geochemical investigations in the campi flegrei caldera - IEAGHG
GEOCHEMICAL INVESTIGATIONS IN THE CAMPI FLEGREI CALDERA Voltattorni N., Pizzino L., Cinti D., Galli G., Quattrocchi F.
Istituto Nazionale di Geofisica e Vulcanologia
Istituto Nazionale di Geofisica e Vulcanologia
INGV – National Institute of Geophysic and Vulcanology – Roma 1, Fluid Geochemistry Laboratory – Via di Vigna Murata, 605 – Rome, Italy e-mail: [email protected], [email protected]
ABSTRACT Solfatara volcano, located in the central part of Campi Flegrei caldera (Naples, southern Italy), is characterized by intense and diffusive fumarolic and hydrothermal activity confirming that magmatic system is still active. During summer 2002, geochemical investigations were performed in the Solfatara and surrounding areas. Flux measurements were performed using the accumulation chamber and a portable gas-cromatographer. Furthermore, soil gas samples were collected and stored in metallic containers for laboratory analysis. Results from soil gas samples analyzed both in the field and in the laboratory are in agreement with gas flux results that highlighted a clear correspondence between gaseous emanation and local tectonic. Water samples from springs, wells and gaseous pools were collected in order to emphasize the origin of the discharging fluids, to quantify the various degree of the gas-steam-rock interaction and the geochemical processes accounting for their final chemical features. The geochemical classification of the sampled ground-waters highlighted four families: Na-Cl, Na-HCO3, sulphate-acid and Ca-SO4 waters. MAIN GOALS Geochemical investigations were performed in the Solfatara and surrounding areas (Pozzuoli, Cuma-Cigliano, Agnano, Bagnoli e Astroni) in order to: • 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; • quantify geochemical processes accounting for their final chemical features. GENERAL SETTINGS Campi Flegrei caldera is the result of two large collapses related to the Campanian Ignimbrite and to the Neapolitan Yellow Tuff eruptions. The Campi Flegrei magmatic system is still active and it is affected by NW-SE and NE-SW faults (typical of the Campanian Plain). Fumaroles and thermal springs occur in different sectors of the caldera. In particular, fumarolic activity occurs along the coast south of Pozzuoli and in the Mofete area and concentrates in the Solfatara area.
Radon soil gas (Bq/m3) 4520600 30 000 4520400
4520500
Detailed survey
4520000
4520200
20 000
4520000
15 000
4519800
10 000
Large scale survey
Main Main statistics statistics of of large large scale scale survey survey results results Gas
4518500
n° samples
Min value
Max value
Mean
Radon (Bq/ m3)
85
0
42000
4984,89
Thoron (ppm)
85
0
19600
3360,96
427000
427500
428000
428500
429000
429500
La fangaia
0
400m
WORK DONE Soil gas surveys: large scale survey (85 radon and thoron measurements all over the Campi Flegrei area); detailed survey (32 soil gas samples collected and analysed in the laboratory and the same number of radon measurements performed in loco); flux measurements (32 sampled points) in the Solfatara area. Groundwater survey: 35 sampled points (springs and wells). Performed analysis: physical-chemical parameters (pH, Eh, electrical conductivity); HCO3 content (by 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 isotopes (18O, D, 13C).
11000 10000 9000 8000 7000 6000 5000 4000 3000 0
0.4km
2000 1000
400
n° samples
Min value
Max value
Mean
32
83.3
5287.2
1127.3
CH4 (mgr/ m2*d)
32
0
1525.0
361.5
32
0
390.2
28.3
32
0
92763.9
18234.5
m2*d)
Rn (Bq/ m2*d)
0.2km
350
1400
300
1200
250
1000
200
800
150
600
La fangaia
100
BN BG
400
BN BG
50
200 0
Rn (Bq/m2*d)
CO2 (gr/m2*d) 5000
90000
Le stufe
Le stufe
80000
4500 4000
70000
3500 60000
3000
50000
2500
40000
Soil Soil gas gas concentrations concentrations Gas
La fangaia
n° samples
Min value
Max value
Mean
CO2 (%,v/v)
32
0.0038
7.3
3.9
CH4 (ppm)
32
0
165.5
85.1
H2S (%, v/v)
32
0
2.62
0.5
Rn (Bq/ m3)
32
0
33767
5504.4
He (ppm)
32
0
9.0
3.5
H2S (%, v/v)
0
1600
Le stufe
0
CO2 (gr/ m2*d) H2S (gr/ Fumaroles Diffusive degassing structure (Cardellini 2003) (Chiodini et al., 2001) Crater rim Fault Fracture
12000
CH4 (mgr/m2*d)
Le stufe
0
Flux Flux data data Gas
Legend
0
H2 S (gr/m2*day1)
4518000 426500
Thoron soil gas (Bq/m3)
426800 427000 427200 427400 427600 427800 428000 428200
Camp i Flegre i (Na)
La fangaia
50 00
Figure 1 – Large scale surveys 4520600 of radon and thoron distribution maps. 4520400 Highest radon values are present along the northern margin of the caldera. 4520200 Anomalous distributions are elongated 4520000 following the local tectonic (NW-SE direction). Thoron distribution (that 4519800 has a shallow origin) is strictly related to high density of houses located 4519600 on the boundary of the caldera.
4519500
4519000
0.4km
42 6800 42 7000 42 7200 427400 4276 00 4278 00 4280 00 428 200
Main Main statistics statistics of of detailed detailed surveys surveys results results
Bocca nuova Bocca grande
0
4519600
N
Le stufe
25 000
2000
30000 20000
BN BG
1500
La fangaia
BN BG
10000
1000 500 0
0
Figure 2 – Soil gas flux measurements in the inter crater sector of Solfatara area. CO 2 flux is mainly 1127.32 gr/m2*d although highest flux values were found in the “La fangaia” and near the “bocca grande” (BG) fumarolas. Radon flux distribution is very similar to the CO2 one: both gases have a dominant flux in the NE-SW direction and in minor part in E-W and NW-SE directions. The H2 S flux measurements highlighted a NW-SE anomalous trend and a local spot with values > 100 gr/ m2*d, in front of the “ Le stufe” area. CH4 flux shows a trend quite different from the other gases: it is possible to distinguish NW-SE, N-S and E-W anomalous trends with local fluxes > 1000 gr/ m2*d.
CH4 (%, v/v)
He (ppm)
CO2 (%, v/v)
Rn (Bq/m3)
140
2.4
30000
9
Le stufe 31
32 Astroni - Lago 19 grande 18
23
27
Tempio Serapide
Cuma 22 Cigliano Solfatara 1
3
4
6
2
30
17 14 21 8 10 11 9 13 Ippodromo di 24 Agnano 28 25 29 26 15 16
Terme di Agnano
20 7 5
Pozzuoli
12
Terme Puteolane
33
Bagnoli
31
Astroni - Lago grande 32 19 18
23 22
27
3
Tempio Serapide
Cuma Cigliano Solfatara 1
4
6
2
30 17 14 21 8 10 11 9 13 Ippodromo di 24 Agnano 2825 29 26 15 16
11000
9000
7000
Terme di Agnano
20 7 5
Pozzuoli
85 80 75 70 65 60 55 50 45 40 35 30 25 20 15
5000
12 3000
Terme Puteolane
33
Bagnoli
Le stufe
23
27
Cuma 22 Cigliano
3
Tempio Serapide
Solfatara 1 2
4
6
30 17 14 21 8 10 11 9 13 Ippodromo di 24 Agnano 28 25 29 26 15 16
Terme di Agnano
20 7 5
Pozzuoli
260 220 180 140 100 60 20 -20
12
-60
Terme Puteolane
Bagnoli
-100 -140
Astroni - Lago 19 grande 32 18
23
27
Tempio Serapide
Cuma 22 Cigliano
3
Solfatara 1 2
4
6
4000
30 17 14 21 8 10 11 9 13 Ippodromo di 24 Agnano 28 25 29 26 15 16
Terme di Agnano
20 7 5
Pozzuoli
31
3600 3200 2800 2400 2000 1600 1200
12
800
Terme Puteolane
33
Bagnoli
400
Astroni - Lago 19 grande 32 18 23
27
Tempio Serapide
Cuma 22 Cigliano
3
Solfatara 1
4
6
2
Pozzuoli
50
40
30
Terme di Agnano
20 7 5
20
12
Terme Puteolane
10
Bagnoli
33 0
20000
Figure 4 – Water temperature distribution map. Hottest areas (high thermalism) are directly connected to magmatic chamber
Figure 5 – Water electrical conductivity (µS/cm) distribution map. Highest electrical conductivity values are found in proximity of the coast suggesting sea water mixing phenomena: •Terme Puteolane :12000 mS/cm •Tempio Serapide: 20000 mS/cm
0
BN BG
Figure 6 – Water redox potential (Eh) distribution map. Negative values highlight three well defined areas characterised by highest H2S values. Positive values could be due to the sea water influence (along the coast), to the presence of shallow waters and/or to the absence of fractures that control CO 2 flux.
3
La fangaia
0.6
0.1km
0
BN BG
0.1km
0
15000
60
4
La fangaia
4
5
1.2
La fangaia
BN BG
2 1
0
0
10000
La fangaia
BN BG
20
0.1km
0
2
La fangaia
40
0.1km
BN BG 0
0
0
0
Na/1000
Cl + SO4
50
Tortorelli
Hotel Tennis seawater
Tufano Terme Puteolane A gnano Sprudel
Tempio di
Serapide
From Caprarelli et al., 1997
Full equilibrium
Carannante
%-Na
Capriccio
25
Figure 7 – Water total CO2 content (ppm) distribution map. Highest values are found in the Agnano spa/race-course, Cuma/Cigliano area and along the coast. In the Solfatara area, steam dilutes CO 2 and H2 S contents except in the “La fangaia” zone.
MF-2
Legend
Solfatara-Agnano Agnano Bagnoli
Fangaia Pisciarelli
MF-1
CF 3
CF 6 CF 16
Pozzuoli
CF 27
SW CF 12
IMMATURE WATERS
Cuma-Cigliano Astroni
Solfatara well
PARTIALLY EQUILIBRATED WATERS
CF 7
K/100
other samples
0
Figure 8 – Water radon (Bq/l) distribution map. Radon is random distributed and there is, apparently, some correlations with the other species: it is possible to distinguish some anomalous spots where CO2 content is low suggesting “stripping” effects.
5000
0.1km
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 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, 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 species: helium, methane and carbon dioxide have the same distribution, with highest values near the “La fangaia” fumarola. Anomalous distributions are elongated in the NESW 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 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 proximity of the two main degassing zone: the “bocca nuova” (BN) and the “bocca grande” (BG) fumarolas.
0
30 17 14 21 8 10 11 9 13 Ippodromo di 24 Agnano 28 25 29 26 15 16
7
80
0
31
100
Ca + Mg
33
Le stufe
6
6
Na + K
Astroni - Lago 19 grande 32 18
Le stufe
120
25000
1.8
1000
31
Le stufe
8
25
50
HCO3
Figure 9 - Ludwing-Langelier diagram. It is possible to distinguish four main chemical families: 1.Solfatara-Agnano family: interaction between shallow waters and acid and reducing gases. 2.Agnano family: interaction between deep CO2 and volcanic rocks. 3.Cuma-Cigliano family: high CO2 content. 4.Pozzuoli area: mixing between sea-waters and deep brines.
Figure 10 - Giggenbach diagram. Most part of samples fall in the “immature waters” area excepting: i) “CF3” sample (Tortorelli well) due to a mixing between a mature water and a pure term (end-member); ii) “CF6” sample (Tennis Hotel) close to the deep end-member (brines). The chemical composition of “CF6” sample is the result of the equilibrium between circulating fluids and rocks in the reservoir.
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), 123139. •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)
Istituto Nazionale di Geofisica e Vulcanologia
Istituto Nazionale di Geofisica e Vulcanologia
INGV – National Institute of Geophysic and Vulcanology – Roma 1, Fluid Geochemistry Laboratory – Via di Vigna Murata, 605 – Rome, Italy e-mail: [email protected], [email protected]
ABSTRACT Solfatara volcano, located in the central part of Campi Flegrei caldera (Naples, southern Italy), is characterized by intense and diffusive fumarolic and hydrothermal activity confirming that magmatic system is still active. During summer 2002, geochemical investigations were performed in the Solfatara and surrounding areas. Flux measurements were performed using the accumulation chamber and a portable gas-cromatographer. Furthermore, soil gas samples were collected and stored in metallic containers for laboratory analysis. Results from soil gas samples analyzed both in the field and in the laboratory are in agreement with gas flux results that highlighted a clear correspondence between gaseous emanation and local tectonic. Water samples from springs, wells and gaseous pools were collected in order to emphasize the origin of the discharging fluids, to quantify the various degree of the gas-steam-rock interaction and the geochemical processes accounting for their final chemical features. The geochemical classification of the sampled ground-waters highlighted four families: Na-Cl, Na-HCO3, sulphate-acid and Ca-SO4 waters. MAIN GOALS Geochemical investigations were performed in the Solfatara and surrounding areas (Pozzuoli, Cuma-Cigliano, Agnano, Bagnoli e Astroni) in order to: • 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; • quantify geochemical processes accounting for their final chemical features. GENERAL SETTINGS Campi Flegrei caldera is the result of two large collapses related to the Campanian Ignimbrite and to the Neapolitan Yellow Tuff eruptions. The Campi Flegrei magmatic system is still active and it is affected by NW-SE and NE-SW faults (typical of the Campanian Plain). Fumaroles and thermal springs occur in different sectors of the caldera. In particular, fumarolic activity occurs along the coast south of Pozzuoli and in the Mofete area and concentrates in the Solfatara area.
Radon soil gas (Bq/m3) 4520600 30 000 4520400
4520500
Detailed survey
4520000
4520200
20 000
4520000
15 000
4519800
10 000
Large scale survey
Main Main statistics statistics of of large large scale scale survey survey results results Gas
4518500
n° samples
Min value
Max value
Mean
Radon (Bq/ m3)
85
0
42000
4984,89
Thoron (ppm)
85
0
19600
3360,96
427000
427500
428000
428500
429000
429500
La fangaia
0
400m
WORK DONE Soil gas surveys: large scale survey (85 radon and thoron measurements all over the Campi Flegrei area); detailed survey (32 soil gas samples collected and analysed in the laboratory and the same number of radon measurements performed in loco); flux measurements (32 sampled points) in the Solfatara area. Groundwater survey: 35 sampled points (springs and wells). Performed analysis: physical-chemical parameters (pH, Eh, electrical conductivity); HCO3 content (by 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 isotopes (18O, D, 13C).
11000 10000 9000 8000 7000 6000 5000 4000 3000 0
0.4km
2000 1000
400
n° samples
Min value
Max value
Mean
32
83.3
5287.2
1127.3
CH4 (mgr/ m2*d)
32
0
1525.0
361.5
32
0
390.2
28.3
32
0
92763.9
18234.5
m2*d)
Rn (Bq/ m2*d)
0.2km
350
1400
300
1200
250
1000
200
800
150
600
La fangaia
100
BN BG
400
BN BG
50
200 0
Rn (Bq/m2*d)
CO2 (gr/m2*d) 5000
90000
Le stufe
Le stufe
80000
4500 4000
70000
3500 60000
3000
50000
2500
40000
Soil Soil gas gas concentrations concentrations Gas
La fangaia
n° samples
Min value
Max value
Mean
CO2 (%,v/v)
32
0.0038
7.3
3.9
CH4 (ppm)
32
0
165.5
85.1
H2S (%, v/v)
32
0
2.62
0.5
Rn (Bq/ m3)
32
0
33767
5504.4
He (ppm)
32
0
9.0
3.5
H2S (%, v/v)
0
1600
Le stufe
0
CO2 (gr/ m2*d) H2S (gr/ Fumaroles Diffusive degassing structure (Cardellini 2003) (Chiodini et al., 2001) Crater rim Fault Fracture
12000
CH4 (mgr/m2*d)
Le stufe
0
Flux Flux data data Gas
Legend
0
H2 S (gr/m2*day1)
4518000 426500
Thoron soil gas (Bq/m3)
426800 427000 427200 427400 427600 427800 428000 428200
Camp i Flegre i (Na)
La fangaia
50 00
Figure 1 – Large scale surveys 4520600 of radon and thoron distribution maps. 4520400 Highest radon values are present along the northern margin of the caldera. 4520200 Anomalous distributions are elongated 4520000 following the local tectonic (NW-SE direction). Thoron distribution (that 4519800 has a shallow origin) is strictly related to high density of houses located 4519600 on the boundary of the caldera.
4519500
4519000
0.4km
42 6800 42 7000 42 7200 427400 4276 00 4278 00 4280 00 428 200
Main Main statistics statistics of of detailed detailed surveys surveys results results
Bocca nuova Bocca grande
0
4519600
N
Le stufe
25 000
2000
30000 20000
BN BG
1500
La fangaia
BN BG
10000
1000 500 0
0
Figure 2 – Soil gas flux measurements in the inter crater sector of Solfatara area. CO 2 flux is mainly 1127.32 gr/m2*d although highest flux values were found in the “La fangaia” and near the “bocca grande” (BG) fumarolas. Radon flux distribution is very similar to the CO2 one: both gases have a dominant flux in the NE-SW direction and in minor part in E-W and NW-SE directions. The H2 S flux measurements highlighted a NW-SE anomalous trend and a local spot with values > 100 gr/ m2*d, in front of the “ Le stufe” area. CH4 flux shows a trend quite different from the other gases: it is possible to distinguish NW-SE, N-S and E-W anomalous trends with local fluxes > 1000 gr/ m2*d.
CH4 (%, v/v)
He (ppm)
CO2 (%, v/v)
Rn (Bq/m3)
140
2.4
30000
9
Le stufe 31
32 Astroni - Lago 19 grande 18
23
27
Tempio Serapide
Cuma 22 Cigliano Solfatara 1
3
4
6
2
30
17 14 21 8 10 11 9 13 Ippodromo di 24 Agnano 28 25 29 26 15 16
Terme di Agnano
20 7 5
Pozzuoli
12
Terme Puteolane
33
Bagnoli
31
Astroni - Lago grande 32 19 18
23 22
27
3
Tempio Serapide
Cuma Cigliano Solfatara 1
4
6
2
30 17 14 21 8 10 11 9 13 Ippodromo di 24 Agnano 2825 29 26 15 16
11000
9000
7000
Terme di Agnano
20 7 5
Pozzuoli
85 80 75 70 65 60 55 50 45 40 35 30 25 20 15
5000
12 3000
Terme Puteolane
33
Bagnoli
Le stufe
23
27
Cuma 22 Cigliano
3
Tempio Serapide
Solfatara 1 2
4
6
30 17 14 21 8 10 11 9 13 Ippodromo di 24 Agnano 28 25 29 26 15 16
Terme di Agnano
20 7 5
Pozzuoli
260 220 180 140 100 60 20 -20
12
-60
Terme Puteolane
Bagnoli
-100 -140
Astroni - Lago 19 grande 32 18
23
27
Tempio Serapide
Cuma 22 Cigliano
3
Solfatara 1 2
4
6
4000
30 17 14 21 8 10 11 9 13 Ippodromo di 24 Agnano 28 25 29 26 15 16
Terme di Agnano
20 7 5
Pozzuoli
31
3600 3200 2800 2400 2000 1600 1200
12
800
Terme Puteolane
33
Bagnoli
400
Astroni - Lago 19 grande 32 18 23
27
Tempio Serapide
Cuma 22 Cigliano
3
Solfatara 1
4
6
2
Pozzuoli
50
40
30
Terme di Agnano
20 7 5
20
12
Terme Puteolane
10
Bagnoli
33 0
20000
Figure 4 – Water temperature distribution map. Hottest areas (high thermalism) are directly connected to magmatic chamber
Figure 5 – Water electrical conductivity (µS/cm) distribution map. Highest electrical conductivity values are found in proximity of the coast suggesting sea water mixing phenomena: •Terme Puteolane :12000 mS/cm •Tempio Serapide: 20000 mS/cm
0
BN BG
Figure 6 – Water redox potential (Eh) distribution map. Negative values highlight three well defined areas characterised by highest H2S values. Positive values could be due to the sea water influence (along the coast), to the presence of shallow waters and/or to the absence of fractures that control CO 2 flux.
3
La fangaia
0.6
0.1km
0
BN BG
0.1km
0
15000
60
4
La fangaia
4
5
1.2
La fangaia
BN BG
2 1
0
0
10000
La fangaia
BN BG
20
0.1km
0
2
La fangaia
40
0.1km
BN BG 0
0
0
0
Na/1000
Cl + SO4
50
Tortorelli
Hotel Tennis seawater
Tufano Terme Puteolane A gnano Sprudel
Tempio di
Serapide
From Caprarelli et al., 1997
Full equilibrium
Carannante
%-Na
Capriccio
25
Figure 7 – Water total CO2 content (ppm) distribution map. Highest values are found in the Agnano spa/race-course, Cuma/Cigliano area and along the coast. In the Solfatara area, steam dilutes CO 2 and H2 S contents except in the “La fangaia” zone.
MF-2
Legend
Solfatara-Agnano Agnano Bagnoli
Fangaia Pisciarelli
MF-1
CF 3
CF 6 CF 16
Pozzuoli
CF 27
SW CF 12
IMMATURE WATERS
Cuma-Cigliano Astroni
Solfatara well
PARTIALLY EQUILIBRATED WATERS
CF 7
K/100
other samples
0
Figure 8 – Water radon (Bq/l) distribution map. Radon is random distributed and there is, apparently, some correlations with the other species: it is possible to distinguish some anomalous spots where CO2 content is low suggesting “stripping” effects.
5000
0.1km
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 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, 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 species: helium, methane and carbon dioxide have the same distribution, with highest values near the “La fangaia” fumarola. Anomalous distributions are elongated in the NESW 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 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 proximity of the two main degassing zone: the “bocca nuova” (BN) and the “bocca grande” (BG) fumarolas.
0
30 17 14 21 8 10 11 9 13 Ippodromo di 24 Agnano 28 25 29 26 15 16
7
80
0
31
100
Ca + Mg
33
Le stufe
6
6
Na + K
Astroni - Lago 19 grande 32 18
Le stufe
120
25000
1.8
1000
31
Le stufe
8
25
50
HCO3
Figure 9 - Ludwing-Langelier diagram. It is possible to distinguish four main chemical families: 1.Solfatara-Agnano family: interaction between shallow waters and acid and reducing gases. 2.Agnano family: interaction between deep CO2 and volcanic rocks. 3.Cuma-Cigliano family: high CO2 content. 4.Pozzuoli area: mixing between sea-waters and deep brines.
Figure 10 - Giggenbach diagram. Most part of samples fall in the “immature waters” area excepting: i) “CF3” sample (Tortorelli well) due to a mixing between a mature water and a pure term (end-member); ii) “CF6” sample (Tennis Hotel) close to the deep end-member (brines). The chemical composition of “CF6” sample is the result of the equilibrium between circulating fluids and rocks in the reservoir.
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), 123139. •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)
