PII S0016-7037(01)00813-4

Magma-derived gas influx and water-rock interactions in the volcanic aquifer ofMt. Vesuvius, Italy

C. FEDERICO,2 A. AIUPPA,1 P. ALLARD,4 S. BELLOMO,1 P. JEAN-BAPTISTE,3 F. PARELLO,1 and M. VALENZA1

1Dipartimento di Chimica e Fisica della Terra, Universita di Palermo, via Archirafi 36, 90123 Palermo, Italy2Instituto Nazionale di Geofisica e Vulcanologia—Sezione di Palermo, via U. La Malfa 153, 90146 Palermo, Italy

3Laboratoire des Sciences du Climat et de l’Environment, Commissariat a l’Energie Atomique–Centre National de la Recherche Scientifique,CE-Saclay, 91191 Gif-sur-Yvette, France

4Laboratoire Pierre-Sue, CNRS-CEA, CE-Saclay, 91191 Gif-sur-Yvette, France

(Received March 23, 2001; accepted in revised form August 15, 2001)

Abstract—We report in this paper a systematic investigation of the chemical and isotopic composition ofgroundwaters flowing in the volcanic aquifer of Mt. Vesuvius during its current phase of dormancy, includingthe first data on dissolved helium isotope composition and tritium content. The relevant results on dissolvedHe and C presented in this paper reveal that an extensive interaction between rising magmatic volatiles andgroundwaters currently takes place at Vesuvius.

Vesuvius groundwaters are dilute (mean TDS � 2800 mg/L) hypothermal fluids (mean T � 17.7°C) witha prevalent alkaline-bicarbonate composition. Calcium-bicarbonate groundwaters normally occur on thesurrounding Campanian Plain, likely recharged from the Apennines. �D and �18O data evidence an essentiallymeteoric origin of Vesuvius groundwaters, the contribution from either Tyrrhenian seawater or 18O-enrichedthermal water appearing to be small or negligible. However, the dissolution of CO2-rich gases at depthpromotes acid alteration and isochemical leaching of the permeable volcanic rocks, which explains thegenerally low pH and high total carbon content of waters. Attainment of chemical equilibrium between therock and the weathering solutions is prevented by commonly low temperature (10 to 28°C) and acid-reducingconditions.

The chemical and isotope (C and He) composition of dissolved gases highlights the magmatic origin of thegas phase feeding the aquifer. We show that although the pristine magmatic composition may vary upon gasascent because of either dilution by a soil-atmospheric component or fractionation processes during interactionwith the aquifer, both 13C/12C and 3He/4He measurements indicate the contribution of a magmatic componentwith a �13C � 0‰ and R/Ra of �2.7, which is consistent with data from Vesuvius fumaroles and phenocrystmelt inclusions in olivine phenocrysts.

A main control of tectonics on gas ascent is revealed by data presented in this paper. For example, two areasof high CO2 release and enhanced rock leaching are recognized on the western (Torre del Greco) andsouthwestern (Torre Annunziata–Pompeii) flanks of Vesuvius, where important NE-SW and NW-SE tectonicstructures are recognized. In contrast, waters flowing through the northern sector of the volcano are generallycolder, less saline, and CO2 depleted, despite in some cases containing significant concentrations of magma-derived helium. The remarkable differences among the various sectors of the volcano are reconciled in ageochemical interpretative model, which is consistent with recent structural and geophysical evidences on thestructure of Somma-Vesuvius volcanic complex. Copyright © 2002 Elsevier Science Ltd

1. INTRODUCTION

Mt. Vesuvius (1281 m high above sea level and 10 km wide),located 15 km southeast of Naples, is the southernmost activecenter of K-rich magmatism (leucite-bearing tephrites) in cen-tral Italy. Continental alkaline volcanism in this region hasdeveloped in response to tectonic movements associated withsubduction of the African Plate margin beneath the Europeanblock and the opening of the Tyrrhenian Sea basin since the lateTertiary (Scandone, 1978). Vesuvius is actually a young vol-cano whose oldest products are dated at 25,100 yr (Delibrias etal., 1979). Six major Plinian eruptions, each marking the onsetof a new eruptive cycle (Delibrias et al., 1979), have occurredin the last 17,000 yr, the most famous one being the AD 79Plinian eruption, which destroyed the Roman settlements of

Pompeii and Herculaneum. Over the last three centuries (1631to 1944), Vesuvius displayed semipersistent eruptive activity,with intermittent lava fountains and lava flow emitted fromboth summit and lateral vents (Santacroce, 1983; Arno et al.,1987). Since 1944, Vesuvius has been dormant, displayinggradually decreasing thermal emissions in its summit crater andminor tectonic activity. On several occasions, eruptions ofVesuvius caused many casualties because of the proximity ofthe inhabited surrounding areas and the violence of someexplosive events. Today, �1 million people live around thebase and on the western-southern lower flanks of the volcano.Therefore, any new unrest may raise dramatic issues for thesafety of this dense population and thus enhances the urgentneed for better knowledge and active monitoring of the vol-cano.

The geochemical survey of groundwaters is one approachthat can provide useful information on the evolution of adormant volcano such as Vesuvius. Indeed, volcanic aquifers

* Author to whom correspondence should be addressed (federico@pa.ingv.it).

Pergamon

Geochimica et Cosmochimica Acta, Vol. 66, No. 6, pp. 963–981, 2002Copyright © 2002 Elsevier Science LtdPrinted in the USA. All rights reserved

0016-7037/02 $22.00 � .00

963

commonly trap the main soluble components of fluids releasedfrom magma chambers (CO2, SO2, HCl, HF, etc.) and can thusprovide sensitive indications of temporal and spatial changes inheat and gas transfer at a volcano. In particular, the interactionsof magmatic gas with a volcanic aquifer can be preciselytracked from the isotopic ratios of helium and carbon, aspreviously shown at various dormant and erupting volcanoes(e.g., Sorey et al., 1993, 1998; Allard et al., 1997; Parello et al.,2000). Therefore, studying the aquifer of Vesuvius duringdormancy is of great interest (a) to determine the backgroundlevel of magmatic gas input in the aquifer during quiescentperiods and its relationship with the structural setting of thevolcano, (b) to model the chemical and physical process con-trolling the evolution of groundwaters as they flow through theedifice, and (c) to assess the nature and extent of geochemicalchanges that could herald a future eruption.

Recent seismic tomographic studies do not detect moltenmagma at depths shallower than �10 km beneath Vesuvius(Zollo et al., 1998). However, isotopic studies of the summitcrater fumaroles (Allard et al., 1988; Tedesco et al., 1991,1998) have revealed that magma-derived helium, similar to thattrapped in recent lavas (Graham et al., 1993), is still beingemitted by the volcano. There have been no equivalent data forthe groundwater system with which to evaluate the possibleextent and spatial distribution of present-day magmatic gasinputs into the volcanic aquifer. Nevertheless, previous hydro-geochemical studies of Vesuvius groundwaters (Avino et al.,1994; Caliro et al., 1998; Celico et al., 1998) have suggestedhigh CO2 contents in some waters on the southern volcanoflank, pointing to an input of CO2-rich gas, possibly of mag-matic derivation.

Here we report an extensive investigation of the chemistry(major and minor ions, dissolved gases) and isotopic compo-sition (H, O, He, and C) of groundwaters sampled at 56different sites on and around the Vesuvius volcano. In partic-ular, we provide the first data for helium isotopes and tritium inVesuvian groundwaters. On the basis of both chemical andisotopic results, we propose a geochemical model of ground-water circulation and magmatic gas-water-rock interactionsunder the present noneruptive condition of activity that couldserve as a baseline for future geochemical monitoring of thevolcano.

2. STUDY AREA

The birth and buildup of the Vesuvius volcano is intimatelyrelated to block faulting of the Apennines thrust belt sincePlio-Pleistocene times (Pescatore and Sgrosso, 1973). TheSomma-Vesuvius complex formed within the southern part ofthe Campanian Plain, a depression related to NW-SE stretchingof the crust and to the backward retreat of the Italian peninsula(Di Maio et al., 1998) at the intersection of two main faultsystems of regional stress NW-SE/NNW-SSE and NNE-SSW/NE-SW (Fig. 1). The Campanian Plain is bordered by Tertiaryand Mesozoic carbonate massifs and filled with Quaternaryvolcanoclastic and sedimentary deposits. A wide negativegravimetric anomaly in its central part (Scandone et al., 1991)marks an important subsidence of the buried carbonate base-ment (Carrara et al., 1973). Vesuvius developed along thesouthern edge of this graben (Marzocchi et al., 1993) and

stands some 2 km above the carbonate basement (Berrino et al.,1998).

The active volcanic structures are clearly controlled by re-gional tectonics (Fig. 1). A main NW-SE fault system is rec-ognized on the northern flank of the former Somma volcanoand possibly belongs to the same SW-dipping fault systemdissecting the carbonate basement (Bianco et al., 1998). An-other main fault system, trending NE-SW/NNE-SSW, runsfrom the northeastern (near Somma-Vesuviana-Ottaviano) tothe southwestern flank of the volcano (Torre del Greco area).The 1794 and 1861 “en echelon” eruptive fractures and thesubmarine volcanic vents offshore from Torre del Greco (Fi-netti and Morelli, 1974) are aligned on this same fault system.The entire volcanic complex is affected by NW-SE and NE-SWvolcano-tectonic lineaments (Bianco et al., 1998).

The Somma-Vesuvius complex is characterized by an asym-metric topographic profile: The northern flank of the Somma iswell preserved, and its caldera rim stands �400 m higher thanits southern and western rims, covered by post–AD 79 deposits.According to Ventura et al. (1999), the S and W flanks of theSomma define an amphitheater-shaped collapse structure thatformed between 20–25 and 18 ka as a result of flank failure ofthe volcanic edifice, probably triggered by dike intrusions alongthe NW-SE fault system.

Water circulation in and around Vesuvius occurs in two mainhydrogeological features (Celico, 1978, 1983; Corniello et al.,1990; Celico et al., 1998): (a) a deep carbonate aquifer hostedin the buried Mesozoic series beneath the Campanian Plain and(b) a shallower volcanic aquifer (the Vesuvius aquifer) hostedby fractured lavas and coarse-grained pyroclastic deposits pro-duced by eruptions of both the nearby Phlegrean Fields and theSomma-Vesuvius complex.

The Apennine chain constitutes the recharge area for thecarbonate aquifer underlying the volcanoclastic and sedimen-tary deposits of the Campanian Plain. In this deep aquifer,water circulation is generally quite slow, and hydraulic gradi-ents are low (Corniello et al., 1990). In the most fractured areas,upward water circulation is more rapid. This typically occurs inthe area between Castellammare di Stabia and Torre Annun-ziata (Fig. 1), where a lower thickness of the volcanic coverfacilitates exchanges between the deep and the shallow waterbodies. In this area, the fractured carbonate aquifer lies at only500 m deep (Celico et al., 1998).

According to available data for the volcanic-hosted flowsystem, water circulation in the fractured edifice of Vesuviusoccurs in several discrete flow zones separated by impermeablepyroclastic layers that are interposed with lava flows in thevolcanic sequence. At a large scale, this complex hydrologicalsystem, composed of several superposed water bodies, may beviewed as single aquifer whose geometry and structure tend tomimic the morphology of the volcano itself and whose flowlines follow a broad radial pattern (Celico et al., 1998). Thetransmissivity values range from 10�4 to 10�1 m2/s (Celico etal., 1998), the highest values (10�2 to 10�1 m2/s) being foundon the southern flank of the volcanic apparatus. The estimatedthickness of the volcanic aquifer is 600 to 700 m. The hydraulicconductivity decreases in correspondence with the presence ofclayey marine deposits interbedded with the volcanics (Ippolitoet al., 1973; Aprile and Ortolani, 1979). Approaching the baseof the volcano, the “volcanic” groundwaters can mix with

964 C. Federico et al.

waters circulating in the Campanian Plain, mainly rechargedfrom the surrounding carbonate massifs.

3. SAMPLING AND ANALYTICAL METHODS

Figure 1 shows the location of our sampling sites. Sampleswere collected from 1997 to 1998 from two springs and 54drilled wells commonly used for irrigation, ranging in depthfrom 20 to 260 m. Temperature, pH, Eh, and conductivity ofthe waters were determined upon sampling, while samples forlaboratory analysis were collected and stored in high-densitypolyethylene flacons. Alkalinity was determined immediatelyby titration with HCl (0.1 N), using a mixed indicator (methylred–bromcresol green). The concentrations of major elementswere measured at Instituto Nazionale di Geofisica e Vulcano-logia (INGV)–Consiglio Nazionale delle Richerche (CNR) andDipartimento di Chimica e Fisica della Terra (CFTA), Univer-sity of Palermo, by ion chromatography on either unfiltered (Cl,F, Br, NO3, SO4) or filtered acidified (Na, K, Ca, Mg) samples.Boron was determined by the azometine-H colorimetricmethod proposed by Trujillo et al. (1982), and silicon was alsodetermined by conventional colorimetric techniques (Spectro-

quant–Merck kit). These analyses were carried out at INGV–Sezione di Palermo (PA) (ex Istituto di Geochimica dei Fluidi–CNR) with a Shimadzu UV 1601 spectrophotometer, operatingat � � 415 nm for B and � � 650 nm for Si. Sr and Fe weredetermined by inductively coupled plasma mass spectrometryat Laboratoire Pierre-Sue, Commissariat a l’Energie Atom-ique–Centre National de la Recherche Scientifique, followingthe procedures described by Aiuppa et al. (2000).

The D/H and 18O/16O ratios of waters were determined onseparate samples, using a Finnigan MAT 252 and LODO massspectrometers operated in routine at Laboratoire des Sciencesdu Climat et de l’Environment (LSCE). The results are reportedin � units per mil vs. Vienna standard mean ocean water, witha respective precision of �0.5‰ on �D and 0.05‰ on �18Ovalues. The latter are measured after equilibration with CO2 at25°C (Epstein and Mayeda, 1953). The tritium content of thewaters was determined by mass spectrometry, following aroutine procedure (Jean-Baptiste et al., 1992), after previousdegassing of the samples under a pure argon flow and subse-quent measurement of the accumulating amount of tritiogenic3He (MAP spectrometer).

Fig. 1. Schematic map of the Somma-Vesuvius area, showing the locations of water sampling sites and main towns(squares). Three different symbols are used for samples collected in the northern (white circles), western (grey circles), andsouthern (black circles) sectors. Also indicated are the main tectonic faults cutting the Campanian Plain (white dashed lines)and the volcanic complex (black dashed lines), along with the U-shaped collapse structure affecting the southern flank ofthe pile (black solid line). Modified from Bianco et al. (1998) and Ventura et al. (1999).

965Magma-derived gas influx and WRI processes at Vesuvius

The dissolved gases (O2, N2, CH4 and CO2) were analyzedby gas chromatography (Perkin-Elmer 8500 equipped with 4-mcarbosieve II columns and hot wire and flame ionization de-tectors and using Ar as the carrier gas) after equilibration in ahost gas (pure argon) and extraction, following the proceduredescribed by Capasso and Inguaggiato (1998). The concentra-tion of each gas (cm3/L of water at STP) was computed from itsmolar fraction in the gas phase, the total volume of extractedgas, and the respective solubility coefficients (from Whitfield,1978). The analytical uncertainty is �5%.

Carbon isotope ratios of total dissolved carbon (TDC) weredetermined by mass spectrometry analyzing the CO2 releasedby acidification of water samples (0.5 mL of HCl 37%; Favaraet al., in press). 13C/12C ratios, reported in �13C units (�0.1‰)with respect to Vienna Peedee belemnite (V-PDB), were mea-sured at both CFTA and INGV-PA with Finnigan MAT DeltaS and Finnigan Delta Plus mass spectrometers, respectively,after purification of CO2 in a vacuum line. For He isotopeanalysis, waters were sampled in special copper tubes withclips, from which the dissolved helium was subsequently ex-tracted under vacuum, following a routine procedure (Jean-Baptiste et al., 1992). He isotope measurements were made atLSCE with a VG 3000 mass spectrometer connected to ahigh-vacuum inlet line. The spectrometer is equipped with adouble collector for the simultaneous measurements of 3He�

and 4He� ions. 3He/4He ratios (R) were determined against anair standard and are referred to the atmospheric ratio (1.38 �

10�6) as R/Ra. The overall uncertainty on R/Ra is �3%.

Dissolved He contents are expressed in cubic centimeters perliter at STP, with an overall uncertainty of �1%.

4. RESULTS

4.1. �D-�18O and Tritium Content of Groundwaters

The analyzed groundwaters have �D and �18O values rang-ing between �39.4 and �32‰ and �6.8 and �5.5‰, respec-tively (Table 1). These values are comparable to the isotopiccomposition of groundwaters from the Campanian Plain andthe Apennines (Celico et al., 1980; Duchi et al., 1995). In a�D-�18O diagram (Fig. 2), Vesuvius groundwaters plot close tothe local meteoric water line (�D � 8�18O � 13) proposed byCaliro et al. (1998), in intermediate position between the re-spective domains for worldwide meteoric waters (�D �8�18O � 10; Craig, 1961) and precipitations in East Mediter-ranean Sea (�D � 8�18O � 22; Gat and Carmi, 1970). Thisdemonstrates their predominantly meteoric origin. Otherwise, aslight positive 18O shift may explain some scatter in the �D-�18O diagram, especially for groundwaters from the southernsector of Vesuvius (black dots). Such a very limited 18O shiftwith respect to the local meteoric water line may suggest minorlow-temperature water-rock exchanges, in agreement with thegeneral features of most investigated groundwaters (i.e., T �29°C).

The distribution map of �D values (Fig. 3a) shows thatgroundwaters flowing through the northern flank of the volcanoare the most depleted in deuterium. The lowest �D values

Fig. 2. (a) �D-�18O scatter diagram for Vesuvian groundwaters. Symbols refer to sample locations on the northern (whitecircles), western (gray circles), and southern (black circles) flanks of the volcano (Table 1). Also plotted is the water vaporcompositional field of Vesuvius crater fumaroles (V.F.; Osservatorio Vesuviano and Instituto Nazionale di Geofisica eVulcanologia, unpublished data). The three lines delineate the isotopic domains for local meteoric waters (LMWL; �D �8�18O � 13; Caliro et al., 1998), worldwide meteoric waters (MWL; �D � 8�18O � 10; Craig, 1961), and precipitationsin the eastern Mediterranean Sea (EMWL; �D � 8�18O � 22; Gat and Carmi, 1970). (b) Extended scale that permitscomparison of the �D-�18O values of Vesuvius groundwaters with the isotopic composition of seawater (SW), basalticmagmatic water (1; Allard, 1983), and andesitic water (2; Giggenbach, 1992).

966 C. Federico et al.

Table 1. Major element chemistry and isotope composition of Vesuvius groundwaters. Concentrations are in mg/L, except for boron, strontium,and iron (�g/L). Temperature (�0.1°C), pH (�0.05 pH units), conductivity at 20°C (�2%) and redox potential (Eh, �5%) of the waters are alsolisted. �D and �18O values are in per mil vs. Vienna standard mean ocean water. Measured tritium contents are expressed in tritium units (1 TUcorresponds to a 3H/H ratio of 10�18).

SI Date TypeT

(°C) pHEh

(mV)C20

(�S/cm)Na

(mg/L)K

(mg/L)Ca

(mg/L)Mg

(mg/L)Cl

(mg/L)SO4

(mg/L)HCO3

(mg/L)SiO2

(mg/L)B

(�g/L)Fe

(�g/L)Sr

(�g/L)�18O(%o)

�D(%o)

Tritium(TU)

Northern flank1 12/01/98 d.w. 16.5 6.6 –64 1900 124 121 228 54 145 241 877 72 262 16,500 1078 –6.07 –35.4 4.8 � 0.112 20/10/97 d.w. 15.8 6.8 298 1251 70 127 112 49 114 183 427 57 282 709 498 –5.81 –35.6 n.m.3 01/10/97 d.w. 16.4 6.3 182 1786 117 204 124 91 187 196 866 66 452 809 828 –6.29 –36.2 0 � 0.054 23/10/97 d.w. 16.8 6.6 115 1409 97 175 117 60 155 145 626 57 431 710 774 –5.79 n.m. 0 � 0.065 20/10/97 d.w. 15.8 6.5 213 1260 75 127 155 67 70 148 814 60 269 928 876 –6.18 –35.3 7.4 � 0.12

39 10/01/98 d.w. 15.9 7.7 n.m 1116 51 83 115 24 79 108 342 36 151 722 460 –6.38 –37 5.1 � 0.3640 10/01/98 d.w. 15.5 7.3 n.m 895 48 70 145 26 74 157 351 51 96 840 441 n.m. n.m. 4 � 0.241 22/10/97 d.w. 15 7.5 163 1129 53 86 103 28 84 106 378 38 141 581 495 –6.39 –36.6 n.m.42 09/01/98 d.w. 15.4 7.5 150 1152 55 78 174 29 83 206 376 46 �100 873 719 –6.02 –35.1 7.3 � 0.3443 08/01/98 d.w. 14.9 7.4 170 879 49 76 121 22 77 91 342 37 289 715 445 –6.25 –37.3 6.8 � 0.2244 09/01/98 d.w. 15 7.1 135 1040 61 103 84 38 78 124 372 37 153 464 525 –6.16 –36.7 7.5 � 0.1645 19/05/98 d.w. 14.7 8.0 21 748 30 78 37 32 53 77 232 32 128 265 204 –6.65 –36.1 5.9 � 0.1246 10/01/98 d.w. 14.7 7.5 n.m. 1048 64 99 65 51 101 111 384 27 152 400 425 –6.76 –39.1 0 � 0.0647 18/05/98 s. 13.2 8.0 107 679 50 72 78 22 36 52 397 40 �100 525 519 –6.52 –37.5 0 � 0.1448 19/05/98 d.w. 14.8 7.7 –145 673 37 51 58 23 80 72 217 40 115 659 355 n.m. –37.7 0 � 0.1749 19/05/98 d.w. 15.3 7.4 88 797 51 89 87 31 77 98 360 38 185 359 511 –6.5 –37.4 8.6 � 0.2450 18/05/98 d.w. 15.4 7.2 106 1012 53 92 88 33 72 93 415 39 197 465 599 –6.64 –37.4 8.6 � 0.1751 12/01/98 d.w. 15.2 7.5 n.m 1029 58 94 84 24 78 95 315 34 208 525 505 –6.43 –37.1 9 � 0.152 18/05/98 d.w. 14.1 7.1 68 948 55 87 84 35 92 92 369 37 209 460 645 –6.92 –39.4 4.5 � 0.1253 12/01/98 d.w. 15.9 6.3 n.m 777 55 91 103 18 82 68 393 46 455 640 544 n.m. –38.7 13.1 � 0.1954 18/05/98 d.w. 17.4 7.1 142 1056 56 101 81 29 91 94 381 46 222 730 666 –6.73 –38.6 6.4 � 0.4255 12/01/98 d.w. 11 7.5 n.m 1186 65 129 96 35 93 143 376 45 215 608 504 –6.05 –35.5 6.9 � 0.1656 18/05/98 d.w. 16.2 7.1 126 1330 66 117 115 30 107 100 378 52 253 1196 585 –6.41 –36.7 9.6 � 0.47Southern flank6 23/10/97 d.w. 16.8 5.8 –85 1990 145 253 62 87 199 35 1013 84 840 368 430 –6.56 –35.9 4.7 � 0.087 20/10/97 d.w. 17.8 6.5 214 2030 131 219 135 102 248 224 839 61 569 807 947 –6.5 –36.6 8.1 � 0.148 23/10/97 d.w. 17.1 6.5 155 2020 150 171 123 103 255 195 860 56 500 475 866 –6.57 –37.2 9.8 � 0.19 06/11/98 d.w. 21.1 6.7 102 1338 46 60 168 30 80 87 576 55 206 n.m. n.m. n.m. n.m. n.m.

10 20/10/97 d.w. 18.2 6.3 171 2020 144 220 150 112 178 180 1055 60 528 855 1065 –5.78 –36.8 5.9 � 1.2811 20/10/97 d.w. 16.8 6.1 –26 2900 213 227 367 261 318 109 1876 75 781 1967 1794 –6.1 –36.7 1.7 � 0.0712 23/10/97 d.w. 17.1 6.8 137 1478 92 156 138 74 167 172 708 61 405 783 781 –6.47 –37.3 7.4 � 0.0613 23/10/97 s. 22.7 6.1 –74 2530 192 361 181 107 364 264 1129 62 908 1833 1066 –6.58 –38.2 7.3 � 0.1714 17/05/98 d.w. 23.3 6.3 –168 8900 533 410 616 763 2001 88 4060 100 3970 30,800 6830 –6.57 –35.6 0.8 � 0.1415 23/10/97 d.w. 27.8 6.2 241 3920 320 591 121 137 708 359 1110 81 1489 548 1058 –6.47 –36.5 7.7 � 0.1416 23/10/97 d.w. 22.2 6.2 84 3970 332 563 113 192 556 234 1739 77 1369 825 1107 –6.22 –36.7 1.2 � 0.0717 23/10/97 d.w. 22.2 6.3 263 3160 260 459 115 162 430 250 1379 72 981 579 1091 n.m. n.m. 4.3 � 0.06Western flank18 20/10/97 d.w. 21.7 6.2 54 2460 195 329 358 100 340 433 1429 75 622 2038 2074 –5.91 –34.4 7.4 � 0.0719 23/10/97 d.w. 21.2 6.4 158 2900 230 407 115 140 308 205 1269 71 934 724 953 –6.5 –37.5 5 � 0.0620 23/10/97 d.w. 20.4 6.7 220 2430 205 300 196 113 240 521 1074 49 759 977 1412 –5.88 –33.9 6.4 � 0.0921 23/10/97 d.w. 19.4 7.1 216 1833 138 224 122 66 166 216 663 n.m. n.m. 594 1017 –6.11 –35.3 4.4 � 0.2122 23/10/97 d.w. 19.4 6.8 237 2170 137 244 104 104 181 264 784 52 675 432 635 –6.16 –35.3 3.6 � 0.0823 23/10/97 d.w. 19.7 6.7 232 1987 140 244 91 98 165 215 889 57 662 394 671 –6.3 –35.6 4.5 � 0.0824 22/10/97 d.w. 19.2 6.6 199 2410 176 316 114 133 177 250 1302 60 846 702 855 –6.34 –36.4 2.5 � 0.1325 23/10/97 d.w. 17.4 6.7 155 1759 114 197 96 82 174 271 623 53 572 567 535 –6.3 –35.7 4.2 � 0.4426 20/10/97 d.w. 19.3 6.6 189 1893 155 287 92 78 107 196 1023 57 771 449 932 –6.12 –34.6 1.8 � 0.1227 20/10/97 d.w. 18.7 6.2 198 3220 192 372 56 242 182 126 1452 65 762 766 1118 –6.54 –36.5 1.8 � 0.1228 23/10/97 d.w. 19.9 6.8 191 1763 127 210 80 78 137 218 727 55 723 439 421 –6.51 –36.7 4.3 � 0.129 18/05/98 d.w. 21.7 7.0 –30 1825 163 225 73 68 153 241 732 60 1009 868 589 –6.44 –36.3 3.1 � 0.2930 16/05/98 d.w. 19.4 7.3 –67 2290 187 296 95 127 173 292 1144 51 1405 1234 1163 –6.42 –35.2 0 � 0.231 16/05/98 d.w. 19 7.0 –90 3140 309 285 141 191 383 173 1653 60 3011 2260 1191 –6.42 –33.6 0 � 0.2932 16/05/98 d.w. 20.5 7.1 8 2510 322 257 72 79 243 258 1064 66 2370 451 490 –6.42 –35.7 0.2 � 0.1333 18/05/98 d.w. 18.1 6.7 155 1671 117 212 60 95 116 168 778 70 660 538 394 –6.5 –36.4 n.m.34 16/05/98 d.w. 16.5 6.4 44 2840 254 374 62 136 331 297 1080 58 1480 447 436 –6.22 –34.6 2.6 � 0.7535 08/01/98 d.w. 17.8 6.3 n.m 1890 132 261 81 107 142 449 674 62 n.m. 1378 391 –6.42 –36.3 3 � 0.0736 09/01/98 d.w. 15 7.3 n.m 1056 54 89 115 33 84 117 369 38 158 666 469 –6.48 –36.7 6.8 � 0.0837 20/10/97 d.w. 17.2 6.5 n.m 1450 68 126 69 77 145 187 428 56 515 397 384 –6.49 –36.8 5.2 � 0.1438 09/01/98 d.w. 13.3 7.6 n.m 1916 124 203 210 38 133 349 421 31 465 1305 1320 –5.46 –32 7.6 � 0.3

SI � sample identity; TU � tritium units; d.w. � drilled well; n.m. � not measured; S. � spring.

967Magma-derived gas influx and WRI processes at Vesuvius

(�39.4 to �39.1‰) measured near Ottaviano (samples 45 and52) are almost identical to that of rainwater collected in thesame area (�D of �40.0 � 0.5; Federico, unpublished data).Waters collected on the western and eastern flanks of Vesuviushave higher D/H and 18O/16O isotopic ratios. The highest �D(�32‰) and �18O (�5.46‰) values, measured at a well (sam-ple 38) close to the Tyrrhenian seaside, are representative ofprecipitations in coastal areas of the Neapolitan region(Longinelli, personal communication, 1999). This pattern to-ward more negative �D-�18O values from the south to the northof Vesuvius likely reflects a more continental origin of precip-itations on the northern flank than on the seaside of the moun-tain, as also suggested by Caliro et al. (1998). The same authorsascribed the intermediate isotopic values displayed by ground-waters in the Boscotrecase area (S flank) to a partial mixing of

coastal-type water with deeper water derived from the Apen-nine chain and flowing through the carbonate basement.

The tritium content of Vesuvius groundwaters is highlyvariable, ranging from 0 to 13 tritium units (TU) (Table 1), andtherefore suggests different origins and/or variable flow ratesand residence times of waters in the different sectors of theaquifer. Higher tritium contents (�7 TU) characterize waterssampled from shallow wells on the lower slopes of the volcano(Fig. 3b), suggesting their feeding by meteoric waters thatrapidly infiltrate into the permeable deposits of the CampanianPlain (N) and the coastal areas (S and W). The highest tritiumcontent, close to that of present-day local rain water (15 TU;Federico, unpublished data), is found at site 53 on the north-eastern base of the Somma. Samples collected at higher eleva-tions (but often in deeper wells) on the volcano contain less

Fig. 3. Spatial distribution of some selected variables in the Vesuvian aquifer: (a) �D (‰), (b) tritium (tritium units), (c)temperature (°C), and (d) chlorine (mg/L).

968 C. Federico et al.

tritium (�7 TU) and thus may involve deeper flow paths and aless direct recharge by recent precipitations. This feature isconsistent with a deeper water level, limiting the exchangeswith the superficial meteoric recharge. Especially, we find nilor very low tritium (3 TU) in waters circulating in the area ofTorre del Greco (samples 30, 31, and 32), in that of Pompeii–Torre Annunziata (samples 3, 4, 11, 14, and 16) and also at afew elevated sites at on the northern flank (samples 46, 47, and48).

4.2. Main Hydrochemical Features

Groundwaters hosted in the volcanic aquifer of Vesuvius aredominantly dilute hypothermal fluids, with TDS contents from657 to 12,500 mg/L (average � 2835 mg/L) and temperaturesvarying between 11.0 and 27.8°C (average � 17.7°C). How-ever, there is a significant contrast between groundwaters flow-ing through the northern and the S-SW flanks of the volcano,respectively (Figs. 3c and 3d): The latter are warmer (13 to28°C and an average of 19.4°C compared to 11 to 17°C and anaverage of 14.4°C), more saline, and richer in chlorine than theformer. The most saline water is found at site 14, at TorreAnnunziata (Fig. 1); it has a TDS of 12,500 � 500 mg/L, atemperature of 23.3 � 0.1°C, and a chloride content of 2000 �400 mg/L, far above the average Cl content of 240 mg/L for theVesuvius aquifer. The positive correlation between temperatureand TDS is also seen for temperature vs. chloride (Fig. 4a) andboron (not shown), two elements that share a conservativebehavior in cold groundwater systems (Arnorsson and An-dresdottir, 1995). The departure of the B-Cl-rich water sample14 from the main trend might result from conductive coolingupon rise. The variations of Cl with �18O (Fig. 4b) highlight awide �18O range for the Cl-poor waters collected in the north-ern and eastern sectors of Vesuvius. This likely reflects avariable mixing of two meteoric components, here identified asM1 and M2. M1 stands for precipitation on the volcanic coneof Vesuvius, whose �18O (�6.9‰) is computed using the local18O vertical gradient of 0.2‰/100 m (Longinelli, personalcommunication, 1999); M2 is taken to be representative ofprecipitation on the Tyrrhenian coast (�18O � �5.5‰;Longinelli, personal communication, 1999). The higher chlo-ride content of waters collected in the southern and westernsectors of Vesuvius, particularly obvious at well 14, furtherclaims for the contribution of a saline component whose originhas to be clarified. By and large, an input of chlorine-rich fluidsderiving from the deep-seated hydrothermal aquifer could rep-resent an important source of volatile elements in this water.

One prominent feature of Vesuvius groundwaters is theirwide pH range (5.8 to 8.0, average � 6.8) and high total carboncontent (TDC � 5.2 to 68.2 mmol/L). An inverse correlationbetween alkalinity and pH is observed (i.e., the lower the pH,the higher the bicarbonate content in the solution). A Cl-SO4-HCO3 triangular diagram (Fig. 5a) shows that bicarbonate isthe dominant anion species in solution. The Cl/SO4 ratio isquite variable, from 0.32 to 27, though it is lower than 5 in 95%of the waters. The highest sulfate concentrations are foundalong the coastal area between Torre Annunziata, where sulfur-rich thermal waters (23°C) are discharged at the surface, andTorre del Greco, where persistent submarine gas manifesta-tions, likely fed by boiling thermal waters, do occur (Tedesco

et al., 1991). These high sulfate contents can be attributed toeither the oxidation of hydrothermal sulfur-bearing minerals(i.e., pyrite) or the dissolution of a H2S-bearing gas phase bythe circulating solutions. Only a few warm and highly saline(Cl-rich) waters discharged around Boscotrecase are markedlydepleted in sulfate (�2 meq/L), which we attribute to precip-itation of solid sulfides in the prevailing reducing conditions, assupported by the computed saturation indexes (SI) (e.g., SIFeS2

from �4 to �12; Federico, 1999).The fair covariation of bicarbonate with the cation content,

as exemplified for Mg (Fig. 6a), reveals an increasing extent ofrock leaching by acid carbon-rich groundwaters. Obviously, theefficiency of rock leaching is also dependent on other factors,such as the redox potential of the solution. For instance, ironcontents as high as 30 mg/L are produced during rock weath-ering in reducing environments (Table 1).

Because of the alkali-rich composition of the host volcanicrocks at Vesuvius, Na-K-HCO3 groundwaters are produced bywater-rock interaction (Fig. 5b). As the “volcanic” groundwa-ters flow down gradient and further enter the Campanian Plain,they locally mix with cold dilute Ca-HCO3 waters derived fromthe surrounding Apennine carbonate massifs or rising from theunderlying carbonate basement. This mixing process between“purely volcanic” (alkali-rich) and “purely carbonate” (Ca-

Fig. 4. Scatter diagram of (a) chlorine vs. temperature and (b)chlorine vs. �18O. In (a) higher temperature and higher Cl contents areevidenced for groundwaters flowing in the southern sector of Vesuvius.M1 and M2 are chemical and isotopic end-members for local precip-itations on the cone of Vesuvius and on the Tyrrhenian coast, respec-tively. The former is derived from the local vertical isotopic gradient of0.2‰/100 m (Longinelli, personal communication, 1999), while thelatter is characterized by a �18O of �5.5‰ (Longinelli, personalcommunication, 1999).

969Magma-derived gas influx and WRI processes at Vesuvius

rich) waters is clearly evidenced by the linear trend observed inFigure 5b and by the variations of Ca/Sr ratios between thevalues typical for carbonate rocks (400 to 500) and the localvolcanics (50 to 70; Fig. 6b). The highest Ca contents andCa/Sr ratios are actually encountered in samples collected inthe plain surrounding Vesuvius, in agreement with the inferredflow paths of waters discharged from the carbonate massifs(Celico et al., 1998).

4.3. Chemical Composition of Dissolved Gases

Table 2 reports the chemical composition of dissolved gasesin groundwaters. In many groundwaters, the dissolved gas

phase is dominated by air-derived N2 (6.5 to 17 cm3/L at STP)and O2 (0.1 to 8 cm3/L at STP). This is especially the case inwater samples collected from the northern and western sectorsof Vesuvius.

In contrast, medium to high concentrations of carbon dioxide(50 to 1046 cm3/L at STP) characterize waters flowing throughthe southern flank of the volcano (with maxima between TorreAnnunziata and Boscotrecase and north of Pompeii) and partlyon its western flank (Fig. 7a). East of Vesuvius, another local-ized outflow of CO2-rich waters (107 to 419 cm3/L at STP) isidentified in proximity to San Giuseppe Vesuviano village.

A N2-O2-CO2 triangular diagram (Fig. 8a) shows that dis-solved gases in most Vesuvius groundwaters are variable mix-tures of a CO2-rich component with dissolved air. The slightincrease in the N2/O2 ratio compared to the air saturated waterobserved at several sites can be ascribed to O2 consumptionduring gas-water-rock reactions occurring in volcanic and/orsedimentary settings.

Dissolved methane was detected in 23 samples, with con-centrations from 10�4 to 0.498 cm3/L at STP, and was belowthe detection limit (10�5 cm3/L at STP) in the remaining 33samples. The highest methane contents (up to 0.498 cm3/L at

Fig. 5. (a) Cl-SO4-HCO3 triangular diagram showing that mostwaters plot in the field of peripheral, bicarbonate groundwaters definedby Giggenbach (1991). Samples from the southern sector of Vesuvius(14 and 15) are enriched in chlorine and depleted in sulphate withrespect to average groundwater composition. (b) Cation triangulardiagram. Samples display a linear array from Ca rich (sample 9) toalkali rich (samples 15 and 16) waters. A few water samples (11, 14,and 27), characterized by low pHs, display a Mg-rich compositionresulting from enhanced leaching of ferro-magnesian minerals.

Fig. 6. (a) Mg-HCO3 scatter diagram. The linear trend demonstratesthat the increasing acidity of Vesuvius groundwaters, related to deepCO2 dissolution in the aquifer, enhances rock leaching and metalcontent in solution. (b) Variations of Sr vs. Ca content in groundwaters.The solid lines represent the average Sr/Ca ratios in local volcanicrocks (Ayuso et al., 1998) and mesozoic limestones. Open squares referto waters circulating in the carbonate massifs bordering the CampanianPlain (Celico, 1980; Duchi et al., 1995).

970 C. Federico et al.

Table 2. Chemical and isotope composition of dissolved gases in Vesuvius groundwaters. Concentrations are expressed in cubic centimeters perliter at STP. 13C/12C ratios of total dissolved carbon and are expressed in the per mil notation versus Peedee belemnite. �13C of equilibrium CO2(g)

was computed according to Eq. 1 (Zhang et al., 1995). 3He/4He ratios in groundwater samples (R) are normalized to the atmospheric ratio (Ra �1.38 � 10�6).

SI CO2 N2 O2 CH4 - 10–3 He -10–5 �13CTDC �13CCO2 R/Ra

Northern flank1 419 12.7 0.3 9.1 138.2 1.5 –4.9 2.52 32 11.0 4.5 �0.01 5.9 –0.6 –6.5 1.23 205 9.5 3.8 �0.01 8.7 –0.2 –3.5 1.94 107 10.2 2.3 �0.01 9.2 –2.1 –6.8 1.85 88 6.5 2.5 �0.01 n.d. n.d. n.d. 1.939 65 11.2 4.9 �0.01 6.1 –9.2 –17.7 1.140 18 9.3 3.6 �0.01 6.7 –7.8 –15.5 1.441 8 14.9 6.4 0.4 6.8 –7.1 –15.5 1.242 21 8.7 3.5 �0.01 5.4 –12.6 –20.9 1.243 4 14.8 6.6 �0.01 n.d. –9.4 –17.4 n.d.44 12 14.4 7.7 �0.01 11.6 –4.8 –11.9 1.645 22 12.4 4.2 �0.01 5.6 –10.9 –19.7 1.246 7 15.1 5.7 0.2 n.d. –4.9 –13.2 n.d.47 8 7.2 3.4 3.3 4.6 –12.2 –21.7 1.048 5 11.9 2.8 �0.01 590.0 –8.4 –16.9 2.749 7 15.2 7.7 0.5 n.d. –8.1 –16.1 n.d.50 5 16.4 6.8 0.1 16.8 –7.3 –14.8 2.151 8 13.7 6.5 �0.01 10.6 –5.3 –13.5 1.852 9 7.9 4.0 �0.01 16.2 –4.6 –12.0 2.153 5 8.4 4.3 �0.01 5.3 –11.8 –15.2 1.154 18 9.8 5.3 �0.01 n.d. –3.8 –10.8 n.d.55 9 17.4 7.8 �0.01 5.1 –2.1 –10.7 1.056 7 15.4 2.2 �0.01 5.0 –4.0 –11.1 1.1

Southern flank6 951 4.6 0.1 498 60.6 0.0 –1.1 2.67 143 8.3 2.6 0.02 8.8 –0.9 –5.3 1.48 151 9.7 4.9 0.25 n.d. –0.9 –5.1 n.d.9 84 11.3 1.5 �0.01 86.9 –6.8 –11.9 2.010 277 11.0 8.0 �0.01 10.3 0.0 –3.2 2.011 1046 2.7 0.1 65.8 n.d. n.d. n.d. 1.712 81 9.6 4.2 �0.01 20.2 1.5 –4.6 2.113 383 6.3 2.8 2.3 7.2 1.4 –0.6 2.014 689 4.0 1.2 81.0 10.1 1.3 –1.9 2.514 504 5.6 0.4 12.9 4.5 2.8 –0.4 2.315 284 10.1 3.3 �0.01 21.7 0.3 –2.1 2.416 409 5.2 1.4 0.6 46.6 n.d. n.d. 2.417 71 13.5 6.1 �0.01 35.0 1.0 –1.9 2.5

Western flank18 104 7.5 2.8 �0.01 6.4 0.5 –2.1 2.019 161 17.0 6.6 �0.01 21.3 0.9 –2.7 2.320 100 11.2 4.4 0.4 n.d. 0.2 –5.4 n.d.21 41 9.1 3.2 �0.01 17.4 0.1 –7.0 2.222 93 15.9 6.6 �0.01 n.d. n.d. n.d. 1.423 107 10.7 4.8 �0.01 10.1 –8.1 –13.7 1.624 39 12.7 5.3 �0.01 n.d. 1.0 –3.7 2.225 55 13.8 5.2 �0.01 10.4 –2.9 –8.3 1.526 128 9.7 1.2 �0.01 9.5 1.8 –3.0 n.d.27 299 8.9 4.2 0.2 n.d. 0.3 –2.4 2.328 62 11.0 4.7 0.1 53.1 –2.3 –8.3 2.529 57 11.0 3.9 0.6 15.5 –1.2 –8.1 2.130 52 9.4 4.3 �0.01 15.5 0.1 –7.4 2.131 139 10.2 0.1 293 152.4 2.0 –4.5 1.632 60 8.7 0.1 0.3 n.d. –0.8 –7.8 n.d.33 146 11.7 3.8 0.04 11.7 2.0 –3.3 1.734 51 10.8 5.7 �0.01 7.1 1.4 –2.3 1.535 207 10.8 0.3 1.3 20.4 2.1 –0.7 1.537 15 16.4 6.5 0.1 12.4 n.d. n.d. 1.038 22 9.0 4.5 �0.01 5.8 –10.9 –19.5 1

SI � sample identity; TDC � total dissolved carbon; n.d. � not determined.

971Magma-derived gas influx and WRI processes at Vesuvius

STP) occur in the CO2-rich groundwaters emerging in thesouthern sector of Vesuvius, which are also characterized byCH4/CO2 ratios close to or slightly higher than those typical ofthe crater fumaroles (Chiodini et al., 1999a). The remainingwater samples show methane concentrations in the range ofstandard air (COESA, 1976).

Helium contents are quite variable (4.5 � 10�5 to 5.9 � 10�3

cm3/L at STP). A strictly atmospheric origin is suggested inseveral samples, whose helium concentrations equal that ofair-saturated groundwaters (4 � 10�5 cm3/L at STP; Weiss,1971). In contrast, a considerable He enrichment is found atother sites, with He contents as high as 5.9 � 10�3 cm3/L atSTP. Such a significant He excess with respect to the air is

paralleled by 3He/4He ratios up to 2.7 Ra, which argues for theinvolvement of a mantle- or magma-derived He component(see below).

4.4. Helium Isotopes

The measured 3He/4He ratios of dissolved helium range from1.0 to 2.7 Ra (Table 2). Plotting 3He vs. 4He concentrations(Fig. 9) reveals a dominant mixing trend between dissolved airand a deep component with R/Ra of 2.7. The isotopic ratio of2.7 is characteristic of local magmatic helium, as indicated bysimilar values measured in both Vesuvius crater fumaroles (2.2

Fig. 7. Distribution maps of (a) dissolved CO2 (cm3/L at STP) and(b) R/Ra ratios. See text for discussion.

Fig. 8. (a) N2-O2-CO2 triangular diagram showing the two-compo-nent mixing between atmospheric and magmatic end-members; dataare expressed in cubic centimeters per liter at STP. (b) CO2-3He-4Hetriangular diagram highlighting the presence of a third component(C), characterized by magmatic R/Ra signature and CO2-depletedcomposition; data are expressed in cubic centimeters per liter atSTP. The composition of air-saturated water at 20°C is indicated bya square.

972 C. Federico et al.

to 2.7 Ra; Allard et al., 1988; Tedesco et al., 1991) and olivineand pyroxene phenocrysts of the lavas erupted over the lastthree centuries (2.3 to 2.7 Ra; Graham et al., 1993). This“magmatic” value also plots within the He isotopic range fornearby Phlegrean Fields caldera (2 to 3.2 Ra; Sano et al., 1989;Tedesco et al., 1990) and Ischia Island fluid manifestations (1.6to 3.7 Ra; Inguaggiato et al., 2000). The low tritium content ofthe waters, especially in the S and SW sectors of Vesuvius (0to 5 TU), totally excludes any significant contribution of tritio-genic 3He to the measured 3He/4He ratios. The spatial distri-bution of R/Ra ratios (Fig. 7b) evidences the contribution of amagma-derived 3He-rich component in some preferential areasof Vesuvius, in particular the southern (Torre Annunziata–Pompeii area), the western (Torre del Greco), and the northernsectors (Ottaviano village). The variations of He isotopes withdissolved CO2 (Fig. 8b) define a three-component mixing do-main between: (a) a pure CO2 gas with R/Ra of 2.7 (A); (b) asoil-air component with R/Ra � 1 and a He/CO2 ratio of�10�5 (B) and (c) a He-rich but CO2-depleted end-member(He/CO2 � 0.0012) and R/Ra � 2.7, exemplified by watersample 48 (C).

4.5. Carbon Isotopes

The �13C of gaseous CO2 in two bubbling water samplescollected in Torre Annunziata village (samples 13 and 14) are�0.6 and �0.4‰ vs. V-PDB, respectively, quite similar to the�13C of fumarolic CO2 (0 to 0.1‰) emitted at the bottom ofVesuvius crater (Allard et al., 1988; Chiodini et al., 2001) andof CO2-rich fluid inclusions in recent eruptive ejecta (A.Sbrana, personal communication, 1999). Such an isotopic sim-ilarity, along with the coexistence with magma-derived helium,strongly supports a deep, presumably magmatic derivation ofthe carbon dioxide. Note, however, that marble xenoliths de-rived from the metamorphosed carbonate basement and ejectedduring Vesuvius eruptions have an almost indistinguishable�13C value (0.3‰; Allard, 1983).

The �13C values of TDC exhibit a wider range, from –12.6‰to �2.8‰ vs. V-PDB (Table 2). Part of this variability can be

due to temperature- and pH-dependent isotopic fractionationeffects during water-gas interaction. Therefore, to assess theoriginal �13C of the gas at each site, we computed the theoret-ical equilibrium �13C of gaseous CO2 (Table 2) from thefollowing equation (e.g., Zhang et al., 1995):

�13C(CO2)g � �13C(TDC) �H2CO3

TDC�(H2CO3 � CO2)

�HCO3

�

TDC�(HCO3

� � CO2) �CO3

��

TDC�(CO3

�� � CO2). (1)

Eqn. 1 takes into account the measured �13C of TDC, theequilibrium molar ratios of aqueous carbon species at samplingtemperature and pH, computed with the PHREEQC program(Parkhurst, 1995), and the isotope enrichment factors (�) be-tween dissolved carbon species and gaseous CO2 under thesame conditions (Zhang et al., 1995). The computed �13Cvalues range from –21.7‰ to –0.4‰ (Table 2) and display amarked dependency on dissolved CO2 concentrations (Fig. 10).The highest �13C values, relative to the gas-rich groundwaterscollected on the southern (Boscotrecase–Torre Annunziata–Pompeii) and southwestern (Ercolano–Torre del Greco) sectorsof the volcano, approach carbon isotope ratios measured in boththe bubbling gases and the crater fumaroles. On the contrary,the more negative �13C values associated with decreasing CO2

contents can be related to an increasing contribution of soil gas(organic plus atmospheric) CO2 (0.1 to 1 vol.%, �13C � �20to �30‰). The theoretical mixing curves between magmaticand soil components are plotted in Figure 10, as calculatedfrom the following equation:

(�CO2�13C)mix � y(�CO2�

13C)org � (1 � y)(�CO2�13C)d, (2)

Fig. 9. 3He-vs.-4He scatter plot for Vesuvius groundwaters (data arein cubic centimeters per liter at STP; symbols as elsewhere). Data forVesuvius crater fumaroles (triangles; Allard et al., 1988) are alsoshown.

Fig. 10. �13C (‰ vs. PDB) of gaseous CO2 in equilibrium withgroundwaters (computed from the isotope composition of total dis-solved carbon, Eqn. 1), plotted vs. dissolved CO2 content (expressed asmole fraction in the gas phase in equilibrium with each water sample atsampling T). The theoretical mixing lines between the hypothesizedlocal magmatic end-member (Allard et al., 1988; Osservatorio Vesu-viano and Instituto Nazionale di Geofisica e Vulcanologia, unpublisheddata) and two soil-air component (�13CCO2

� �25‰; [CO2] � 0.1 and1%, respectively) are drawn (solid lines). Dashed lines represent thetheoretical Rayleigh’s fractionation curves computed from Eqn. 4and 5 for the conditions T � 20°C and pH from 6.0 to 6.5. The plotdemonstrates that fractionation processes involving the rising mag-matic CO2-rich gas phase can account for departures from thecalculated mixing curves between soil air and magmatic compo-nents.

973Magma-derived gas influx and WRI processes at Vesuvius

where � and �13C indicate the mole fractions and carbonisotope compositions of CO2 in the mixture (mix) and in theorganic (org) and deep (d) end-members, respectively; and y isthe fraction of organic CO2 in the mixture. The computation isperformed for two different soil CO2 concentrations (respec-tively, 0.1 and 1%), covering the whole range of background(biogenic) soil CO2 concentrations measured in the Vesuviusarea (Aiuppa et al., subm.). It is evident from the plot that datapoints qualitatively fit with the calculated mixing curves; how-ever, their quite large scattering additionally suggests the ex-istence of fractionation processes involving the rising mag-matic CO2-rich gas phase (see below).

5. DISCUSSION

5.1. Gas-Water-Rock Interactions at Vesuvius

Two prominent chemical features of Vesuvius groundwatersare their high total carbon content and their generally low pH(35% of water samples have pH � 6.5). These features areindicative of a high reactivity and “chemical immaturity” of thegroundwaters, implying that magmatic gas-water-rock interac-tions proceed under low temperature conditions. This is con-sistent with the generally low temperatures (�20°C) of thegroundwaters and with a strikingly low geothermal gradient(25°C/km) measured in the 2-km-deep Trecase geothermaldrilling (Bernasconi et al., 1981) south of the volcano (Fig. 1).Such a low gradient could also be due to localized downwardgroundwater flow masking the regional conductive gradient. Insuch conditions, thermodynamic equilibrium between ground-waters and the host rocks during weathering is not achieved,and isochemical leaching prevails. In fact, the waters analyzedin this study display a near constant Na/K ratio, very similar tothat of the host volcanic rocks. This feature is typical of coldCO2-rich groundwaters flowing in shallow volcanic aquifers(e.g., Arnorsson, 1983) and/or along the margins of volcanicedifices (“peripheral waters”; Giggenbach, 1991), whose Na/Kratio is principally determined by rock composition. Further-more, Figure 5b demonstrates the peculiar magnesium enrich-ment of the waters with highest bicarbonate content and lowestpH (samples 14, 11, 27). This is typical of a very immaturestage of weathering (Giggenbach, 1988), during which intensedissolution of ferro-magnesian minerals occurs in a shallow,acidic chemical environment.

The described chemical features suggest either that Vesuviusgroundwaters do not interact with the deep hydrothermal sys-tem feeding the crater fumaroles (Chiodini et al., 2001) or thatany thermal signature is masked by shallower processes such asconductive cooling and dilution by cold meteoric water. Aspointed out by Giggenbach (1988), the application of classicalcation geothermometers to such “immature” waters is virtuallyuseless. For example, applying the Na/K geothermometer(Fournier, 1991) would yield unrealistic equilibrium tempera-tures of 600 to 1100°C for the waters analyzed here. Moreover,this ratio also depends on the complex assemblage of K-richminerals (leucite, K-feldspar) in Vesuvius lavas.

The carbon dioxide content of hydrothermal aquifers is gen-erally controlled by a reaction involving a Ca-Al-silicate, K-feldspar, mica, and calcite (Giggenbach, 1988). The full equi-librium line resulting from this reaction is reported in Figure11 as a field inside which calcite is formed. Bicarbonate

groundwaters at Vesuvius plot far above this equilibriumline, in the field of immature waters and along the so-called“average crust dissolution line” (Giggenbach, 1988). Thisconfirms that their chemistry is essentially governed byisochemical rock leaching and that the fugacity of CO2 isexternally controlled by the amount of gas entering theaquifer from below. The presence of this “CO2 excess”implies that a Ca-Al-silicate cannot be among the stablesecondary mineral phases at Vesuvius and that weatheringmoves towards the formation of calcite (Chiodini et al.,1991), this latter being actually close to saturation in mostanalyzed groundwaters (Gurrieri et al., 2001).

5.2. Processes Governing the Chemical and IsotopeComposition of Dissolved Gases

Figure 8a reveals that the chemistry of gases dissolved inVesuvius groundwaters is best explained by a variable mixingof two main components: a CO2-rich gas phase of deep originand dissolved air. By and large, the relative proportion of eachcomponent in the mixture reflects a variable supply of the deepgas at different locations and/or differences in flow paths.However, a more complex framework is suggested when minorspecies such as He and CH4 are considered. In fact, the heter-ogeneity of He and CH4 contents, along with the differentHe/CO2 and CH4/CO2 ratios measured in groundwaters, cannotbe reconciled with a simple dilution of rising magmatic gasesby air. In fact, whatever chemical model is proposed to accountfor the chemistry of dissolved gases in groundwaters has to fitthe peculiar chemical and isotope composition of end-memberC (typified by sample 48). This source is characterized by amagmatic 3He/4He signature (Fig. 8b) and enhanced He andCH4 contents despite a significant CO2 depletion. The involve-ment of processes other than mixing between soil air and risingmagmatic gas is further supported by the carbon isotope com-position of dissolved CO2 (Fig. 10). A He/CO2-vs.-�13C(CO2)

scatter diagram sheds light on this topic (Fig. 12). This diagramevidences a clustering of several data points along the theoret-

Fig. 11. Lkm (log c2K�

/cMg2�) vs. Lkc (log c2K�

/cCa2�) scatter diagram,modified from Giggenbach (1988). The concentration of each species(ci) is in milligrams per liter. Also plotted are two subparallel linescorresponding to the upper and lower boundaries for calcite equilib-rium formation from thermal fluids, and the theoretical line for fluidsderived from isochemical leaching of average crustal rocks. The com-position of bicarbonate groundwaters from Vesuvius suggests an im-mature stage of weathering, in a cold, “fluid-dominated” environment(Giggenbach, 1988).

974 C. Federico et al.

ical mixing curves between a soil-air component and crater-likefumarole gas phase but also highlights the He-rich and 13C-depleted composition of both sample 48 and a few other watersfrom the northern sector of the volcano. These latter featurescan be best interpreted as due to the fractionation processesaffecting the rising gas phase upon interaction with the ground-water system. Given the contrasted solubility of the two gasesin liquid water (Henry’s constant � KH(He) � 141,000 [atm ���1], and KH(CO2)

� 1500 [atm � ��1] at 20°C; Whitfield,1978), a He-CO2 gas phase is expected to become graduallydepleted in CO2 on partial dissolution in the aquifer. At Vesu-vius, the existence of several superposed water bodies, both inthe volcanic and in the sedimentary sequences, guarantees theefficiency of this CO2 removal, much of the gas being trappedin deeper aquifers. In contrast, the less soluble helium will beable to reach the shallowest parts of the groundwater system, astapped by the wells. Moreover, partial dissolution of the mag-ma-derived CO2 into groundwaters can also significantly affectthe �13C in the gas phase (Capasso et al., 1997). In fact,because of the preferential partitioning of 13C in the dissolvedcarbon species with respect to gaseous CO2 (Mook et al.,1974), the original isotopic signature of magmatic CO2 may bepartially or completely masked by fractional dissolution in theaquifer.

A Rayleigh-type fractionation process is considered to modelboth the chemical and isotopic changes experienced by the gasphase upon interaction with liquid water (Federico, 1999),according to the following equations:

(He/CO2)r � (He/CO2)i � FKCO2

KHe (3)

�13C(CO2)r � (�13C(CO2)i � 1000) � F��1 � 1000 (4)

(CO2)r � (CO2)i � F. (5)

F is the fraction of residual to initial gas after each step ofpartial dissolution in water (ranging from 1 to 0); (He/CO2)i

and (He/CO2)r stand for the mass ratios of the He and CO2 inthe original and the residual gas phase for each F value,respectively; KCO2

/KHe is the ratio of Henry’s solubility con-stants for CO2 and He; and � is the 13C fractionation factorbetween TDC and gaseous CO2. The composition of Vesuviuscrater fumaroles (Allard et al., 1988; Chiodini and Marini,1998; Chiodini et al., 1999a, 2001) is chosen as representativeof the deep, nonfractionated gas. Combining Eqn. 3, 4, and 5,theoretical Rayleigh’s fractionation curves were computed fordifferent pH and temperature values and plotted in Figures 10and 12. We observe (Fig. 12) that the composition of sample 48is consistent with extensive fractionation (F � 0.0001) upondissolution in water, with pH � 6 and T � 15°C, while samples47 and 50 require slightly higher pH conditions. Figure 12 alsoconfirms that the CO2-rich groundwaters emerging in thesouthern flank of the volcano (samples 6, 11, and 14) andcharacterized by �13C and He/CO2 similar to those of craterfumaroles, are slightly affected by such a fractionation process,their representative sample points actually fitting theoreticalfractionation curves for F � 0.9, pH between 5.5 and 6, andT � 20°C.

Partial gas dissolution in water can also explain the widerange in methane content and CH4/CO2 ratios measured inVesuvius groundwaters. Although many samples display atypical atmospheric signature (the molar CH4/CO2 ratio in thegas phase in equilibrium with the collected groundwaters being�10�4), the gas-rich groundwaters emerging at Boscotrecaseare characterized by CH4/CO2 ratios similar to but systemati-cally higher than the reference ratio in Vesuvius crater fuma-roles (5 � 10�4; Chiodini et al., 2001), which may again beascribed to preferential enrichment of less soluble methane inthe ascending gas phase. For instance, given a Henry’s constantKH(CH4)

� 36,150 (atm � ��1), a partial dissolution (F � 0.3 to0.9) of the pristine magmatic gas phase at 20°C and pH � 6would produce an increase of the CH4/CO2 ratio from theoriginal value to 3.3 � 10�3 (sample 14) or more (1.3 � 10�2,sample 6, ratios in the gas phase in equilibrium with ground-waters).

However, it should be noted that the variable methane tocarbon dioxide ratios are likely to be also controlled by otherprocesses (e.g., oxidative processes, degradation of organicmatter) whose effects are difficult to assess.

5.3. The C and He Isotope Signature ofVesuvius Volatiles

In the CO2-rich waters (western and southwestern sectors),the �13C(CO2)

values calculated from the �13C(TDC) at samplingpH and temperature and those measured in bubbling gasesindicate a maximum value of �0‰ for the deep gas, consistentwith data of the crater fumaroles (Allard et al., 1988; Chiodini

Fig. 12. He/CO2 ratios vs. �13C(CO2)g in Vesuvius groundwaters. The

helium to carbon dioxide molar ratios in the gas phase in equilibriumwith groundwaters (computed from data in Table 2 and Henry’s con-stants) are reported. Also reported in the diagram are the theoreticalmixing lines (solid lines) between the magmatic end-member (triangle)typified by crater fumarole composition (Allard, 1983, Chiodini andMarini, 1998; Chiodini et al., 1999a, 2001) and the two soil compo-nents (diamonds, �13CCO2

� �25‰; CCO2� 0.1 and 1%, respectively).

The theoretical Rayleigh fractionation curves (dashed lines) computedcombining Eqn. 3 and 4 are also reported, along with pHs and tem-peratures used in the calculations. The figure demonstrates that thechemical and isotope composition of dissolved gases can be bestdescribed by two-component mixing between magmatic and atmo-spheric sources, though Rayleigh-type fractionation mechanisms needto be considered to explain the He-rich, CO2-poor composition of somewater samples collected on the northern sector of Somma-Vesuvius(samples 47, 48, and 50).

975Magma-derived gas influx and WRI processes at Vesuvius

et al., 2001). Such a value is much higher than those commonlyencountered for magmatic CO2 (�13C � �5 � 1‰; Pineau andJavoy, 1983; Allard, 1983; Trull et al., 1993) and very similarto the �13C values of CO2 produced by thermal alteration ofcarbonate rocks. High �13C values are characteristic of variouscarbon dioxide emissions in central and southern Italy whoseorigins have long remained a matter of intense debate (Panichiand Tongiorgi, 1975; Ferrara and Stefani, 1977; Chiodini et al.,1995; Chiodini et al., 1999b; Inguaggiato et al., 2000). Panichiand Tongiorgi (1975) proposed that 13C-rich CO2 degassing inthis region results from the hydrolysis of carbonate rocks in thecrust at 100 to 300°C rather than from decarbonation at highertemperature, because the latter would produce CO2 enriched by2 to 3‰ with respect to the original limestone (Bottinga, 1969).Based on a correlation between CO2 degassing and depth ofMoho’s discontinuity beneath central Italy, Minissale (1991)suggested that the emitted CO2 be of mantle origin but has beencontaminated by subducted pelagic carbonates. Contact meta-morphism of the thick carbonate formations extending at a fewkilometers depth beneath Vesuvius and at temperatures of afew hundreds of degrees Celsius is another possible process forsupplying 13C-rich CO2 (Duchi et al., 1992; Marini and Chio-dini, 1994). The presence of marble xenoliths and Mg andCa-skarn derived from the Mesozoic basement of the volcaniccomplex (�2 to 3 km deep) in the products of the mostexplosive eruptions of Mt. Vesuvius provides evidence of mag-ma-carbonate interactions, during which CO2 can be releasedfrom metamorphic decarbonation (Barberi and Leoni, 1980;Belkin et al., 1993; Fulignati et al., 1998). However, anylarge-scale assimilation of carbonate metasedimentary rocksbeneath Vesuvius has been generally dismissed (Taylor et al.,1979; Civetta et al., 1981; Cortini and Hermes, 1981; Ayuso etal., 1998). As inferred from CO2/3He values, we suggest thatmantle-derived CO2 may mix with sedimentary CO2, mostlyproduced from carbonate sediments, during magma ascent andaccumulation in magmatic chambers located within the 5-km-thick carbonate sequence. In fact, CO2/3He ratios as high as1.2 � 1011 are found in CO2-rich waters from Vesuvius, depart-ing from the accepted CO2/3He range (1.6 to 3 � 109) typical ofmidocean ridge basalts (MORB) (Marty and Jambon, 1987).

3He/4He measurements in Vesuvius groundwaters demon-strate the contribution of magma-derived helium with R/Ra

ratio of 2.7, which is consistent with data from Vesuviusfumaroles and crystal crushing (Graham et al., 1993). The3He/4He ratios at both Vesuvius and other active campanianvolcanoes (Tedesco et al., 1990), is much lower than the typicalMORB range (8 � 1; e.g. Craig and Lupton, 1976) and plotswithin a now well-documented regional isotopic pattern occur-ring along the African-European plate collision boundary (Al-lard et al., 1997; Marty et al., 1994; Parello et al., 2000). Thispattern has been attributed to crustal contamination affectingmantle beneath the Campania region (Tedesco et al., 1990;Graham et al., 1993; Minissale et al., 1997; Italiano et al., 2001)or the ascending magmas (Hooker et al., 1985; Sano et al.,1989; Italiano et al., 2000). We propose that both He and CO2

emitted at Vesuvius probably originated in a contaminatedmantle but that the CO2 can be additionally diluted (and en-riched in 13C) by shallower sedimentary formations.

6. MAGMA DEGASSING AND CHEMISTRY OFGROUNDWATERS: RELATIONS WITH THESTRUCTURAL FRAMEWORK OF VESUVIUS

Our data allow us to better constrain the hydrogeochemistryof the Vesuvius aquifer and the different regimes of ground-water flow prevailing in different sectors of the volcano (Fig.13). One relevant feature is the contrast in tritium contentbetween groundwaters from the Campanian Plain and thosecollected at a higher altitude both on the northern and southernflanks of Somma-Vesuvius. The former, with tritium contentshigher than 7 TU, are essentially representative of shallowgroundwater recharged from the surrounding carbonate mas-sifs, as suggested by several features, including their prevalentCa-HCO3 chemistry (Fig. 5b) and Ca/Sr ratios (Fig. 6b). Thelatter instead display much lower tritium contents, which to-gether with �D-�18O ratios imply deeper infiltration and longerresidence times. A very slight 18O shift, typical of some triti-um-depleted waters from the southern sector of Vesuvius (Fig.2), is consistent with a prolonged interaction at low temperature(T 100°C) with 18O-enriched crustal (volcanic or carbonate)rocks. To reconcile the low tritium content with the generallyhigh flow rates typical of fractured volcanic aquifers (Celico etal., 1998) and with the short flow distances from the rechargearea to the sampling locations, we propose the addition ofwaters rising from the underlying carbonate basement. In fact,an active upward water flow from the deep carbonate to theshallow volcanic aquifer has been already hypothesized byother authors (Caliro et al., 1998; Celico et al., 1998) on thebasis of hydrological considerations. The presence of deepcrustal faults no doubt facilitates the upward migration offluids. Both low velocity and long flow paths from the Apen-nines recharge area toward Vesuvius could account for the lowtritium content of groundwaters flowing in the deep carbonateaquifer. The time-invariant �D and �18O of groundwaters (Fe-derico et al., subm.) seems to support such a hydrologicalscenario.

The diagram in Figure 13 includes areas where fluid chem-istry would be influenced by interaction between hot ground-waters and magma-derived volatiles. The spatial distribution ofmajor ions and dissolved gases in Vesuvius groundwaters re-veals a marked heterogeneity, which is likely to reflect differ-ences in gas supply and extent of gas-water-rock interactionbetween each sector of the volcano. Figure 13 highlights thatthe NW-SE and NE-SW volcano-tectonic structures, cuttingboth the Somma-Vesuvius complex and the underlying sedi-mentary basement (Bianco et al., 1998; Bruno et al., 1998),could act as preferential pathways for the ascent of magma-derived fluids up to their final dissolution into the shallowvolcanic aquifers. This structural control upon gas ascent isconsistent with the finding that groundwaters enriched in mag-ma-derived volatiles are located along the main NW-SE re-gional “Apenninic” fault (Torre Annunziata–Boscotrecase sec-tor; samples 13, 14, and 15), and along the NE-SW fracturezone running from Ottaviano (samples 1 and 48) to Torre delGreco (samples 31, 32, 33, and 34; Fig. 13a). Also located inthis latter sector is a spring (sample 47), which experiencedrelevant chemical and isotopic variations during a seismic crisisin November 1999 at Vesuvius (Federico et al., 2001), and islocated on the NW-SE trending fault running on the Sommavolcano close to S. Anastasia village.

976 C. Federico et al.

Fig. 13. Interpretative geochemical model of groundwater circulation in the Somma-Vesuvius complex, combined withsome relevant structural and geophysical data (Di Maio et al., 1998; Bianco et al., 1998; Ventura et al., 1999). (a) Schematicmap showing the spatial distribution of the different groundwater types observed at Vesuvius: (i) tritium-rich shallowgroundwaters flowing in the plain bordering the volcanic complex; (ii) cold dilute waters, mainly collected in the Sommasector, that are variably tritium depleted depending on their pathflow; and (iii) southward-flowing Vesuvius groundwaters,including (iv) hypothermal saline Cl-rich groundwaters. (b) Schematic cross-section depicting the interaction processesbetween shallow meteoric waters and deep rising magmatic volatiles. The variable composition of the gas phase feedingVesuvius groundwaters is interpreted in light of the fractionation processes taking place upon interaction with the deepcarbonate aquifer. A CO2-depleted signature is typical of fluids rising on the northern flank of Vesuvius, whereas CO2-richgroundwaters emerging in the southern flank of the volcano, characterized by �13C and He/CO2 similar to those typical ofcrater fumaroles, are slightly affected by the fractionation process. The interaction with thermal fluids, whose presence inthe central conduit area is suggested by the chemistry of crater fumaroles (Chiodini and Marini, 1998) and by self-potentialand seismic data (Di Maio et al., 1998; Ventura and Vilardo, 1999), may explain the higher temperature, TDS, Cl, and Bcontent of southward-flowing waters. See text for further details.

977Magma-derived gas influx and WRI processes at Vesuvius

It is noteworthy that the southern and northern branches ofthe above-mentioned fault structures are marked by geochemi-cal contrast (Fig. 13a). On their southern sides, a CO2-richmagma-derived gas phase containing appreciable He and CH4

reaches the shallow groundwaters; conversely, along the north-ern portions of the fault systems, a He-rich but CO2-depleted,with otherwise magmatic 3He/4He ratio, is degassed (compo-nent C in Fig. 8b). As described in previous sections, thesedifferences can be reconciled with a common magmatic source,but suggest a more extensive gas trapping by fractionationprocesses occurring in the northern sector. This, in turn, indi-cates either a lower gas supply under this sector of the volcanoand/or a more extensive CO2 removal through preferentialdissolution into deeply circulating waters.

On the basis of the available geochemical, structural, andhydrogeological data, we propose two alternative models, in-tegrated in the interpretative cross section of Figure 13b. In thefirst model, we emphasize the possible role played by the deepcarbonate aquifer in controlling the chemistry of Vesuviusgroundwaters. While flowing southward from the Apennines tothe Tyrrhenian Sea, carbonate groundwaters can act as a dom-inant filter on rising magmatic volatiles by trapping at depth asignificant portion of the more soluble carbon dioxide whilebeing more transparent to less soluble He. On their flow be-neath Somma-Vesuvius, groundwaters flowing inside the car-bonate sequence interact with the central conduit system.There, they become heated and gas-charged, by the upstream ofhot fluids sustained by deep magma degassing. Further south,CO2-rich carbonate waters flowing underneath the southernflank of Vesuvius may became unable to dissolve additionalCO2 rising through deep faults. Conversely, these CO2-chargedcarbonate groundwaters may represent a CO2 source for theshallow volcanic aquifer, for example in the Torre Annunziataarea, where the carbonate aquifer lies at only 500 m deep(Celico et al., 1998). The fault-controlled vertical movement ofwarm carbonate waters may also explain the higher tempera-ture and higher concentrations of Na, Cl, and B, which char-acterize groundwaters in this southern area (e.g., samples 13,14, and 15).

The second alternative model assumes that the processesgiving rise to the peculiar chemistry of Vesuvius groundwatersoccur exclusively within the volcanic sequence. In that case,groundwater chemistry under the northern flank of Vesuviuscould simply result from chemical and isotopic fractionation ofthe magma-derived gas upon interaction with deep water bod-ies hosted in the volcanic layers of the ancient Somma volcano.The occurrence of CO2-rich, warm, and saline groundwaters inthe southern sector of Vesuvius instead could be related to thestructural framework defined by Ventura et al. (1999). Accord-ing to these authors, a main control upon groundwater circula-tion at Vesuvius is exerted by the asymmetric topography of theancient Somma volcanic apparatus (Di Maio et al., 1998;Ventura and Vilardo, 1999; Ventura et al., 1999), characterizedby the presence of a S-SW dipping “Somma paleo-depression.”As a result, infiltrating precipitation on the recent cone ofVesuvius is forced to flow southward, while becoming hotter,more saline, and more gas rich because of the interaction withthe hydrothermal envelope feeding the crater fumaroles (Chio-dini et al., 2001). The high chlorine and boron content ofsoutherly flowing groundwaters can then be ascribed to either

(a) an input of high-temperature vapors or thermal brines risingfrom the hydrothermal reservoir or (b) leaching of Cl-richsublimates deposited inside the crater area during previousstages of activity of the volcano. However, in order to reconcilethe above interaction with the typically meteoric H and Oisotopic ratios of the groundwaters, the hydrothermal contribu-tion must be very small. For illustration, mixing only 5% of an18O-rich thermal fluid (say, �18O � 6‰) with 95% of localmeteoric water with �18O of �5‰ leads to a very minorisotopic shift in the resulting groundwater, well within theobserved 18O range. For the same mixing percentage, we wouldexpect a Cl content of �1000 mg/L, which is in the range ofobserved values.

Based on the available data, the more likely of the twoalternative models cannot be resolved. However, the low tri-tium contents and the constancy of chemical and isotope com-position of groundwaters (Federico et al., subm.) support theimplication that water from the deep carbonate aquifer contrib-utes to the chemistry of the sampled fluids. New hydrologicaldata, essential for an accurate assessment of the water budget inthe Vesuvius area, may help delineate between these alternativemodels.

7. CONCLUSIONS

The geochemical investigations of Vesuvius groundwaterspresented here provide a detailed baseline for the chemical andisotope composition of the groundwaters hosted by the Somma-Vesuvius volcanic complex during the present period of dor-mancy. As pointed out by our systematic survey on He and Cisotope composition of dissolved gases, the Vesuvius ground-water system is affected by upflow of magma-derived volatiles,even during quiescence. A structural control on the gas supplyto the shallow aquifer is revealed by geochemical mapping.Upon ascent toward the surface, the chemistry and isotopecomposition of the magma-derived gas phase dissolving ingroundwaters appears to change from its pristine magmaticcomposition as a result of various processes, such as dilution byatmospheric and soil gases and solubility-controlled fraction-ation mechanisms. The influence of these processes appears tobe stronger in the northern sector of Vesuvius than in thesouthern one. To the south, the gases dissolved in groundwatersare chemically and isotopically more representative of the localmagmatic end-member. Moreover, the systematically higher Cland B contents and the high temperature and TDS of southerlyflowing groundwaters further evidence the contrasting struc-tural and hydrological features of northern and southern sectorsof Somma-Vesuvius volcanic complex. Two alternative geo-chemical models accounting for such a relevant difference inchemical and isotopic composition are tentatively illustrated inthis paper.

More generally, the present study highlights that waterchemistry at Vesuvius, as at other active volcanoes, is stronglyinfluenced by the input of CO2-rich volcanic gases. Upondissolution, volcanic volatiles increase the acidity of infiltratingmeteoric waters, favoring an intense alteration of the host rocksand the release of chemicals. At Vesuvius, chemical equilib-rium between the rock and the weathering solution is far frombeing reached, because of dominantly low temperature and pHconditions. Isochemical rock dissolution, leading to groundwa-

978 C. Federico et al.

ters with a dominantly Na-K-HCO3 composition, is the prev-alent process controlling the water chemistry.

Finally, although the complex interplay of processes deter-mining the chemistry of Vesuvius groundwaters still needsfurther study, our results clearly show that groundwaters in theS-SW sector of Vesuvius are the most closely linked to themain degassing system of the volcano and should therefore bethe most suitable for continuous geochemical modeling.

Acknowledgments—We are grateful to M. L. Sorey, G. Chiodini, andan anonymous reviewer for their helpful comments on this manuscript.This study, part of the Ph.D. thesis of Dr. Federico, was carried out withthe financial support of the European Union, Ministero dell’Universita’e della Ricerca Scientifica e Tecnologica, and CNR–Gruppo Nazionaleper la Vulcanologia.

Associate editor: M. A. McKibben

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981Magma-derived gas influx and WRI processes at Vesuvius