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Chemical and isotopic variations in fumarolic discharge and thermal waters at Vulcano Island (Aeolian Islands, Italy) during 1996: evidence of resumed volcanic activity
Journal of Volcanology and Geothermal Research 88 Ž1999. 167–175
Chemical and isotopic variations in fumarolic discharge and thermal waters at Vulcano Island žAeolian Islands, Italy/ during 1996: evidence of resumed volcanic activity Giorgio Capasso, Rocco Favara, Salvatore Francofonte, Salvatore Inguaggiato
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Istituto di Geochimica dei Fluidi-CNR-Via Ugo La Malfa 153-90146 Palermo, Italy Received 3 April 1998; accepted 12 October 1998
Abstract Gas samples from some fumaroles at ‘La Fossa’ crater and Baia di Levante on Vulcano Island and from a diffuse soil gas emission were analysed during 1995–1996, along with water samples from thermal wells in the area of Vulcano Porto. During 1996, we observed a significant increase both in the gasrsteam ratio and in the CO 2 concentration, as well as strong variations in d13C CO 2 , d D H 2 O and d18 O H 2 O of fumarolic gases. These variations are probably related to an increased inflow of deep fluids of magmatic origin. The temperatures of fumaroles did not show remarkable variations except for fumarole F11. In this case, temperature increased by about 808C from February to August 1996. During the same period, remarkable variations in temperature, phreatic level and chemical and isotopic composition of water were also recorded in one of the geothermal wells in the Vulcano Porto area ŽCamping Sicilia; T ; 608C.. The observed variations in this well are probably related to a pressure build-up, occurring at least in the surficial part of the system, because of increased gas flux andror decreased permeability of the fumarolic degassing system. Chemical and isotopic composition of the water showed that during this evolutionary phase, the content of fumarolic condensate in this well was about 80 to 90%. Based on the observation of physical and chemical variables of the Camping Sicilia fluids, during this phase of activity, it is concluded that this area is affected by a phreatic eruption hazard if a volcanic episode with high energy discharge in a limited time span occurs. It follows that this well may be considered as a preferential point for volcanic activity monitoring, both in the case of normal routine surveillance and in the case of inaccessibility to the crater area. q 1999 Elsevier Science B.V. All rights reserved. Keywords: chemical and isotopic variations; fumarolic discharge and thermal waters; Vulcano Island
1. Introduction Vulcano is the southernmost of the seven islands forming the Aeolian Archipelago ŽSouthern Tyrrhenian Sea, Italy, Fig. 1.. Vulcano and Stromboli are )
Corresponding author. Fax: q39-091-6809449; E-mail: [email protected]
two of the most active volcanoes in this area. Volcanic activity on Vulcano is connected to a tectonic lineament directed NW–SE. This lineament is the surficial expression of the Aeolian Islands–Malta escarpment regional lithospheric discontinuity ŽGabbianelli et al., 1991.. Since the last eruption, in 1888–1890, Vulcano has experienced only fumarolic activity, which con-
0377-0273r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 7 - 0 2 7 3 Ž 9 8 . 0 0 1 1 1 - 5
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G. Capasso et al.r Journal of Volcanology and Geothermal Research 88 (1999) 167–175
Fig. 1. Sketch map of Vulcano Island. Location of sampling sites. M.P. stands for Mud Pool.
centrated near the active edifice of ‘La Fossa’ crater in the northern part of the island. A detailed description of the geological settings of Vulcano Island is given by Keller Ž1980.. La Fossa is a 391-m-high
cone, with a base diameter of 1 km. Five eruptive cycles have been described in its activity ŽFrazzetta et al., 1984., the most recent of which ended with the last eruption. These eruptive cycles show a char-
G. Capasso et al.r Journal of Volcanology and Geothermal Research 88 (1999) 167–175 Table 1 Žcontinued.
Table 1 Analytical results of sampled gaseous emissions Sampler date
T Ž8C.
GrS
13
169
18
d C ŽCO 2 .
dD ŽH 2 O.
d O ŽH 2 O.
FA 08r02r95 18r04r95 03r05r95 27r07r95 23r09r95 14r11r95 22r02r96 04r06r96 12r08r96 20r09r96 11r12r96
515 495 490 490 430 435 430 410 411 437 421
0.10 0.11 0.11 0.10 0.07 0.09 0.06 0.26 0.24 0.16 0.10
y0.4 y0.8 y0.8 y1.1 y1.5 y2.8 y2.3 0.4 0.3 0.0 y0.9
n.d. n.d. n.d. n.d. n.d. n.d. y0.4 y0.8 n.d. y2.0 y4.0
n.d. n.d. n.d. n.d. n.d. n.d. 3.2 5.6 n.d. 5.4 3.7
F11 08r02r95 03r05r95 27r07r95 23r09r95 14r11r95 22r02r96 04r06r96 12r08r96 20r09r96 11r12r96
463 473 445 450 450 440 487 522 520 489
0.08 0.07 0.06 0.05 0.04 0.03 0.21 0.22 0.14 0.08
y1.7 n.d. y1.8 y2.5 y2.8 y2.8 0.1 0.3 y0.2 y0.2
n.d. n.d. n.d. n.d. n.d. 2.7 1.5 n.d. y0.4 y2.6
n.d. n.d. n.d. n.d. n.d. 4.5 3.8 n.d. 4.3 4.4
F58 18r04r95 03r05r95 04r06r96
575 543 447
0.11 0.11 0.27
y0.4 y0.7 0.2
n.d. n.d. n.d.
n.d. n.d. n.d.
F0 03r05r95 04r06r96 11r12r96
n.d. 345 342
0.09 0.25 0.09
y1.3 y0.1 y0.8
n.d. n.d. n.d.
n.d. n.d. n.d.
F5AT 03r05r95 04r06r96 11r12r96
515 523 498
0.10 0.24 0.09
y2.0 0.2 y1.0
n.d. n.d. n.d.
n.d. n.d. n.d.
Mud pool 08r02r95 03r05r95 27r07r95 14r11r95 22r02r96 04r06r96 12r08r96 20r09r96 11r12r96
100 98 100 100 99 100 100 99 99
– – – – – – – – –
y3.1 y3.0 y2.8 y3.0 y3.5 y3.4 y2.9 y2.6 y2.8
n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.
n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.
Sampler date P4Max 08r02r95 03r05r95 27r07r95 14r11r95 22r02r96 04r06r96 12r08r96 20r09r96
T Ž8C.
GrS
d13 C ŽCO 2 .
dD ŽH 2 O.
d18 O ŽH 2 O.
n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.
– – – – – – – –
y2.8 y1.8 y1.8 y2.9 y3.0 y2.9 y2.5 y3.1
n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.
n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.
GrS stands for gasrsteam ratio; n.d.s not determined. d13 C CO 2 values are expressed in d units ‰ vs. PDB, while d D H 2 O and d18 O H 2 O in d units ‰ vs. V-SMOW.
acteristic transition from phreatomagmatic to magmatic activity that often ended with the emplacement of viscous mixed Žtrachytic–rhyolitic. lava flows. During the last 10 years, volcanic activity greatly increased, showing signs of a potential reactivation. The most important evolutionary phases were observed in 1988 ŽBadalamenti et al., 1991a; Barberi et al., 1991. and in 1996 Žthis work.. Since 1988 in fact, remarkable variations in many Žphysico-chemical. geochemical parameters were recorded. In particular, fumarole temperatures generally increased from 3008C in 1987 to 6708C in 1992 and then decreased to 5708C in 1995. Also, the output of steam at the crater fumaroles increased ŽItaliano et al., 1998. and marked variations were observed in the chemical and isotopic compositions of fumarole gases and thermal groundwaters as well. Furthermore, during the same period, the opening of new fractures was recorded, which can account for the increased inflow of hotter and deeper fluids ŽBadalamenti et al., 1991a; Barberi et al., 1991.. At present, volcanic activity on Vulcano is limited to an extensive fumarole field in the northern part of La Fossa crater Ž1008C - T - 5008C., low-temperature fumaroles ŽT ; 1008C. in the Baia Di Levante area, and widespread manifestations in the soil in the Vulcano Porto area and around the volcanic cone Žsee Fig. 1.. Previous studies on the latter manifestations have indicated zones of anomalous degassing of CO 2 and He that are concentrated in the ‘Faraglioni’ area Žnear Mud Pool. and in some lower zones
G. Capasso et al.r Journal of Volcanology and Geothermal Research 88 (1999) 167–175
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Table 2 Physico-chemical values of Camping Sicilia well water Date
Depth
T
pH
Na
K
Ca
Mg
HCO 3
Cl
SO4
F
Br
d18 O
dD
PCO 2
TDS
08r02r95 03r05r95 05r06r95 27r07r95 23r09r95 14r11r95 22r02r96 04r06r96 16r07r96 12r08r96 20r09r96 11r12r96
n.d. n.d. 19.56 19.54 19.49 19.43 19.45 19.50 n.d. 19.50 18.90 19.43
51.4 51.1 52.3 52.3 54.0 51.6 52.0 51.0 53.0 56.0 55.5 55.5
7.04 7.31 7.11 7.38 7.10 7.14 7.12 7.25 7.21 6.64 6.70 7.03
62.5 59.9 59.7 64.0 70.9 64.0 74.5 74.8 87.5 103.5 111.3 107.5
11.5 12.0 11.4 11.1 11.5 12.4 13.9 13.7 17.6 16.1 15.9 16.2
6.7 8.4 7.3 5.1 5.8 5.6 7.6 8.0 8.5 13.9 12.3 12.8
8.0 5.4 5.2 6.6 8.1 7.0 13.6 11.2 12.8 19.0 20.1 21.9
12.5 10.4 10.0 9.8 9.8 9.6 8.6 8.0 14.5 9.1 8.6 7.5
40.0 38.8 37.9 44.2 51.3 45.3 56.6 57.9 76.7 89.3 96.6 97.7
33.2 33.4 32.9 35.6 38.1 35.5 46.3 42.0 47.0 49.2 54.1 55.3
0.24 0.33 0.64 0.36 0.29 0.36 0.52 0.53 0.81 0.23 0.29 0.30
0.05 0.05 0.06 0.04 n.d. 0.05 0.05 0.06 n.d. n.d. 0.11 0.11
n.d. n.d. n.d. n.d. n.d. n.d. 0.7 0.4 0.8 1.3 1.9 2.4
n.d. n.d. n.d. n.d. n.d. n.d. y9.6 y10.3 y8.3 y3.7 y0.8 0.3
0.09 0.04 0.06 0.03 0.06 0.05 0.05 0.03 0.06 0.15 0.13 0.05
5895 5699 5572 5964 6544 6054 7332 7120 9211 9600 10 223 10 208
Temperatures are expressed in 8C, groundwater level ŽDepth. in meters below ground surface, concentrations in meqrl, salinity ŽTDS. in mgrl and PCO 2 in atm PCO 2 values are calculated by the dissolved carbonate species equilibria ŽAppelo, 1988.. The isotopic composition of water is reported in d units ‰ vs. V-SMOW. n.d.s not determined.
of La Fossa crater ŽBadalamenti et al., 1984, 1988, 1991b; Baubron et al., 1990; Carapezza and Diliberto, 1993.. Moreover, the presence in these zones of water wells with major thermal andror geochemical anomalies ŽCarapezza et al., 1983; Dongarra` et al., 1988; Capasso et al., 1991, 1992. confirms the existence of structural weaknesses in the volcanic edifice that serve as preferential pathways for uprising fumarolic fluids. Also, Capasso et al. Ž1996, 1997a,b. pointed out that the variations in time and space of the isotopic and chemical composition of pericrateric manifestations are related to the presence of active tectonic structures and to the variations in the outgassing activity of the crater. In this work, the relationships between the main degassing system Žcrater fumaroles. and the peripheral systems have been checked based on the chemical and isotopic change of fumarolic and soil gases along with thermal groundwaters. The measurements were carried out on samples from some fumaroles of ‘La Fossa’ crater ŽFA, F11, F58, F5, F0., from a low temperature fumarole ŽT s 1008C. in the ‘Baia di Levante’ ŽMud Pool., from a soil gas emission point ŽP4Max., and from the Camping Sicilia well water ŽFig. 1 and Tables 1 and 2.. 2. Sampling and analytical methods Temperature and pH values of water samples were measured directly in the field. Bicarbonate was
also analysed by acid base titration in the field. The major and minor components were determined in the laboratory by ion chromatography with a mean relative error of about "2%. The depth of the groundwater surface was measured with a precision of "1 cm. Various sampling techniques were used, depending on the type of discharge. Steam from crater fumaroles was condensed in an ethyl–ether condenser, whereas dry gases for CO 2 isotopic analyses were collected in a tube connected to the condenser. Bubbling gases from Mud Pool were collected into an inverted funnel, connected to a two-way stopcock, syringe and sampling bottle. At the soil sampling point ŽP4Max, Fig. 1., gases were collected with a modified version of the same apparatus using a metal tube inserted to a depth of about 50 cm into the soil. Condensate and water samples were reacted with zinc ŽKendall and Coplen, 1985. for DrH measurements, and 18 Or16 O measurements were carried out after isotopic equilibration with CO 2 ŽEpstein and Mayeda, 1953.. Isotopic ratio measurements were made on Varian MAT 250 and Finnigan Delta S mass spectrometers. The results are reported as d per mil relative to the V-SMOW standard ŽDrH and 18 Or16 O of water. and relative to PDB Ž13 Cr12 C ratios.. The standard deviation of DrH isotope ratios is close to "1‰ Ž1 s . and that of 18 Or16 O ratios is about "0.2‰ Ž1 s .. The standard deviation of 13 Cr12 C ratios is about "0.2‰ Ž1 s ..
G. Capasso et al.r Journal of Volcanology and Geothermal Research 88 (1999) 167–175
171
3. Results and discussion 3.1. Fumarole discharge and soil gases During the first months of 1996, a new evolutionary phase of the volcanic activity was observed at Vulcano. At the beginning, this activity was characterized by variations in the chemical and isotopic composition of fumarole discharge of ‘La Fossa’ crater. Later on, marked variations in T Ž8C., conductivity and pH were also recorded in the thermal groundwater at Vulcano Porto ŽFig. 1.. During the period March–September, an important increase of the gasrsteam ratio and d13 C CO 2 ŽFig. 2. was observed in the crater fumaroles. Capasso et al. Ž1996, 1997a,b. pointed out that the fumaroles located in the inner part of the crater, representative of the deep system ŽFA, F58., were characterized by d13 C CO 2 values close to 0‰, while the fumaroles located at the crater rim ŽF11, F5, F5AT, F0. were characterized by lighter d13 C CO 2 values Žy2‰.. Interaction
Fig. 2. Variations of gasrsteam ratios and d13 C CO 2 Žvs. PDB. at FA and F11 fumaroles during 1995–1996.
Fig. 3. d13 C CO 2 vs. gasrsteam ratio diagram of crater fumaroles. Dashed lines bound the field values of Vulcano magmatic CO 2 composition ŽCapasso et al., 1996, 1997a,b.. During the evolutionary phase of 1996, all the fumaroles tend to homogenize their chemical and isotopic composition, while they display different values in a period of normal degassing activity, probably caused by shallow hydrothermal interaction phenomena ŽCapasso et al., 1996, 1997a,b..
phenomena between deep gases and the shallow hydrothermal system were likely to be responsible for the different isotopic composition of the rim fumaroles with respect to those inside the crater ŽCapasso et al., 1996, 1997a.. Since June 1995, a general decrease of d13 C CO 2 in both the FA and F11 fumaroles was observed, reaching the value of y2.5‰ between December 1995 and February 1996. The decrease in d13 C CO 2 probably resulted from both a temporary decrease in the magmatic gases input and a greater interaction between deep fluids and the shallow hydrothermal system. In June 1996, d13 C CO 2 in both fumaroles reached values close to 0‰, which are thought to be typical of the magmatic component at Vulcano Island ŽCapasso et al., 1996, 1997a.. This is consistent with the high values of helium isotopic ratio measured in fumarolic gases Ž5.5 - RrRa - 6.2. ŽItaliano and Nuccio, 1996.. The d13 C CO variations 2 were observed both in fumaroles of the inner part of the crater and in the crater rim fumaroles, and are well correlated with the increase of the gasrsteam ratio ŽFig. 3.. The fumarole discharge also showed variations in isotopic composition, more evident in the case of oxygen than in the case of hydrogen ŽFig. 4.. The
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G. Capasso et al.r Journal of Volcanology and Geothermal Research 88 (1999) 167–175
Fig. 4. d D– d18 O diagram of fumarolic steam from 1978 to 1996 and of Camping Sicilia well water from 1987 to 1996. The 1978–1994 data of fumaroles and 1987–1989 data of Camping Sicilia are taken from the works of Bolognesi and D’Amore Ž1993. and Capasso et al. Ž1992, 1997a,b.. The compositional fields relative to ‘Andesitic Water’ ŽGiggenbach, 1992. and Vulcano magmatic water ŽBolognesi and D’Amore, 1993; Capasso et al., 1997a,b. are also reported.
observed increase of the d18 O H 2 O values suggests a larger contribution of deep fluids to the fumarole discharge with values close to the typically magmatic ones Ž d18 O H 2 O q5‰ to q8‰.. The moderate variation in d D H 2 O values seems to exclude a similarity between the values of the Vulcano magmatic water and those of Andesitic water ŽGiggenbach, 1992. supporting, as already suggested ŽBolognesi and D’Amore, 1993; Capasso et al., 1996, 1997a., the hypothesis of more positive values for the Vulcano water. The increase of the gasrsteam ratio, along with the variations in the isotopic composition of CO 2 and of fumarole discharges were probably caused by a renewed input of magmatic fluids large enough to homogenize, isotopically and chemically, the entire fumarolic field. 3.2. Thermal groundwaters Throughout 1996, almost all the sampled wells in the Vulcano Porto area showed only normal seasonal oscillations in measured physical and chemical variables ŽCapasso et al., 1998.. Only one of the warmest
wells, Camping Sicilia ŽFig. 5., showed significant variations in temperature Žan increase of about 58C between June and August.. Episodes of increasing water temperature, from 588C up to 628C, were recorded in August 1988 at this well ŽDongarra` et al., 1988; Capasso et al., 1991., along with an increase in the outgassing activity of the volcano ŽBadalamenti et al., 1991a,b; Barberi et al., 1991.. An increase of the bicarbonate content was observed in July 1996 ŽFig. 5. together with the rise of temperature, probably due to a greater contribution of CO 2-rich gases to well waters. This is consistent with the recorded increase of the CO 2 flux from the soil which took place, in the same period, in the area surrounding the crater ŽBadalamenti et al., 1997.. Later on, the increased CO 2 caused the lowering of pH and a shift of the equilibrium of carbonate species towards dissolved CO 2 ŽFig. 5.. A large increase of salinity was observed ŽFig. 5. from the first months of 1996 reaching values of about 10 grl, essentially due to greater concentrations of Na, Cl and SO4 . The increase in SO4 and Cl, recorded between June and September 1996, corresponds to a decrease of the pH values and, as
G. Capasso et al.r Journal of Volcanology and Geothermal Research 88 (1999) 167–175
Fig. 5. Variations of some physico-chemical parameters recorded in Camping Sicilia well water during 1995–1996. Salinity values ŽTDS. are expressed in mgrl, HCOy 3 in meqrl, PCO 2 in atm and depth of groundwater level in m from well head.
previously observed, to a temperature increase. It follows that the increase in Cl content cannot be related only to a greater sea water contribution. Assuming that the Br content is essentially of sea water origin, a contribution of about 10% of this component can be calculated. However, taking into consideration a possible fumarolic condensate origin for a part of the Br content, the calculated contribution of sea water is estimated to be well below 10%. The excess content of Cl and SO4 , after scaling the concentrations related to the sea water contribution, could be due to a greater contribution of deep fluids ŽCl contents of fumaroles condensates from 8 to 60 meqrl. to the well water. The presence of a considerable amount of fumarolic condensate in the
173
Camping Sicilia well is also suggested by the high boron content in this water ŽB s 10–16 ppm; Capasso et al., 1997b.. Previous studies of the isotopic composition of the waters and of the fumarolic condensate highlighted a close relationship between the DrH ratio of the water of the Camping Sicilia well and the fumarolic condensate ŽCapasso et al., 1992.. During June–September 1996, the isotope values of the Camping Sicilia well water ŽFig. 5. show a d D increase of about 11‰ and a d18 O increase of 2‰, reaching d D values heavier than those of fumarolic steam. Recalculating the d D of the Camping Sicilia well water as a function of the sea water contribution previously estimated from chemical data, values very close to those of the crater fumaroles are obtained. From these considerations, it is apparent that during this evolutionary phase, the contribution of fumarolic condensate to the groundwater of Camping Sicilia reaches 80 to 90%. The remaining 10 to 20% may be ascribed to sea water, while the meteoric component is negligible. The supply of the fumarolic condensate to the well waters is probably related to a preferential drainage structure that conveys the condensed fumarolic vapour within some section of ‘La Fossa’ edifice ŽChiodini et al., 1996. to the groundwater of the Camping Sicilia. In September 1996, a marked rise Žabout 60 cm. of the groundwater level was recorded at the Camping Sicilia well ŽFig. 5., this rise being the largest one ever observed in the Vulcano wells. From a hydrological point of view, such a large variation cannot be attributed to recharge by meteoric waters, since it was detected at the end of the dry season when the Vulcano Porto groundwater is intensely exploited ŽDongarra` et al., 1988; Capasso et al., 1991.. This conclusion is also supported by hydrological data that locate an abnormally high phreatic level in the area of the Camping Sicilia relative to both the Vulcano Porto water table and the meteoric recharge coming from the Vulcano Piano area ŽMadonia, P., personal communication.. Finally, the temperature and salinity increase rules out the hypothesis of a meteoric water supply. In this context, the observed variations of the phreatic surface can be plausibly explained by the temporary formation of a permeable conduit located along degassing tectonic structures. The accumulation of gases in the soil due to higher degassing rates causes a decrease in both
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G. Capasso et al.r Journal of Volcanology and Geothermal Research 88 (1999) 167–175
permeability and groundwater discharge and, as a direct consequence, an increase of the phreatic level. In addition, the increased interaction between fumarolic gases and groundwater caused the observed variations in chemical and isotopic parameters. This hypothesis is also compatible with the existence of a very thin groundwater layer in this area which may be strongly influenced by the supply of fumarolic fluids. Volcanic gases could easily pass through it when degassing increases along tectonic zones. Such a phenomenon has been observed repeatedly in this area with the occasional appearance of fumarolic emissions ŽT f 100 C8. or mofettes. The large variations of rock resistivity recorded in the Camping Sicilia area Žfour orders of magnitude in a time span of a few months, Di Maio et al., 1994. seem to support this hypothesis. This interpretation has important implications for hazard evaluation in Vulcano where phreatic eruptions have occurred through the recent history of the island. A combined effort in order to relate geophysical monitoring Žresistivity, deformation. with the ingress of deep fluids into the volcanic system is highly important. The aim of such a study would primarily be the quantification of the volcanic impact in terms of fluids and affected rock volumes.
4. Conclusions The increase of the gasrsteam ratio as well as the variations in the isotopic composition of CO 2 and steam observed in 1996 were probably caused by an input of fluids of magmatic origin during a new evolutionary phase of volcanic activity. The marked variations of some physico-chemical parameters such as temperature, conductivity and pH in the water of the Camping Sicilia well confirm its importance in volcanic surveillance in Vulcano. Nevertheless, the first variations in this well were observed about 2 months later than those recorded at the crater fumaroles. This delay is possibly due to delayed pressure rise in the peripheral part of the system. The increase of pressure, probably related to variations in the discharge rate of the deep fluids towards the surface Žincrease of the gas flux andror decrease of the permeability of the fumarolic outgassing system., determined the draining of the flu-
ids towards the peripheral systems, causing the observed variations at the Camping Sicilia well and the increase of CO 2 fluxes from the soil in the area surrounding the crater ŽBadalamenti et al., 1997.. Under normal conditions, the fluids are mainly drained by the fumaroles of ‘La Fossa’ crater, and, to a lesser degree, by the system of fractures related to the ‘Baia di Levante’ fumaroles and to the P4Max area. The time span of this evolution is well defined by three parameters which can be easily measured: temperature, pH and groundwater level. In fact, while the beginning of the evolution ŽJuly 1996. was marked by the temperature increase, its ending was marked by the increase of pH and by lowering of the groundwater level which occurred in December 1996 ŽFig. 5.. The relative amount of fumarolic condensate in the Camping Sicilia well water clearly indicates interactions between deep fluids and groundwater. This observable phenomenon is thus, indirectly an indication of the risk of the phreatic eruption hazard that affects this area.
Acknowledgements We thank Yuri Taran, Niels Oskarsson and Antonio Longinelli for helpful suggestions and reviews of early versions of this paper. This study was performed with the financial support of the C.N.R.-National Group for Volcanology, Italy.
References Appelo, C.A.J., 1988. WATEQP program. Inst. Voor Aardweten Schappen, Vrije Universiteit, Amsterdam. Badalamenti, B., Gurrieri, S., Hauser, S., Tonani, F., Valenza, M., 1984. Considerazioni sulla concentrazione e sulla composizione isotopica della CO 2 presente nelle manifestazioni naturali e nell’atmosfera dell’isola di Vulcano. Rend. Soc. It. Miner. Petrol. 39, 367–378. Badalamenti, B., Gurrieri, S., Hauser, S., Parello, F., Valenza, M., 1988. Soil CO 2 output in the island of Vulcano during the period 1984–1988: surveillance of gas hazard and volcanic activity. Rend. Soc. It. Miner. Petrol. 43, 893–899. Badalamenti, B., Chiodini, G., Cioni, R., Favara, R., Francofonte, S., Gurrieri, S., Hauser, S., Inguaggiato, S., Italiano, F., Magro, G., Nuccio, P.M., Parello, F., Pennisi, M., Romeo, L., Russo, M., Sortino, F., Valenza, M., Vurro, F., 1991a. Special
G. Capasso et al.r Journal of Volcanology and Geothermal Research 88 (1999) 167–175 Field Workshop at Vulcano ŽAeolian Islands. during summer 1988: geochemical results. Acta Vulcanol. 1, 223–228. Badalamenti, B., Gurrieri, S., Hauser, S., Parello, F., Valenza, M., 1991b. Change in CO 2 output at Vulcano island during summer 1988. Acta Vulcanol. 1, 219–221. Badalamenti, B., Diliberto, I.S., Giammanco, S., Gurrieri, S., Valenza, M., 1997. Soil CO 2 degassing in some active volcanic areas of Italy. IAVCEI, General Ass. ’97. Puerto Vallarta, Mexico, Jan. 19–24. Volcanic activity and the environment. Abstracts Vol., p. 77. Barberi, F., Neri, G., Valenza, M., Villari, L., 1991. 1987–1990 unrest at Vulcano. Acta Vulcanol. 1, 95–106. Baubron, C., Allard, P., Toutain, J.P., 1990. Diffuse volcanic emissions of carbon dioxide from Vulcano Island, Italy. Nature 344, 51–53. Bolognesi, L., D’Amore, F., 1993. Isotopic variation of the hydrothermal system on Vulcano Island. Italy Geochim. Cosmochim. Acta 57, 2069–2082. Capasso, G., Dongarra, ` G., Hauser, S., Favara, R., Valenza, M., 1991. Chemical changes in waters from Vulcano island: an update. Acta Vulcanol. 1, 199–209. Capasso, G., Dongarra, ` G., Hauser, S., Favara, R., Valenza, M., 1992. Isotope composition of rain water, well water and fumarole steam on the island of Vulcano, and their implications for volcanic surveillance. J. Volcanol. Geotherm. Res. 49, 147–155. Capasso, G., Favara, R., Inguaggiato, S., 1996. Chemical and isotopic characterization of gaseous emissions on Vulcano Island ŽAeolian Islands, Italy.. Proc. fourth Intern. Symp. on the Geochem. of the Earth’s Surf., Ilkley, England, 22–28 July. Abstract Vol., pp. 554–558. Capasso, G., Favara, R., Inguaggiato, S., 1997a. Chemical features and isotopic composition of gaseous manifestations on Vulcano Island ŽAeolian Islands, Italy.: an interpretative model of fluid circulation. Geochim. Cosmochim. Acta 61, 3425– 3440. Capasso, G., D’Alessandro, W., Favara, R., Inguaggiato, S., Parello, F., 1997b. Interaction between deep fluids and the shallow groundwater on the Vulcano Island ŽItaly.. IAVCEI, General Ass. ’97. Volcanic activity and the environment, Puerto Vallarta, Mexico, Jan. 19–24. Abstracts Vol., p. 46. Capasso, G., Favara, R., Francofonte, S., Inguaggiato, S., 1998. Geochemical surveillance of thermal waters and gaseous emis-
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sions at Vulcano Island ŽAeolian Islands, Italy.. Acta Vulcanol., in press. Carapezza, M.L., Diliberto, I.S., 1993. Helium and CO 2 soil degassing. In: ‘Data related to eruptive activity, unrest phenomena and other observation on the Italian active volcanoes in 1991. Vulcano and Stromboli’. Acta Vulcanol. 3, pp. 273–276. Carapezza, M., Dongarra, ` G., Hauser, S., Longinelli, A., 1983. Preliminary isotopic investigations on thermal waters from Vulcano Island, Italy. Miner. Petrogr. Acta 27, 221–232. Chiodini, G., Frondini, F., Raco, B., 1996. Diffuse emission of CO 2 from the Fossa crater, Vulcano Island ŽItaly.. Bull. Volcanol. 58, 41–50. Di Maio, R., Mauriello, P., Patella, D., Petrillo, Z., Piscitelli, S., Siniscalchi, A., Veneruso, M., 1994. Self-potential and dipole–dipole geoelectrical measurements ŽVulcano.. Acta Vulcanol. 6, 64–66. Dongarra, ` G., Hauser, S., Capasso, G., Favara, R., 1988. Characteristics of variations in water chemistry of some wells from Vulcano island. Rend. Soc. It. Miner. Petrol. 43, 1123–1131. Epstein, S., Mayeda, T., 1953. Variations of 18 O content of water from natural sources. Geochim. Cosmochim. Acta. 4, 213–224. Frazzetta, G., Gillot, P.Y., La Volpe, L., Sheridan, M.F., 1984. Volcanic hazard at Fossa of Vulcano: data from the last 6000 years. Bull. Volcanol. 47, 105–124. Gabbianelli, G., Romagnoli, C., Rossi, P.L., Calanchi, N., Lucchini, F., 1991. Submarine morphology and tectonics of Vulcano ŽAeolian Islands, southeastern Tyrrhenian Sea.. Acta Vulcanol. 1, 135–141. Giggenbach, W.F., 1992. Isotopic shifts in waters from geothermal and volcanic systems along convergent plate boundaries and their origin. Earth Planet. Sci. Lett. 113, 495–510. Italiano, F., Nuccio, P.M., 1996. Isotopic ratios of helium in fumaroles from Vulcano Island. Acta Vulcanol. 8 Ž2., 212–215. Italiano, F., Nuccio, P.M., Pecoraino, G., 1998. Steam output from fumaroles of an active volcano: tectonic and magmatichydrothermal controls on the degassing system at Vulcano ŽAeolian arc.. J. Geophys. Res., in press. Keller, J., 1980. The island of Vulcano. Rend. Soc. It. Miner. Petrol. 36, 489–584. Kendall, C., Coplen, T.B., 1985. Multisample conversion of water to hydrogen by zinc for stable isotope determination. Anal. Chem. 57, 1437–1440.
Chemical and isotopic variations in fumarolic discharge and thermal waters at Vulcano Island žAeolian Islands, Italy/ during 1996: evidence of resumed volcanic activity Giorgio Capasso, Rocco Favara, Salvatore Francofonte, Salvatore Inguaggiato
)
Istituto di Geochimica dei Fluidi-CNR-Via Ugo La Malfa 153-90146 Palermo, Italy Received 3 April 1998; accepted 12 October 1998
Abstract Gas samples from some fumaroles at ‘La Fossa’ crater and Baia di Levante on Vulcano Island and from a diffuse soil gas emission were analysed during 1995–1996, along with water samples from thermal wells in the area of Vulcano Porto. During 1996, we observed a significant increase both in the gasrsteam ratio and in the CO 2 concentration, as well as strong variations in d13C CO 2 , d D H 2 O and d18 O H 2 O of fumarolic gases. These variations are probably related to an increased inflow of deep fluids of magmatic origin. The temperatures of fumaroles did not show remarkable variations except for fumarole F11. In this case, temperature increased by about 808C from February to August 1996. During the same period, remarkable variations in temperature, phreatic level and chemical and isotopic composition of water were also recorded in one of the geothermal wells in the Vulcano Porto area ŽCamping Sicilia; T ; 608C.. The observed variations in this well are probably related to a pressure build-up, occurring at least in the surficial part of the system, because of increased gas flux andror decreased permeability of the fumarolic degassing system. Chemical and isotopic composition of the water showed that during this evolutionary phase, the content of fumarolic condensate in this well was about 80 to 90%. Based on the observation of physical and chemical variables of the Camping Sicilia fluids, during this phase of activity, it is concluded that this area is affected by a phreatic eruption hazard if a volcanic episode with high energy discharge in a limited time span occurs. It follows that this well may be considered as a preferential point for volcanic activity monitoring, both in the case of normal routine surveillance and in the case of inaccessibility to the crater area. q 1999 Elsevier Science B.V. All rights reserved. Keywords: chemical and isotopic variations; fumarolic discharge and thermal waters; Vulcano Island
1. Introduction Vulcano is the southernmost of the seven islands forming the Aeolian Archipelago ŽSouthern Tyrrhenian Sea, Italy, Fig. 1.. Vulcano and Stromboli are )
Corresponding author. Fax: q39-091-6809449; E-mail: [email protected]
two of the most active volcanoes in this area. Volcanic activity on Vulcano is connected to a tectonic lineament directed NW–SE. This lineament is the surficial expression of the Aeolian Islands–Malta escarpment regional lithospheric discontinuity ŽGabbianelli et al., 1991.. Since the last eruption, in 1888–1890, Vulcano has experienced only fumarolic activity, which con-
0377-0273r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 7 - 0 2 7 3 Ž 9 8 . 0 0 1 1 1 - 5
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G. Capasso et al.r Journal of Volcanology and Geothermal Research 88 (1999) 167–175
Fig. 1. Sketch map of Vulcano Island. Location of sampling sites. M.P. stands for Mud Pool.
centrated near the active edifice of ‘La Fossa’ crater in the northern part of the island. A detailed description of the geological settings of Vulcano Island is given by Keller Ž1980.. La Fossa is a 391-m-high
cone, with a base diameter of 1 km. Five eruptive cycles have been described in its activity ŽFrazzetta et al., 1984., the most recent of which ended with the last eruption. These eruptive cycles show a char-
G. Capasso et al.r Journal of Volcanology and Geothermal Research 88 (1999) 167–175 Table 1 Žcontinued.
Table 1 Analytical results of sampled gaseous emissions Sampler date
T Ž8C.
GrS
13
169
18
d C ŽCO 2 .
dD ŽH 2 O.
d O ŽH 2 O.
FA 08r02r95 18r04r95 03r05r95 27r07r95 23r09r95 14r11r95 22r02r96 04r06r96 12r08r96 20r09r96 11r12r96
515 495 490 490 430 435 430 410 411 437 421
0.10 0.11 0.11 0.10 0.07 0.09 0.06 0.26 0.24 0.16 0.10
y0.4 y0.8 y0.8 y1.1 y1.5 y2.8 y2.3 0.4 0.3 0.0 y0.9
n.d. n.d. n.d. n.d. n.d. n.d. y0.4 y0.8 n.d. y2.0 y4.0
n.d. n.d. n.d. n.d. n.d. n.d. 3.2 5.6 n.d. 5.4 3.7
F11 08r02r95 03r05r95 27r07r95 23r09r95 14r11r95 22r02r96 04r06r96 12r08r96 20r09r96 11r12r96
463 473 445 450 450 440 487 522 520 489
0.08 0.07 0.06 0.05 0.04 0.03 0.21 0.22 0.14 0.08
y1.7 n.d. y1.8 y2.5 y2.8 y2.8 0.1 0.3 y0.2 y0.2
n.d. n.d. n.d. n.d. n.d. 2.7 1.5 n.d. y0.4 y2.6
n.d. n.d. n.d. n.d. n.d. 4.5 3.8 n.d. 4.3 4.4
F58 18r04r95 03r05r95 04r06r96
575 543 447
0.11 0.11 0.27
y0.4 y0.7 0.2
n.d. n.d. n.d.
n.d. n.d. n.d.
F0 03r05r95 04r06r96 11r12r96
n.d. 345 342
0.09 0.25 0.09
y1.3 y0.1 y0.8
n.d. n.d. n.d.
n.d. n.d. n.d.
F5AT 03r05r95 04r06r96 11r12r96
515 523 498
0.10 0.24 0.09
y2.0 0.2 y1.0
n.d. n.d. n.d.
n.d. n.d. n.d.
Mud pool 08r02r95 03r05r95 27r07r95 14r11r95 22r02r96 04r06r96 12r08r96 20r09r96 11r12r96
100 98 100 100 99 100 100 99 99
– – – – – – – – –
y3.1 y3.0 y2.8 y3.0 y3.5 y3.4 y2.9 y2.6 y2.8
n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.
n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.
Sampler date P4Max 08r02r95 03r05r95 27r07r95 14r11r95 22r02r96 04r06r96 12r08r96 20r09r96
T Ž8C.
GrS
d13 C ŽCO 2 .
dD ŽH 2 O.
d18 O ŽH 2 O.
n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.
– – – – – – – –
y2.8 y1.8 y1.8 y2.9 y3.0 y2.9 y2.5 y3.1
n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.
n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.
GrS stands for gasrsteam ratio; n.d.s not determined. d13 C CO 2 values are expressed in d units ‰ vs. PDB, while d D H 2 O and d18 O H 2 O in d units ‰ vs. V-SMOW.
acteristic transition from phreatomagmatic to magmatic activity that often ended with the emplacement of viscous mixed Žtrachytic–rhyolitic. lava flows. During the last 10 years, volcanic activity greatly increased, showing signs of a potential reactivation. The most important evolutionary phases were observed in 1988 ŽBadalamenti et al., 1991a; Barberi et al., 1991. and in 1996 Žthis work.. Since 1988 in fact, remarkable variations in many Žphysico-chemical. geochemical parameters were recorded. In particular, fumarole temperatures generally increased from 3008C in 1987 to 6708C in 1992 and then decreased to 5708C in 1995. Also, the output of steam at the crater fumaroles increased ŽItaliano et al., 1998. and marked variations were observed in the chemical and isotopic compositions of fumarole gases and thermal groundwaters as well. Furthermore, during the same period, the opening of new fractures was recorded, which can account for the increased inflow of hotter and deeper fluids ŽBadalamenti et al., 1991a; Barberi et al., 1991.. At present, volcanic activity on Vulcano is limited to an extensive fumarole field in the northern part of La Fossa crater Ž1008C - T - 5008C., low-temperature fumaroles ŽT ; 1008C. in the Baia Di Levante area, and widespread manifestations in the soil in the Vulcano Porto area and around the volcanic cone Žsee Fig. 1.. Previous studies on the latter manifestations have indicated zones of anomalous degassing of CO 2 and He that are concentrated in the ‘Faraglioni’ area Žnear Mud Pool. and in some lower zones
G. Capasso et al.r Journal of Volcanology and Geothermal Research 88 (1999) 167–175
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Table 2 Physico-chemical values of Camping Sicilia well water Date
Depth
T
pH
Na
K
Ca
Mg
HCO 3
Cl
SO4
F
Br
d18 O
dD
PCO 2
TDS
08r02r95 03r05r95 05r06r95 27r07r95 23r09r95 14r11r95 22r02r96 04r06r96 16r07r96 12r08r96 20r09r96 11r12r96
n.d. n.d. 19.56 19.54 19.49 19.43 19.45 19.50 n.d. 19.50 18.90 19.43
51.4 51.1 52.3 52.3 54.0 51.6 52.0 51.0 53.0 56.0 55.5 55.5
7.04 7.31 7.11 7.38 7.10 7.14 7.12 7.25 7.21 6.64 6.70 7.03
62.5 59.9 59.7 64.0 70.9 64.0 74.5 74.8 87.5 103.5 111.3 107.5
11.5 12.0 11.4 11.1 11.5 12.4 13.9 13.7 17.6 16.1 15.9 16.2
6.7 8.4 7.3 5.1 5.8 5.6 7.6 8.0 8.5 13.9 12.3 12.8
8.0 5.4 5.2 6.6 8.1 7.0 13.6 11.2 12.8 19.0 20.1 21.9
12.5 10.4 10.0 9.8 9.8 9.6 8.6 8.0 14.5 9.1 8.6 7.5
40.0 38.8 37.9 44.2 51.3 45.3 56.6 57.9 76.7 89.3 96.6 97.7
33.2 33.4 32.9 35.6 38.1 35.5 46.3 42.0 47.0 49.2 54.1 55.3
0.24 0.33 0.64 0.36 0.29 0.36 0.52 0.53 0.81 0.23 0.29 0.30
0.05 0.05 0.06 0.04 n.d. 0.05 0.05 0.06 n.d. n.d. 0.11 0.11
n.d. n.d. n.d. n.d. n.d. n.d. 0.7 0.4 0.8 1.3 1.9 2.4
n.d. n.d. n.d. n.d. n.d. n.d. y9.6 y10.3 y8.3 y3.7 y0.8 0.3
0.09 0.04 0.06 0.03 0.06 0.05 0.05 0.03 0.06 0.15 0.13 0.05
5895 5699 5572 5964 6544 6054 7332 7120 9211 9600 10 223 10 208
Temperatures are expressed in 8C, groundwater level ŽDepth. in meters below ground surface, concentrations in meqrl, salinity ŽTDS. in mgrl and PCO 2 in atm PCO 2 values are calculated by the dissolved carbonate species equilibria ŽAppelo, 1988.. The isotopic composition of water is reported in d units ‰ vs. V-SMOW. n.d.s not determined.
of La Fossa crater ŽBadalamenti et al., 1984, 1988, 1991b; Baubron et al., 1990; Carapezza and Diliberto, 1993.. Moreover, the presence in these zones of water wells with major thermal andror geochemical anomalies ŽCarapezza et al., 1983; Dongarra` et al., 1988; Capasso et al., 1991, 1992. confirms the existence of structural weaknesses in the volcanic edifice that serve as preferential pathways for uprising fumarolic fluids. Also, Capasso et al. Ž1996, 1997a,b. pointed out that the variations in time and space of the isotopic and chemical composition of pericrateric manifestations are related to the presence of active tectonic structures and to the variations in the outgassing activity of the crater. In this work, the relationships between the main degassing system Žcrater fumaroles. and the peripheral systems have been checked based on the chemical and isotopic change of fumarolic and soil gases along with thermal groundwaters. The measurements were carried out on samples from some fumaroles of ‘La Fossa’ crater ŽFA, F11, F58, F5, F0., from a low temperature fumarole ŽT s 1008C. in the ‘Baia di Levante’ ŽMud Pool., from a soil gas emission point ŽP4Max., and from the Camping Sicilia well water ŽFig. 1 and Tables 1 and 2.. 2. Sampling and analytical methods Temperature and pH values of water samples were measured directly in the field. Bicarbonate was
also analysed by acid base titration in the field. The major and minor components were determined in the laboratory by ion chromatography with a mean relative error of about "2%. The depth of the groundwater surface was measured with a precision of "1 cm. Various sampling techniques were used, depending on the type of discharge. Steam from crater fumaroles was condensed in an ethyl–ether condenser, whereas dry gases for CO 2 isotopic analyses were collected in a tube connected to the condenser. Bubbling gases from Mud Pool were collected into an inverted funnel, connected to a two-way stopcock, syringe and sampling bottle. At the soil sampling point ŽP4Max, Fig. 1., gases were collected with a modified version of the same apparatus using a metal tube inserted to a depth of about 50 cm into the soil. Condensate and water samples were reacted with zinc ŽKendall and Coplen, 1985. for DrH measurements, and 18 Or16 O measurements were carried out after isotopic equilibration with CO 2 ŽEpstein and Mayeda, 1953.. Isotopic ratio measurements were made on Varian MAT 250 and Finnigan Delta S mass spectrometers. The results are reported as d per mil relative to the V-SMOW standard ŽDrH and 18 Or16 O of water. and relative to PDB Ž13 Cr12 C ratios.. The standard deviation of DrH isotope ratios is close to "1‰ Ž1 s . and that of 18 Or16 O ratios is about "0.2‰ Ž1 s .. The standard deviation of 13 Cr12 C ratios is about "0.2‰ Ž1 s ..
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171
3. Results and discussion 3.1. Fumarole discharge and soil gases During the first months of 1996, a new evolutionary phase of the volcanic activity was observed at Vulcano. At the beginning, this activity was characterized by variations in the chemical and isotopic composition of fumarole discharge of ‘La Fossa’ crater. Later on, marked variations in T Ž8C., conductivity and pH were also recorded in the thermal groundwater at Vulcano Porto ŽFig. 1.. During the period March–September, an important increase of the gasrsteam ratio and d13 C CO 2 ŽFig. 2. was observed in the crater fumaroles. Capasso et al. Ž1996, 1997a,b. pointed out that the fumaroles located in the inner part of the crater, representative of the deep system ŽFA, F58., were characterized by d13 C CO 2 values close to 0‰, while the fumaroles located at the crater rim ŽF11, F5, F5AT, F0. were characterized by lighter d13 C CO 2 values Žy2‰.. Interaction
Fig. 2. Variations of gasrsteam ratios and d13 C CO 2 Žvs. PDB. at FA and F11 fumaroles during 1995–1996.
Fig. 3. d13 C CO 2 vs. gasrsteam ratio diagram of crater fumaroles. Dashed lines bound the field values of Vulcano magmatic CO 2 composition ŽCapasso et al., 1996, 1997a,b.. During the evolutionary phase of 1996, all the fumaroles tend to homogenize their chemical and isotopic composition, while they display different values in a period of normal degassing activity, probably caused by shallow hydrothermal interaction phenomena ŽCapasso et al., 1996, 1997a,b..
phenomena between deep gases and the shallow hydrothermal system were likely to be responsible for the different isotopic composition of the rim fumaroles with respect to those inside the crater ŽCapasso et al., 1996, 1997a.. Since June 1995, a general decrease of d13 C CO 2 in both the FA and F11 fumaroles was observed, reaching the value of y2.5‰ between December 1995 and February 1996. The decrease in d13 C CO 2 probably resulted from both a temporary decrease in the magmatic gases input and a greater interaction between deep fluids and the shallow hydrothermal system. In June 1996, d13 C CO 2 in both fumaroles reached values close to 0‰, which are thought to be typical of the magmatic component at Vulcano Island ŽCapasso et al., 1996, 1997a.. This is consistent with the high values of helium isotopic ratio measured in fumarolic gases Ž5.5 - RrRa - 6.2. ŽItaliano and Nuccio, 1996.. The d13 C CO variations 2 were observed both in fumaroles of the inner part of the crater and in the crater rim fumaroles, and are well correlated with the increase of the gasrsteam ratio ŽFig. 3.. The fumarole discharge also showed variations in isotopic composition, more evident in the case of oxygen than in the case of hydrogen ŽFig. 4.. The
172
G. Capasso et al.r Journal of Volcanology and Geothermal Research 88 (1999) 167–175
Fig. 4. d D– d18 O diagram of fumarolic steam from 1978 to 1996 and of Camping Sicilia well water from 1987 to 1996. The 1978–1994 data of fumaroles and 1987–1989 data of Camping Sicilia are taken from the works of Bolognesi and D’Amore Ž1993. and Capasso et al. Ž1992, 1997a,b.. The compositional fields relative to ‘Andesitic Water’ ŽGiggenbach, 1992. and Vulcano magmatic water ŽBolognesi and D’Amore, 1993; Capasso et al., 1997a,b. are also reported.
observed increase of the d18 O H 2 O values suggests a larger contribution of deep fluids to the fumarole discharge with values close to the typically magmatic ones Ž d18 O H 2 O q5‰ to q8‰.. The moderate variation in d D H 2 O values seems to exclude a similarity between the values of the Vulcano magmatic water and those of Andesitic water ŽGiggenbach, 1992. supporting, as already suggested ŽBolognesi and D’Amore, 1993; Capasso et al., 1996, 1997a., the hypothesis of more positive values for the Vulcano water. The increase of the gasrsteam ratio, along with the variations in the isotopic composition of CO 2 and of fumarole discharges were probably caused by a renewed input of magmatic fluids large enough to homogenize, isotopically and chemically, the entire fumarolic field. 3.2. Thermal groundwaters Throughout 1996, almost all the sampled wells in the Vulcano Porto area showed only normal seasonal oscillations in measured physical and chemical variables ŽCapasso et al., 1998.. Only one of the warmest
wells, Camping Sicilia ŽFig. 5., showed significant variations in temperature Žan increase of about 58C between June and August.. Episodes of increasing water temperature, from 588C up to 628C, were recorded in August 1988 at this well ŽDongarra` et al., 1988; Capasso et al., 1991., along with an increase in the outgassing activity of the volcano ŽBadalamenti et al., 1991a,b; Barberi et al., 1991.. An increase of the bicarbonate content was observed in July 1996 ŽFig. 5. together with the rise of temperature, probably due to a greater contribution of CO 2-rich gases to well waters. This is consistent with the recorded increase of the CO 2 flux from the soil which took place, in the same period, in the area surrounding the crater ŽBadalamenti et al., 1997.. Later on, the increased CO 2 caused the lowering of pH and a shift of the equilibrium of carbonate species towards dissolved CO 2 ŽFig. 5.. A large increase of salinity was observed ŽFig. 5. from the first months of 1996 reaching values of about 10 grl, essentially due to greater concentrations of Na, Cl and SO4 . The increase in SO4 and Cl, recorded between June and September 1996, corresponds to a decrease of the pH values and, as
G. Capasso et al.r Journal of Volcanology and Geothermal Research 88 (1999) 167–175
Fig. 5. Variations of some physico-chemical parameters recorded in Camping Sicilia well water during 1995–1996. Salinity values ŽTDS. are expressed in mgrl, HCOy 3 in meqrl, PCO 2 in atm and depth of groundwater level in m from well head.
previously observed, to a temperature increase. It follows that the increase in Cl content cannot be related only to a greater sea water contribution. Assuming that the Br content is essentially of sea water origin, a contribution of about 10% of this component can be calculated. However, taking into consideration a possible fumarolic condensate origin for a part of the Br content, the calculated contribution of sea water is estimated to be well below 10%. The excess content of Cl and SO4 , after scaling the concentrations related to the sea water contribution, could be due to a greater contribution of deep fluids ŽCl contents of fumaroles condensates from 8 to 60 meqrl. to the well water. The presence of a considerable amount of fumarolic condensate in the
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Camping Sicilia well is also suggested by the high boron content in this water ŽB s 10–16 ppm; Capasso et al., 1997b.. Previous studies of the isotopic composition of the waters and of the fumarolic condensate highlighted a close relationship between the DrH ratio of the water of the Camping Sicilia well and the fumarolic condensate ŽCapasso et al., 1992.. During June–September 1996, the isotope values of the Camping Sicilia well water ŽFig. 5. show a d D increase of about 11‰ and a d18 O increase of 2‰, reaching d D values heavier than those of fumarolic steam. Recalculating the d D of the Camping Sicilia well water as a function of the sea water contribution previously estimated from chemical data, values very close to those of the crater fumaroles are obtained. From these considerations, it is apparent that during this evolutionary phase, the contribution of fumarolic condensate to the groundwater of Camping Sicilia reaches 80 to 90%. The remaining 10 to 20% may be ascribed to sea water, while the meteoric component is negligible. The supply of the fumarolic condensate to the well waters is probably related to a preferential drainage structure that conveys the condensed fumarolic vapour within some section of ‘La Fossa’ edifice ŽChiodini et al., 1996. to the groundwater of the Camping Sicilia. In September 1996, a marked rise Žabout 60 cm. of the groundwater level was recorded at the Camping Sicilia well ŽFig. 5., this rise being the largest one ever observed in the Vulcano wells. From a hydrological point of view, such a large variation cannot be attributed to recharge by meteoric waters, since it was detected at the end of the dry season when the Vulcano Porto groundwater is intensely exploited ŽDongarra` et al., 1988; Capasso et al., 1991.. This conclusion is also supported by hydrological data that locate an abnormally high phreatic level in the area of the Camping Sicilia relative to both the Vulcano Porto water table and the meteoric recharge coming from the Vulcano Piano area ŽMadonia, P., personal communication.. Finally, the temperature and salinity increase rules out the hypothesis of a meteoric water supply. In this context, the observed variations of the phreatic surface can be plausibly explained by the temporary formation of a permeable conduit located along degassing tectonic structures. The accumulation of gases in the soil due to higher degassing rates causes a decrease in both
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permeability and groundwater discharge and, as a direct consequence, an increase of the phreatic level. In addition, the increased interaction between fumarolic gases and groundwater caused the observed variations in chemical and isotopic parameters. This hypothesis is also compatible with the existence of a very thin groundwater layer in this area which may be strongly influenced by the supply of fumarolic fluids. Volcanic gases could easily pass through it when degassing increases along tectonic zones. Such a phenomenon has been observed repeatedly in this area with the occasional appearance of fumarolic emissions ŽT f 100 C8. or mofettes. The large variations of rock resistivity recorded in the Camping Sicilia area Žfour orders of magnitude in a time span of a few months, Di Maio et al., 1994. seem to support this hypothesis. This interpretation has important implications for hazard evaluation in Vulcano where phreatic eruptions have occurred through the recent history of the island. A combined effort in order to relate geophysical monitoring Žresistivity, deformation. with the ingress of deep fluids into the volcanic system is highly important. The aim of such a study would primarily be the quantification of the volcanic impact in terms of fluids and affected rock volumes.
4. Conclusions The increase of the gasrsteam ratio as well as the variations in the isotopic composition of CO 2 and steam observed in 1996 were probably caused by an input of fluids of magmatic origin during a new evolutionary phase of volcanic activity. The marked variations of some physico-chemical parameters such as temperature, conductivity and pH in the water of the Camping Sicilia well confirm its importance in volcanic surveillance in Vulcano. Nevertheless, the first variations in this well were observed about 2 months later than those recorded at the crater fumaroles. This delay is possibly due to delayed pressure rise in the peripheral part of the system. The increase of pressure, probably related to variations in the discharge rate of the deep fluids towards the surface Žincrease of the gas flux andror decrease of the permeability of the fumarolic outgassing system., determined the draining of the flu-
ids towards the peripheral systems, causing the observed variations at the Camping Sicilia well and the increase of CO 2 fluxes from the soil in the area surrounding the crater ŽBadalamenti et al., 1997.. Under normal conditions, the fluids are mainly drained by the fumaroles of ‘La Fossa’ crater, and, to a lesser degree, by the system of fractures related to the ‘Baia di Levante’ fumaroles and to the P4Max area. The time span of this evolution is well defined by three parameters which can be easily measured: temperature, pH and groundwater level. In fact, while the beginning of the evolution ŽJuly 1996. was marked by the temperature increase, its ending was marked by the increase of pH and by lowering of the groundwater level which occurred in December 1996 ŽFig. 5.. The relative amount of fumarolic condensate in the Camping Sicilia well water clearly indicates interactions between deep fluids and groundwater. This observable phenomenon is thus, indirectly an indication of the risk of the phreatic eruption hazard that affects this area.
Acknowledgements We thank Yuri Taran, Niels Oskarsson and Antonio Longinelli for helpful suggestions and reviews of early versions of this paper. This study was performed with the financial support of the C.N.R.-National Group for Volcanology, Italy.
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