Geochemical Monitoring System II prototype (GMS II) installation at the “Acqua Difesa” well,...

Geochemical Monitoring System II prototype (GMS II) installationat the “Acqua Difesa” well, within the Etna region: first data during

the 1999 volcanic crisis

F. Quattrocchi* , G. Di Stefano, L. Pizzino, F. Pongetti, G. Romeo, P. Scarlato,U. Sciacca, G. Urbini

Istituto Nazionale di Geofisica, Via di Vigna Murata 605, 00143, Rome, Italy

Received 6 April 1999; received in revised form 20 January 2000; accepted 20 January 2000

Abstract

The Geochemical Monitoring System II (GMS II) prototype was designed, assembled, tested and installed at theAcquaDifesa test site, near Belpasso (CT), within the Etna region, in the frame of an EC funded program namedAutomaticGeochemical Monitoring of Volcanoes. The easily changeable and versatile configuration of the remote station provides actualdata every 10 min, recording: groundwater temperature, electrical conductivity, pH, redox potential, dissolved CO2, He,atmospheric pressure and air temperature. The radon sensor was assembled despite no jet added to the remote station. Anautomatic sampler collects every 72 h (3 days) two bottles of water, acidified and not, for further laboratory analyses.

The choice of the Acqua Difesa test site arises from preliminary volcanological studies, deserving special interest, i.e. for thehigh CO2 degassing, for the high He isotopic values, exhibiting a direct mantle signature of the volcanic gases and for thepeculiar sensitivity to volcanic activity as tested by discrete monitoring in the past and confirmed by the present study.

During the discussed monitoring period (October 1998–June 1999) an enhanced volcanic activity of Etna was observed; itstarted on July 1998 with enhanced plume degassing and, since September 1998, with periodical lava fountains as well as withepisodic low-energy earthquakes and seismic swarms�Mmax� 3:1 till June 1999). The activity culminated with a summit craterfracture involving a sub-terminal eruption on 04/02/1999. The summit crater fracture was not accompanied by enhancedseismicity, despite moderate seismic energy release occurring in the succeeding months. In that monitoring period, theAcqua Difesa GMS II station recorded a few geochemical anomalies, mostly in temperature, pH, CO2, electrical conductivityand Eh as well as in the concentration of some major elements trend, mostly in Mg and HCO3. The geochemical continuous anddiscrete data-set is discussed in comparison with the available information on the ongoing volcanic and seismic activitytentatively enhancing the comprehension of the aquifers mass–heat transport properties of the volcano during crises. Theexperimental observations may be defined, at present, the “forerunners” of the Etna eruptions affecting the single-out AcquaDifesa well. It is nowadays a “self consistent” site with its own response to the pre-eruptive processes, definedsensitiveto theEtna aquifer modifications induced by the volcanic activity, in a recurrent way. Thus, we defined the “typical answer” of thisspecific test-site to the pre-eruptive processes and to the associated seismicity. Further data may modify this statement. Thestation prototype configuration and the results may give hints for future volcanic eruption and seismic activity forecasting.q 2000 Elsevier Science B.V. All rights reserved.

Keywords: Geochemical Monitoring System II; Etna region; Acqua Difesa well

Journal of Volcanology and Geothermal Research 101 (2000) 273–306

0377-0273/00/$ - see front matterq 2000 Elsevier Science B.V. All rights reserved.PII: S0377-0273(00)00177-3

www.elsevier.nl/locate/jvolgeores

* Corresponding author. Fax:1 39-06-504-1181.E-mail address:[email protected] (F. Quattrocchi).

1. Introduction

The monitoring of active volcanoes and forecastingof volcanic eruptions are among the main topics of themajor international earth science and hazard reductionprogrammes exploited in the last decades (Tazieff andSabroux, 1983; McGuire et al., 1993; EVOP, 1994AGMV EC Program 1996–1998, see Quattrocchi etal., 1997b). A large quantity of discrete monitoringdata have been collected and theoretical modelsformulated since the 1970s, which tried to explainthe volcanic structures and the volcano-tectonicprocesses. The researchers are now ready to addressmajor efforts for continuous monitoring and surveil-lance. This strategy arises mainly as a consequence ofthe drop in the costs/benefits ratio of automaticdevices (i.e. PC-IBM compatible, A/D converter elec-tronics, telecommunication systems, etc.) and theimpending needs and management of the ItalianCivil Protection Department (i.e. GNV 1999–2001Program, CNR-GNV, 1997).

In particular, geochemical discrete monitoring ofmany active volcanoes produced extensive baselinedata, revealing thebackgroundgeochemical beha-viour of the volcanoes during quiescent periods.These data manipulations contribute to build theworking modelof the volcanoes, mainly when accom-plished in a multi-parametric approach (Stix et al.,1997; Williams et al., 1998). Many studies exist deal-ing with the exploitation of fluid geochemistry inunderstanding the volcano-tectonic processes, speci-fically addressed to volcanic eruption forecasting(Giggenbach, 1975, 1987; Manyailov, 1975; Oskars-son, 1978; Giggenbach et al., 1990; Williams et al.,1990; McGuire et al., 1993; Fischer et al., 1994, 1996,1997; Dall’Aglio et al., 1994; Tedesco, 1995; Stauda-cher et al., 1998; Capasso et al., 1999). However, themost noteworthy results in the field of volcanic andearthquake prediction and in the understanding of therole of fluids in the volcanic structures (Fournier,1987; Nur and Walder, 1990; Hedenquist, 1992;Carroll and Holloway, 1994) have been gathered bycontinuous monitoring (Noguchi and Kamiya, 1963;Baubron et al., 1991; Notsu et al., 1991a,b; Sato et al.,1991; Quattrocchi and Calcara, 1998).

For a long time we stressed the lack of a full multi-parametric approach in continuous monitoring(Dall’Aglio et al., 1991; Quattrocchi et al., 1996):

often, no more than two or three sensors have beenused at the same time in each monitoring station (i.e.radon, temperature, water level, electrical conductiv-ity). For example, the active volcano Piton de LaFournaise was equipped by a dense and permanent222Rn monitoring network which started to worksince 1994 (Clippertonw sensors, by usinga particlesdetection adopting picture electric cells, with1 datum/h). This mono-parametric network gave thegood opportunity to observe Rn pulses (50 times thebackground) in 6 stations at the same time, before twoseismic crises occurred in November 1996 and August1997 (Staudacher et al., 1998): in this case thepresence of a multi-parametric approach shouldhave been exploited more to know the volcanologicalsignificance of these anomalies.

Groundwater seems to be more promising than soilsfor continuous monitoring, as a consequence of thenoteworthy quantity of external factors influencingthe soil-gas variations. For example, cases of positiveand negative correlation between222Rn and CO2 insoils and onset of volcanic–seismic activity wererecorded, however CO2 in soil-gas is not always anindicator for the presence of magmatic gases (Heilig-mann et al., 1997).

In almost all cases, the remote stations weredesigned without the possibility to change the typeof sensor during the research/surveillance monitoring,despite the fact that the sensor market development isvery rapid since the 1980s.

These deficiencies point out a serious limit regard-ing the versatility of the remote stations, either atgroundwater or gas emission sites. The exponentialdevelopment of different techniques to measuregeochemical–hydrologic variables and the adoptionof a geochemical network strategynowadays permitsthe gathering of new scientific results in the field ofvolcanology, but a versatile hardware/softwareconfiguration of the system is strongly recommendedas the main prerequisite.

The choice of the Acqua Difesa test site issues fromgeochemical, hydrogeological and volcanologicalstudies performed by the Istituto Nazionale di Geofi-sica (ING) (Dall’Aglio et al., 1994) and by otherItalian research groups (Aureli, 1973; Ferrara, 1990;Anzaet al., 1989; Bonfanti et al., 1996a,b; Chiodini etal., 1996; Aiuppa et al., 1997; Allard et al., 1997,1998; Giammanco et al., 1998; Brusca et al., 1999):

F. Quattrocchi et al. / Journal Volcanology and Geothermal Research 101 (2000) 273–306274

in particular, the rise of deep gases, characterised by aclear He-mantle signature and the information onepisodic variations in chemistry and temperature inoccurrence of volcanic activity, deserved specialinterest in the final choice of this site. An accurateknowledge of the geochemical and hydrogeologicalframework of the test-site, as reported in the text, isa prerequisite for a correct interpretation of the moni-toring data.

During this first geochemical monitoring periodenhanced volcanic activity of the Etna volcano wasobserved, which started on July 1998 with increasedplume degassing, episodic low-energy seismicity andperiodical (strombolian) lava fountains (23 episodesup to date since September 1998, Privitera, personalcommunication, 1999). The volcanic activity parox-ysmal phase was a summit crater outbreak and aneffusive eruption, that started on 04/02/1999. As a

whole, the summit crater fracturing was not accom-panied by enhanced seismic activity (ING Data Bankand Privitera, personal communication, 1999),although a few seismic swarms withMmax� 3:1were recorded in the succeeding months.

We show in this paper the entire continuous andsemi-discrete monitoring data-set gathered fromOctober 1998 to June 1999. We correlate thegeochemical anomalies with the ongoing seismicand volcanic activity and attempt to explain theobserved correlation by using a volcanologicalapproach.

2. Methods: the GMS II prototype

The GMS II design strategy is based upon theexperience gathered by ING since 1990, managing

F. Quattrocchi et al. / Journal Volcanology and Geothermal Research 101 (2000) 273–306 275

Fig. 1. Scheme of the GMS II prototype design, installed at the Acqua Difesa well. The encoder are the Signal Conditioning System for eachsensor/instrument. The Data collector is the Serial Collector–Multiplexer, as described in the text.

the first prototype (GMS I, still on line at theBarozzeWell within the Colli Albani quiescent volcano),equipped with a multi-parametricIdronaut probe,measuring temperature, pH, Eh, electrical conductiv-ity, dissolved CO2, and other atmospheric sensors(Dall’Aglio et al., 1990, 1991); episodically a Rnmonitor was on-line separately, as a consequence ofthe non-versatile configuration of the GMS I proto-type (Quattrocchi et al., 1997b). The innovationsbrought to the GMS II prototype with respect to thefirst are mainly referred to a separate sensors manage-ment, discarding the use of pre-equipped and closed-system multi-parametric probes, which are typicallyassembled by the standard marketing exploited for theenvironmental risks assessment. They are assembledin such a way that they prevent an easy access to thesignal path as a whole, indispensable for single sensorrenewal, maintenance and management, mainly ifconsidering the early state-of-the-art of this research.In fact, in our GMS II design, the entire path of signalis accessible, considering the acquisition channels asthe base units, on which the entire system has to bebuilt up. In this way, the GMS II remote station mayassume different configuration (admitting ground-water, soil, fumaroles under monitoring) with regardto the sensor/instruments choice, although a uniqueacquisition-software located in the single remotestations (SOFTSENS and SOFTLINK codes, seebelow) records the data. They remain accessible byphone with a shared archive-software (SOFTINGcode, see below) located in a central station (futureINGV seats). This last topic allows the managementof the geochemical–geophysical network in real-timealso from remote erupting/shocked regions duringemergency periods, by using a remote station ascentral base.

2.1. Sensors

The GMS II sensors/instruments, once selected,may be partially or completely renewed, followingboth the continuous monitoring evolution and thecurrent remote-site needs: in the course of monitoring,a new scientific task may be envisaged for a correcttime-series interpretation, requiring the implementa-tion and/or removing of sensors, also for geophysicalparameters.

The basic GMS II design (Fig. 1) foresees the

presence of the following sensors/instruments:temperature of water, pH, Eh, electrical conductivity,as well as dissolved CO2, Rn, He and H2S; other chan-nels were provided for groundwater static level andatmospheric parameters (P, T, rain, humidity). Thehelium continuous monitoring device was alsoassembled, and it was added to the GMS II stationon March 1999, considering the importance of He involcanic activity monitoring (Sano et al., 1984;Fischer et al., 1997). In recent years, differentRadon continuous monitoring devices have beendesigned, improving the performances of the proto-types (Quattrocchi et al., 1997a; Galli et al., 2000), butthe prototype reserved for the Acqua Difesa aquifer isstill under testing, after calibration step, and tempora-rily an Alphaguarde continuous monitor wasinstalled (November 1999). The dissolved H2S at theAcqua Difesa well is under the Minimum DetectionLimit (MDL � 5 ppm), rendering useless the newlymanufactured dissolved H2S amperometric sensor(thus destined to the Tivoli–Rome remote station):it was developed in collaboration with the WolfsonElectroanalytical Sensor Group of the University ofNew Castle Upon Tyne and manufactured by the ESTLTD e (New Castle, UK). It provides auto-calibrationroutines, a final analog output of the dissolved H2Sconcentration in groundwater, once the temperature iscompensated.

Finally, eight sensors are on-line nowadays at theAcqua Difesa station (Photos 1 and 2), here describedin more detail: groundwater temperature, electricalconductivity, pH, redox potential, dissolved CO2,He, atmospheric pressure and air temperature; anentire record is filed every 10 min with date andhour. The selected sensors for the pH (temperaturecompensated) and Eh measurement are fromEndress1 Hausere and ORIONe, respectively. Inparticular the pH-measuring glass electrode isequipped with a separate long-life reference electrode(Endress1 Hausere ORBISINT CP13) and a micro-processor pH-transmitter device (Endress1 HausereMycom CPM 121 P), providing two 4–20 mA analogoutputs (temperature and pH signals) connected to itsspecific Signal Conditioning System(see hardwaresection and Photo 1; Fig. 1). The precision (taken aserror) is expected around 0.02 pH units. The pHsensor calibration is performed monthly by pH 7and pH 4 buffer solutions: the sensor has not exhibited


any drift up to now. The electrical conductivity cell�K � 1:0; temperature compensated), equipped with aspecific signal transmitter as 4–20 mA analog output,is manufactured by the B&C Electronicse; thenominal precision is 10mS/cm (taken as error). Thesensor for the dissolved CO2 measurement in ground-water was customised by Idronaute (Brugherio, MI,Italy); it is the same sensor used for the GMS I proto-type, on-line since 1991; it is a gas-perviousmembrane ion-selective electrode, indirectly cali-brated using an ORIONe CO2 ion-selective electrodein two different solutions; we inferred an error of 15%in the data handling. The air-temperature, barometricpressure and humidity sensors have been taken fromMTXe; in particular the atmospheric pressure sensor(Model APR 011 C) is a piezo-resistive one, with arange of measurement from 850 to 1050 hPa, a preci-

sion (at 228C) of 0.5 hPa (taken as error). The airtemperature sensor (Model TAM-020 C) is a linearthermistor, with a range of measurement from230to 1 508C, a precision of 0.28C (taken as error). Weselected the ALCATELe ASM 100 HDS He LeakDetector (Photo 1) as the continuous He monitoringdevice, it being a selective mass spectrometerconceived as portable He-leak detector. It provides a0–5 V analog output, measuring the He flux rangingfrom 1027 to 1 mbar l/s (manufactured so that theatmospheric He concentration of 5.3 ppm correspondsto a flux of 5.0× 1026 mbar l/s). The He-detector wascalibrated by using this reference point and an He-calibrated leak of 1024 mbar l/s (ALCATELe furn-ished), selecting the window of instrumental responseto proportionate theSignal Conditioning Systemelec-tronics only within the range between 1 and 1000 ppm


Photo 1. GMS II prototype final assembly inside the remote station. From left to right: the cellular phone on the wall, the remote station PC-IBMcompatible computer, the Serial Collector–Multiplexer on the big table, the boxes of Signal Conditioning System on the little table, the bufferbatteries and the stabiliser–inverter assemblage under the big table, the sensors inside the lavatory, feed by the pumped groundwater. On theother flank of the room: the He mass spectrometer on-line with the gas-stripping device and the ISCO automatic sampler below. Solutions forcleaning and calibrations are always ready available for the operator during discrete sampling and maintenance, conceiving the station aslaboratory.

of He. The dissolved gases that arrive to the massspectrometer, after passing through a silica-gel dehu-midification trap, are extracted from groundwater by atwo-heads MASTERFLEXe peristaltic pump, as yetused for the222Rn stripping (see Quattrocchi et al.,1997a). The entire He-monitoring system has aninstrumental response time of 90 s. The calibrationof the He facility is still lacking as a whole: it requiresa complicate calculation involving exact knowledgeof air/water volumes, He stripping efficiency, He-solubility, etc., but if all these parameters are main-tained constant, relative measurements in mbar l/sunits are equally sound. The nominal sensitivity of0.1 ppm is scheduled for gaseous phase under moni-toring (i.e. fumaroles, soils, etc.): the Acqua Difesasite having an He concentration of 40 cc=g�STP� × 1028

(measured by Aiuppa et al., 1997), we expected adetectablebackgroundsignal and meaningful time-series during monitoring of groundwater too; other-

wise the He facility will be transferred to another soil/fumarole future station.

An auto-sampler (ISCOe) equipped by a micropro-cessor and 24 bottles is currently on-line, collecting twobottles of 500 ml every 72 h (3 days), either acidified (topH 2.0) or not. A 1mm long-life Milliporee filtrationmodule will be tested in the impending future despitethe enhanced possibility of more recurrent interrup-tions of water circulation. This situation is similar tothat of the GMS I prototype installed in the ColliAlbani area, where CO2-rich groundwater involveshydroxides–oxides progressive precipitation withinthe filters. All the cations, together with SO4 and Cl,have been analysed at ING by DIONEX HPLC(double columns type, standard DIONEX proce-dures), while HCO3 was analysed by acidic titration(1/20 N HCl). The NH4 was measured by an ORIONeion-selective electrode, but all the samples were foundto be below the MDL (0.7 ppm).


Photo 2. The Acqua Difesa well site located in the Southern sector of the Etna volcano edifice, just near the Belpasso city, 3 km west from theBorrello village.

2.2. Power configuration and communication

With regards to the electrical power supply config-uration of the Acqua Difesa remote station, a staticelectrical unit was selected; it has been properly modi-fied getting the alternate current (220 V) with a suita-ble degree of protection against lack and failures inthe main supply (6 h of subsistence of the system as awhole, considering the actual loads). Two 160 A hbatteries feed the inverter providing current to theloads. A stabiliser was added to the inverter to assurea good quality current. The auto-sampler, workingwith internal buffer batteries with 1 day autonomy,is linked to no-protection 220 V.

Nowadays, the data link to the central station(SCING) is accomplished by an Ericssone cellulartelephone (Model GH388), connected to the PC viaa PCMCIA modem card. The data rate is 9600 baud.Automatic call to the ING centre is performed every2 h, and a real-time data routine may be launchedevery time, to check the ongoing GMS II remoteroutines. Scheduled stand-by position of the centralSCING computer allows to receive the alarm filesfrom the remote stations.

2.3. Hardware—data acquisition system

The data acquisition system was designed andrealised by ING as a completely modular hardware(Fig. 1; Photo 1), as a three layer system made of : aDedicated Analog Conditioning Modulus (ADAN-MOD), as front end to each geochemical sensor/instrument, an Analog to Digital Converter for eachConditioned Sensor (ADINTER) and finally a SingleSerial Collector–Multiplexer (DATAMIXER), feed-ing all the acquired data to the data logger. Two arethe main characteristic of the GMS II hardware (acomplete technical manual is in progress) namely:(i) all the sensors are connected to the PC-IBMcompatible through a Serial Collector–Multiplexer(DATAMIXER), which allows several serial portsto be collected to a single serial port. This last devicetemporarily downloads data into its internal buffer,places a sensor recognising label to each data stringand then sends it out as soon as the output linebecomes available; (ii) each measurement isperformed by a sensor and by specially designed elec-tronics for the Signal Conditioning System, enclosed

in shielded rough boxes and placed as close as possi-ble to the geochemical sensor to increase signal tonoise performance. By this device, each sensor is digi-tised by its own A/D converter (ADINTER), insteadof using a unique multiplexed one as it is preferred inthe most common used techniques: this noveltyavoids the introduction of some electronics on theanalog signal path that may decrease the converterperformance. The common characteristics of theSignal Conditioning System (ADANMOD, ADIN-TER) are listed below. The analog to digital conver-sion board is capable of 16 or 20 bit resolution in a^2.5 V range (or 0–1 2.5 V range), employing asigma–delta technology. Sampled data output is setat about 20 sps (up to 1900 sps are possible). It exhi-bits excellent linearity and a quantified noise belowthe Less Significant Bit (LSB). The PC data loggingsoftware will decimate the signal to attain the desiredamount of data. Digital output is in plain ASCII,RS232, so each unit may be used as a stand-alonesystem and the output can be easily recorded using aPC or a data logger. Internal logic is attained by PLDlogic (Programmable Logic Device), avoiding the useof microprocessors to improve system reliability andimmunity to electromagnetic interference.

The converter and the geochemical sensors arepowered separately from the rest of the electronicsand data logger, and they are galvanically isolatedusing DC/DC converters: this will ensure less systemfailures and increased noise reduction especially forhigh impedance sensors (typically adopted in fluidgeochemistry monitoring). The Common ModeRejection Ratio (CMRR) has been tested to be greaterthan 136 dB. The analog front-end (ADANMOD) tothe geochemical sensors, if needed, is a piggy-backmodule installed on the converter board and capableto interface to multiple standard sensor outputs aswell as custom needs (i.e. current to voltage conver-sion, signal amplification, signal offsetting, signalsumming, etc.).

2.4. Software

The software codes written to manage the GMS IIwas outlined in other ING technical reports (Sciaccaand Quattrocchi, 1998; Sciacca, 2000 unpublishedsoftware operator manual). Briefly, the SOFTINGcode runs at the PC IBM-compatible of the central


station (SCING): it has the aim of managing the trans-fer of the data coming from the remote stations and ofarranging them in files suitable for being accessedeasily by operators. Otherwise, two codes are runningsimultaneously at the PC IBM-compatible of theremote stations: the first one, called SOFTSENS, isreserved for the acquisition of the data coming fromthe sensors linked to the Signal Conditioning System—DATAMIXER; the second one, called SOFT-LINK, manages both the storage of the acquireddata on the local hard disk and the communicationstowards the SCING. Among the other auxiliaryroutines available, also remotely launched, are: themodification of operating parameters, the calibrationof the system and the check on the data to verifypossible alarm situations.

3. Overall site description and expected results

A research program aimed at monitoring volcanicand seismic activity necessarily implies the evaluationof the most powerful sites (i.e.sensitivesites in litera-ture) to attain reliable and meaningful results.

The Acqua Difesa well, chosen for the GMS II firstinstallation is located in the Southern sector of theEtna volcano edifice (Photo 2), just near Belpassocity, 3 km west from Borrello village (UTM location33SVVB957625,h� 600 m a.s.l.). Its flow capacityis around 50 l/s. Two pumps were positioned withinthe well, one—the biggest (PG, 150 CV)—at 165 mand the other—smaller (PP)—at 193 m depth; thestatic level is,500 m a.s.l.

The choice of this particular well arises from therecognised relationships between geodynamicalpatterns and fluids evolution of the Etna region, asdescribed in previous papers, specifically addressedto rework the backgroundfluid geochemistry andisotopic chemistry (Malinconico, 1979; Chester etal., 1985; Allard et al., 1991; Edner et al., 1994;Marty et al., 1994; D’Alessandro et al., 1992, 1996,1997, 1999; Allard, 1997; Aiuppa et al., 1997; Hauletet al., 1977; Brusca et al., 1999): the Southern andEastern sectors of the volcano were discovered asthe most intriguing areas to understand the differentsources and components of the uprising fluids, mostlyas a consequence of the high magmatic gases flux, notaffected by dilution with shallower components. The

most anomalous magmatic signature is mainly locatedthroughout the Paterno` –Belpasso area (Badalamentiet al., 1994; Giammanco et al., 1995; Chiodini et al.,1996; D’Alessandro et al., 1996; Allard, 1997; Allardet al., 1997). The deep gas chemistry is represented, inthe Southern sector by the Salinelle di Paterno` site,where a reservoir temperature of 100–1508C wasinferred (Chiodini et al., 1996): this site may beconsidered theend memberof the deeper gaseousinput, with the presence of all the magmatic gaseousspecies, and enrichment in major, minor and traceelements (i.e. B up to 200 ppm, Li, SiO2, etc.) path-finders of tectonic and geothermal processes.

A large number of papers exist on the Etna volcano-tectonic patterns, reworking the historical volcanicactivity (Nadge, 1978; Romano and Sturiale, 1982),the seismic activity and the deformation associatedwith volcanic activity (Patane` et al., 1984, 1994;Ferrucci and Patane`, 1993; Murray, 1994; Lanari etal., 1998). These studies pointed out the peculiar rela-tionships between carbonate basement, cover andvolcanic edifice in the Southern and Eastern sectorsof the volcano.

Considering these two most anomalous sectors ofthe Etna volcano (Zafferana Etnea and Paterno` –Belpasso areas) the Southern sector is affected bymore strict relationships with the sedimentary-carbo-nate basement underlying the volcanic and the clayeyNeogenic strata, than the Eastern one. In particular theformer sector seems to be characterised by two aqui-fers: the deeper (not intercepted by wells in the Etnaarea), typical of a geothermal evolution of a carbonateaquifer and the shallower, typically showing a fastwater circulation in volcanic rocks. Some differencesdistinguish the two sectors, i.e. the H2S–SO2 signature(and consequently SO4 in solution) in groundwater isless pronounced in the Southern sector than in theZafferana Etnea–S. Venerina–Acireale area (i.e.here the SO4 content was strongly modified beforeand after the paroxysmal phase of the 1991 eruption,see Dall’Aglio et al., 1994). Thus, different resultsamong the two areas during a continuous geochemicalmonitoring are expected. In view of a continuousmonitoring network strategy, a few sites just in theSouthern and Eastern sectors of the volcano wereselected, as suited for discrete geochemical surveil-lance of the volcano since 1980 (Anza` et al., 1989;Dall’Aglio et al., 1994; Allard et al., 1997; Aiuppa


et al., 1997, 1998; D’Alessandro et al., 1997;Giammanco et al., 1998; Brusca et al., 1999), alsotaking in consideration the hydrogeological informa-tion (Aureli, 1973; Ferrara, 1990).

In this framework, the physico-chemical para-meters and the major, minor and trace elements ofthe Acqua Difesa well were analysed (Dall’Aglio etal., 1994; Bonfanti et al., 1996a,b); the main resultspointed out a clear deep magmatic input, i.e.,PCO2

upto 1.2 atm, mantle3He/4He signature (Marty et al.,1994; Allard 1997; D’Alessandro et al., 1997) anddifferential deep input in occurrence of the ongoingvolcano-tectonic activity. The well is particularlyanomalous with regards to variables such as watertemperature, pH, Mg, Fe, Mn, SiO2, taking into con-sideration the Etna groundwater collected up to now.

Here, the Salinelle di Paterno` “end member” (onlya few kilometres away) may be mixed locally andepisodically with the meteoric circulation. For exam-ple, the noteworthy B enrichment at the Salinelle diPaterno` allows to foresee a B increase, also within themain aquifer at the Acqua Difesa site, as a conse-quence of the episodic uprising of a B-rich gasphase, originating from a boiling hydrothermal aqui-fer, probably interacting with the shallow aquifer(normally with around 0.3 ppm of B), as suggestedby the Cl/B ratio in the Paterno` –Belpasso–Adranoarea, which is lower than the rock ratio (Chiodini etal., 1996).

The lack of a background positive oxygen isotopicshift, the lack of background H2S or SO4 anomaliesand the normal average radon concentration (around10–15 Bq/l) may deserve special interest to monitortheir variations as a function of changing volcano-tectonic conditions, i.e. fracture opening with releaseof vapour spikes, fluids richer in Rn, He, CO2, H2S andSO2, possibly mixed with the deep geothermal-carbo-nate aquifer, being of different isotopic composition.

In view of its interest, the Acqua Difesa well wasalready selected also for thePoseidon Projectcontin-uous geochemical surveillance of the volcano(ISMES, 1992), but the recorded data are not reliableas a consequence of lack of maintenance to thestations and have never been processed. Despite thelack of continuous monitoring data, a discrete moni-toring on a monthly basis is currently performed(Aiuppa et al., 1997).

In conclusion, these previous studies pointed out

that the best strategy for the Etna volcano surveillanceis to continuously monitor the main factor controllingthe chemistry of the Etna groundwater, i.e. the deepgaseous inputs. They are mainly composed by acidicand reducing gases (CO2, H2S, SO2, HCl, HF, H2,CH4, etc.) that are able to enhance the leachingpower of the circulating meteoric groundwater, invol-ving an increase in solution of major and some minorand trace elements and compounds (Mg, Na, Li, B,SiO2, As, Hg, Fe, Mn, Sr, F, ClNH4, etc.) and gaseoustrace elements (222Rn, He, etc.). The jointed discreteand continuous monitoring in groundwater mayrecord in real-time the variation of this deep gaseousinput, possibly associated with the ongoing volcano-tectonic processes, as a consequence of the variationin chemistry of some parameters directly linked to thedifferential gaseous release in occurrence of volcaniccrises. The main parameters are temperature (heat fluxand vapour flux), SO4 (SO2 and H2S gaseous input), Cl(HCl and deep geothermal reservoir), HCO3 and pH(CO2 gaseous input), Mg, Na, B, Li, As, F and SiO2

(geothermal reservoir equilibrium and rate of leach-ing) and redox potential (H2S and other reducingagents).

The expected results should confirm the inferencereported throughout other active volcanoes. Forexample, three of the largest eruptions of the Galerasvolcano (1989–1995 period) were immediatelyfollowed by intense periods of seismicity and bycomparatively large SO2 fluxes recorded by remotesensing on daily and weekly basis and interpreted asrapid degassing of partially solidified magma. SO2,H2S, HCl decreased significantly weeks before theJuly 16, 1992 Galeras eruption (Fischer et al., 1997),caused by selective adsorption of more water-solublegases, total S and HCl into the hydrothermal system ofthe volcano before the eruption (Fischer et al., 1996).The increased adsorption was related to the sealing ofgas conduits, as proposed by Fischer et al. (1994).

Data on the chemistry and the emission rates ofgases and their evolution with time were used to fore-cast eruptive behaviour and to infer conditions withinthe magma chamber. Changes in concentrations ofgases and their rates of emission indicate the extentto which the volatiles have been removed from themagma and the energy available to power gas-drivenvolcanic activity (Casadevall et al., 1983, 1994;Zapata et al., 1997).


In particular, the strict link between CO2 as maingas carrier and other carried trace gases as222Rn andHe, enriched by enhanced leaching and differentialsolubility during uprising (Dall’Aglio et al., 1994;Aiuppa et al., 1997), allow to predict the capabilityof the multi-variable monitoring, comprising raregases, to the modelling of the reservoir evolutionduring the ongoing seismo-volcanic activity.

At the Etna volcano we expected, as in other volca-noes, that the CO2 may be the best indicator of theongoing magmatic processes. As observed at theGaleras Volcano in Colombia, discharged acidicgases allowed to define a clear change in the degas-sing patterns during 1989 through 1994, probablyassociated with pressure build-up. In this occasion,the temporal response in gas composition, particularlyCO2, suggested three possibilities (Alfaro and Zapata,1997): (1) magma resurgence as a result of new gascontribution and its relatively low solubility in themagma; (2) solidification of magma; (3) contributionof a deeper hydrothermal system, produced by watersaturation and/or temperature increase that wouldreduce CO2 solubility in water. Bicarbonate water atthe Galeras volcano in Colombia was considered(Fischer et al., 1997) as direct absorption of magmaticgases into shallow groundwater (supported by R/Raand stable isotope data): the same situation occurs inthe Southern sector of Etna volcano, just where themain aquifer is cut by the Acqua Difesa well.

4. Results

The GMS II prototype started to run on 17 October1998, recording one complete record (10 variables)every 10 min; we show and discuss (Figs. 2–6) thedata up to June 31, 1999. The activated sensors arepresently measuring: groundwater temperature, elec-trical conductivity (EC onwards), pH, redox potential

(Eh), dissolved CO2, atmospheric pressure and airtemperature (downfall on 30/10/1998); from 17/03/99 to 06/05/99 the He continuous measurementswere recorded too. Lack of data recording occurredfrom 20/10/98 to 26/10/98, from 16/11/98 to 30/11/98, from 17/12/98 to 26/01/99, from 23/02/99 to 01/03/99 and from 06/05/99 to 18/05/99, as a conse-quence of prolonged power failure/maintenance andsoftware failing in restarting the system automatically(now the software have been modified but prolonged220 V power or Serial Collector Multiplexer failuresmay be crucial for long data lacking).

The discrete geochemical data available up todate, (52 samples every three days for a period of9 months, from 15/10/98 to 30/06/99) are relatedonly to the major cations and anions (Fig. 7), whilea few minor and trace elements as well as selectedisotopic ratios analyses are still in progress. A fullstudy should comprise: As, Se, Hg, Mn, Al, Fe, Mn,Ni, Cd, V, Cr, Sb, Ag, Co, Cs, Au, Br and selectedisotopic ratios as Cl, B, C and He (Tazieff andSabroux, 1983).

The seismic and volcanic activity informationthroughout the Etna volcano during the consideredperiod come from the data available by ING SeismicBulletins, and by IIV Catania (Privitera and Condar-elli, personal communication, 1999) as reported inTable 1.

4.1. Variations related to well-pumps

In this initial monitoring period we tried to discri-minate the intrinsic processes and phenomena char-acterising the “well-life” such as pump stops, diurnalvariations, recurrent spectral components, aquifercharacteristic behaviour, for a better comprehensionof the found anomalies, cutting thefalse anomalies.

In the first period of the monitoring (period A, from16/10/98 to 16/11/98), only the deeper little pump (PP)


Fig. 2. Temporal trend of the GMS II prototype located at the Acqua Difesa test-site remote station: (a) from 16/10/1998 to 18/12/1998; (b) from26/01/1999 to 07/02/1999; (c) from 07/02/1999 to 23/02/1999; (d) from 01/03/1999 to 31/03/1999; (e) from 01/04/1999 to 30/04/1999; (f) from01/05/1999 to 31/05/1999; (g) from 01/06/1999 to 30/06/1999. The sensors on line are water temperature, pH, Eh, EC, dissolved CO2, He, airtemperature, barometric pressure. The dotted lines with small arrows correspond to power failure at the well, with long-period data recordinglack (see text). Different lines refer to different kind of episodes: fine dotted lines areas are referred to “stop of pump” (SP); bulk dotted linesareas refer to the most apparent anomalies (A1–A12), as explained in the text, while the numbers within the little boxes below refer to seismic orvolcanic events listed in Table 1 (see Ref. no. column). The symbolMsymbol has the meaning of operator maintenance at the remote station.The daily variation of water temperature, pH, EC and dissolved CO2, that follow the air temperature daily variation, are well recognisable. Theevent no. 8 refers to the fracture opening with lava flow which occurred at the summit crater (2900 m) on 04/02/1999.


Fig. 2. (continued)

was running: during this period only randomly the PP

stopped (SP events in Fig. 2a up to mid-November1998) and the bigger shallower pump (PG) was alter-natively used. After this period only the PG wasrunning (period B) and it was stopped only randomly(SP events in Fig. 2b and c); in these cases the PP wasalternatively used, up to the end of February 1999.From the beginning of March 1999 up to the end ofthe discussed monitoring (period C) the PG has had noproblems and inside the remote station we receiveonly water pertaining to the PG. The data clearlyshow that when, in the period A the PP was stopped(Mr Martinello, daily reports) the following falseanomalies occur (fine dotted lines areas with SPsignals in Fig. 2a): the EC drops by about 150mS/cm and the pH increases abruptly by about 0.15 pHunits. This evidence may infer that when the PP stops,running the shallower PG pump, the input of a deeperaquifer is decreased, rendering groundwater lesssaline and less acidic (i.e. CO2 uprising decreases).On the other hand, during the period B, the dataclearly show that when the PG was stopped, runningthe PP (Mr Martinello, daily reports) the following

false anomalies occur (fine dotted lines areas withSP signals in Fig. 2b and c): the EC rises by about150mS/cm and the pH decreases abruptly by about0.15 pH units. From this evidence one may infer thatwhen the PG stops the input of a deeper aquifer rises,rendering the groundwater more saline and moreacidic (i.e. CO2 flow increases). This kind of describedfalse anomalies are recurrent any time the PG isswitched off, leaving only the PP running, and vice-versa allowing to discriminate these man-mainte-nance events from the possibletrueanomalies, relatedto volcanic processes.

In conclusion, two aquifers exist, normally sepa-rated (i.e. by clayey tuffs or by impervious lavabody), cutting the well: the deeper more saline(,1200mS/cm), more acidic (pH,5.6), more redu-cing (around 50–100 mV more negative) and richer inCO2, the shallower less saline (,800mS/cm), lessacidic (pH ,5.8–6.0), more oxidant and poorer inCO2. So that when the actual routine-pump PG is notswitched, only some external cause may mix the twoaquifers, i.e. a tectonic/stress sources, gaseous inputfrom below, accompanied by groundwater mixing


Fig. 3. Detailed temporal trend of the A1 anomaly (see text for explanation), recorded by the GMS II prototype located at the Acqua Difesa test-site remote station, verified on 08/12/98 (10:42–11:13 Italian time), as example of a spike-like short period anomaly at the station.

(valve opening mechanism), pore-pressure uprisingwithin the deeper artesian aquifer, etc.

4.2. Long period variations during period A(pre-eruptive)

Taking into consideration the continuous monitor-ing data set as a whole,long periodvariations morethan short ones were verified during the first period A,as follows:

1. At the beginning of the monitoring, after anincreasing trend of the Eh signal, an apparent Ehdecrease from 27/10/98 to 11/11/98 from 410 to325 mV (1 245 mV are added to the reportedsignal, to refer the used electrode to the hydrogenelectrode) was observed. This trend may be relatedto an input of reducing agents within the aquiferduring the mentioned period, starting from 27/10/98, involving minimum Eh values on 11/11/1998

and maximum negative derivative from 27 to 31/10/1998. It may be mentioned that in this period asensor of reducing capacity (mainly linked to H2

and CH4 input), installed at a sensitive soil-gasstation in the vicinity of the crater, recorded anapparent positive anomaly just at the end of Octo-ber 1999 (J.P. Sabroux, personal communication,1999, AGMV Final Meeting, data not yetpublished). This event may be possibly connectedwith reducing agents recorded in the main Etnaaquifer.

2. At the same time, a decreasing trend in the EC wasobserved, starting on 30/10/1998: it exhibited adrop from about 1200mS/cm (i.e. the deeper aqui-fer) to values around 750mS/cm, typical of theshallower aquifer. After that, when the PG routinepumping was switched on following one of theperiods of lack in data (17–30/11/1998), the ECsignal remained constant. The above long-period


Fig. 4. Complete data set (10/1998–6/1999) of the water temperature recorded by the GMS II prototype located at the Acqua Difesa test-siteremote station. For the A9 geochemical anomaly see text, Figs. 2e and 6: starting from 25/04/1999 a notable increase of water temperature fromaround 16 to around 228C was recorded, exceeding apparently the 2s value for the data set as a whole. It occurred just in correspondence of thehighest released seismic energy during the monitoring period, within 30 km from the well.

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Fig. 5. Continuous monitoring of He dissolved in groundwater recorded by the GMS II prototype spanning from 17/03/199 to 06/05/199 at the Acqua Difesatest-site remote station(see text for the method and discussion). M� operator maintenance at the remote station.

decrease of the EC (usually the sensor shift isinstead slightly positive, induced by a unhurriedred moisture precipitation on the cell body) maybe related to a variation in mixing processesbetween the two aquifers, possibly linked to thesame cause of the slight decrease of the dissolvedCO2 and to the SO4 decrease. The latter occurredafter an apparent peak in the SO4 input (24/10/98discrete sample, Fig. 7), as a possible consequenceof SO2–H2S spike-input, rapidly converted to SO4.It possibly rendered slowly the groundwater morereducing, involving the observed long period Ehdecrease. So finally, the possible overall interpreta-tion of these end-October 1998 anomalies may bereferred to prolonged dilution of magmatic gaseousbatches into the aquifer, without heat signature.Restored conditions were verified after around amonth (recorded only after the power failure/lackof data and pumps changing). This long periodmulti-parametric anomaly occurred just beforeand during re-activated seismicity (Table 1, events1–2), after a relatively seismic quiescence insidethe area, corresponding to the period of lava foun-tains at the crater, which was accompanied byenhanced plume degassing.

After this seismic sequence in the Belpasso area, noother significant seismic events were recorded in theEtna region up to 05/12/98 withMmax� 3:1 (event 3in Table 1, Fig. 6). This earthquake and the associatedlow-energy sequence occurred just three days beforethe first short-period geochemical anomalies listedbelow.

4.3. Short–medium period variations during theperiod B (pre-sin-eruptive)

We do not envisage the opportunity to describe indetail all the short-term anomalies, being shown inFig. 2a–g, possibly having a meaning to testify varia-tions within the aquifer.

The A1 anomaly (Fig. 2a detailed in Fig. 3) wasrecorded on 08/12/98, contemporaneously exhibitinga sharp decrease of Eh, a pH increase, an EC decrease,sharply returning to values around 850mS/cm. Thewater temperature sharp increased (from 13.6 to14.98C) remaining at the higher mean value in thesubsequent period; moreover starting from thisepisode, the temperature diurnal excursion changed

apparently, decreasing in amplitude, despite theunchanged spectral signal of the air temperature.This A1 anomaly may be interpreted as a probableinput of a less mineralised groundwater, slightly redu-cing, associated with a slight heat signature, charac-terised by less dissolved CO2 and consequentlymarked by increased pH values. This input may beinduced by a fracture opening at depth, allowing amixing episode, as a spike-like connection (valvemechanism) with a normally sealed aquifer at depth,sharply masked by the huge groundwater volume ofthe shallow aquifer.

During the subsequent seismic events on 31/12/98,located within the Southern slope of the volcano, theGMS II prototype was switched off (17/12/98–26/01/99 lack of data), so that it was not possible to verify bythe continuous monitoring data the same correlationamong seismicity and geochemical anomalies.Anyway, the available discrete monitoring data forthat period exhibited an increase in HCO3 and Mg(Fig. 7), which has been very significant mostlyafter 30/12/1998.

The A2 anomaly (Fig. 2b) started on 31/01/99showing a sharp decrease of pH from 6.00 to 5.75;the water temperature increased by a mean value of28C and the dissolved CO2 signal, that was following aslightly decreasing trend, started to remain constant(variation in derivative); the EC showed a spike-likevariation in the signal frequency. This A2 anomalywas less pronounced with respect to the othersobserved, but in this step of the beginning monitoringwe prefer to detail and discriminate all the possiblyanomalous phenomena, to better distinguish thefalseanomalies. The A2 anomaly may testify a slight inputfrom depth of a mixed fluid slightly hot and moreacidic. It is remarkable that just on 31/01/1999 therewas also the highest crater area background seismicity(Table 1), preceding the eruption fracture (04/02/1999).

The A3 and A4 anomalies (see Fig. 2b) started on03/02/99 and on 04/02/99, respectively: the latter, afew hours before the eruption. They are characterisedby the following anomalies, very similar to the SPfalse anomalies (despite no pump-stops or mainte-nance accounting these anomalies have to be consid-ered): water temperature increased by around 28C, pHdecreased by 0.2 units; EC sharply increased by300mS/cm and the Eh slightly decreased. Differently,


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Fig. 6. Temporal trend of the seismic energy release in the Etna region (E� seismic energy to be multiplied for× 108 J) and the geochemical anomalies recorded at the GMS IIprototype located at the Acqua Difesa remote station, expressed in a Quality Factor (QF). This was roughly evaluated on the basis of quality, reliability, duration, multiparametricanomalous behaviour, possible external influences, etc. The numbers 1–4 of they-axis indicate a progressively better assigned QF: see Fig. 2a–g to observe the real shape, durationand engaged parameters. The seismic events and geochemical anomalies reference numbers match with those assigned in Fig. 2 and Table 1. We reported only earthquakes selectedby a circular extraction from the ING data-bank, within 30 km from the Belpasso co-ordinates (lat.� 37.68N and long.� 15.08E) and by the Etna region seismicity reported by theIstituto Internazionale di Vulcanologia (IIV) seismic network rough manipulation. The seismic energy was calculated on the basis of the algorithm:log E �J� � 4:8 1 1:5M (Tazieffand Sabroux, 1983), whereMd is the earthquake duration Magnitudo (we used theMd available from the ING bulletins and the available information from the IIV). The seismicenergy was cumulative in the case of more events or seismic swarm, the clusters are reported in Table 1. For each earthquake energy a logarithmic distance weight was assigned.

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Fig. 6. (continued)

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Fig. 7. Temporal trends from 15/10/1998 to 30/06/1999 of major elements dissolved in the groundwater of the Acqua Difesa remote station. Previous literature data are fromDall’Aglio et al., 1994 (1992 data), from Aiuppa et al., 1997 (1994–1995 data average values) and from Brusca et al., 1999 (1996–1997 data). The numbers inside the boxes belowmatch to the events listed in Table 1 (see Ref. no. column).

the dissolved CO2 signal sharply dropped by 100–200 ppm. Anyway, the Quality Factor (QF, see Fig.6 for an explanation) of the anomaly, roughly evalu-ated on the basis of shape, duration, reliability andmulti-parametricity was assigned low. Despite theimportance of these two pre-eruption anomalies (A3

and A4), possibly associated with two spike-likemixing episodes with a hotter fluid, more mineralised,more acidic, more reducing and CO2 diluting (i.e. avapour gushing, sharply mixed with the huge mass ofshallow groundwater), the most significant observa-tion is amedium-periodincrease in water temperatureobserved from 02/02/99 to 07/02/99. The increasingtrend of groundwater temperature (see the thick arrowin Fig. 2b) spanned between 12 and 148C: this anom-aly of 28C trend does not exceed the 2s value of thewater temperature calculated for the 9-month data-setas a whole (Fig. 4), but the derivative clearly showed aprogressive entrance in the shallower aquifer of hotterwater, pertaining possibly to the deeper aquifer, whichexhibited (Fig. 2a) an average temperature of 28Chigher. The water temperature remained constantafter 07/02/1999.

Starting from 04/02/99, a fracture opening at thesummit crater occurred (event 8 in Table 1) accom-panied by lava flow and subsequently by episodes ofenhanced seismicity.

The CO2 uprising of the A5 anomaly was verifiedjust during the maximum initial lava flow (06/02/1999); and the same CO2 uprising was observedbetween 9th and 11th February 1999, despite thatthe month of February 1999 (period B) was affectedby recurrent PG switching off and PP switching on,rendering difficult thetrue anomalydiscrimination(Fig. 2c). The February 1999 period was characterisedby seismic events (Table 1, events 9–12), but no reli-able geochemical anomalies (low QF) have beenobserved.

4.4. Short-term variations strictly correlated withseismicity during period C (sin-post-eruptive)

After the low-energy seismic sequence occurredmainly along the Northern slope of the volcano, on1–2 March and 12–13 March 1999 (events 13 and 14in Table 1), not associated to any apparent geochem-ical anomaly, an increasing of CO2 (A6 anomaly, Fig.2d) was contemporaneous to a swarm of 12 seismic

events (event 15 in Table 1), recorded inside 10 kmaway from the well (Nicolosi area). The same CO2

increase (A7 in Fig. 2d) was verified less apparentlyon 22/03/1999, in occurrence of a seismic event(event 16 in Table 1). This CO2 increase is well corre-lated with one of the most significant He anomalies(Fig. 5). On the contrary, in concomitance of the seis-mic event of 31/03/1999�Md � 2:6; central craterarea, event 17) the co-seismic CO2 increase wasvery slim, while an He anomaly was more clear, butrecorded only on 2–3 April 1999. The short He datarecording available does not allow a refined interpre-tation of the He time-series, also induced by the note-worthy maintenance of the prototypal He massspectrometer, during the discussed period (M pointsin Fig. 5), assigning a very low quality factor to the Heanomalies.

Starting from April 1999 up to June 1999 (Fig. 2e,period C corresponding to the PG pump’s good perfor-mance, as mentioned) we observed a very strict rela-tionship between seismic events inside the volcanicstructure and geochemical anomalies in groundwater(Fig. 6b). In particular the seismic event on 08/04/1999 �Md � 2:6� was recorded in occurrence of thefollowing anomaly (A8 in Fig. 2): a CO2 increase, aslight water temperature variance of the signal, an Ehminimum as well the beginning of a period of a pHnegative derivative, spanning from 08/04/1999 to 18/04/1999. All these observed anomalies may be relatedto an acidic and slightly reducing episodic gaseousinput (i.e. CO2, H2S, H2, SO2, CH4) slowly changingthe Eh and pH of groundwater, and disturbing thenormal volcanic aquifer leakage.

Starting from 23/04/1999 a relatively high seismicenergy swarm occurred, with 16 events, along thesouth-western slope of the Etna volcano (Table 1,Fig. 6), comprising aMd � 2:9 event occurred only4 km away from the well (26/04/99, 21:06 local time)and six events occurred on 29/04/1999. All theseevents have been clustered in event 19 in Table 1.The seismic energy associated to this swarm (around20× 108 J� was the highest evaluated during themonitoring period as a whole around the well, exclud-ing the energy release associated with the volcaniceruption of 04/02/1999. On the other hand, the energyroughly calculated for the seismic events reported inTable 1 may also be managed with caution. Theproblem is complicated by the fact that the significance


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298Table 1Seismic data recorded by the ING-RSNC National Network attained by a circular extraction from ING data-bank, within 30 km around the Belpasso area (seat of the monitoringwell) having co-ordinates: lat.� 37.68N and long.� 15.08E. Other information was made available by the IIV of Catania, despite the full data; elaboration is not yet complete atpresent.Md is the duration Magnitudo

Date (d/m/y) Hour (local time) Long. (ING) Lat. (ING) Md (ING) Distance (km) Epicentral area (ING) Note (ING, IIV information) N.RIF

26/10/98 12:54 37.689 15.014 2.6 12 Southern slope 130/10/98 Morning Background seismicity 231/10/98 Afternoon Background seismicity 231/10/98 17:15 37.652 14.981 2.5 7 Ragalna–Belpasso Start sequence 231/10/98 17:32 37.653 14.996 2.5 7 Ragalna–Belpasso Seismic swarm 231/10/98 21:05 37.645 14.980 2.5 6 Ragalna–Belpasso End sequence 202/11/98 Afternoon Background seismicity03/11/98 16:26 37.603 15.073 2.5 9 Pedara05/11/98 19:58 37.699 15.090 2.5 16 Petrulli14/11/98 22:07 37.767 14.957 2.5 20 Northern slope01/12/98 20:01 37.300 14.730 3.1 46 Mineo CO2 site in Etna borders03/12/98 Night Background seismicity05/12/98 05:38 37.615 15.115 3.1 12 Aci Bonaccorsi Start sequence 305/12/98 07:51 37.620 15.107 2.7 12 Aci Bonaccorsi End sequence 331/12/98 11:01 37.704 14.993 2.6 13 Southern slope Start sequence 431/12/98 11:14 37.705 14.972 2.9 13 Southern slope 401/01/99 00:56 37.698 14.974 2.7 12 Western slope End sequence 413/01/99 08:40 37.670 14.980 2.3 8.0 Southern slope Seismic tremor before

increased tremor5

20/01/99 12:4023/01/99 Morning Background seismicity31/01/99 16:25 37.740 15.030 2.6 17 Crater area Seismic swarm 603/02/99 05:06 37.734 14.987 2.5 14 Crater area 7

04/02/99 Early afternoon Increased tremor, lava fountain 804/02/99 Afternoon Fracture with lava flow, no

seismicity8

08/02/99 18:24 37.658 14.991 2.9 7 Southern slope Seismic swarm, 30 events 911/02/99 22:58 37.65 15.05 3.1 4.0 Tarderia Seismic swarm, 10 events 1016/02/99 08:45 37.626 15.093 2.8 13 Southeastern crater Seismic swarm, 56 events 1116/02/99 09:54 37.60 15.08 2.4 12.1 Pedara, Aci Castello Background seismicity 1122/02/99 03:57 37.750 14.797 2.6 21 Western slope Background seismicity 1223/02/99 02:41 37.622 14.978 2.5 4 Southern slope Background seismicity 1201/03/99 01:16 37.808 15.177 2.4 30 Eastern slope Seismic swarm 1302/03/99 00:03 37.769 15.234 2.6 30 Piedimeonte Etneo Seismic Swarm 1302/03/99 09:31 37.776 15.207 3.0 30 Eastern slope 1302/03/99 19:37 37.272 14.991 3.0 15 Crater area 1312/03/99 00:56 37.823 15.088 2.5 26 Northeastern slope Seismic swarm, 12 events 1412/03/99 20:37 37.706 15.062 2.5 15 Southern slope 14

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Table 1 (continued)

Date (d/m/y) Hour (local time) Long. (ING) Lat. (ING) Md (ING) Distance (km) Epicentral area (ING) Note (ING, IIV information) N.RIF

13/03/99 00:44 37.719 15.014 3.0 14 Central crater 1413/03/99 10:19 37.790 15.146 2.6 30 Presa Background seismicity 1418/03/99 11:05 37.626 15.038 2.3 7 Nicolosi Seismic swarm, 8 events 1518/03/99 11:47 37.745 15.087 3.1 18 Western slope 1522/03/99 17:19 37.704 15.099 2.4 16 Petrulli 1631/03/99 17:07 37.708 15.000 2.6 12 Central Crater 1708/04/99 00:22 37.655 14.809 2.6 14 Western slope 1823/04/99 23:50 37.454 14.838 2.6 20 Southwestern slope Seismic swarm, 16 events 1926/04/99 21:04 37.586 14.931 2.9 3.9 Southwestern slope 1929/04/99 02:28 37.699 14.997 2.5 10 5 km SW from crater Seismic swarm, 6 events 1923/05/99 14:33 37.683 14.988 2.8 9 Southern slope 2002/06/99 20:14 37.702 15.000 2.8 11 Southern slope Seismic swarm, 10 events 2121/06/99 02:07 37.679 15.029 2.7 6.0 Southern slope 22

of volcanic seismicity is usually rather difficult tounderstand and quantify: volcanic earthquakes aresupposed frequently to by triggered by the splittingopening of channels in the rocks of the crust, but theymay be induced also by pressure gradients linked tomagma batches injection and by release of over-pres-sured underground volatiles or even to undergroundstreaming of fluid magma.

This period of relatively high seismic energyrelease is significantly correlated with the co-seismicand post-seismic A9 geochemical anomaly (Fig. 2e):starting from 25/04/1999 an increase of watertemperature from around 16 to around 228C wasrecorded, exceeding apparently the 2s value (Fig. 4,T12s � 19:3 calculated by Sigmaplot code, selectingthe entire Twater data-set). The water temperatureanomaly reached its maximum around 30/04/1999,returning to normal values only on 06/05/1999. Atthe same time, pH increased up to,6.1, returningto previous values (,5.8) only on 02/06/1999, theEC slightly increased, at the same time Eh and CO2

remained constant. This A9 anomaly was the mostapparent and prolonged of the entire data-set of conti-nuos monitoring: an explanation may be found recal-ling an episode of heat input (i.e. a vapour spikediluted in the huge mass of groundwater), withoutapparent mass-transport (i.e. cations and anions leach-ing in solution remained almost constant, see Fig. 7).The correspondence between the highest cumulativeseismic energy released in the vicinity of the well (i.e.very close activated fracturing field) and the subse-quent highest heat signature in the aquifer is veryintriguing. This is not the first example in literatureabout temperature anomaly in fluids throughout avolcanic structure in occurrence of the main clusterof volcanic earthquakes: the temperature recorded bya telemetry buoy during 1976 at the Ruapahu (NewZealand) crater lake exhibited (Hurst, 1980) the morepronounced peaks (from around 30 to 408C) just afterthe main clusters of volcanic earthquakes withM ,3:0:

In the course of May 1999 only one geochemicalanomaly was observed: it affected the CO2 sensor,with a very well-pronounced increase of the signal(A10 in Fig. 2f). It occurred just a few hours beforetheMd � 2:8 seismic event, located along the South-ern slope of the volcano, only 9 km from the well. Theother time-series remained constant, despite the fact

that since 06/05/1999 the Eh data are lacking and agap in the EC time-series has been verified.

On 02/06/1999, a noteworthy CO2 increasing hasbeen observed (A11 in Fig. 2g), with the pH signalreturning to values around 5.9; this anomaly wasrecorded in occurrence of a seismic swarm (event21 in Table 1, and Fig. 6) characterised by 10 low-energy events�Mmax� 2:8� inside 10 km from thewell.

After that, only on 20/06/1999 a slight geochemicalanomaly (A12 in Fig. 2g) was discerned within thespectral analysis of pH, water temperature and CO2

signals. This anomaly occurred only 12 h before theseismic event of 21/06/99 withMd � 2:7 locatedaround 5 km away from the well.

4.5. Long–medium-term variations of major cationsand anions

With regard to the semi-discrete monitoring data(Fig. 7) gathered by analysing the automaticallysampled bottles, we may state the following items:

1. A long-period trend exists, exhibiting increasedMg, Na (less apparently) and HCO3 very wellcorrelated to each other, starting from the begin-ning of the monitoring (15/10/98) with an increaseof the derivative after the end of December 1998(corresponding to a period of seismicity, see Table1); this multivariable anomaly is partially corre-lated with the Cl increase at the beginning of themonitoring. The explanation of these anomaliesmay be searched in an increase of the groundwaterleaching power throughout Mg-rich host rocks, as aconsequence of the enhanced input of acidic andreducing gases within the aquifer. Also a slightmixing with a deeper-sealed Na–Mg–Cl aquifermay be awakened.

2. Two peaks exist in the SO4 content: around theend of October 1998 and at the end of January1999 (theaverage lines are omitted in Fig. 7,for graphical reasons). Both have a possibleconnection with enhanced H2S–SO2 input insidethe aquifer, in good agreement with the Ehlong-period continuous decreasing previouslydescribed (27/10/98–11/11/98). The formeranomaly occurred before the starting of theabove-mentioned low energy seismicity (end ofOctober 1998) and the latter, just 6 days before


the beginning of the fracture opening with lavaflow (beginning of February 1999).

3. Just after the fracture opening on February 1999, anapparent increase of HCO3, Mg, Na and Cloccurred, inferring a possible variation in thedeep water–rock interaction processes during themagma uprising in the surroundings and gas exha-lation toward surface. In particular, the Mg content,element typically enriched in the Etna lava, ispeculiarly higher at the Acqua Difesa site thanthe other Etna groundwater as a whole.

Our data confirm the evidence yet reported in litera-ture that mainly SO4 and Cl may vary as a conse-quence of enhanced activity of volcanoes. It hassince been shown (Fischer et al., 1997) that the SO4

concentration of the Pandiaco bicarbonate spring,located around 10 km from the central crater of theGaleras Volcano, apparently increased (from 0.13 to23.3 ppm from January 1993 to April 1993). Thisvariation may be related to the increased volcanicactivity as a consequence of SO4 and Cl uprising,attributable to the dome eruption on July 16, 1992and to the series of eruptions in 1993. The condensa-tion and adsorption of magmatic S, Cl and C (HCO3)into meteoric groundwater results in more acidic solu-tions, i.e. acid-sulphate springs, able to readilydissolve rocks and to leach cations; this process mayvary in time, as we observed. Also Fischer et al.(1997) found that the similarity of S/Cl ratios betweengases and groundwater supports the assumption thatthe S and Cl enrichment in liquid phase resulted by adirect exhalation/absorption of magmatic gases intoaquifers. During open degassing among differenteruptions, the S/Cl ratio in fumaroles tend to belower than during the period shortly prior to andfollowing the eruptions; the same behaviour maypossibly occur in the aquifers surrounding the crater.Prior the eruption when the gas pathways are blocked,the hydrothermal system tends to dominate the gascomposition possibly resulting in increased HCl parti-tioning into the hydrothermal system and thus higherS/Cl ratio of the fumaroles gases as well as dissolveddeep components in groundwater.

Giggenbach and Glower (1975) after an extensiveinvestigation and 10 years of monitoring studying theeffect of volcanic activity on the Ruapehu crater lake(New Zealand) have since shown that there are two

main indicators representing the processes affectingthe crater lake chemistry: (i) the Cl content, whosevariations in time are assumed to be proportional tothe steam required to bring about the observedchanges in temperature; thus it was revealed that itis sensible to monitor the rate of injection of steamin fumaroles (or groundwater) and (ii) the Mg content,which found suitable to assess the degree of interac-tion of lake waters with high temperature andesiticmaterial during or before volcanic crises (eruptiveoutbreaks).

5. Discussion

Considering the geochemical and geophysicalavailable data-sets as a whole, we may state thatthey are correlated to each other, in particular beforeand during the fracture opening with magma flow inthe vicinity of the summit crater, which started on 04/02/1999 and in occurrence of the most energetic seis-mic swarms, which occurred along the Southern slopeof the volcano. The almost simultaneity of the twoprocesses, i.e. stress release and gaseous release,recorded by seismicity and by the physico-chemicalparameters in groundwater respectively, suggested acommon cause for both and cause/effect triggering foreach other, i.e. hydrofracturing with gas uprising andfaulting induced by local-stress field driven by theexpansion/compression stages during the eruptiveprocess of the active volcano. Volcanic earthquakeshave been frequently supposed to result from channelsopening inside the volcanic structure and basement,from pressure gradients linked with magma batchesinjection, from release of over-pressured magmaticvolatiles and from streaming of fluid magma. On thecontrary, it seems that the seismic events and swarmsoccurring in the Northern slope of the volcano do notaffect the geochemistry of the main aquifer sectorlocated along the Southern slope (cutting the AcquaDifesa well). This hypothesis is strengthened by theradial hydrogeology of the volcano verified byprevious studies (Ferrara, 1990). We may state thatvolcano faulting, stress-field changes and hydro-frac-turing that trigger seismicity along a singled out slopeof the volcano, create possibly gaseous/heat inputalong pathways to surface, without affecting theother slopes of the volcano. If the seismicity is instead


widespread, energetic and prolonged in a peculiarperiod (i.e. event 19, associated with the A9 anomaly)also different sectors/aquifers of the volcano may beaffected by clear heat and mass transport at thesurface. More data and more stations may confirmthese evidences attained by only one station.

The main geochemical anomalies, either ofshortperiod (A2–A4) or of medium–long periodtype (i.e.the slight but apparent groundwater temperatureincrease on 24/02/99), occurred starting just aroundone day before the beginning of the fracture openingwith lava flow at the summit crater, may be consid-ered, at present, the “forerunners” of the Etna erup-tions affecting this single-out Acqua Difesa well. Itmay be considered here as a “self consistent” sitewith its own response to the pre-eruptive processesand possiblysensitiveto the Etna aquifer modifica-tions induced by the volcanic activity, in a recurrentway. Thus, the experimental observation collectedbefore this peculiar eruption may be considered, atpresent, as the “typical answer” of this specific test-site to the pre-eruptive processes and to the associatedseismicity. Further data may modify this statement.

An increase in deep hot-reducing-acidic agents(such as SO2, H2S, CH4) may be explained by avalve openingepisode during the volcano’s expansionand degassing induced by the impending magmauprising, before the eruption. These processes mayallow a spike-like communication between the shal-low and the deep artesian-sealed aquifer, whichcontains a higher concentration of volcanic gases.An increase in stress/deformation accumulation,induced by the lava rising, with consequent seismicand volcanic activity, may be the forerunner tomagma eruption (see also Notsu et al., 1991a,b; Satoet al., 1991; Bonfanti et al., 1996a,b), with readyconsequences on aquifers, mostly if stratified as inthe case of the Acqua Difesa hydrogeologicalpatterns.

A full multidisciplinary data manipulation, still inprogress, has to be attained to better understand whatreally happened during the discussed period ofenhanced volcanic activity. In fact, these phenomena,if recorded in a multidisciplinary task-force, mayconfirm experimentally the “chain reaction” of thevolcanic eruption onset, which have been widelydescribed and modelled (i.e. Tazieff and Sabroux,1983 and references therein): the magma during

impending rise gets its buoyancy from the presenceof volatiles, which originally were dissolved withinthe magmatic liquid. When this liquid become over-saturated by volatiles, the originally dissolved gasesstart to vesciculate. A chain reaction starts as soon asbubbles appear, increasing the magma buoyancy,helping its rise, decreasing at the same time the hydro-static pressure, which decreases the gas solubility inthe magma, and finally further enhancing vescicula-tion and gas exhalation up to the surface, through newopened fractures, which triggers seismicity and aqui-fers/fluid reservoirs modifications. This process maybe less pronounced if the gas bubbles do not play apreponderant role in the magma ascent toward thesurface: so that geophysical and geochemicalnetworks exploited together may give elucidation tothe magma ascent processes for each volcano. On theother hand, gases are also preponderant during theentire eruptive process: thus it is important toknow—also by indirect methods such as geochemicalmonitoring—the physico-chemical characteristics ofthe ascending gas phase, recording its output and thepossible evolution. Aquifers, despite the presence ofdilution processes, are very good and spatially wide-spread systems to receive ready and reliable answersabout it, when the quantity of exhaled gases and theirpressure is enough to modify the groundwaterdissolved gas composition and physic-chemical para-meters.

6. Conclusions

The following primary objectives of the GMS IIprototype design were accomplished:

1. To enhance within the frame of seismic hazard,volcanic hazard and gas hazard assessments theuse of fluid geochemistry continuous monitoring,either in liquid phase (groundwater) or in gaseousphase (soils, fumaroles, gas gushing sites), with thefinal goal to model time-series of a wide spectrumof geochemical and hydrologic parameters as toolfor a better comprehension of the ongoing seismo-volcano-tectonic processes.

2. To understand, considering the contribution of fluidgeochemistry continuous monitoring, the pre-seis-mic and pre-eruption processes, the earthquakesand seismic tremor sources, the role of fluids during


the ongoing activity, the water–rock interactionprocesses variation during volcanic crises: thegoal of deterministic prediction remains challen-ging and ambitious.

3. To develop a multidisciplinary approach to thenatural hazards assessment, exploiting a reallymultivariable network strategy, either in software/hardware as well as a fully versatile multi-para-metric sensor/instrument selection (i.e. addinglow dynamic—16 bits maximum—geophysicalparameters). The configuration design may betaken up for environmental surveillance as well(i.e. biogeochemical risk assessment and gashazard assessment), among sites with differentlogistics and available environments/phases.

The more soluble species of the uprising magmaticgases, such as SO2, CO2, H2S and HCl may be dilutedwithin the large mass of meteoric groundwater circu-lation inside the volcanic rocks, producing an increasein solution of SO4, Cl, Mg, HCO3, with changes intemperature, redox potential, EC and pH, as a conse-quence of the heat and mass transfer brought into theaquifer by the hot, acidic and reducing magmaticgases. These anomalies, different in shape, durationand reliability were observed during the first monthsof monitoring at the first GMS II station installed atthe Acqua Difesa well, just in correspondence of the1999 enhanced volcanic activity accompanied bybackground low-energy seismicity, mostly if recordedalong the Southern slope of the volcano, where thewell is located, confirming the radial hydro-geology.

New batches-injections of magmatic gases, with aspike-like duration, into the shallower aquifer inter-sected by the Acqua Difesa well are masked by thehuge meteoric circulation. Nevertheless they may bestill recorded by a slight increase in the reservoirtemperature as signatures of an higher steam/liquidwater ratio in the ascending fluid, by an increase inthe concentration of the dissolved species (Mg, Na,HCO3, SO4 mainly) brought up by the typicalmagmatic species and then by a rising in the equili-brium temperature and leaching power of the gasesthemselves. Uprising volcanic gases have beenconfirmed to be the main factor controlling thewater–rock interaction processes in the Etna ground-water. Moreover, spike-like signatures of hot deepreducing and acidic agents may be explained by a

valve opening mechanism episode along fractures,during degassing of the magma, before and duringthe eruption. These expansion and valve openingepisodes may allow a spike-like communicationbetween the shallow and the deep artesian-sealedaquifer, richer in volcanic gases, as verified at theAcqua Difesa well.

The recorded geochemical anomalies were accompa-nied, preceded or followed by seismic activity in thealmost the totality of the cases. This fact spur us tohypothesise a common cause for both the stress releaseand the gaseous release recorded by seismicity and bythe physic-chemical parameters in groundwater, respec-tively. Hydro-fracturing with consequent gas uprisingand aquifer mixing induced by active stress-fielddriven by the expansion stages of the eruptive process,may account for the observed correlation.

The main recorded geochemical anomalies occurredjust a few days/hours before the beginning of the frac-ture opening with lava flow at the summit crater andduring the main seismic swarm may be considered, atpresent, the “typical forerunners” and “own response”of the Acqua Difesa well to the Etna the ongoing volca-nic activity. This well demonstrated to besensitive to theEtna aquifer modifications induced by volcano-tectonicprocesses. Finally, the experimental observations inoccurrence of this peculiar eruption and seismic activitymay be considered, at present, the “typical answer” ofthis peculiar well. Also typical and recurrent falseanomalies at this test-site were discriminated andsingled out. Future continuous monitoring may modifyor confirm these statements.

A rapid development of a geochemical networkthroughout active volcanoes is strongly recommended:geochemical surveillance makes significant contribu-tion to understanding volcanic activity, magma degas-sing and the source of magmatic and hydrothermalvolatiles, and contributes to better understanding volca-nic hazard and eruption forecasting.

Acknowledgements

We thank the CEC-DGXII for funding the programAutomatic Geochemical Monitoring of Volcanoes(Contract no. ENV4-CT96-0289). Thanks to theanonymous referees for hints and corrections, allow-ing to drastically improve the volcanological


approach of the work. We wish to thank in specialmanner Mimmo Condarelli working at IIV-INGV ofCatania for the field work during the GMS II stationinstallation and periodical remote station mainte-nance. We are grateful to Dr Privitera working atthe IIV-INGV of Catania for the information aboutthe ongoing volcanic activity. Many thanks also toIng. Guglielmino and to Mr Martinello of theBelpasso Municipality for the help to manage theAcqua Difesa well site. Finally, thanks to the INGtechnician C. Salvaterra for mounting the SignalConditioning Systems and to Piccolini L. for theING Seismic Bulletin management.

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Geochemical Monitoring System II prototype (GMS II) installation at the “Acqua Difesa” well,...

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