Carbon dioxide degassing from the Albani Hills volcanic region, Central Italy
Introduction
The Albani Hills (Fig. 1), located 20 km southeast of Rome, has been subject to volcanic activity, involving mainly pyroclastic eruptions, from about 0.7 to about 0.01 Ma (Voltaggio and Barbieri, 1995). During this period, pyroclastic flows covered an area of about 1500 km2. The Albani Hills volcanism developed in an extensional tectonic regime and its evolution is directly controlled by primary regional structures connected to the recent geodynamic evolution of the Tyrrhenian basin–Apennine Chain system (De Rita et al., 1995a). The position of the Albani Hills volcano coincides with a structural high of the sedimentary basement, where an NE trending extensional fault intersects an NW trending normal fault. All the volcanic products are characteristic of an alkaline potassic series with an SiO2 content lower than 50% (Fornaseri et al., 1963).
At present, the area is characterised by almost continuous low-level seismic activity Amato et al., 1991, Amato et al., 1994 and by the presence of an intense deep CO2 degassing process, which causes widespread gas emissions (CO2 and H2S), and numerous soda water discharges. On the basis of the chemical and isotopic composition of a relatively small number of groundwater samples, previous geochemical studies recognized the presence of deeply derived gases in the shallow groundwater Duchi et al., 1991, Giggenbach et al., 1988.
On the morning of November 2, 1995, a large area of the Albani Hills was affected by a sudden release of CO2 from the soil and shallow water wells. The phenomenon affected a densely inhabited zone near Rome, alarming the Roman municipal authorities and the Italian Civil Defence. Numerous official documents and newspaper articles (i.e. ‘Il Tempo’ November 3, 1995) refer to the November 2 anomalous degassing. From 2:00 pm of November 2, 1995, some private citizens started to phone the Civil Defence and the Rome Fireguards. They spoke about noisy emissions of pressurized gas from the head of shallow water wells (less than 60-m deep) and about the anomalous presence of gas in the underground cellars. From 3:00 pm of November 2, 1995, field surveys have been carried out by researchers of the National Institute of Geophysics (ING; Quattrocchi and Calcara, 1995). At Ciampino, they observed a water-well discharging such a high amount of CO2 as to make the air unbreathable in the first 2-m of height. The water had a pH of 5.48 and an alkalinity of 15.83 mmol l−1, which roughly corresponds to a pCO2 of 2.5 bar. The same well, on November 3, 1995, did not show any anomalous degassing process and the water was characterised by a pH of 6.0 and an alkalinity of 15.0 mmol l−1 which corresponds to a pCO2 of about 0.7 bar. The cellars of a nearby building were inaccessible from the morning until the evening of November 2 because of high CO2 concentrations, but the cellars were accessible on November 3. Other wells, at Ciampino area, showed similar, but less strong, CO2 degassing phenomena on November 2, 1995, while no evident anomalies were found during the inspections carried out on November 3, 1995. Other CO2 degassing phenomena at Rocca di Papa and near the Albano Lake, were reported on November 2, but were not directly observed by the ING researchers. At 8:25 am of the same day, a low magnitude earthquake (3.2) occurred near Terni (70-km north of the study area), while another low magnitude earthquake occurred near Tivoli at 3:22 pm of November 3, 1995.
From the available data, it is impossible to understand the origin of the degassing event, the relations with the earthquakes, or to quantify the amount of the gas released. The only certain evidence is that an area of 25 km2, between Ciampino and Rocca di Papa (Fig. 1), was affected by the diffuse CO2 degassing phenomenon which lasted for no more than 1 day. However, other stronger but similar events occurred in the Albani Hills area during historic times. In fact, historic chronicles register several episodes of gas and steam emissions occurring from the Roman age until 1989 Voltaggio and Barbieri, 1995, Funiciello et al., 1992. Furthermore, the occurrence of some explosive activity, probably similar to hydrothermal eruptions, can be deduced from Roman documents, which describe the occurrence of ‘rain of stones’ (Andretta and Voltaggio, 1988).
The possibility of sudden gas releases, both from the subsoil and from the volcanic lakes, such as those which occurred catastrophically at Lakes Nyos and Monoun, Cameroon Giggenbach, 1990, Giggenbach et al., 1991, Sigurdsson et al., 1987 and of Dieng Volcanic Complex, Indonesia Le Guern et al., 1982, Allard et al., 1989, suggests the necessity for an assessment of the gas hazard of the region.
The high level of gas hazard at Albani Hills is pointed out by several lethal accidents which involved both humans and animals, the last occurred in October 1999 and in March 2000 when, respectively, 30 cows and five sheep died from the gas expulsed by the Cava dei Selci gas emission.
As a result, the main aim of this work is to investigate the CO2 degassing processes, which are present at Albani Hills, attempting both to estimate the total amount of gas involved in the process and to recognize the geological structures, if any, from which the CO2 anomaly originates.
To estimate the total amount of deep CO2 discharged at Albani Hills, both the CO2 directly discharged from gas manifestations and the deeply derived CO2 dissolved by groundwater, have been considered. The CO2 flux from gas manifestations is determined by direct measurements performed during summer 1996, concurrently with a campaign of gas sampling. Influx of deep CO2 into groundwater is estimated from the carbon balance of the volcanic aquifer based on the data listed in a 1981 AGIP–ENEL survey in which one of the authors participated. The analytical results are available in the GEOCH data bank (Principe et al., 1994).
Section snippets
Gas flux measurements
The chemical and isotopic composition of the six known gas vents in the Albani Hills are reported in Table 1. The main component of each gas emission is CO2, followed by variable contents of H2S, N2, CH4, Ar, He and H2. The high δ13C value of the CO2 (from −3.5 to +0.9 δ13C vs. PDB) excludes a shallow organic derivation and is compatible with an original high temperature source of the gas. Such high temperature CO2 producing systems, both mantle and/or metamorphic decarbonation, are present at
Buried structures degassing CO2
The map in Fig. 8 shows the result of the detailed pCO2 prospecting in the groundwaters of the Albani Hills. The northern and the western sectors of the study area have pCO2 values generally above the mean value, while the southeastern sector is characterised by a large zone with low pCO2 values. The geographic distribution of the higher values identifies the following anomalous zones: (1) a broad area N and NW of Albano lake (Rome, Ciampino and Albano anomaly); (2) a small maximum NE of Nemi
Discussion and conclusion
At Albani Hills, the combination of in situ CO2 flux measurements and regional geochemical studies, allows an estimation of the amount of CO2 discharged and the recognition of the deep CO2 degassing structures which generates the process.
The total carbon discharged by the Albani Hills region is 4.2×109 mol year−1. This value has to be considered a minimum estimate, because part of the gas arrives at the surface without being dissolved by groundwaters, and because the contribution from the gas
References (46)
- et al.
Origin of the carbon dioxide emanation from the 1979 Dieng eruption, Indonesia; implications for the origin of the 1986 Nyos catastrophe
J. Volcanol. Geotherm. Res.
(1989) - et al.
The 1989–1990 seismic swarm in the Alban Hills volcanic area, central Italy
J. Volcanol. Geotherm. Res.
(1994) - et al.
Deep structures and carbon dioxide degassing in Central Italy
Geothermics
(1995) - et al.
Soil CO2 flux measurements in volcanic and geothermal areas
Appl. Geochem.
(1998) - et al.
Geochemical study on natural gas and water discharges in the Southern Latium (Italy): circulation, evolution of fluids and geothermal potential in the region
J. Volcanol. Geotherm. Res.
(1991) - et al.
Chemical relationship between discharging fluids in the Siena–Radicofani graben and the deep fluids produced by the geothermal fields of Mt. Amiata, Torre Alfina and Latera (Central Italy)
Geothermics
(1992) - et al.
The Latera geothermal system (Italy): chemical composition of the geothermal fluid and hypotheses on its origin
Geothermics
(1989) Geothermal solute equilibria. Derivation of Na–K–Mg–Ca geoindicators
Geochim. Cosmochim. Acta
(1988)Water and gas chemistry of Lake Nyos and its bearing on the eruptive process
J. Volcanol. Geotherm. Res.
(1990)- et al.
Isotopic and chemical assessment of geothermal potential of the Colli Albani area, Latium region, Italy
Appl. Geochem.
(1988)
CO2-rich gases from Lakes Nyos and Monoun, Cameroon; Laacher See, Germany; Dieng, Indonesia and Mt. Gambier, Australia
J. Volcanol. Geotherm. Res.
Helium-3 anomalies and crust–mantle interaction in Italy
Geochim. Cosmochim. Acta
Convective hydrothermal CO2 emission from high heat flow regions
Chem. Geol.
Results of analyses on fumarolic gases from F-1 and F-5 fumaroles of Vulcano, Italy
Geothermics
Origin of the lethal gas burst from Lake Monoun, Cameroon
J. Volcanol. Geotherm. Res.
Selection of threshold values in geochemical data using probability graphs
J. Geochem. Explor.
Silica separation from reinjection brines at Monte Amiata geothermal plants, Italy
Geothermics
Data from a (broad-band) digital seismic network in the Albani Hills Volcanic Region (Latium, Italy)
La cronologia recente del vulcanismo dei Colli Albani
Le Scienze
Studio chimico ed isotopico della Solfatara di Pozzuoli
Rend. Soc. Ital. Mineral. Petrol.
Indagine idrogeologica e geochimica sui rapporti tra ciclo carsico e circuito idrotermale della Pianura Pontina (Lazio meridionale)
Geol. Appl. Idrogeol.
Hydrogeology, fluid geochemistry and thermalism
Ricerca ed esplorazione nell'area geotermica di Torre Alfina (Lazio-Umbria)
Boll. Soc. Geol. Ital.
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