Carbon dioxide degassing from the Albani Hills volcanic region, Central Italy

Abstract

The Albani Hills are affected by strong CO₂ degassing processes. The amount of CO₂, which rises from the depths and subsequently dissolves into shallow groundwater, has been estimated to be more than 4.2×10⁹ mol year⁻¹. Most of the gas is released in localised anomalous spots (Lavinio, Solforata, and Ciampino–Albano–Nemi areas), generated by fluid leakage processes from buried pressurized reservoirs hosted by structural highs of the calcareous–siliceous marly basement. The groundwater circulating in the anomalous areas becomes CO₂ oversaturated, generating observable gas manifestations whose composition is mainly controlled by gas–liquid separation processes. A gas flow rate of 6.1×10⁸ mol year⁻¹ has been measured from the two larger manifestations using the accumulation chamber technique. The presence at shallow level of CO₂ oversaturated groundwater can explain several episodes of sudden release of gas which was documented by historical chronicles from Roman times, until now.

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).