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Despite its known reconstructed volcanic history, the structural setting and present state of the Astroni Volcano of the Campi Flegrei Caldera in Italy are still poorly defined. Through structural, geophysical, and geochemical investigations, we elucidate the structure and present volcanic activity of the Astroni Volcano, hosting which hosts tuff cones, scoriae cones, lava domes, and lakes on the crater floor. A volcano-tectonic analysis focused on the entire volcano edifice, coupled with electrical resistivity tomography of the shallower part of the Astroni crater, revealed the main rock formations, faults, and possible fluid patterns within the first 100 m depth. Two main NE–SW and NW–SE strike-fault sets were imaged using electrical-resistivity modeling and measurements along the wall of the volcanic edifice; they likely delimit a maar-like structure resulting from the highest energetic subplinian Astroni 6 eruption event and acted as magma pathways during the late eruptive activity stage. A 3D view of the reconstructed resistivity model revealed both deep root-conduit-like structures and shallower dome-like shapes for volcanic edifices on the crater floor. Gas and carbon compositions in the NNE sectors of the Astroni Lago Grande are similar to those of the Solfatara fumarole fluids, suggesting common hydrothermal origin and a possible link with a deep hydrothermal reservoir. This fluid-emission area along the border of the younger volcanic structure exhibits a +40 °C maximum soil-temperature anomaly. The proposed volcano-tectonic architecture should improve the unrest scenarios in case of reactivation in this Campi Flegrei Caldera sector and the monitoring strategies for the Astroni Volcano.

Comprehensive hydrogeochemical studies have been conducted in the Campi Flegrei volcanic aquifer since late 20th century due to the volcanic unrest. In the last decade, groundwater samples were grouped based on the dominant anion species (i.e. bicarbonate, sulfate and chloride) to explain the general hydrogeochemical processes. In this article, 44 groundwater samples are collected from Campi Flegrei aquifer to geochemically and spatially capture the main characteristics of the groundwater body. The hierarchical clustering algorithm is then performed on proportion of bicarbonate, sulfate and chloride, and the optimum number of clusters are determined regarding the results of deep hydrogeochemical investigations published in the past. The collected samples are categorized in the following groups: (1) bicarbonate-rich groundwater; (2) chlorine-rich groundwater; (3) sulfate-rich groundwater; and (4) mixed groundwater. The first group (As = 158.2 ± 169 μg/l, electric conductivity = 1,732.1 ± 1,086 μS/cm and temperature = 25.6 ± 8 °C) is mainly derived from poor arsenic meteoric water, but there is significant thermal/seawater contribution in the second one (As = 1,457.8 ± 2,210 μg/l, electric conductivity = 20,118.3 ± 11,139 μS/cm and temperature = 37.1 ± 20 °C). Interaction of the bicarbonate-rich groundwater and hydrothermal vapors gives rise to the sulfate-rich groundwater (As = 847.2 ± 679 μg/l, electric conductivity = 3,940.0 ± 540 μS/cm and temperature = 82.8 ± 3 °C) around Solfatara volcano. The mixed groundwater (As = 451.4 ± 388 μg/l, electric conductivity = 4,482.9 ± 4,027 μS/cm and temperature = 37.1 ± 16 °C) is observed where the three main groundwater groups undergo a mixing process, depending on the hydrogeology of the volcanic aquifer. Contrary to the bicarbonate- and sulfate-rich groundwater, the chlorine-rich and mixed groundwater generally occurs at low piezometric levels (approximately

Fluids supplied by stored magma at depth are causal factors of volcanic unrest, as they can cause pressurization/heating of hydrothermal systems. However, evidence for links between hydrothermal pressurization, CO2 emission and volcano seismicity have remained elusive. Here, we use recent (2010−2020) observations at Campi Flegrei caldera (CFc) to show hydrothermal pressure, gas emission and seismicity at CFc share common source areas and well-matching temporal evolutions. We interpret the recent escalation in seismicity and surface gas emissions as caused by pressure-temperature increase at the top of a vertically elongated (0.3–2 km deep) gas front. Using mass (steam) balance considerations, we show hydrothermal pressurization is causing energy transfer from the fluids to the host rocks, ultimately triggering low magnitude earthquakes within a seismogenetic volume containing the hydrothermal system. This mechanism is probably common to other worldwide calderas in similar hydrothermal activity state., Journal of Volcanology and Geothermal Research, 414, ISSN:0377-0273

This short communication aims at providing an updated report on degassing activity and ground deformation variations observed during the ongoing (2012–2019) Campi Flegrei caldera unrest, with a particular focus on Pisciarelli, currently its most active fumarolic field. We show that the CO2 flux from the main Pisciarelli fumarolic vent (referred as “Soffione”) has increased by a factor > 3 since 2012, reaching in 2018–2019 levels (>600 tons/day) that are comparable to those typical of a medium-sized erupting arc volcano. A substantial widening of the degassing vents and bubbling pools, and a further increase in CO2 concentrations in ambient air (up to 6000 ppm), have also been detected since mid-2018. We interpret this escalating CO2 degassing activity using a multidisciplinary dataset that includes thermodynamically estimated pressures for the source hydrothermal system, seismic and ground deformation data. From this analysis, we show that degassing, deformation and seismicity have all reached in 2018–2019 levels never observed since the onset of the unrest in 2005, with an overall uplift of ~57 cm and ~448 seismic events in the last year. The calculated pressure of the Campi Flegrei hydrothermal system has reached ~44 bar and is rapidly increasing. Our results raise concern on the possible evolution of the Campi Flegrei unrest and reinforce the need for careful monitoring of the degassing activity at Pisciarelli, hopefully with the deployment of additional permanent gas monitoring units.

Hydrothermal systems with active surface expressions are important natural source of atmospheric mercury. Here we report on the first simultaneous assessment of gaseous elemental mercury (GEM) and major volatiles (H2S and CO2) fluxes from the fumarolic system of Pisciarelli, currently the most active at the Campi Flegrei caldera (CFc), Naples (Italy). This was achieved via a GPS-synchronized Lumex and MultiGAS survey which extends similar investigations reported elsewhere. GEM concentrations measured in the fumarolic emissions were consistently above background air level close to the degassing area (mean ~ 8 ± 3 ng m−3 on average) and ranged up to 12,000 ng m−3. Our data evidenced pulsed sequences of GEM increases in the fumarole plume, closely matched by temporally consistent increases in CO2 and H2S (r2 = 0.9), supporting the idea that major volatiles, such as CO2 acts as potential carrier in transporting GEM within the magmatic/hydrothermal systems. The slope of the best fit calculated for the dispersion of our data provides a GEM/CO2 molar ratio of 1.1 × 10−8 and a GEM/H2S of 5 × 10−6, respectively. These ratios are comparable to those reported for both low (~ 100 °C) and high-T (~ 250 °C) fumaroles from non-explosive volcanic/hydrothermal degassing systems elsewhere. We adopted an ad-hoc method that combines video footages and gas measurements to obtain high precision concentration maps of gas emissions used to estimate the total atmospheric GEM, CO2 and H2S flux of about 0.0113, 225059 and 511 t y−1, respectively. The human health risk assessment related to the GEM emissions at Pisciarelli, confirms that, except for the degassing fumarolic area, all the main sites affected by the plume dispersion, always remain well below the safe threshold recommended by the health authorities.

The Campi Flegrei caldera is located in the metropolitan area of Naples (Italy) and has been undergoing different stages of unrest since 1950, evidenced by episodes of significant ground uplift followed by minor subsidence, increasing and fluctuating emission strengths of water vapor and CO2 from fumaroles, and periodic seismic crises. We deployed a scanning laser remote-sensing spectrometer (LARSS) that measured path-integrated CO2 concentrations in the Pisciarelli area in May 2017. The resulting mean CO2 flux is 578 ± 246 t d−1. Our data suggest a significant increase in CO2 flux at this site since 2015. Together with recent geophysical observations, this suggests a greater contribution of the magmatic source to the degassing and/or an increase in permeability at shallow levels. Thanks to the integrated path soundings, LARSS may help to give representative measurements from large regions containing different CO2 sources, including fumaroles, low-temperature vents, and degassing soils, helping to constrain the contribution of deep gases and their migration mechanisms towards the surface.

Detailed surveys of diffuse CO2 flux, soil temperature, thermal gradients, and sampling of high-T fumaroles were carried out in the Favare area and Lake Specchio di Venere on Pantelleria Island. Spatial patterns of diffuse CO2 emissions in the Favare area reflect structural discontinuities (faults, fractures or cracks in the soil) associated with the volcano-tectonic structures of the young Monastero Caldera (NNE–SSW to NE–SW trending). The estimated diffuse CO2 output from two adjacent sites in the Favare area (~ 93,000 m2) is 7.8 t d− 1 (equivalent to 2.62 kt a− 1), whereas that from the west shore of the lake (450 m2) is 0.041 t d− 1 (or 0.015 kt a− 1). The extrapolation of diffuse CO2 fluxes across the entire altered area of Favare suggests that CO2 emissions are ~ 19.3 t d− 1. The diffuse CO2 flux correlates with shallow soil temperatures, indicating a similar source for both the heat and volatiles from the underlying geothermal reservoir. Gas equilibria applied to fumarolic effluents define P–T conditions for this reservoir at 2–6 bar and 120–160 °C, in good agreement with measurements from exploratory wells in these areas (e.g., 135 °C at a depth of 290 m). Using the CO2 flux as a tracer for steam output, and consequently for heat flow, the calculated thermal energy for the shallow reservoir is 10–12 MW; this represents the minimum geothermal potential of the reservoir on Pantelleria island.

Our study is aimed to develop a 3D physical model of the Campi Flegrei geothermal system, in order to achieve a more accurate and comprehensive representation of the hydrothermal processes occurring in the caldera. The new model, developed by using the TOUGH2 code simulator, accounts for the caldera rocks' physical properties, bathymetry and water table topography. In particular, the computational domain is constrained by density values obtained by tomographic inversion of gravity data collected during several surveys at CF both onshore and offshore the caldera. Empirical relations between density and porosity and between porosity and permeability, derived by published data on samples cored in deep wells or collected in outcrops, allowed us to characterize the main rocks physical parameters controlling the dynamic of the CF geothermal system. We have performed and compared several simulations investigating the effects of the injection at depth, underneath Solfatara crater, of a hot gaseous mixture rich in CO 2 . We show that, with respect to the available literature on 2D axisymmetric models, the effects of the water table topography together with the bathymetry and the heterogeneous distribution of the rocks' physical properties, lead to important differences in the hydrothermal circulation of fluids at CF. These constraints allow the activation of convective cells with different behaviors, which produce variable patterns of temperature inside the hydrothermal system. As a consequence, the predominant effect is again represented by a central plume below the Solfatara crater, but with a non-axisymmetric structure and a wider extension. Additionally, high temperature zones are present near the coastline and in the middle part of the submerged area of the caldera with a SE–NW alignment. Moreover, our results indicate that, the submerged part of the CF caldera would deserve a more accurate study and survey, being affected by phenomenon of heating and degassing. This information could be very useful in terms of hazard assessment and volcanic risk mitigation in such an active and densely inhabited volcanic and geothermal area.

SUMMARY The Solfatara is one of the major volcanoes of the Phlegrean Fields (Campi Flegrei) volcanic complex, and it is located in a densely populated area a few kilometres west of the city of Naples. It is an active resurgent caldera that has been characterized by a rich history of surface–ground deformation and soil diffuse degassing and fumarolic emissions, which are indications of the top of a hydrothermal plume. A seismic survey was completed in May 2009 for the characterization of the main subsurface features of the Solfatara. Using the complete data set, we have carried out surface wave inversion with high spatial resolution. A classical minimization of a least-squares objective function was first computed to retrieve the dispersion curves of the surface waves. Then, the fitting procedure between the data and a three-sediment-layer forward model was carried out (to a depth of 7 m), using an improved version of the neighbourhood algorithm. The inversion results indicate a NE-SW fault, which is not visible at the surface. This was confirmed by a temperature survey conducted in 2010. A passive seismic experiment localized the ambient noise sources that correlate well with the areas of high CO2 flux and high soil temperatures. Finally, considering that the intrinsic attenuation is proportional to the frequency, a centroid analysis provides an overview of the attenuation of the seismic waves, which is closely linked to the petrophysical properties of the rock. These different approaches that merge complete active and passive seismic data with soil temperature and CO2 flux maps confirm the presence of the hydrothermal system plume. Some properties of the top of the plume are indicated and localized.

Carbon dioxide flux from the soil is regularly monitored in selected areas of Vesuvio and Solfatara (Campi Flegrei, Pozzuoli) with the twofold aim of i) monitoring spatial and temporal variations of the degassing process and ii) investigating if the surface phenomena could provide information about the processes occurring at depth. At present, the surveyed areas include 15 fixed points around the rim of Vesuvio and 71 fixed points in the floor of Solfatara crater. Soil CO2 flux has been measured since 1998, at least once a month, in both areas. In addition, two automatic permanent stations, located at Vesuvio and Solfatara, measure the CO2 flux and some environmental parameters that can potentially influence the CO2 diffuse degassing. Series acquired by continuous stations are characterized by an annual periodicity that is related to the typical periodicities of some meteorological parameters. Conversely, series of CO2 flux data arising from periodic measurements over the arrays of Vesuvio and Solfatara are less dependent on external factors such as meteorological parameters, local soil properties (porosity, hydraulic conductivity) and topographic effects (high or low ground). Therefore we argue that the long-term trend of this signal contains the “best” possible representation of the endogenous signal related to the upflow of deep hydrothermal fluids.

Lake Averno is situated in the homonymous crater in the northwestern sector of the Campi Flegrei active volcanic system in Campania region, Italy. In February 2005 a fish kill event was observed in the lake, prompting a geochemical survey to ascertain the possible cause. In February 2005 a geochemical survey revealed that the lake water was unstratified chemically and isotopically, presumably, as a result of lake overturn. This fish kill phenomenon was recorded at least two other times in the past. In contrast to the February 2005 results, data collected in October 2005, shows the Lake Averno to be stratified, with an oxic epilimnion (surface to 6 m) and an anoxic hypolimnion (6 m to lake bottom at about 33 m). Chemical and isotopic compositions of Lake Averno waters suggest an origin by mixing of shallow waters with a Na–Cl hydrothermal component coupled with an active evaporation process. The isotopic composition of Dissolved Inorganic Carbon, as well as the composition of the non-reactive dissolved gas species again supports the occurrence of this mixing process. Decreasing levels of SO 4 and increasing levels of H 2 S and CH 4 contents in lake water with depth, strongly suggests anaerobic bacterial processes are occurring through decomposition of organic matter under anoxic conditions in the sediment and in the water column. Sulfate reduction and methanogenesis processes coexist and play a pivotal role in the anaerobic environment of the Lake Averno. The sulfate reducing bacterial activity has been estimated in the range of 14–22 μmol m − 2 day − 1 . Total gas pressure of dissolved gases ranges between 800 and 1400 mbar, well below the hydrostatic pressure throughout the water column, excluding the possibility, at least at the survey time, of a limnic eruption. Vertical changes in the density of lake waters indicate that overturn may be triggered by cooling of epilimnetic waters below 7 °C. This is a possible phenomenon in winter periods if atmospheric temperatures remain frosty for enough time, as occurred in February 2005. The bulk of these results strongly support the hypothesis that fish kill was caused by a series of events that began with the cooling of the epilimnetic waters with breaking of the thermal stratification, followed by lake overturn and the rise of toxic levels of H 2 S from the reduced waters near the lake bottom.

This paper documents arsenic concentrations in 157 groundwater samples from the island of Ischia and the Phlegrean Fields, two of the most active volcano-hosted hydrothermal systems from the Campanian Volcanic Province (Southern Italy), in an attempt to identify the environmental conditions and mineral-solution reactions governing arsenic aqueous cycling. On Ischia and in the Phlegrean Fields, groundwaters range in composition from NaCl brines, which we interpret as the surface discharge of deep reservoir fluids, to shallow-depth circulating fluids, the latter ranging from acid-sulphate steam-heated to hypothermal, cold, bicarbonate groundwaters. Arsenic concentrations range from 1.6 to 6900 μg·l- 1 and from 2.6 to 3800 μg·l- 1 in the Phlegrean Fields and on Ischia, respectively. They increase with increasing water temperature and chlorine contents, and in the sequence bicarbonate groundwaters < steam-heated groundwaters < NaCl brines. According to thermochemical modeling, we propose that high As concentrations in NaCl brines form after prolonged water-rock interactions at reservoir T, fO2 and fH2S conditions, and under the buffering action of an arsenopyrite + pyrite + pyrrhotite rock assemblage. On their ascent toward the surface, NaCl brines become diluted by As-depleted meteoric-derived bicarbonate groundwaters, giving rise to hybrid water types with intermediate to low As contents. Steam-heated groundwaters give their intermediate to high As concentrations to extensive rock leaching promoted by interaction with As-bearing hydrothermal steam. © 2005 Elsevier B.V. All rights reserved.

The first measurements of volcanic/hydrothermal water vapor and heat flux using eddy covariance (EC) were made at Solfatara crater, Italy, June 8–25, 2001. Deployment at six different locations within the crater allowed areas of focused gas venting to be variably included in the measured flux. Turbulent (EC) fluxes of water vapor varied between 680 and 11 200 g H2O m− 2 d− 1. Heat fluxes varied diurnally with the solar input, and the volcanic component of sensible heat ranged from ∼25 to 238 W m− 2. The highest measurements of both sensible and latent heat flux were made downwind of hot soil regions and degassing pools and during mid-day. The ratio of average volcanic heat (both latent and sensible) to CO2 flux resulted in an equivalent H2O/CO2 flux ratio of 2.2 by weight, which reflects the deep source H2O/CO2 gas ratio. The amount latent heat flux/evaporation was determined to be consistent both with what would be expected from the magnitude of CO2 fluxes and the fumarolic H2O/CO2 ratio, as well as with observed surface temperatures and wind speeds given a moist soil. This suggests that the water vapor that condenses in the shallow subsurface is remobilized at the soil–atmosphere interface through variable evaporation dependent on the deep heat flux and surface temperature. The results suggest that EC provides a quick and easy method to monitor average H2O/CO2 ratios continuously in volcanic regions, providing another important tool for volcanic hazards monitoring.

A geochemical model is proposed for water evolution at Somma–Vesuvio, based on the chemical and isotopic composition of groundwaters, submarine gas emission and chemical composition of the dissolved gases. The active degassing processes, present in the highest part of the volcano edifice, strongly influence the groundwater evolution. The geological–volcanological setting of the volcano forces the waters infiltrating at Somma–Vesuvio caldera, enriched in volcanic gases, to flow towards the southern sector to an area of high pCO2 groundwaters. Reaction path modelling applied to this conceptual model, involving gas–water–rock interaction, highlights an intense degassing process in the aquifer controlling the chemical and isotopic composition of dissolved gases, total dissolved inorganic C (TDIC) and submarine gas emission. Mapping of TDIC shows a unique area of high values situated SSE of Vesuvio volcano with an average TDIC value of 0.039 mol/L, i.e., one order of magnitude higher than groundwaters from other sectors of the volcano. On the basis of TDIC values, the amount of CO2 transported by Vesuvio groundwaters was estimated at about 150 t/d. This estimate does not take into account the fraction of gas loss by degassing, however, it represents a relevant part of the CO2 emitted in this quiescent period by the Vesuvio volcanic system, being of the same order of magnitude as the CO2 diffusely degassed from the crater area.

Cuticle micromorphology together with epidermal and epistomatal wax, in both current- and first-year-old needles of conifer Pinus halepensis (Aleppo pine) trees growing under volcanic gas fumigation was analysed in Pisciarelli area, Campi Flegrei, Southern Italy. As a control, current- and first-year-old needles growing far from volcanic gas emission were also sampled. Using a multidisciplinary approach with SEM, TEM and X-ray, volcanic gases were shown to cause degradation on epicuticular and epistomatal waxes. Significant statistical variations of ultrastructural components of the cuticle, with 30 measurements, including total thickness of the cuticle, and details and proportions of all different layers, and use of confidence interval, revealed a high degree of sensitivity of Aleppo pine to this extreme environment. In the present study, non-significant thickness variations of the cell wall plus cuticle among current- and first-year-old needles of both fumigated and non fumigated trees have been found. However, at the ultrastructural level, significant variations in cell wall and total cuticle thickness, especially within the three zones of B1 fibrillar layer, revealed different equilibria for each of the four types of material. Using energy dispersive X-ray microanalysis, no sulphur was found in either cuticle or epidermal cells, but the presence of H 2 S in the fumarole gas is suspected to cause indirect and/or direct cuticle alterations of wax structure. Ultrastructural characters of plant cuticles related to emission of volcanic gases during the geological past are also discussed. Among these considerations, an identification key enabling distinction between non fumigated and fumigated materials with 9 characters, provides a good tool detecting the influence of volcanism for extant and fossil plants.

Fumarolic fluid compositions and diffuse soil emissions of hydrothermal fluids of the Donna Rachele area (0.86 km 2 , western flank of Mt. Epomeo, Ischia Island) have been studied in order to develop a conceptual geochemical model of the hydrothermal system. The degassing area was mapped and the total release of hydrothermal gas and heat associated with the diffuse emission of hydrothermal fluids was estimated. A mesostructural study was carried out in order to investigate the relations between the brittle structures and the main pathways of the uprising vapor. The fumarolic compositions are typical of hydrothermal fluids and water (>99%) represents the major component. All gas species in the H 2 O–H 2 –CO–CH 4 –CO 2 –H 2 S system are close to equilibrium concentrations at temperatures of ∼300°C and at redox conditions slightly more oxidizing than expected. The compositions of the Donna Rachele fumarolic gas approach the pure liquid equilibrium composition. This indicates a high fraction of separated vapor and suggests the presence of a highly energetic hydrothermal system at depth. The pure liquid equilibrium compositions of the Donna Rachele fumaroles, along with the historical records of shallow seismicity, the ‘explosion’ of a well in 1995, the occurrence of intense acoustic phenomena and of shallow wells discharging vapor indicate that the internal pressure of the hydrothermal system is occasionally larger than the hydrostatic pressure. To quantify the energy dissipated in the Donna Rachele area by the emission of fumarolic fluids, the hydrothermal diffuse degassing was studied by means of 336 soil CO 2 flux measurements. The highest CO 2 fluxes were measured in hydrothermally altered areas along the faults that border Mt. Epomeo. Structural data indicate that the vapor rises up along NW–SE striking normal faults related to gravity-induced stresses and affecting highly fractured lavas. The older faults, which are related to the Mt. Epomeo resurgence, act as a permeability barrier and bound the Donna Rachele diffuse degassing structure. The total hydrothermal CO 2 output was estimated to be ∼9 t d −1 . Assuming that the H 2 O/CO 2 ratio of the fluids that feed the diffuse degassing is the same as that of fumarolic effluents, the calculated heat flux is ∼40 MW. This value, which represents an important energy release, is only a part of the total thermal energy release of Ischia, where other fumarolic areas occur. The results obtained at Ischia indicate the importance of thermal energy released by diffuse degassing structures in the energy balance of quiescent volcanoes. Values of the thermal energy release from the Ischia hydrothermal system are comparable with those estimated on other quiescent volcanoes.

An eddy covariance (EC) station was deployed at Solfatara crater, Italy, June 8–25, 2001 to assess if EC could reliably monitor CO2 fluxes continuously at this site. Deployment at six different locations within the crater allowed areas of focused gas venting to be variably included in the measured flux. Turbulent (EC) fluxes calculated in 30-min averages varied between 950 and 4460 g CO2 m−2 d−1; the highest measurements were made downwind of degassing pools. Comparing turbulent fluxes with chamber measurements of surface fluxes using footprint models in diffuse degassing regions yielded an average difference of 0% (±4%), indicating that EC measurements are representative of surface fluxes at this volcanic site. Similar comparisons made downwind of degassing pools yielded emission rates from 12 to 27 t CO2 d−1 for these features. Reliable EC measurements (i.e. measurements with sufficient and stationary turbulence) were obtained primarily during daytime hours (08:00 and 20:00 local time) when the wind speed exceeded 2 m s−1. Daily average EC fluxes varied by ±50% and variations were likely correlated to changes in atmospheric pressure. Variations in CO2 emissions due to volcanic processes at depth would have to be on the same order of magnitude as the measured diurnal variability in order to be useful in predicting volcanic hazard. First-order models of magma emplacement suggest that emissions could exceed this rate for reasonable assumptions of magma movement. EC therefore provides a useful method of monitoring volcanic hazard at Solfatara. Further, EC can monitor significantly larger areas than can be monitored by previous methods.

Measurements of soil fluxes of hydrothermal gases, with special emphasis on C 6 H 6 , as well as chemical composition of mono-aromatic compounds in fumaroles and air, were carried out in April 2012 at the Solfatara crater (Campi Flegrei, Southern Italy) to investigate the distribution and behavior of these species as they migrate through the soil from their deep source to the atmosphere. Soil fluxes of CO 2 , CH 4 and C 6 H 6 exhibit good spatial correlation, suggesting that diffuse degassing is mainly controlled by local fractures. The calculated total output of diffuse C 6 H 6 from Solfatara is 0.10 kg day −1 , whereas fluxes of CO 2 and CH 4 are 79 × 10 3 and 1.04 kg day −1 , respectively. A comparison between soil gas fluxes and fumarole composition reveals that within the crater soil CH 4 is significantly affected by oxidation processes, which are more efficient for low gas fluxes, being dependent on the residence time of the uprising hydrothermal gases at shallow depth. Benzene degradation, mainly proceeding through oxidation via benzoate, seems to be strongly controlled by the presence of a shallow SO 4 2 - -rich aquifer located in the central and southwestern sectors of the crater, suggesting that the process is particularly efficient when SO 4 2 - acts as terminal electron acceptor (SO 4 reduction). Relatively high C 6 H 6 /C 7 H 8 ratios, typical of hydrothermal fluids, were measured in air close to the main fumarolic field of Solfatara crater. Here, C 6 H 6 concentrations, whose detection limit is ∼0.1 μg m −3 , are more than one order of magnitude higher than the limit value for ambient air (5 μg m −3 ). This suggests that hydrothermal fluids have a strong impact on air quality in the immediate surroundings of the fumarolic vents. Significant concentrations of endogenous mono-aromatics were also detected in air samples collected from the northern and western sides of the crater, where these gas compounds are mostly fed by diffuse degassing through the crater bottom soil.

We describe analytical details and uncertainty evaluation of a simple technique for the measurement of the carbon isotopic composition of CO2 in volcanic plumes. Data collected at Solfatara and Vulcano, where plumes are fed by fumaroles which are accessible for direct sampling, were first used to validate the technique. For both volcanoes, the plume-derived carbon isotopic compositions are in good agreement with the fumarolic compositions, thus providing confidence on the method, and allowing its application at volcanoes where the volcanic component is inaccessible to direct sampling. As a notable example, we applied the same method to Mount Etna where we derived a δ13C of volcanic CO2 between −0.9 ± 0.27‰ and −1.41 ± 0.27‰ (Bocca Nuova and Voragine craters). The comparison of our measurements to data reported in previous work highlights a temporal trend of systematic increase of δ13C values of Etna CO2 from ~ −4‰, in the 1970’s and the 1980’s, to ~ −1‰ at the present time (2009). This shift toward more positive δ13C values matches a concurrent change in magma composition and an increase in the eruption frequency and energy. We discuss such variations in terms of two possible processes: magma carbonate assimilation and carbon isotopic fractionation due to magma degassing along the Etna plumbing system. Finally, our results highlight potential of systematic measurements of the carbon isotopic composition of the CO2 emitted by volcanic plumes for a better understanding of volcanic processes and for improved surveillance of volcanic activity.

[1] Long-duration time series of the chemical composition of fumaroles and of soil CO2 flux reveal that important variations in the activity of the Solfatara fumarolic field, the most important hydrothermal site of Campi Flegrei, occurred in the 2000–2008 period. A continuous increase of the CO2 concentrations and a general decrease of the CH4 concentrations are interpreted to be the consequence of the increment of the relative amount of magmatic fluids, rich in CO2 and poor in CH4, hosted by the hydrothermal system. Contemporaneously, the H2O-CO2-He-N2 gas system shows remarkable compositional variations in the samples collected after July 2000 with respect to the previous ones, indicating the progressive arrival at the surface of a magmatic component different from that involved in the 1983–1984 episode of volcanic unrest (1983–1984 bradyseism). The change starts in 2000, concurrently with the occurrence of relatively deep, long-period seismic events which were the indicator of the opening of an easy ascent pathway for the transfer of magmatic fluids toward the shallower, brittle domain hosting the hydrothermal system. Since 2000, this magmatic gas source is active and causes ground deformations and seismicity as well as the expansion of the area affected by soil degassing of deeply derived CO2. Even though the activity will most probably be limited to the expulsion of large amounts of gases and thermal energy, as observed in other volcanoes and in the past activity of Campi Flegrei, the behavior of the system in the future is, at the moment, unpredictable.

Several nonvolcanic sources in central Italy emit a large amount of carbon dioxide (CO2). Under stable atmospheric conditions and/or in the presence of topographic depressions, the concentration of CO2, which has a molecular mass greater than that of air, can reach high values that are lethal to humans or animals. Several episodes of this phenomenon were recorded in central Italy and elsewhere. In order to validate a model for the dispersion of a heavy gas and to assess the consequent hazard, we applied and tested the code TWODEE-2, an improved version of the established TWODEE model, which is based on a shallow-layer approach that uses depth-averaged variables to describe the flow behavior of dense gas over complex topography. We present results for a vented CO2 release at Caldara di Manziana in central Italy. We find that the model gives reliable results when the input quantity can be properly defined. Moreover, we show that the model can be a useful tool for gas hazard assessment by evaluating where and when lethal concentrations for humans and animals are reached.

A new method combining measurements of soil CO 2 flux and determinations of the carbon isotopic composition of soil CO 2 efflux was developed in order to qualitatively and quantitatively characterise the CO 2 source feeding the soil CO 2 diffuse degassing. The method was tested in March 2007 at the Solfatara of Pozzuoli volcano degassing area (Naples, Italy) where more than 300 measurements of soil CO 2 flux and determinations of the carbon isotopic composition of soil CO 2 efflux were performed, surveying Solfatara crater and its surroundings. The wide range of CO 2 flux and CO 2 isotopic composition values (from 8.4 g m − 2 d − 1 to 28,834 g m − 2 d − 1 , and from 0.73‰ to − 33.54‰, respectively), together with their statistical distributions suggests the occurrence of multiple CO 2 sources feeding soil degassing. The combined interpretation of flux and isotopic data allows us to identify and characterise two distinct gas sources: a hydrothermal and a biogenic source. The soil CO 2 from the hydrothermal source is characterised by a mean δ 13 C CO 2 of − 2.3‰ ± 0.9‰, hence close to the isotopic composition of the fumarolic CO 2 ( δ 13 C CO 2 = − 1.48‰ ± 0.22‰) and by a mean CO 2 flux of 2875 g m − 2 d − 1 . The CO 2 from the biogenic source is characterised by a mean δ 13 C CO 2 of − 19.4‰ ± 2.1‰, and by a mean CO 2 flux of 26 g m − 2 d − 1 , which are both in the range of the typical values for biologic CO 2 soil degassing. This reliable characterisation of the biogenic CO 2 flux would not have been possible by solely applying a statistical analysis of the CO 2 flux values, which is commonly applied in volcanological studies for the partitioning between background fluxes and anomalous CO 2 fluxes. A map of the Solfatara diffuse degassing structure was derived from the estimated threshold for the biogenic CO 2 flux, highlighting that soil degassing of hydrothermal CO 2 mixed in different proportion with biogenic CO 2 occurs over a large area (~ 0.8 km 2 ), which extends over the inner part of the Solfatara crater as well as the eastern periphery, corresponding with a NW–SE fault system. The presented method and data analysis are important means of surveillance of the volcanic activity.

[1] We present a reliable methodology to estimate the energy associated with the subaerial diffuse degassing of volcanic-hydrothermal fluids. The fumaroles of 15 diffuse degassing structures (DDSs) located in eight volcanic systems in the world were sampled and analyzed. Furthermore, each area was measured for soil temperature gradients and for soil CO2 fluxes. The results show that each hydrothermal or volcanic system is characterized by a typical source fluid which feeds both the fumaroles and diffuse degassing through the soil. Experimental data and the results of physical numerical modeling of the process demonstrate that the heat released by condensation of steam at depth is almost totally transferred by conduction in the uppermost part of the soil. A linear relationship is observed between the log of the steam/gas ratio measured in the fumaroles and the log of the ratio between soil thermal gradient and soil-gas flux. The main parameter controlling this relation is the thermal conductivity of the soil (Kc). For each area, we computed the values of Kc which range from 0.4 to 2.3 W m−1 °C−1. Using the CO2 soil fluxes as a tracer of the deep fluids, we estimated that the total heat released by steam condensation in the systems considered varies from 1 to 100 MW.

Carbon dioxide soil flux was continuously measured during 4 years (1998-2002) inside the crater of Solfatara by using the 'time 0, depth 0' accumulation chamber method. The CO2 soil flux (φCO2) is strongly influenced by external factors, such as the barometric pressure, the air and soil temperature and humidity, the wind speed, the amount of rain, and so on. Here, we apply a two-step filtering technique to remove the contribution of these external factors from the raw data and to highlight variations in gas flow from depth. In the first step we apply multiple regression and a best-subset search procedure to determine the minimal number of parameters to insert in the regression model. In the second step we apply time filtering on the residuals of the previous analysis through an ARIMA (integrated autoregressive moving average) model which allows us to quantify long-term trends and short-term periodicities. The statistical analysis showed that (1) the highest frequency fluctuations are due to variations of environmental parameters (particularly soil humidity and air temperature) and (2) the long-term trend of the filtered data is correlated with the ground deformation. This correlation is enhanced by back-shifting the CO2 flux signal by 3 months. These observations, along with the likelihood that the ground deformation at Phlegraean Fields is controlled by fluid pressure within the hydrothermal system, indicate that the long-term trend in soil CO2 flux is related to fluid pressure changes at depth. The delay between the soil CO2 flux and the ground deformation is most probably due to the inertia of the gas moving in the subsoil. © 2003 Elsevier Science B.V. All rights reserved.

Monitoring of the Campi Flegrei caldera has been undertaken since 1983 by measuring radon content (by track etch method), pH and temperature in three thermal water wells. Collection of data started after the area was affected by a dramatic phase of volcano unrest, with a high rate of uplift and increased seismicity. Results show that the radon content is different from well to well; low in the cold well and at least one order of magnitude greater in the two hot wells. Variations in radon content in the three wells sometimes occurred simultaneously and indicate an influence from the same phenomena and feeding source. Three main events, (i) rain fall, (ii) seismic activity and (iii) volcano unrest have been identified to correlate with variations in the radon content, with pH and temperature changes at the same time.