The seismic sequence of 30 May - 9 June 2016 in the geothermal site of Torre Alfina (central Italy) and related variations in soil gas emissions

In the framework of a medium-enthalpy geothermal exploitation project, seismicity and soil gas emissions have been monitored in the area of Castel Giorgio-Torre Alfina since 2014. A dedicated local seismic network deepened the knowledge of the natural local seismicity in terms of source mechanisms, high-quality event localization and magnitude estimation. From November 2014 to May 2016, 846 seismic events were recorded, with a magnitude range of Md 0.1-2.8 and hypocentres 4-8 km depth. On 30th May 2016 a Mw 4.3 earthquake occurred near Castel Giorgio, followed by almost 1700 aftershocks; the moment tensor solution depicts a WNW-ESE oriented normal fault. An overview of the epicentral distributions since 2014, highlights that the active tectonic structures are NE-SW and WNW-ESE orientated. The diffuse soil CO2 flux is monitored since 2013 in six target areas located around the future production and reinjection wells, in order to assess the level of background natural degassing. In all target areas the maximum value of soil CO2 flux has been recorded during the 2016 seismic sequence. However, the values of δ13C of the emitted CO2 indicated a shallow biological origin of the gas. At Torre Alfina, the Solfanare natural gas emission, with a CO2 dominated gas, has same composition of the gas hosted in the geothermal reservoir. Here, high values of diffuse soil CO2 flux were recorded. During the 2016 seismic sequence, the Solfanare gas was continuously analysed by an automatic gas- chromatographic station. Results show that apart from small perturbations, no significant compositional variations were recorded. The significant contribution of CLVD and isotropic components suggest a possible opening of fluid cracks below the geothermal reservoir hosted in fractured Mesozoic limestones. The seismo-tectonic scenario indicates that the Solfanare fault was not activated. Kinematics and orientation of the activated faults suggest a relationship with the Bolsena caldera collapsePublishedNapoli, Italy1IT. Reti di monitoraggi

Accidental gas emission from shallow pressurized aquifers at Alban Hills volcano (Rome, Italy): Geochemical evidence of magmatic degassing?

Recent studies suggested that Alban Hills (Rome) is a quiescent and not an extinct volcano, as it produced Holocene eruptions and several lahars until Roman times by water overflow from the Albano crater lake. Alban Hills are presently characterized by high PCO2 in groundwaters and by several cold gas emissions usually in sites where excavations removed the superficial impervious cover. Gas consists mostly of CO2 with minor H2S and the diffuse CO2 soil flux is locally very high. Accidental gas blowouts, occurred during shallow well drillings (tens to hundreds m depth) in zones with no surface gas manifestations, indicate the presence of gas pressurized aquifers confined underneath impermeable layers, within both the volcanic rock pile and the underlying Pleistocene loose sediments. Degassing mostly occurs in correspondence of bordering faults of buried horsts cut in the Mesozoic carbonate basement, hosting the main aquifer. Carbon isotopic composition (δ13CCO2) suggests that CO2 is at least partly originated by thermal decarbonation of these limestones. 3He/4He isotopic ratio of the gas (up to 1.9 Ra) is the same or even slightly higher than that of olivine and clinopyroxene fluid inclusions of the Alban Hills volcanic products, indicating a possible magmatic source for the gas. Low R/Ra values, compared to MORB and island arc magmas, are characteristic of the potassic Roman Comagmatic Province and reflect a deep involvement of crustal material in the magma genesis. The lack of high temperature fumaroles can be explained by an efficient meteoric cold water penetration and circulation in the volcano permeable terrains

Atmospheric dispersion modelling of CO2 emission in the Colli Albani volcanic district (Central Italy)

Carbon dioxide is a gas denser than air, and its point-source ground emission from natural systems or from areas impacted by CO2injection underground may result in hazardous accumulation, especially in topographically-depressed sites. The use of atmospheric dispersion numerical models helps predicting the dispersion of the CO2-enriched gas plume once emitted from underground and allows an accurate map of hazard level through time under particular meteorological conditions. In this study, the accuracy of atmospheric dispersion simulations has been tested using a natural system of CO2emission to atmosphere from underground in an area called Solforata di Pomezia, near the city of Rome in central Italy. This area is located in the Alban Hills, which underwent volcanic activity during the Quaternary, and is characterised by low permeability volcanic and sedimentary formations that allow the accumulation of gas at shallow depths and below surface. This site has been long investigated in terms of soil CO2emission rates, which range from 44 to 95 tonâ\u88\u99day-1. Using the TWODEE2 numerical code, a number of simulations were performed considering a set of combined CO2soil flux emission and meteorological (wind, temperature) from literature. The results fit well in the range of measured CO2concentration in air at distinct heights in the site. The model does not predict lethal gas concentration at heights 1 and 2 m above the ground based on actual soil emission rate (95 tonâ\u88\u99day-1). Two probabilistic models were developed with emission rate five (500 tonâ\u88\u99day-1) and ten (1000 tonâ\u88\u99day-1times bigger than nowadays but still no hazardous levels were predicted

Geochemistry of the Albano crater lake

Albano Lake is within the youngest polygenetic crater of Colli Albani, from which several lahar-generating water overflows occurred up to early Roman times. The area has anomalous gas emissions and is affected by seismicity and uplift. The geochemistry of the lake have been systematically investigated since 2003 by measuring physico-chemical parameters along vertical profiles with a multiparametric probe and by collecting water samples for chemical and isotopic analyses. The lake is thermally and chemically stratified, with an anoxic hypolimnion from 270 m to the bottom (2167 m). The isotopic composition of dissolved helium and total carbon is similar to that of the main gas emissions of Colli Albani and of the phenocryst inclusions of the Alban volcanics, suggesting that an endogenous gas of deep provenance is injected into the lake water. The dissolved CO2 content is, however, far from saturation, and no Nyos-type hazardous gas cloud emission may presently occur in the lake. Temperature and chemical time variations indicate that water rollover episodes occur in harsh rainy winters when the surface lake temperature cools below 8.5 8C. Such rollovers tend to homogenize the physico-chemistry of the lake water and reduce the dissolved CO2 content. They may cause an environmental hazard because of related toxic algal blooms

Geochemistry of the Albano and Nemi crater lakes in the volcanic district of Alban Hills (Rome, Italy)

Lake Albano, located 20 km to the SE of Rome, is hosted within the most recent crater of the quiescent Alban Hills volcanic complex that produced hydromagmatic eruptions in Holocene times. Stratigraphic, archaeological and historical evidence indicates that the lake level underwent important variations in the Bronze Age. Before the IV century B.C. several lahars were generated by water overflows from the lake and in the IV century B.C. Romans excavated a drainage tunnel. The lake is located above a buried carbonate horst that contains a pressurized medium-enthalpy geothermal reservoir from which fluids escape to the surface to produce many important gas manifestations of mostly CO2. Previous studies recognized the presence of gas emissions also from the crater bottom. In 1997 the possibility of a Nyos-type event triggered by a lake rollover was considered very low, because the CO2 water concentration at depth was found to be far from saturation. However, considering the high population density nearby, the Italian Civil Protection Department recommended that periodical monitoring be carried out. To this scope we initiated in 2001 a systematic geochemical study of the lake. Thirteen vertical profiles have been repeatedly carried out in 2001–2006, especially in the deepest part of the lake (167 m in 2006), measuring T, pH, dissolved O2 and electrical conductivity. Water samples were collected from various depths and chemically and isotopically analysed. Two similar profiles have been measured also in the nearby Nemi crater lake. Results indicate that in the 4.5 years of monitoring the pressure of gas dissolved in the Lake Albano deep waters remained much lower than the hydrostatic pressure. A CO2 soil survey carried out on the borders of the two lakes, indicates the presence of some zones of anomalous degassing of likely magmatic origin. A water overturn or a heavy mixing of deep and shallow waters likely occurred in winter 2003–2004, when cold rainfall cooled the surface water below 8.5 °C. Such overturns cause only a limited gas exsolution from the lake when the deep water is brought to a few meters depth but can explain the observed decrease with time of dissolved CO2 at depth and related water pH increase. A gas hazard could occur in the case of a sudden injection through the lake bottom of a huge quantity of CO2-rich fluids, which might be caused by earthquake induced fracturing of the rock pile beneath the lake. A limnic gas eruption might also occur should CO2 concentration build up within the lake for a long time

Active degassing structures of Stromboli and variations in diffuse CO2 output related to the volcanic activity

The main CO2 diffuse degassing structures (DDS) of Stromboli were identified through extensive CO2 soil flux investigations, with 3600 measurements by an accumulation chamber. These DDS extend from the nearby crater area of Pizzo sopra la Fossa (Pizzo) to the coastal area of Pizzillo and are all associated with NE–SW deep fractures, corresponding to the main volcano-tectonic axis of the island, some of which produced flank eruptions in prehistoric times. In each of the four main DDS, a target area was defined covering the zone with the highest CO2 soil flux, where periodic CO2 flux surveys were carried out. The highest CO2 release was observed during the 2007 eruption and high flux values were recorded at both Pizzo and Pizzillo also in moments of high prolonged Strombolian activity (high number of daily explosions observed from the craters and/or high frequency of VLP seismic events). In order to better investigate the rate of diffuse CO2 degassing in relation to volcanic activity, an automatic station hourly measuring CO2 soil flux and environmental parameters (atmospheric T, P and humidity, soil moisture and T, wind speed and direction) was installed in March 2007 at Nel Cannestrà and Rina Grande DDS. Unusual positive correlations were found at Nel Cannestrà between gas flux and SE wind speed and at Rina Grande between gas flux and soil moisture, which are explained by the local conditions, which favour respectively a Venturi effect and the increase in gas flux toward the station during rains. Ten months of continuous recording confirmed the strong influence of environmental conditions on the CO2 soil flux, but statistical data processing made it possible to recognize clear positive anomalies expressing high rates of deep magmatic CO2 degassing. Comparison with seismic data indicates that high CO2 fluxes are apparently correlated with increases in volcanic activity, such as higher explosion frequency and VLP amplitude. Particularly promising is the temporal coincidence of highest recorded flux anomaly with a major explosion that occurred during the observation period. Data confirm that the two continuously monitored DDS are preferentially deep degassing sites, where anomalous increases of CO2 release could represent a geochemical precursor for either high energy explosions from the craters or the opening of flank eruptive fissures that might threaten the village of Stromboli

Active degassing structures of Stromboli and variations in diffuse CO 2 output related to the volcanic activity

The main CO2 diffuse degassing structures (DDS) of Stromboli were identified through extensive CO2 soil flux investigations, with 3600 measurements by an accumulation chamber. These DDS extend from the nearby crater area of Pizzo sopra la Fossa (Pizzo) to the coastal area of Pizzillo and are all associated with NE–SW deep fractures, corresponding to the main volcano-tectonic axis of the island, some of which produced flank eruptions in prehistoric times. In each of the four main DDS, a target area was defined covering the zone with the highest CO2 soil flux, where periodic CO2 flux surveys were carried out. The highest CO2 release was observed during the 2007 eruption and high flux values were recorded at both Pizzo and Pizzillo also in moments of high prolonged Strombolian activity (high number of daily explosions observed from the craters and/or high frequency of VLP seismic events). In order to better investigate the rate of diffuse CO2 degassing in relation to volcanic activity, an automatic station hourly measuring CO2 soil flux and environmental parameters (atmospheric T, P and humidity, soil moisture and T, wind speed and direction) was installed in March 2007 at Nel Cannestrà and Rina Grande DDS. Unusual positive correlations were found at Nel Cannestrà between gas flux and SE wind speed and at Rina Grande between gas flux and soil moisture, which are explained by the local conditions, which favour respectively a Venturi effect and the increase in gas flux toward the station during rains. Ten months of continuous recording confirmed the strong influence of environmental conditions on the CO2 soil flux, but statistical data processing made it possible to recognize clear positive anomalies expressing high rates of deep magmatic CO2 degassing. Comparison with seismic data indicates that high CO2 fluxes are apparently correlated with increases in volcanic activity, such as higher explosion frequency and VLP amplitude. Particularly promising is the temporal coincidence of highest recorded flux anomaly with a major explosion that occurred during the observation period. Data confirm that the two continuously monitored DDS are preferentially deep degassing sites, where anomalous increases of CO2 release could represent a geochemical precursor for either high energy explosions from the craters or the opening of flank eruptive fissures that might threaten the village of Stromboli

Diffuse degassing of carbon dioxide on the NW sector of Colli Albani volcanic complex (Rome, Italy)

systematic CO2soil flux surveys carried out at cava dei Selcion the Colli Albani volcano (28 seasonal surveys since the year 2000), have shown a significant variation of CO2 diffuse release, with a marked decrease, from 25 to 4 tons/day, from May 2000 to August 2004, followed by a new increase. In the same time CO2 flux halved at S. Maria delle Mole (16.8 tons/day in 2000 and 8.3 tons/day in 2006). Also the total quantity of CO2 dissolved in the deep waters of the Albano crater lake decreased by one order in the period 1997-2006. The high CO2 flux values could represent the “tail” of a strong degassing episode recorded at Colli Albani in 1995 and related to local earthquakes. The following decrease of CO2 release could reflect a permeability decrease caused by hydrothermal calcite precipitation favoured by PCO2 reduction in the deep sourc

Carbon dioxide circulation and diffuse degassing within the Roman volcanic province (Colli Albani, Italy). IAVCEI General Assembly, Pucon, Chile Novembre 2004.The 2002-2003 submarine gas eruption at Panarea (Aeolian Islands, Italy): study of the seafloor and implications for volcanic hazard assessment.

The Colli Albani is a quiescent Quaternary volcano located nearby the city of Roma, characterized by moderate seismic activity, ground deformation, and by a remarkable CO2 degassing through the soil that, at some locations, induces significant health hazard. The circulation and ascent of deep seated CO2 in the Colli Albani region has been simulated by applying a two-phase, two-component heat and fluid flow model (TOUGH2), describing the circulation of water and carbon dioxide through an heterogeneous porous medium. The model accounts for phase changes, and for CO2 dissolution in liquid water, with the associated latent heat and dissolution enthalpy effects. All the available geological, geophysical, geochemical and hydrogeological information available for the Colli Albani region have been implemented into the model to provide a reliable description of the underground rock sequence, with the most relevant porous domains, and to define initial and boundary conditions for the simulation. Simulations are performed to evaluate conditions favoring the ascent and exsolution of carbon dioxide from the carbonatic basement up to the shallow volcanic series hosting the water table. Structural highs in the carbonatic basement, regions of enhanced vertical permeability, or periods of increased CO2 supply from depth are all features going to affect shallow CO2 degassing that will be taken into account by the simulations. Early results show that overall fluid circulation and surface CO2 emissions are highly affected by the physical properties of the carbonatic basement, and its thickness

Estimation of the geothermal potential of the Caldara di Manziana site in the Mts Sabatini Volcanic District (Central Italy) by integrating geochemical data and 3D-GIS modelling

The Tyrrhenian margin of central Italy is an area characterized by crustal thinning (300 mW/m2), which makes it attractive for medium to high-enthalpy geothermal projects. The main difficulties encountered in the geothermal exploitation of the area are not related to the thermal conditions, but rather to the lack of an adequate rock permeability at depth. The medium-high enthalpy geothermal reservoirs of central Italy are hosted in Mesozoic carbonate-evaporitic rocks. These rocks exhibit secondary fracture permeability, locally reduced by self-sealing processes, especially in areas of low seismicity. Also, a low partial pressure of CO2 (PCO2) may facilitate the complete sealing of the reservoir fractures, preventing the ascent of hot fluids and resulting in a low CO2 flux at the surface. Conversely, a high CO2 flux reflects a high pressure of CO2 at depth, that is suggestive of the presence of an active geothermal reservoir. Despite the possibility that also part of CO2 be dissolved into groundwater, a large amount of this non-condensable gas will reach the surface being emitted into the atmosphere by discrete manifestations (gas vents) or through diffuse soil emissions. This is particularly true in sites -such as Caldara di Manziana, hereafter (CM) characterized by cold-gas emissions, which have CO2 concentrations up to 97 vol. %. In this paper we present the results of a study carried out in the western zone of Sabatini Volcanic District (SVD; north of Rome, Italy) that hosts CM (and also Solfatara di Manziana-SM), one of the most spectacular CO2 gas manifestations of central Italy. This study estimated the temperature and pressure conditions of the reservoir and the depth of its top using geological, geochemical and geophysical data and the TIN (triangulated irregular networks) interpolator provided in the ArcGIS for Desktop software. A new structural setting of the Mesozoic carbonates in the CM site is proposed, and an estimation of its geothermal potential is presented on the base of the total (diffuse and viscous) CO2 release.PublishedWien, Austria6A. Geochimica per l'ambient