GEOCHEMICAL INVESTIGATIONS IN THE CAMPI FLEGREI CALDERA

Istituto Nazionale di Voltattorni N., Pizzino L., Cinti D., Galli G., Quattrocchi F. Istituto Nazionale di Geofisica e Geofisica e Vulcanologia Vulcanologia INGV – National Institute of Geophysic and Vulcanology – Roma 1, Fluid Geochemistry Laboratory – Via di Vigna Murata, 605 – Rome, e-mail: [email protected], [email protected]

ABSTRACT 3 Solfatara , located in the central part of Campi Flegrei caldera (, southern Radon soil gas (Bq/m ) 4520600 Italy), is characterized by intense and diffusive fumarolic and hydrothermal activity 30 000 confirming that magmatic system is still active. During summer 2002, geochemical 4520400 45 205 00 25 000 investigations were performed in the Solfatara and surrounding areas. Flux measurements Detailed survey 4520200 were performed using the accumulation chamber and a portable gas-cromatographer. 20 000 Furthermore, soil gas samples were collected and stored in metallic containers for 4520000 15 000 laboratory analysis. Results from soil gas samples analyzed both in the field and in the 45 200 00 4519800 10 000 laboratory are in agreement with gas flux results that highlighted a clear correspondence 0 0.4km 50 00 4519600 between gaseous emanation and local tectonic. Water samples from springs, wells and Thoron soil gas (Bq/m3) 42 6800 42 7000 42 7200 427400 4276 00 4278 00 4280 00 428200 0 Large scale survey 12000 gaseous pools were collected in order to emphasize the origin of the discharging fluids, to Figure 1 – Large scale surveys 45of 20 60 0 45 195 00 11000 quantify the various degree of the gas-steam-rock interaction and the geochemical radon and thoron distribution maps. 10000 45 20 40 0 processes accounting for their final chemical features. The geochemical classification of Highest radon values are present along 9000 8000 the northern margin of the caldera.45 20 20 0 the sampled ground-waters highlighted four families: Na-Cl, Na-HCO3, sulphate-acid and Anomalous distributions are elongated 7000 Ca-SO waters. 6000 4 45 190 00 following the local tectonic (NW-SE45 20 00 0 MainMain statisticsstatistics ofof largelarge scalescale surveysurvey resultsresults 5000 direction). Thoron distribution (that 4000 Gas n° samples Min value Max value Mean 45 19 80 0 MAIN GOALS has a shallow origin) is strictly related 3000 0 0.4km Radon (Bq/ m3) 85 0 42000 4984,89 to high density of houses located on 2000 Geochemical investigations were performed in the Solfatara and surrounding areas 45 19 60 0 the boundary of the caldera. 1000 (, Cuma-Cigliano, Agnano, Bagnoli e Astroni) in order to: 45 185 00 Thoron (ppm) 85 0 19600 3360,96 426800 427000 427200 427400 427600 427800 428000 428200 0 •evaluate CO2, H2S, CH4, radon and helium degassing phenomena; • emphasise the origin of the discharging fluids; • quantify the various degree of the gas-steam-rock interaction; H S (gr/m2*day1) CH (mgr/m2*d) • quantify geochemical processes accounting for their final chemical features. 45 180 00 2 4 4 265 00 427000 427500 428000 428500 429000 429500 400 1600 Le stufe Le stufe 350 1400

GENERAL SETTINGS 300 1200 Campi Flegrei caldera is the result of two 250 1000 large collapses related to the Campanian 200 800 150 600 Ignimbrite and to the Neapolitan Yellow La fangaia La fangaia N MainMain statisticsstatistics ofof detaileddetailed surveyssurveys resultsresults BN 100 BN 400 Camp i Flegre i (Na) BG BG Tuff eruptions. 0 0.2km 50 200 The Campi Flegrei magmatic system is FluxFlux data data 0 0 still active and it is affected by Gas n° samples Min value Max value Mean Rn (Bq/m2*d) CO (gr/m2*d) NW-SE and NE-SW faults (typical of 2 2 CO2 (gr/ m *d) 32 83.3 5287.2 1127.3 5000 Le stufe 90000 the Campanian Plain). and 2 Le stufe Le stufe 4500 CH4 (mgr/ m *d) 32 0 1525.0 361.5 80000 4000 thermal springs occur in different 2 70000 H2S (gr/ m *d) 32 0 390.2 28.3 3500 Bocca nuova 60000 sectors of the caldera. In particular, La fangaia 2 3000 Bocca grande Rn (Bq/ m d) 32 0 92763.9 18234.5 50000 Legend * 2500 fumarolic activity occurs along 40000 2000 Fumaroles the coast south of Pozzuoli and in 30000 1500 Dif fusi ve de ga ss ing s truc tur e La fangaia La fangaia SoilSoil gas gas concentrationsconcentrations BN 20000 BN 1000 the Mofete area and concentrates (Cardellini et al., 2003) (Chiodini et al., 2001) BG 10000 BG 500 Crater rim in the Solfatara area. Gas n° samples Min value Max value Mean 0 0 Fault 0 400m Fracture CO2 (%,v/v) 32 0.0038 7.3 3.9 WORK DONE Figure 2 – Soil gas flux measurements in the inter crater sector of Solfatara CH4 (ppm) 32 0 165.5 85.1 2 ™Soil gas surveys: large scale survey (85 radon and thoron measurements all over the area. CO2 flux is mainly 1127.32 gr/m *d although highest flux values were found H S (%, v/v) 32 0 2.62 0.5 Campi Flegrei area); detailed survey (32 soil gas samples collected and analysed in the 2 in the “La fangaia” and near the “bocca grande” (BG) fumarolas. Radon flux 3 Rn (Bq/ m ) 32 0 33767 5504.4 distribution is very similar to the CO2 one: both gases have a dominant flux in laboratory and the same number of radon measurements performed in loco); flux the NE-SW direction and in minor part in E-W and NW-SE directions. The H S He (ppm) 32 0 9.0 3.5 2 measurements (32 sampled points) in the Solfatara area. flux measurements highlighted a NW-SE anomalous trend and a local spot with

™Groundwater survey: 35 sampled points (springs and wells). Performed analysis: values > 100 gr/ m2*d, in front of the “ Le stufe” area. CH4 flux shows a trend physical-chemical parameters (pH, Eh, electrical conductivity); HCO content (by quite different from the other gases: it is possible to distinguish NW-SE, N-S 3 and E-W anomalous trends with local fluxes > 1000 gr/ m2*d. titration); H2S and NH4 content (colorimetric methods); total CO2 content (ion-selective method); major and minor elements (ionic chromatography); 222Rn content (γ spectrometry); trace elements (ICP); dissolved gases (CO2, CH4, H2S, O2, N2); stable 18 13 isotopes ( O, D, C). CH4 (%, v/v) 3 H2S (%, v/v) He (ppm) CO2 (%, v/v) Rn (Bq/m )

140 2.4 30000 9 Le stufe 120 Le stufe Le stufe 8 Le stufe Le stufe 6 Le stufe 25000 31 1.8 32 7 100 Astroni - Lago19 grande 85 18 30 80 17 14 6 20000 23 21 75 Cuma 8 10 80 4 22 11 9 70 Cigliano 13 Ippodromo di 24 1.2 5 65 15000 Agnano 60 6 2825 60 Figure 4 – Water temperature 4 4 29 26 55 1516 3 10000 Solfatara1 50 40 2 27 3 2 distribution map. Hottest areas (high La fangaia 0.6 20 Ter me di Agna no 45 La fangaia La fangaia La fangaia La fangaia La fangaia Te mp io Serap ide BN BN 2 BN BN BN 7 5 40 thermalism) are directly connected to Pozzuoli BG BG BG 20 BG BG 5000 35 12 0 0.1km 0 0.1km 1 0 0.1km 0 0.1km 0 0.1km 30 magmatic chamber 0 0 Terme Puteolane 0 0 33 25 0 Bagnoli 20 15 Figure 3 – Detailed soil gas surveys in the Solfatara area. This campaign was performed in the inter-crater sector of Solfatara diffusive degassing structures. Results from 31 32 soil gas samples analyzed both in the field and in the laboratory for concentration measurements are in agreement with gas flux results (Fig. 2). Local trends are very similar, Astroni - Lago19 grande 18 30 17 14 11000 Figure 5 – Water electrical conductivity 23 21 although soil-gas concentrations show a more diffusive distribution, as it was reasonable to suppose. Soil gas distribution maps highlight the different behaviour of gas Cuma 8 10 22 11 9 Cigliano 13 24 (µS/cm) distribution map. Highest species: helium, methane and carbon dioxide have the same distribution, with highest values near the “La fangaia” fumarola. Anomalous distributions are elongated in the NE- Ippodromo di 9000 Agnano 6 2825 electrical conductivity values are found in SW direction following the orientation of one of the two main fracture strikes (Chiodini et al., 2001). Radon is well distributed where the soil is dry and the concentration 4 29 26 1516 7000 Solfatara1 proximity of the coast suggesting sea are not affected by water or steam. Also radon distribution follows local tectonic (NE-SW direction). Results from H2S highlight that max concentrations (> 2%) are in 27 3 2 Ter me di Agna no Te mp io Serap ide 20 water mixing phenomena: proximity of the two main degassing zone: the “bocca nuova” (BN) and the “bocca grande” (BG) fumarolas. Pozzuoli 7 5 5000 12 •Terme Puteolane :12000 mS/cm Terme Puteolane 3000 •Tempio Serapide: 20000 mS/cm Bagnoli 33 1000 Cl + SO4 Na/1000 50 31 Tortorelli 32 Figure 6 – Water redox potential (Eh) Hotel Tennis Astroni - Lago19 grande 260 18 30 17 14 distribution map. Negative values Tuf ano 23 21 220 seawater Cuma 8 10 22 11 9 Terme P uteolane Cigliano 13 24 180 highlight three well defined areas Ippodromo di A gnano Sprudel 140 Agnano characterised by highest H S values. Tempio di 6 2825 2 4 29 26 100 1516 Serapide Solfatara1 Positive values could be due to the sea 27 3 2 60 20 Ter me di Agna no From Caprarelli et al., 1997 Te mp io Serap ide 20 water influence (along the coast), to the Pozzuoli 7 5 -20 Full equilibrium 12 presence of shallow waters and/or to the Carannante -60 Terme Puteolane absence of fractures that control CO2 Capriccio %-Na Bagnoli 33 -100 C K a -140 flux. + + a

25 M N 31 g 32 4000 Astroni - Lago19 grande 18 30 17 14 3600 23 21 Figure 7 – Water total CO content Legend MF-2 PARTIALLY EQUILIBRATED WATERS Cuma 8 10 2 22 11 9 3200 Cigliano 13 24 (ppm) distribution map. Highest values are Solfatara-Agnano MF-1 Ippodromo di 2800 Fangaia CF 3 Agnano Agnano 6 2825 2400 found in the Agnano spa/race-course, Pisciarelli 4 29 26 SW 1516 Bagnoli CF 6 CF 27 Solfatara1 2000 27 3 2 Cuma/Cigliano area and along the coast. CF 16 CF 12 20 Ter me di Agna no Pozzuoli Te mp io Serap ide 1600 IMMATURE WATERS 7 5 In the Solfatara area, steam dilutes CO2 Pozzuoli 1200 Cuma-Cigliano CF 7 12 and H S contents except in the “La 800 2 Solfatara Astroni other sa mples Terme Puteolane fangaia” zone. well Bagnoli 33 400 K/100 0 0 0 25 50 31 32 Astroni - Lago19 grande 18 30 17 14 HCO 23 21 50 Figure 8 – Water radon (Bq/l) distribution 3 Cuma 8 10 22 11 9 Cigliano 13 24 map. Radon is random distributed and Ippodromo di 40 Figure 9 - Ludwing-Langelier diagram. It is possible to distinguish four main Agnano Figure 10 - Giggenbach diagram. Most part of samples fall in the 6 2825 there is, apparently, some correlations 4 29 26 chemical families: 1516 “immature waters” area excepting: i) “CF3” sample (Tortorelli well) due Solfatara1 30 with the other species: it is possible to 27 3 2 Te mp io Serap ide 20 Ter me di Agna no 1.Solfatara-Agnano family: interaction between shallow waters and acid and to a mixing between a mature water and a pure term (end-member); ii) 7 5 distinguish some anomalous spots where Pozzuoli 20 reducing gases. “CF6” sample (Tennis Hotel) close to the deep end-member (brines). 12 CO2 content is low suggesting “stripp ing” 10 2.Agnano family: interaction between deep CO2 and volcanic rocks. Terme Puteolane The chemical composition of “CF6” sample is the result of the 33 effects. Bagnoli 3.Cuma-Cigliano family: high CO2 content. equilibrium between circulating fluids and rocks in the reservoir. 0 4.Pozzuoli area: mixing between sea-waters and deep brines.

CONCLUSIONS

SOIL GAS SURVEYS The distribution of measured soil gas concentrations and fluxes is not spatially homogeneous: it is strongly affected by structural control. Results from soil gas samples analysed both in the field and in the laboratory are in agreement with gas flux results. Local trends are very similar, although soil-gas concentrations show a more diffusive distribution. Gas flux distribution highlighted a clear correspondence between gaseous emanation and local tectonic: in particular, radon and carbon dioxide have a dominant flux in a NE-SW direction and, in a lesser extent, in a E-W and a NW-SE directions. These directions are in agreement with regional extensional tectonic and with transverse structures considered as transfer faults along which the main regional volcanoes are located (Acocella et al., 1999).

WATER SURVEYS The Ludwig-Langelier diagram highlighted four different chemical groups: Na-Cl waters: in this group we find the samples Hotel Tennis, Tufano, Carannante and Capriccio (belonging to the Solfatara-Agnano family), Puteolane and Serapide (belonging to the Pozzuoli family), as well as some samples of the Agnano family (Agnano sprudel). These waters are characterized by a very high electrical conductivity (up to 20 mS/cm) and high discharge temperatures (up to 85°C, as in the Hotel Tennis well). The only exception is represented by the Tufano well, being less mineralized (electrical conductivity equal to 3 mS/cm) and colder (temperature of 22.4°C) with respect to the above mentioned samples. The origin of these waters may be due to : i)a huge mixing with seawater for the samples located along the Tyrrhenian coast (Tempio di Serapide and Terme Puteolane); ii) various degrees of mixing between cold shallow aquifers and hot deep brines (Agnano-Solfatara area); iii) mixing between deep brines and shallow steam-heated aquifers (Hotel Tennis).

Na-HCO3 waters: in this group we find the bulk of the waters belonging to the Agnano family, samples located in the Cuma-Cigliano, Astroni and Bagnoli areas, and the Tortorelli well of the Pozzuoli family. All samples show relatively high saline contents (values of electrical conductivity ranging from 2 to 5 mS/cm) and temperatures spanning from 18 to 57°C). The origin of these waters may be due to the interaction of CO2-rich fluids with the young vulcanites cropping out extensively in the area. In some cases (Tortorelli sample) the high temperature and the very peculiar chemical features (very low content of Ca and Mg, high bicarbonate content and alkaline pH) are due to the interaction between gas, steam and shallow clayey strata (cationic exchange processes). Sulphate-acid waters: in this group we find samples of the Solfatara-Agnano area (Fangaia and Pisciarelli). These waters shows electrical conductivity values of 3-8 mS/cm and very high discharges temperatures (57-74°C). They are typical acid waters (pH = 2) whose origin is due to the dissolution of steam and reducing gases into shallow aquifers; the sulphate signature is due to the oxidation of the H2S.

Ca-SO4 waters: this chemistry is showed only by the Pozzo Solfatara sample, located inside the homonymous volcano. This water shows an electrical conductivity value of 3 mS/cm and a discharge temperature of 89°C, the hottest in the area. Its chemistry may be due to the mixing between hot steam, reducing gases and Ca-SO4 rich fluids.

IN CLOSING, IT CAN BE SAID THAT BOTH SOIL GAS AND WATER RESULTS CONFIRMED THAT THE SYSTEM IS ACTIVE, STRONGLY CONTROLLED BY LOCAL AND REGIONAL TECTONIC AND AFFECTED BY DIFFERENT GEOCHEMICAL PROCESSES.

References •Acocella V., Salvini R., Funiciello R., Faccenna C. (1999) The role of transfer structures on volcanic activity at Campi Flegrei (Southern Italy). In: special issue “Volcanism in the Campi Flegrei”, Orsi G., Civetta L., Valentie G.A., (Eds). Journ. Volcanol. Geotherm. Res., 91 (2-4), 123- 139. •Chiodini G., Frondini F., Cardellini C., Granieri D.,Marini L., Ventura G. (2001) CO2 degassing and energy release at Solfatara volcano, Campi Flegrei, Italy. Journal of Geophysical research, 106, NO B8, 16213-16221 •Cardellini C., Chiodini G., Frondini F., Granieri D., Lewicki J., Peruzzi L., (2003) Accumulation chamber measurements of methane fluxes: application to volcanic-geothermal areas and landfills. Applied Geochemistry, 18, 45-54

Acknowledgements The authors would like to sincerely acknowledge the efforts of Mr Luigino Piccolini and Dr Fabio Mastino in terms of field work: they were fundamental to this work. This work has received financial support from the GNV project “ Emissioni gassose diffuse in aree vulcaniche. Aspetti geochimici, strutturali e modelli fisici del progetto. Sviluppo di tecniche di monitoraggio.” Project leader: Prof. Giovanni Chiodini (INGV – Osservatorio Vesuviano)