilia (Bolognesi and DAmore, 1993; Bolognesi,

1997 and 2000; Capasso et al, 1999; Cortecci et al., 2001); in this well, the geothermal

endmember mixes with a steam-heated component of meteoric origin (Bolognesi and

DAmore, 1993). Temporary changes in W2 water from geothermal to steam-heated character

were related to stress build-up prior to seismic events and stress reduction afterwards

(Bolognesi, 1997). Direct contribution from magmatic gases to a few shallow groundwaters

was also proposed to occur (Aiuppa et al., 2000; Cortecci et al., 2001). According to Capasso

et al. (1999), rainwater that contributes to the refilling of the shallow waters system is drained

in the subsoil of the La Fossa caldera and the Piano caldera, the latter being located in the

southern part of the island where it reaches an elevation of 400 m a.s.l.. The contribution from

the Piano caldera should be predominant with respect to that from the La Fossa caldera

(Favara et al., 1997).

According to Bolognesi and DAmore (1993) and Cortecci et al. (2001), the involvement of

seawater as contributor to most shallow groundwaters of Vulcano Porto should be excluded.

In the present study, new chemical and isotopic (hydrogen, oxygen tritium and strontium in

water, and sulfur and oxygen in sulfate ions) results on cold and thermal groundwaters from

Vulcano Island are reported and discussed. The chemical and isotopic compositions of

samples collected in June 1995 (Cortecci et al., 2001) are included in the study for

comparison. The main aim of the work was to increase the number of data with time, in order

to check the geochemical variability of the groundwater (phreatic) system and to provide

further constraints on the major sources of water and chemicals. A preliminary discussion of

the whole data set is reported in Cortecci and Boschetti (2001).

2. Sampling and analytical procedures

Water samples were collected in June 1995, June 1996 and January-February 1997 from 22

to 32 wells located in the Vulcano Porto plain (Fig. 1). Sampled wells were numbered

following Bolognesi and DAmore (1993), Bolognesi (1997 and 2000) and Cortecci et al.

(2001). Sampling was carried out using a stainless steel sampler lowered to well bottom or by

an electric pump. Additional samples of local seawater, rainwater from domestic cisterns and

thermal springs (from Vulcano and Lipari islands; see inset in Fig. 1) were also collected.

Temperature, pH, electrical conductivity and carbonate alkalinity were measured in thefield. Sodium, potassium, calcium and magnesium were determined by atomic absorption


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spectrometry following the procedures proposed by Bencini (1977); the relative accuracy of

these methods is in the range of 3 to 5%. Chloride was analysed by Volhards volumetric

method and sulfate by turbidimetry/spectrophotometry with relative accuracies of 7 to

10%. Boric acid was determined by colorimetry using azomethine-H reagent (Bencini,

1985), with an accuracy of 3%. Bromide was analysed by ion chromatography, with an

accuracy of 10-15%.

The concentration of silicic acid was measured by spectrophotometry after filtration through

0.45Milliporeand dilution in the field; in laboratory, water was reacted with ammonium

molybdate and concentration measured with an accuracy of 3-5%.

The 2H and 18O of waters are determined, respectively, by reaction of water with metallic

Zn at 500C (Kendall and Coplen, 1985) and by equilibrating CO 2 with water at 25C

(Epstein and Mayeda, 1953), then analysing H2and CO2in the mass-spectrometer. The results

are relative to the V.SMOW standard. Duplicate preparations and analyses agreed within

1 for hydrogen and 0.1 for oxygen. The tritium (3H) measurementswere carried out

using the method of Cameron (1967). The concentration is reported in tritium units (TU), that

is the number of T atoms relative to 1018H atoms. The analytical accuracy was within 1 TU.

Aliquots of waters were sampled for sulfur and oxygen isotopic analyses of dissolved

sulfate. In the field, a spatula pit of calomel (Hg2Cl2) was added to all samples in order to stop

the sulfate-reducing bacteria. In laboratory, the water samples were filtered through a 0.45 m

Millipore filter. Aqueous sulfate was precipitated as barium sulfate, and then thermally

decomposed to yield sulfur dioxide for the mass-spectrometric sulfur isotope analysis

(Yanagisawa and Sakai, 1983) or reacted with graphite at about 1000C (and using Pt to

oxidize CO) to yield CO2for the mass-spectrometric oxygen isotope analysis (Rafter, 1967).

The results are expressed in terms of 34S and 18O values, in per mill, relative to Canyon

Diablo Troilite (CDT) standard for sulfur and V-SMOW standard for oxygen. Duplicate

preparations and analyses agreed within 0.2 for sulfur and 0.3 for oxygen.

Sr isotopic composition was determined in 1-3 g water evaporated to dryness and

redissolved in 2.5 M HCl. Then, Sr was extracted from the hydrochloric solution by

conventional cation exchange technique. The blank for the whole procedure was of 0.7 ng Sr.

The 87Sr/86Sr ratio was measured by means of a Finnigan MAT 262 mass spectrometer, and

normalized to a 86Sr/88Sr value of 0.1194 in natural strontium. Repeated analyses of NIST

SRM 987 standard during the period of interest yielded an average 87Sr/86Sr value of

0.710224 0.000006 (n =26).


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3. Results and discussion

The results from the chemical and isotopic analyses are reported in Tables 1 to 3. To permit

a direct comparison, the analytical data from our 1995 sampling are reported along with those

from the more recent samplings in 1996 and 1997. Let us begin with the observed time-

variation of temperature in the studied waters.

3.1 Temporal changes in the temperature values

The temperature of water in a number of wells even greatly varied depending on sampling

time. Variations were up to 11C in the shallow waters and up to 7C in the steam-heated

waters. In sulfate-chloride waters, temperature did not vary more than 3C. Large temperature

variations in individual wells are reported also in previous studies (e.g. Capasso et al., 1991

and references therein).

Both lower and higher temperature values were observed in June 1996 with respect to June

1995. Positive variations were not higher than 3C, whereas negative variations were larger

with drops in the range 1 to 8C. Substantial drops of 5 to 8C in W17, W24, W31 and W32

wells were accompanied by a considerable decrease of TDS and lower 18O and 2H values in

W17 well or by nearly constant chemical and isotopic compositions in W31, W32 and W24

wells. These features are difficult to be punctually explained, as many factors are involved

including exploitation of the wells by the population. However, dilution effects should be

excluded in W17 well due to dry weather reasons, whereas a lowering of the volcano-

magmatic component may be invoked.

All but one temperature variations observed in January-February 1997 with respect to June

1996 are negative, with differences of 3 to 11C. On the contrary, temperature rose by 11C

in the water W33, passing from 28 to 39C. In spite of a slightly lower salinity, boron and

bicarbonate concentrations in this well increased from 22 to 48 mg/kg (as H3BO3) and from

340 to 430 mg/kg, respectively. This behaviour suggests enhanced contribution to the aquifer

of hot deep fluids before the sampling date, even if the isotopic composition of water kept

nearly constant. Negative temperature variations were notable in some wells, but univocal

relations are not found with chemical and isotopic variables. In keeping with the winter

season, only the drops of 10-11C observed in W20 and W34 wells can be simply attributed

to dilution effects by infiltrating rain. By these effects, all ions and heavy isotopes are largely

depleted in water W20, whereas comparable isotopic compositions but by an order of

magnitude lower salinity are observed in water W34. In the latter, the high Br content in 1996


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(36 mg/kg) compared to that in 1997 (14 mg/kg) testifies the increased proportion of the

meteoric component relative to the marine component of the water feeding this well.

3.2 Chemical resul ts

The major hydrochemistry basically confirms the results of recent studies (Bolognesi and

DAmore, 1993; Aiuppa et al., 2000; Cortecci et al., 2001; Capasso et al., 2001). Three

groups of groundwaters can be distinguished in terms of HCO3-, SO42- and Cl- relative

concentrations (Fig. 2):

1. Shallow groundwaters (W1, W3, W3b, W5, W5b, W9, W12, W13, W14, W16, W17,

W18, W19, W20, W21, W22, W23, W25, W26, W27, W29, W30, W31, W32, W33, W33b,

W34; T= 20 to 48C in June-July 1995-96, and 14 to 44C in January-February 1997), which

hereafter will be denoted as SGRW; see Cortecci et al., 2001). These waters can be

subdivided in bicarbonate-type (W1, W3, W5, W13, W14, W16, W19, W21, W23, W27,

W29, W30, W31; W33b; TDS = 335 to 2,822 mg/kg), which represent meteoric recharge or

waters affected by uprising anomalous CO2degassing; sulfate/chloride-type (W3b, W5b, W9,

W12, W17, W18, W22, W26, W32; TDS = 967 to 3431 mg/kg), which interact directly with

uprising volcano-magmatic gases; and chloride-type (W34), whose high TDS (9,050 mg/kg in

1997 and 24,017 mg/kg in 1996) is due to lateral seawater contribution. Water W20 changed

from sulfate/chloride in 1995 (TDS = 4,432 mg/kg) to chloride in 1996 (7,201 mg/kg) to

bicarbonate in 1997 (994 mg/kg). Similarly, water W33 was sulfate/chloride in 1996 (TDS =

1,780 mg/kg) and bicarbonate in 1997 (1,576 mg/kg).

2. Sulfate-chloride groundwaters (W0, W2, W4; hereafter reported also as geothermal and

denoted as GGRW), characterized by high temperature (49 to 75C) and high salinity (3,775

to 8,839 mg/kg). The thermal spring water SF (Sorgente Federico; T = 44C; TDS = 7,422

mg/kg) belongs to this group. These waters fit the compositional field of local volcanic

waters, and should be fed by a geothermal reservoir at 200-300 m depth, with temperature of

200-250C, and then mixed with meteoric water (Bolognesi and DAmore, 1993; Cortecci et

al., 2001; Cortecci and Boschetti, 2001). According to these authors, the geothermal fluid

derives from condensation of SO2-HCl rich magmatic fluids like the crater-type fumarolic

ones. These sulfate-chloride waters are located at the base of the volcanic cone, and the

notable chemical (and isotopic) variations, showed especially by water W2 from the 1980s,

may attributed to variable mixing between the deep geothermal component and the shallowwater component, or to temporal fluctuations of the chemical composition (and fluxes) of the


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magmatic fluids feeding the geothermal aquifer at depth. Due to water-rock interaction

effects, the pH of these waters is nearly neutral with values from 6.3 to 7.8.

Acidic sulfate-choloride springs (named Pozza and Istmo; not sampled in this study)

outflow at Vulcano from the Porto di Levante beach along with low-temperature fumaroles.

These springs, fumaroles and other submarine thermal springs lie on a N-S structural

lineament which extends from La Fossa crater to Vulcanello (e.g. Sedwick and Stben, 1996).

Springs are basically fed by local volcanic water, with some contributions of seawater and

steam-heated water; the temperature of the Pozza spring was found to be largely variable with

time in the range 20 to 90C (Fontes et al., 1993; Amend et al., 1998; Aiuppa et al., 2000).

3. Steam-heated groundwaters (W6, W7, W8, W15, W24, W28, W29b; hereafter denoted as

SHGRW), i.e. groundwaters heated by H2S or CO2 rich vapours released from deep boiling

water. Water W29b is included in this group, following the Aiuppa et al.s (2000) study

relative to samples collected in 1997. They are considerably enriched in SO 42-relative to Cl-,

with TDS values of 2,005 to 4,043 mg/kg, and their temperatures are in the range 21 to 57C.

The chloride contents in these waters vary between 134 to 353 mg/kg, that is unusual for this

kind of waters. These concentration values are unusual for steam-heated waters. At Vulcano,

however, shallow and steam-heated groundwaters interact with rocks that may be quite rich in

chloride as observed in rhyolites (1150 to 3000 ppm; Goff and McMurtry, 2000).

As far as the cation composition is concerned, the great majority of waters are alkali-

enriched, with some SGRW (W19 and W25) and W29b) and SHGRW (W8, W24 and W29b)

being calcium-enriched. Wells W8 and W24 are located at the base of the Forgia Vecchia

slope, and may exploit aquifers fed by meteoric water flowing from the volcanic cone and

enriched in calcium and sulfate by dissolution of supergenic gypsum occurrences (see forward

section on sulfur isotopes). The Ca2+to SO42-molar ratio in wells W8 and W24 are close to

unit with values ranging from 0.8 to 1.1. In well W25, the Ca2+/SO42-varied from 0.8 in 1995

to 1.5 in 1996 to 4.2 in 1997, this trend suggesting increasing contribution of shallow water

highly enriched in Ca2+ relative to SO42- like that exploited by the W19 well (molar

Ca2+/SO42- ratio of 11). This component may be present also in the W29b well, thus

explaining the observed Ca2+/SO42-molar ratio of 1.3. Solubility reasons do not preclude for

the majority of studied waters dissolution of gypsum or anhydrite. Only hot waters W8, W24,

W4 (96) and W7(96) are definitively saturated or very near to saturation relative to these

minerals (Boschetti, 1998).


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All groups of waters are far from chemical equilibrium with host rocks both for anions (Fig.

2) and cations (Fig. 3), i.e. all analysed waters are immature. The cation compositions lie

between the isochemical rock dissolution line and the incongruent rock dissolution curve,

testifying the great control of both processes on the chemical composition of the studied

waters. Shallow groundwater W19 is the most immature one, and the sulfate-chloride W0 the

less immature one.

A sodium-chloride plot can reveal the waters contaminated by seawater. In Fig. 4, only

groundwater W34(96) appear to be significantly affected by seawater. The contribution of

seawater to this water calculated from the chloride contents results to be about 55% and 20%

in 1996 and 1997, respectively. Similar contributions are calculated taking into account Na

and Mg and to a lesser extent also SO4. However, we cannot exclude that seawater may have

mixed with another water more saline than W19. Along with the freshwater-seawater mixing

trend, other two main trends can be envisaged from Fig. 4, i.e. shallow groundwaters SGWR

couple with some steam-heated waters with minor sodium (W8, W24 samples), and sulfate-

chloride waters GGWR are tied with more concentrated SHGWR waters.

3.2.1 Classification of the waters by the statistical approach

The Principal Component Analysis (PCA) was applied to the studied waters. This

multivariate analysis reduces the dimensionality of the data set, while retaining the

information present in the data. Previous applications to hydrochemical data (e.g. Razack and

Dazy, 1990; Gler et al., 2002) include geothermal systems (Nicholson, 1993).

The data set of Vulcano refers to eighty samples, and fifteen variables both chemical and

isotopic, i.e. tC, Ca, Na, K, Mg, Cl, SO4, HCO3, H3BO3, Br, SiO2, 18O(H2O), 2H(H2O) and

34S(SO4), were considered in the statistical treatment. The data set appears to be convenient

for the factorial analysis, as it obeys to the Keiser-Meyer-Olkins sampling adequacy and the

Barletts sphericity tests. In addition, the covariance matrix method to extract the principal

components from the data set was preferred to the correlation matrix method; in fact, the

former method explains 97% of the variance with only 2 components, whereas the other one

explains only 86% of the variance with 4 components. The correlation coefficients (factor

loadings or factor weights) of PC1 and PC2 with variables are graphically shown in Fig. 5,

and the factorial scores for individual water samples are reported in the PC1 vs PC2 plot of

Fig. 6. Three groups of waters are defined in the plot, and their classification matches that

based on the ternary diagrams of major anions (Fig. 2). When outliers suspected to be affected

by seawater, i.e. W20(97), W34(96) and W34(97), are removed from the PCA plot, waters


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classified as steam-heated are clearly defined by PC10.0 values. Some waters

(W1, W3 and W25), which are areally close to the steam-heated water zone, lie in the shallow

groundwater group, possibly due to mixing with volcanic condensate (W1) or dilution by

meteoric water (W3, W25). To be noticed that water W29b, sampled from the Bambara well

in 1997, is statistically classified as steam-heated, but it falls out the steam-heated field

depicted in the ternary plot (Fig. 2).

3.2.2 Boron and bromine

Boron in the studied waters positively correlates with temperature pointing out to a close

relation with the outflow of hot crater-type fluids (see also Capasso et al., 2001). In addition,

the B/Cl ratio in thermal waters can be used to investigate mixing between different water

reservoirs. The B vs Cl diagram in Fig. 7 excludes any relationship between groundwaters and

seawater, thus pointing to fumarolic/magmatic condensate and water-rock interaction as

sources of boron and chloride in the studied waters. During the period of survey, a mean B to

Cl by weight ratio of 0.0057 was measured in the crater fumaroles (Martini, 1995, 1997,

1999). Exceptions are the W34 (96 and 97) and W20 (96) waters, for which a substantial

proportion of seawater can be concluded, along with the volcanic proportion. Interestingly,

almost all waters with B > 1 ppm lie on the same trend between a steam-heated endmember

and a fumarolic endmember.

In principle, seawater contamination may be constrained from the Br/Cl ratio of waters (Fig.

8). Waters W34 (96 and 97) and W20 (96) fit a mixing line with seawater as an endmember.

The Br contents (9 to 36 mg/kg) of these cold waters are the highest ones measured in the

studied wells, and in particular they are much higher than those in hot waters like W0, W2

and W4 (0.3 to 5.7 mg/kg), as well as in fumarolic condensate during the same period (about

5 mg/l Br in the highest temperature fumarole FA; Aiuppa et al., 2000). The Br content of

local seawater was measured to be on average 67 mg/kg (Cortecci et al., 2001 and this work).

It is noteworthy that all studied waters lie within a field delimited by the highest and lowest

Br to Cl ratios (5.810-3and 0.610-3) measured in crater fumaroles, the range of values being

in keeping with that observed in andesites (110-3610-3; Fuge, 1974).

3.2.3 Chemical geothermometry

The K/Na thermometer cannot be applied to the groundwaters at Vulcano, as they appear to

be totally immature in the Giggenbachs (1988) diagram of Fig. 3. Immaturity may be

ascribed to Na rather than to K and Mg, the latter ions exchanging rapidly with rock even in


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low temperature hydrothermal systems (Giggenbach, 1988). Therefore, the application of the

K2/Mg-thermometer to uprising geothermal fluids provides usually shallow temperature

estimates (last equilibrium) rather than the reservoir ones. The SiO2 thermometer behaves

similarly. In Fig.9, the studied groundwaters are plotted in the log(SiO2) vs log (K2/Mg)

space, where are compared with the thermometric relations of different forms of SiO 2.

When applied to the 1995, 1996 and 1997 samples of the geochemically less immature W0

water, the K2/Mg-thermometer yields concordant estimates of 187 to 194C. These values are

by 29 to 44C higher than those obtained from the quartz-thermometer (150 and 158C). The

silica thermometric estimates likely feel the effects of silica precipitation during cooling.

Concurrently, the original K2/Mg value in the geothermal reservoir may have been lowered

due to mixing of the rising fluid with steam-heated water (see data from Fontes et al., 1993)

enriched in Mg from advanced argillic alteration (see Cortecci & Boschetti, 2001); this type

of alteration in quite common in the Vulcano Porto area (e.g. Fulignati et al., 1996 and 1999).

Waters W20, W21, W34 depict a trend that may be interpreted as due to mixing of shallow

groundwater with seawater accompanied by interaction with rocks leading to the increase of

the K-content in solution and precipitation of amorphous silica. For instance, water W34(96)

presents a K-excess with respect to the potential mixing between seawater (41%) and water

like spring SF (59%). All these three waters show high PCO2 values of 10-2.6 to 10-1.2 atm

(calculated by PHREEQCI 2.8 software; Parkhurst and Appelo, 1999), that enhance the

incongruent dissolution of K-feldspar and the precipitation of silica. The thermal submarine

springs (110-120C), sampled by Sedwick and Stuben (1996) close to the shoreline at the

beach, match the seawater-SF mixing model; these springs lie on a N-S tectonic lineament

that extends up to the crater.

The data points of the acid sulfate-chloride springs are shifted towards the amorphous silica

line (Fig. 9), as a consequence of water-rock interaction and alteration processes. These

waters are totally immature and the majority of them fits the isochemical dissolution line in

the K-Na-Mg diagram of Fig. 3. Due to its position in the graph of Fig. 9, the SF-spring water

may be supposed to be an intermediate term along the neutralization path of an acid sulfate-

chloride water by interaction with rocks.

The steam-heated waters lie around the chalcedony thermometric line (Fig. 9), the highest

data point corresponding to 140C, i.e. the estimated temperature of this aquifer (Bolognesi

and DAmore, 1993). The other data points correspond to lower temperatures and are

influenced by dilution effects. Exceptions are waters W8 and W24 which are shifted towards


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amorphous silica saturation, and water W28 probably equilibrated with advanced-argillic

alteration minerals.

3.3 Isotopic resul ts

3.3.1 Hydrogen and oxygen isotopes of water

The hydrogen and oxygen isotope compositions of studied well waters show a wide range

of values from 42.6 (W19 well) to2.3 (W2 well) and 7.1 (W19) to +1.9 (W2),

respectively. The only one sample from the thermal spring SF has values of 26.7 for

hydrogen and 4.4 for oxygen. These data are compared in Fig. 10 with those of yearly

average rainwater at Vulcano Porto (Capasso et al., 1992) and rainwater samples from

domestic cisterns (Panichi and Noto, 1992; Goff and McMurtry, 2000; Cortecci et al., 2001

and this work), crater-type fumarolic condensates (e.g. Capasso et al., 1999), beach fumaroles

at Porto di Levante (Chiodini et al., 1995; 1996), and seawater (Cortecci et al., 2001). The

field of values presumed for magmatic water at Vulcano (Bolognesi and DAmore, 1993) is

also shown, along with that of andesitic magmatic water as featured by Taran et al (1989) and

Giggenbach (1992).

The isotopic trend and its interpretation remain basically the same as reported in Cortecci et

al. (2001) on the basis of the 1995 data. In short, groundwaters at Vulcano are mixtures, in

variable proportions, of meteoric water and volcano-magmatic water. Seawater appears to be

involved only in well water W34 sampled in 1996, this indication being in keeping with the

high sodium and chloride contents (and their ratios) and the high bromine content. The 2H-

depletion and the 18O-enrichment in the W34 water sample of 1997, coupled with the

decrease of salinity, can be attributed to dilution effects by rainwater likely related to a heavy

event shortly before sampling.

Waters with a geothermal-magmatic component are enriched in 2H and 18O relative to the

other types of water. The proportion of this geothermal water is highest in the W2 well, where

it increased considerably from 1995 to 1997. The W2 samples fit a mixing line between the

geothermal component and another component, that may be a steam-heated water largely

modified by loss of vapour. A geothermal-magmatic component is present also in W0 and W4

wells, but its proportion is low with respect to W2 well and kept nearly constant during 1995

to 1997. Isotopically, the spring SF falls within the steam-heated water group; however, its

chloride-sulfate chemical composition points to a partially geothermal character of this water.


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3.3.2 3H content of water

Omitting for this moment the set of samples from well W0, the tritium contents in our 1995,

1996 and 1997 samples and those available for the W2 well from other sources even prior to

1995 (Bolognesi and DAmore, 1993; Goff and McMurtry, 2000) range between 3.5 and 15.3

TU. All these values are compared in Fig. 11 with the corresponding chloride contents in the

waters, as well as with the tritium and chloride contents measured in local rainwater and

seawater samples.

Main features from the plot are:

1.) The data from well W2 show a very good correlation (r = 0.96). In keeping with the

conclusions of Bolognesi and DAmore (1993), this correlation suggests that the water

samples are basically binary mixtures, the endmembers being geothermal-magmatic water

with 0 TU and an extrapolated chloride concentration of about 3900 mg/kg, and a steam-

heated water with about 100 mg/kg chloride and 17 TU (see also Cortecci et al., 2001).

2. The steam-heated waters lie within a mixing triangle, the endmembers being represented by

the above geothermal and steam-heated waters, and a third endmember constituted by modern

meteoric water (about 4-5 TU). The classified as geothermal water W4 and the thermal spring

SF are located within this triangle. The former can be interpreted as a ternary mixture,

whereas the latter behaves similarly to W2 and may be considered basically a binary mixture.

3. All but three phreato-volcanic waters are enclosed by the above mixing triangle. Well water

W17 of 1995 lies by far outside the triangle, suggesting that in this case old meteoric water

with about 21 TU is involved, as previously concluded by Cortecci et al. (2001). Well water

W20 of 1996 lies also outside, but its position may be influenced by some contribution of

seawater. Well water W34 represents a peculiar case. It is definitely outside the mixing

triangles: in 1996, this water appears to be substantially contaminated by seawater, the other

endmember being possibly a phreatic water close to W19; in 1997, dilution lowered the

chloride content from 11,083 to 4,321 mg/kg, but kept nearly constant the tritium within 5-6

TU.

Samples from the W0 well occupy a distinct area in Fig. 11. They show a trend from a

shallow water to a geothermal water free of tritium and relatively low in chloride. This

interpretation implies that separate geothermal water bodies may exist at different depths andtemperatures beneath the Vulcano Porto plain.


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3.3.32H vs Cl-and 18O vs Cl

-relations

These relations are shown in Figs. 12 and 13 for our samples, as well as for additional W2

samples from other works (Capasso et al., 1999; Goff and McMurtry, 2000).

Water samples from the W2 well arrange along mixing lines (r = 0.94), by which a 2H of

+4.9 and a 18O of +2.9 are calculated for the geothermal endmember, assuming a

concentration of 3900 mg/kg chloride and no 3H for the latter (see the Cl-3H correlation in

Fig. 11). For the steam-heated endmember, a 2H of30 to -29 and a 18O of3 to 2

are extrapolated in the range 0 to 100 mg/kg chloride. The isotopic values for the geothermal

component are considerably heavier than those (2H of -5 and a 18O of +1) calculated

by Bolognesi and DAmore (1993) on the basis of 1986-1988 samples from the same well.

This isotopic evolution may account for a number of factors, including the condensation in

the geothermal reservoir of fluids with a much higher proportion of the magmatic component,

as already reported by Bolognesi (2000). This interpretation agrees with the increase of the

microseismicity, temperature and CO2degassing recorded at the crater in 1996, and related to

a concurrent increased magmatic activity at depth (La Volpe et al., 1999).

The W0 water data show a vertical array, that may be explained in terms of binary mixtures,

only assuming nearly identical 18

O and 2

H values for the chloride-rich component and the

chloride-depleted component. This seems to be a quite unrealistic possibility. A more realistic

interpretation is to imaging a buried water body which losses steam at temperature higher than

300C and the condensation of the steam in the W0 well. At this temperature, in fact, the

water-steam isotopic fractionation is close to zero for both oxygen and hydrogen (Horita and

Wesolowski, 1994) and the HCl behaves as a weak acid, so that partitioning of HCl into the

vapour can occurs from a low-pH and Cl-rich fluid (Ruaya and Seward, 1987; Simonson and

Palmer, 1993). Alternatively, W0 data trend may be explained by mixing between steam-heated water and fumarolic gas, followed by separation of various proportions of steam. In

both cases, water acidity is neutralized by mineral dissolution processes in the aquifer.

The notable contribution of seawater in the W34 well is further on constrained in the 1996

water sample, the other component being a meteoric water. The dilution undergone by the

W34 well water in 1997 was operated by rainwater comparatively enriched in 18O and

depleted in 2H, and probably related to the heavy rain event occurred just one day before

sampling.

Finally, some seawater would seem to be present in the 1995 and 1996 samples from the

W20 well. In 1997, autumn-winter rain caused strong dilution of this well water.


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3.4 Sulfur and oxygen isotopes of sulfate

3.4.1 34S(SO42-) values

Sulfate from well waters chemically classified as shallow groundwaters (W1, W3, W3b,

W5, W5b, W9, W12, W13, W14, W16, W17, W18, W20, W21, W22, W23 , W25, W26,

W27, W29, W30, W31, W32, W33,W33b, W34) shows 34S values from +2.5 to +12.9,

this large isotopic range suggesting that, along with oxidation of volcano-magmatic gases,

additional sources of sulfate may be appreciably involved in these waters. As reported in

Cortecci et al. (1996), reference 34S values are 2.7 to +3.7 for total sulfur in crater

fumaroles (+2.0 to +6.5 for SO2and7.2 to1.8 for H2S), +7.2 to +16.5 for anhydrite

and11.5 to5.9 for pyrite coexisting in the hydrothermal paragenesis of borehole cuttings

from the VP1 well (see Fig. 1) and +20.2 for seawater sulfate off the Vulcano island.

Additional 34S data (this work) on soluble supergenic sulfate from the Forgia Vecchia site on

the northern flank of the volcanic cone are +3.7 (well crystallized gypsum) to +4 (white

incrustations). Hydrolysis of pyrite would seem to be excluded as significant source of

sulfate, due to the strong depletion in 34S of the mineral relative to crater-type fumarolic

gases. On the other hand, the role of the supergenic sulfate via dissolution by infiltrating

rainwater is problematic to be constrained, the isotopic composition of this sulfate being

nearly identical to that of the crater-fumarolic sulfur. Vein anhydrite should derive from

hydrolysis of hypogene/magmatic SO2, followed by interaction with Ca-minerals in the rocks

(Fulignati et al., 1996) An anhydrite-pyrite mineral pairs from 620 m depth in the borehole

provides an isotopic temperature of 226 25C (using the fractionation factors in Ohmoto

and Rye, 1979), in keeping with the thermometric estimates from other methods (e.g.

Fulignati et al, 1996). Therefore, isotopic equilibrium can be assumed for this specific mineral

pair, and possibly for anhydrite and pyrite on the whole along the drilled core. In the case,

temperature and SO2 to H2S mole ratio in the fluids may be the factors controlling the wide

34S ranges observed for the two minerals (Taran et al., 1996; Kusakabe et al., 2000).

Analogously, mineral sulfates at Forgia Vecchia can be related to near surface oxidation of

crater-type fumarolic sulfur species.

Sulfates from waters classified as sulfate-chloride (W0, W2, W4, SF) and as steam-heated

(W6, W7, W8, W15, W24, W28, W29b) show comparable 34S values within the ranges +0.6

to +5.4 (mean +3.0 0.3) and 0.7 to +3.5 (mean +2.0 0.9), respectively. As

already concluded by Cortecci et al. (2001), these isotopic compositions match those of the

crater fumaroles and support a mostly deep-seated magmatic origin for sulfur and the


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production of aqueous sulfate via hydrolysis of SO2 and oxidation of H2S in subsoil water

bodies. Incidentally, and based on the large depletions in 34S and Cl- relative to waters W0

and W2, water W4 may be considered steam-heated rather than geothermal.

In Fig. 14, the 34S(SO42-) values in well waters are plotted versus the corresponding

Ca2+/SO42- weight ratios, and these data points are compared with those of seawater,

hydrothermal anhydrite and secondary gypsum, as well as with the 34S ranges of total sulfur

(SO2+ H2S) and SO2in the crater fumaroles. The 34S range of crater H2S is omitted, as the

H2S undergoing oxidation in the shallow water system may be considerably enriched in 34S by

addition of H2S deriving from hydrolysis of SO2 in geothermal reservoir(s) at depth (see

Cortecci et al., 2001). The distribution of the repeated sampled sulfates in the graph and the

elaboration carried out can be interpreted as follows:

1. All sulfates lie within a mixing triangle, the endmembers being represented by i) volcano-

magmatic condensate with 34S(SO42-) variable in the range 2.7 to +6.5, depending on

time and space beneath the Vulcano Porto plain, ii) meteoric waters with distinctly high

Ca2+/SO42-ratio, and iii) seawater.

2. Anhydrite and gypsum occurrences are within the triangle, but their sulfur should be totally

volcano-magmatic, with no contribution from seawater.

3. Inside the general mixing triangle, chloride-sulfate, steam-heated and the majority of

shallow groundwaters occupy a sub-space delimited basically by two of the three

endmembers reported above, i.e. meteoric shallow water and volcano-magmatic fluids, the

latter having variable 34S composition in the range 2.7 to +6.5, depending on space and

time. However, we cannot exclude variable Ca2+/SO42- and 34S(SO42-) values also for the

shallow-meteoric water endmember. In addition, gypsum and other supergenic soluble sulfate

may be involved in both steam-heated waters and shallow groundwaters, if their meteoric

recharge occurs from the volcanic cone.

4. The position of the thermal spring SF, which is probably fed by geothermal water, denotes

for dissolved sulfate an origin mostly from disproportionation of volcano-magmatic SO2

substantially enriched in 34S with respect to H2S. This may implies a reservoir at depth fed by

fluids relatively rich in H2S depleted in 32S by isotopic exchange with SO2.


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5. Sulfate in the few shallow groundwaters outside the sub-triangle (W16, W20, W21, W27,

W34) can be interpreted in terms of possible contributions from anhydrite dissolution and/or

seawater ingression. The latter source would seem to be particularly manifest in well water

W34.

3.4.2 34S(SO42-) time-variations

Even if the data points for each water are few, a time-34S plot can give an idea on the

isotopic stability or instability of sulfate in the hydrologic system. In Fig. 15, sulfate-chloride

waters and all but one steam-heated waters show quite constant 34S values within 1.

Astonishing are the nearly identical 34S(SO42-) values of +3.0 0.2 in the W0 well during

eight months from June 1996 to January 1997. Exception is the steam-heated water W7; its

Ca2+/SO42-ratio kept nearly constant between0.28 in 1996 and 0.25 in 1997, and therefore the

observed 34S-enrichment by 1.7 in 1997 with respect to 1995 and 1996 (+1.7 0.1) may

be attributed to a change of the isotopic composition of total sulfur in the magmatic-volcanic

gases entering and escaping from the geothermal water body at depth. Dissolution of vein

anhydrite agrees with the higher 34S(SO42-) value in W7, even if the Ca2+/SO42-ratio is only

slightly lower in 1997 (0.25 by weight) relative to 1996 (0.28 by weight).

Sulfate from the great majority of the shallow groundwaters also show quite constant 34S

values, with variations less than 1. Exceptions are represented by sulfate from wells W20,

W21, W25 and W29, its 34S variations being 3.6, 2.0, 1.5 and 1.4, respectively. Based on

Figs. 14 and 15, the 34S-depletion in sulfate W29 in 1997 with respect to 1996 may be

attributed to a change in the 34S of the gases interacting with the aquifer, whereas the 34S

enrichment of sulfate W21 in 1996 and 1997 with respect to 1995 may be due to some

contribution of heavy sulfate from dissolution of vein anhydrite. The isotopic and chemical

evolution of sulfate with time in well W20 requires a more articulated interpretation, by

which 1) the aquifer was significantly contaminated by seawater in late spring 1996 and 2)

afterwards the aquifer underwent a strong dilution by heavy rains during autumn-winter.

Water W25 displays in 1997 a 34S value by about 1.5 higher than the quite uniform ones

measured in 1995 and 1996 (+3.25 0.05). In addition, its Ca 2+ to SO42-by weight ratio

passed from 0.3 in 1995 to 0.6 in 1996 to 1.7 in 1997. These isotopic and chemical changes,

along with a decreasing TDS during 1995 to 1997, are difficult to be explained, unless

imaging uptake of anhydrite-sulfate enriched in 34S, followed by dilution with shallow

groundwater enriched in Ca2+ relative to SO42- like water W19 with a Ca2+/SO42-by weight

ratio of 4.6 (Fig. 14); the 34S(SO42-) of water W19 may be inferred to be close to zero when


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the isotopic values measured for total sulfur in fumaroles and sulfate from cistern rainwater

are put together. A direct dilution by infiltrating rain would seem to be excluded, as the

Ca2+/SO42-ratio and the 34S(SO42-) value of the meteoric water on the island should be lower

than 1.7 and +4.7, respectively (see Table 1).

The isotopic constancy of sulfate from water W34 (34S = +12.8 0.1), when compared

with the strong dilution undergone in 1997 and the notable proportion of seawater sulfate both

in the 1996 and 1997 water samples (Fig. 16), deserves specific comments. The identical

34S(SO42-) values of 1996 and 1997 are problematic to be explained, considering the mixed

marine/magmatic-volcanic composition of sulfate and the quite different sulfur isotopic

composition of the two components. One may imagine that (1) the aquifer feeding well W34

in June 1996 was closed to further contribution of seawater up to at least February 1997, thus

preserving the 34S(SO42-) signature of 1996, (2) the dilution was promoted by infiltrating

rainwater very depleted in SO42- with respect to the phreatic water. Along with seawater,

another water rich in sulfate (2100-2200 mg/l) should have been present in the W34 water

sample of 1996, this second component being represented by steam-heated or geothermal

water or water like the SF spring. In the latter case, the calculated contribution of 40%

seawater to the aquifer under question represents a minimum estimate (Fig.16; see also

section 3.2.3).

As in the W34 well at Vulcano, a high 34S(SO42-) value of +14.7 was measured in the

AGIP well on the nearby Lipari Island (see Tab. 1). This isotopic signature points to a

significant contamination by seawater, in agreement with the vicinity of the well to the

shoreline and the sodium-chloride composition of the water. Again at Lipari, the 34S(SO42-)

of +4.3 measured in the San Calogero spring supports a mostly deep-seated origin of the

sulfur; the spring is located inland and its water is of sodium-sulfate type.

3.4.3 18O(SO42-) values

Sulfate from the hottest (44 to 73C) waters in 1995 and 1997 was analysed for the oxygen

isotope composition, along with sulfate from selected warm and cold (14 to 33C) waters

(Table 2).

The 18O(SO42-) values show a relatively narrow range between +7.1 to +10.5, without

any relation with temperature, sulfate concentration and 18O(H2O) values. The absence of

isotopic correlation even for the hottest samples can be interpreted in terms of a generalized

isotopic disequilibrium between dissolved sulfate and water, probably due to the mixed

character of the studied waters and the slow oxygen isotope exchange rate between sulfate


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ions and water (Lloyd, 1968; Cortecci, 1974; Chiba and Sakai, 1985). The application of the

18O(SO42--H2O) thermometers of Lloyd (1968) and Mizutani and Rafter (1969) to the hottest

W0 water samples both in 1995 and 1997 yield concordant values of 158 to 173C, which are

significantly lower than the chemical K/Mg thermometric estimates of 187 to 194C. The

influence of the mixing effects on the sulfate-water isotopic fractionation is manifest in water

W2, i.e. its 18O(SO42-) of +9.4 is in equilibrium with water of 18O of +2.9 in the

geothermal reservoir at 254 3C, but subsequent dilution by shallow water during ascent

enlarged the original isotopic fractionation factor leading to a considerably lower

thermometric estimate of 188 3C when the measured 18O(H2O) value of 0.6 is

applied. In spite of the constancy of the 18O(H2O) parameter, the 18O(SO42-) time-variation

in water W0 from +9.4 in 1995 to +8.5 in 1997 appears to be significative, and may be

related to the concurrent lowering of the SO42- concentration, i.e. to dilution by aqueous

sulfate depleted in 18O.

Finally, the 18O(SO42-) of +7.5 in water W34 in 1997 (799 mg/kg SO42-) can be

explained in terms of a mixture of about 40% seawater (Tyrrhenian seawater sulfate: 3170

mg/kg; 18O of +9.5, Cortecci et al., 1974) with about 60% groundwater with sulfate

concentration and sulfate oxygen isotopic composition of about 2000 mg/kg and +5.5,

respectively (see sections 3.2.3 and 3.4.2, and Fig. 16). This mixed water may have been

feeding the W34 well in 1996, and then underwent dilution by very poor in sulfate meteoric

water.

3.5 Strontium isotopes

Sr isotope analyses were performed on water samples from selected wells fed by geothermal

water (W0, 1996; W2, 1996; W4, 1996), steam-heated water (W7, 1996; W28, 1996) and

shallow groundwaters (W17, 1996; W20, 1996; W21, 1996; W23, 1996; W34, 1996 and

1997).

Following the procedure of Pennisi et al. (2000), the 87Sr/86Sr ratios were used in

combination with the 18O signature in order to model the isotopic effects on meteoric water

and fumarolic condensate at depth in the geothermal reservoir due to interaction with the host

volcanic rocks at 250C, and to constrain also by this approach the role of seawater in feeding

the selected wells. In Tab.3 the 18O and 87Sr/86Sr data on analysed waters are reported along

with the reference data used in the modelling for meteoric water, fumarolic condensate and

volcanic rocks, and the applied rock to water oxygen and strontium by weight concentration

ratios (Cr/Cw) and isotopic fractionations. Data sources other than this work are McArthur


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(1994) for the seawater 87Sr/86Sr ratio that is assumed to be equal also for sea-spray and

rainwater; Capasso et al. (1999) and Goff and McMurtry (2000) for the mean 18O value of

high temperature fumarolic condensate during the present survey; Ellam and Harmon (1990)

for the

18

O value averaging most Vulcano basalt samples; De Astis et al. (1997) for the mean87Sr/86Sr ratio of rocks from the caldera La Fossa and the Lentia complex, as an

approximation of the isotopic ratio of the geothermal reservoir; Gioncada et al. (1995) for the

mean 87Sr/86Sr ratio of the fumarolic condensate (approximated from the lava samples). The

strontium Cr/Cwratio was approximated to 100000, as the Sr content in the rocks at Vulcano

averages 1000 ppm (De Astis et al., 1997) and is about 0.01 ppm in rainwater (Pennisi et al.,

2000). The oxygen Cr/Cwratio is that reported in Pennisi et al. (2000; see also Richardson and

McSween, 1989) for the basalt-water system, and the water-rock oxygen isotope fractionation

factor (w-r) is approached by the K-feldspar-water one from ONeil and Taylor (1960). The

Sr isotope fractionation between water and rock is zero. The isotopic effects on water are

described by the following equation, where the water to rock ratio is indicated as N:

w,f= [N w,i+ (Cr/Cw) r,i+ (Cr/Cw) (w-r) ]/[N + (Cr/Cw)]

with = 18O or 87Sr/86Sr; w = water, r = rock, i = initial, f = final.

The main results from the modelling are (Fig. 17): 1) the geothermal water at depth is a

fumarolic condensate exchanged with rocks under a water to rock ratio close to unit, i.e. it is

largely magmatic, and its position on the model curve can be attributed to a shift towards a

depleted 18O value due to water-rock interaction effects (e.g. Giggenbach, 1992 and 1993);

2) the well waters W0, W2 and W4 are mixtures, in variable proportions, of geothermal water

and steam-heated water like W28; and 3) the shallow groundwater W34 appears to be a

mixture of isotopically exchanged meteoric water and normal seawater. In June 1996,

seawater Sr may have been present in the W20 and W17 wells and totally absent in the W21

and W23 wells.

4. Relations between groundwaters and volcanic fluids.

In Fig 18, the sulfate and chloride concentrations of studied groundwaters are compared

with those of crater fumarolic condensates, assuming oxidation of both SO 2and H2S in the

volcanic fluids, and the oxidation of the SO2 alone. The comparison includes also the

available data on the Pozza and Istmo springs (Mazor et al., 1988; Fontes et al., 1993; Aiuppa

et al., 2000). The main feature is the clear indication that all sulfate-chloride waters (SCW)


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both neutral and acid derive from condensation of volcanic fluids, which undergo mixing with

steam-heated water. In details, neutral SCW show a nearly uniform SO4 to Cl ratio, as

expected for binary mixtures between chemically well established endmembers (geothermal

water and steam-heated water). On the other hand, acid SCW are scattered, probably due to

the involvement of additional sources of water like seawater and volcanic condensates

variously enriched in sulfur and chloride.

With respect to the neutral SCW, the acid ones should correspond to a lower maturation

degree in terms of mineral dissolution and related neutralizing effects. Other factors may be

also involved in controlling the pH values and their evolution with time (Mazor et al., 1988;

Fontes et al., 1993; Amend et al., 1998; Aiuppa et al., 2000), like: 1) the size of the aquifer

with respect to the flow of entering volcanic fluids, and 2) the pH of the latter fluids that in

turn depends on the uprising rate and the interaction degree with rocks.

In Fig. 19, the 2H vs 18O field of the steam escaping from the geothermal reservoir (250C;

2H = +4.9;18O = +2.9) as well as from mixtures in different proportions between the

rising geothermal fluids and rainwater at Vulcano Porto (2H = -34, 18O = -6; Capasso et

al., 1992) and Vulcano Piano (2H = -48, 18O = -8; as from data on cistern waters by Goff

and McMurtry, 2000). In the computation, the multistage steam separation method and the

water-steam fractionation factors of Truesdell et al. (1977) were used, assuming 250C for the

geothermal component and 18C for the meteoric component. It is noteworthy that (1) all but

one steam-heated waters lie between the 140 and 160 isotherms, in keeping with the SiO 2-

thermometric estimates (see section 3.2.3); the different behaviour of the steam-heated W28 may

be interpreted in terms of 18O-shift due to interaction with host rocks under low water to rock

ratio, and (2) the steam feeding the beach fumaroles lies on the 140-isotherm, close to the

temperature of 160C estimated by Chiodini et al. (1995; see also Capaccioni et al., 2001) for the

aquifer at depth.

According to Giggenbach and Stewart (1982), the steam-heated water sulfate content can be

related to the H2S content and the steam fraction of the gaseous convoy entering the aquifer,

when H2S is completely oxidized to SO42-in the aquifer:

(SO42-) = 5333 * (H2S) * [Yv/(1-Yv)]

with H2S in mmol/mol, SO42- in mg/kg and Yv being the vapor fraction. Based on this

equation, and applying the highest H2S content of 3 mmol/mol measured in the beach

fumaroles (Chiodini et al., 1995) and a Yvof 0.0421 calculated at 160C in the reservoir, a

SO42- content of 703 mg/kg is obtained for the steam-heated aquifer. This is a minimum


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concentration value for sulfate, as the loss of secondary steam from the aquifer and

dissolution of sulfate minerals (e.g. in water W8; see section 3.2) should be considered. In this

respect, steam-heated waters at Vulcano show sulfate concentrations in the range 700 mg/kg

(W6 in 1996) to 1996 mg/kg (W8 in 1995).

Finally, the only available isotopic data on the acid sulfate-chloride waters refer to the

Pozza spring (Shinohara and Matsuo, 1986). According to Bolognesi and DAmore (1993),

the 18O of +9.4 and 2Hof -1.6 support a magmatic origin for this water.

5. Conclusions

Several concluding remarks derive from the present chemical and isotopic study on the

groundwater system of Vulcano:

1. Independently of the sampling date, all waters are far from chemical equilibrium with host

rocks. Their anion composition ranges from chloride to sulfate-chloride to bicarbonate,

whereas sodium or calcium are the major cations.

2.

Three groups of waters are recognized: (i) shallow groundwaters, which are relatively rich

in chloride and bicarbonate. They derive from direct interaction of shallow meteoric water

with rising crater-type fumarolic/magmatic fluids; (ii) steam-heated waters, enriched in

sulfate and bicarbonate relative to chloride. Depending on the relative abundances, they

can be classified as sulfate-type or bicarbonate-type. These waters originate from

interaction of geothermal steam and gases with shallow aquifers. In turn, the geothermal

reservoir(s) is constituted by fumarolic/magmatic condensate; and (iii) sulfate-chloride

waters, which are mixtures of geothermal water and shallow water, the latter component

being possibly a steam-heated aquifer as shown for the Camping Sicilia well W2. These

waters issue at the base of the volcanic cone.

3. The shallow water component is meteoric both recent and old, that may be contaminated

by seawater. The only significant presence of seawater is testified in the Togo-Togo

Camping well W34 by chemical (sodium and chloride) and isotopic (sulfur and strontium

isotopes) data. Some seawater is suspected in the Scarcella well W20.

4. On the whole, the groundwater system kept rather stable over the period of survey both

chemically and isotopically. Notable chemical and isotopic variations observed in some

wells can be interpreted as due to changes in the mixing proportions of components or to

dilution effects by rainwater.

5.

The sulfur isotopic composition of sulfate was particularly constant, testifying a unique

magmatic major source of sulfur in most aquifers from oxidation of crater-type fumarolic


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sulfur dioxide and hydrogen sulfide. Appreciable variations refer to wells where sulfate

can derive from additional sources like seawater (W20 well) or dissolution in the subsoil

of secondary anhydrite (W16 and W21 wells).

6.

The sulfate-water system in the studied wells is in oxygen isotope disequilibrium, due to

the mixed character of water and sulfate and the slow isotopic exchange rate between

these two compounds. Oxygen isotopes of sulfate in the Camping Sicilia well W2 should

be in equilibrium with water in the geothermal reservoir at the calculated temperature of

254C.

Acknowledgements

Many thanks are due to Sonia Tonarini of the Institute of Geosciences and Georesources

(CNR, Pisa) for the strontium isotope analyses. The research was supported by MURST-ex

60% grants (G. Cortecci) and by the CNR-International Institute for Geothermal Research

1995-1996-1997 ordinary grants (L. Bolognesi).


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Chemical and isotopic compositions of the shallow groundwater system of Vulcano Island, Aeolian Archipelago, Italy: an update