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Accidental gas emission from shallow pressurized aquifers at Alban Hills volcano (Rome, Italy): Geochemical evidence of magmatic degassing?
No full textRecent 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
Geochemistry of the Albano and Nemi crater lakes in the volcanic district of Alban Hills (Rome, Italy)
No full textLake 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
Geochemistry of the Albano crater lake
No full textAlbano 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
Diffuse degassing of carbon dioxide on the NW sector of Colli Albani volcanic complex (Rome, Italy)
No full textsystematic 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
Active degassing structures of Stromboli and variations in diffuse CO 2 output related to the volcanic activity
No full textThe 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
The Albano Maar Lake high resolution bathymetry and dissolved CO2 budget (Colli Albani volcano, Italy): Constrains to hazard evaluation
No full textThe Albano Lake is the deepest volcanic lake in Italy (− 167 m) and fills the youngest maar of the quiescent Colli Albani volcano. The lake has undergone significant level changes and lahar generating overflows occurred about 5800 yrs B.P. and likely in 398 b.C., when Romans excavated a tunnel drain through the maar wall. Hazardous lake rollovers and CO2 release are still possible because the Albano volcano shows active ground deformation, gas emission and periodic seismic swarms. On November 2005, the first high resolution bathymetric survey of the Albano Lake was performed. Here we present the results provided by a Digital Elevation Model and 2-D and 3-D images of the crater lake floor, which is made by coalescent and partly overlapping craters and wide flat surfaces separated by some evident scarps. Submerged shorelines are identified at depths between − 20 m and − 41 m and indicate the occurrence of significant lake level changes, likely between 7.1 and 4.1 ka. The current lake volume is ~ 447.5 × 106 m3 and the total quantity of dissolved CO2 is 6850 t estimated by chemical analyses of samples collected on May 2006. A decrease of nearly one order of magnitude of the CO2 dissolved in the lake water below − 120 m, observed from December 1997 to May 2006 (from 4190 to 465 t respectively), has been attributed to lake water overturn. The observed oscillations of the dissolved CO2 concentrations justify the efforts of monitoring the chemical and physical characteristics of the lake. At present the quantity of dissolved CO2 is very far from saturation and Nyos-type events cannot presently occur
Geochimica delle acque dei Laghi Albano e Nemi
No full textRipetuti episodi esondativi sono avvenuti dal lago craterico di Albano fino probabilmente al 398 a.C. (Funiciello et al., 2002). Sull’esempio di quanto recentemente avvenuto nei laghi africani di Nyos e Monoun (Sigurdsson et al., 1987; Barberi et al., 1989; Rice, 2000) esiste la possibilità che queste esondazioni siano state causate da eruzioni vulcaniche o da improvvise violente immissioni di gas e acque calde sul fondo del lago, oppure ancora dal rovesciamento dell’acqua del lago che avrebbe portato in superficie parti profonde ricche in CO2 con conseguente liberazione di nubi letali di gas. È pertanto importante che la geochimica delle acque dei laghi craterici venga periodicamente controllata, allo scopo in particolare di appurare se vi sono evidenze di immissione di gas profondi o comunque le condizioni perché possa avvenire un accumulo di gas nelle acque profonde
The Albano Maar Lake High resolution bathymetry and dissolved CO2 budget (Colli Albani District, Italy): constrains to hazard evaluation
No full textThe Albano Lake is the deepest volcanic lake in Italy and fills the youngest maar of the quiescent
Colli Albani volcano. The lake has undergone dramatic level changes and overflows about ~5800 yrs B.P.
and likely in 398 b.C., when Romans excavated a tunnel drain through the maar wall. Hazardous lake
rollovers and CO2 release are still possible because the Albano volcano still shows active ground
deformation, gas emissions and periodic seismic swarms. On November 2005, the first very high resolution
bathymetric survey of the Albano Lake was performed. Here we show results provided by a Digital Elevation
Model and 2-D and 3-D images of the crater lake floor which is made by coalescent and partly overlapping
craters. Submerged shorelines have been identified at various depths and indicate the occurrence of lake
level changes. The current lake volume is ~447.5 x 106 m3 and its critical value for outflow is ~895.7 x 106 m3. The total quantity of dissolved CO2 has been estimated to 6846 tons by chemical analyses of samples
collected on May 2006. A decrease of one order of magnitude of the CO2 dissolved in the lake water below -
120 m, observed from December 1997 to May 2006 (from 4187 to 465 tons respectively), has been
attributed to lake water overturn
The sealing capacity of the cap rock above the Torre Alfina geothermal reservoir (Central Italy) revealed by soil CO2 flux investigations
No full textTorre Alfina is a medium enthalpy (T = 140 °C) geothermal field in Central Italy, hosted in buried fractured
Mesozoic limestones, extensively explored in the 1970s and 1980s, but which so far has not been exploited. A
detailed diffuse soil CO2 flux investigation (1336 measurements over a surface of 12.6 km2) and the periodic
monitoring of soil CO2 flux from target areas indicate that in most of the area, even above a pressurized gas
cap existing at the reservoir top, the soil CO2 flux is low and mostly within the background threshold
(48 g m−2 day−1) and is likely generated by biological soil respiration. Anomalous values (up to 30.250 g m−2
day−1) are found only in the proximity of a small zone with gas vents whose composition is identical to that
of the gas produced by the geothermalwells tapping the reservoir. This is the only zonewhere a fault connecting
the deep geothermal reservoir with the surface does exist. These data are compared with those of the near Latera
high enthalpy geothermal field (T = 210 °C), where anomalous soil CO2 flux is recorded above the productive
reservoir. The difference of soil CO2 release in the two fields is attributed to the differences in the impervious
cover (allochthonous flysch deposits) above the carbonate reservoir, which is thin and locally lacking at Latera,
and continuous and thick (N400 m) at Torre Alfina. Results demonstrate that soil CO2 flux investigations are
useful in geothermal exploration, but only high flux values likely indicate the presence of a geothermal reservoir
at depth, whereas low flux values can indicate either the lack of an active geothermal reservoir at depth or the
presence of a very effective impervious cover above the reservoir
Atmospheric dispersion of natural carbon dioxide emissions on Vulcano Island, Italy
No full textLa Fossa quiescent volcano and its surrounding area on the Island of Vulcano (Italy) are characterized by intensive, persistent degassing through both fumaroles and diffuse soil emissions. Periodic degassing crises occur, with marked increase in temperature and steam and gas output (mostly CO2) from crater fumaroles and in CO2 soil diffuse emission from the crater area as well as from the volcano flanks and base. The gas hazard of the most inhabited part of the island, Vulcano Porto, was investigated by simulating the CO2 dispersion in the atmosphere under different wind conditions. The DISGAS (DISpersion of GAS) code, an Eulerian model based on advection-diffusion equations, was used together with the mass-consistent Diagnostic Wind Model. Numerical simulations were validated by measurements of air CO2 concentration inside the village and along the crater's rim by means of a Soil CO2 Automatic Station and a Tunable Diode Laser device. The results show that in the village of Vulcano Porto, the CO2 air concentration is mostly due to local soil degassing, while the contribution from the crater gas emission is negligible at the breathing height for humans and always remains well below the lowest indoor CO2 concentration threshold recommended by the health authorities (1000 ppm). Outdoor excess CO2 maxima up to 200 ppm above local background CO2 air concentration are estimated in the center of the village and up to 100 ppm in other zones. However, in some ground excavations or in basements the health code threshold can be exceeded. In the crater area, because of the combined effect of fumaroles and diffuse soil emissions, CO2 air concentrations can reach 5000–7000 ppm in low-wind conditions and pose a health hazard for visitors
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