AG_57.04.14_NISI_finalonline_Layout 6 ANNALS OF GEOPHYSICS, 57, 4, 2014, S0438; doi:10.4401/ag-6584 S0438 Origin of the gases released from the Acqua Passante and Ermeta wells (Mt. Amiata, central Italy) and possible environmental implications for their closure Barbara Nisi1,*, Orlando Vaselli2,3, Franco Tassi2,3, Javier de Elio4, Marcelo Ortega4, Juan Caballero4, Daniele Rappuoli5, Luis Felipe Mazadiego4 1 CNR-IGG - Istituto di Geoscienze e Georisorse, Pisa, Italy 2 Università di Firenze, Dipartimento di Scienze della Terra, Firenze, Italy 3 CNR-IGG - Istituto di Geoscienze e Georisorse, Firenze, Italy 4 Universidad Politécnica de Madrid, Escuela Técnica Superior de Ingenieros de Minas y Energía, Madrid, Spain 5 Unione dei Comuni Amiata-Val d’Orcia, Castiglion d’Orcia (Siena), Italy ABSTRACT The Mt. Amiata volcano (Tuscany, central Italy) hosts the second largest geothermal field of Italy. Its SW and NE sectors are characterized by the presence of several CO2-rich (>95% by vol.) gas discharges. An intense Hg mining activity had taken place from the 19th century up to the end of the ’70s, particularly close to Abbadia San Salvatore, during which two drillings (Acqua Passante and Ermeta) intercepted a CO2-rich gas fertile horizon. The related gases are emitted in the atmosphere since 1938 and 1959, respectively, causing severe concerns for the local air quality. In this work the results of a geochemical and isotopic survey carried out on these gas emissions from March 2009 to January 2014 are presented. CO2 fluxes from both the two wells and soil from an area of about 653,500 m2 lo- cated between them were measured. The two wells are emitting up to 15,000, 92 and 8 tons y-1 of CO2, CH4 and H2S, respectively, while the computed soil CO2 output was estimated at 4,311 ton y -1. The spatial dis- tribution of the CO2 soil flux suggests the presence of preferential pat- terns, indicating sites of higher permeability. Since the local municipality is evaluating the possibility to plug the Ermeta vent, a temporarily closure should first be carried out to test the possible influence of this operation on the diffuse soil degassing of deep-originated CO2 in the surrounding area. This implies that diffuse soil gases should carefully be monitored be- fore proceeding with its definitive closure. 1. Introduction The occurrence of CO2-rich gas discharges is typ- ical of active, quiescent and recent volcanic activity [e.g. Allard et al. 1991, Burton et al. 2013 and references therein]. Natural subsurface fluid reservoirs accumu- lating CO2 produced by magma degassing and/or thermo-metamorphic reactions of carbonates are in- deed a relatively common geological feature, particu- larly in areas affected by subduction processes or lithospheric thinning where volcanic [e.g. Gerlach 1991] and/or geothermal [e.g. Brombach et al. 2001, Chio- dini et al. 2004a, 2005] systems occur. Carbon dioxide released from these reservoirs is discharged into the at- mosphere through soil diffuse degassing and/or punc- tual gas manifestations [e.g. Chiodini et al. 1999, 2010, Rogie et al. 2001, Minissale 2004]. In the peri-Tyrrhenian Italy, a region characterized by geothermal systems (e.g. Larderello, Mt. Amiata) and Quaternary (Roman Co-magmatic Province) and present (e.g. Vesuvius, Phlegrean Fields, Aeolian Is- lands) volcanic activity related to crustal thinning gen- erated by the opening of the Tyrrhenian Sea [e.g. Zitellini et al. 1986, Bartole 1995, Mauffret et al. 1999, Peccerillo 2003, 2005, Peccerillo and Lustrino 2005], a large amount of deep-originated CO2 is discharged from gas vents and diffuse soil emissions, and is pres- ent as dissolved inorganic carbon in ground waters [e.g. Minissale et al. 1997a, Chiodini et al. 1998, 2004b, Gam- bardella et al. 2004, Minissale 2004]. It has been esti- mated that >25.000 ton CO2 day -1 are naturally discharged by the Tuscan-Roman and Campanian De- gassing Systems (TRDS and CDS, respectively) [Chio- dini et al. 2004a]. In these areas, CO2-rich (>90 % by volume) gases discharged from gas vents located in sub- circular-shaped craters and associated to fissures, frac- Article history Received May 5, 2014; accepted August 15, 2014. Subject classification: Central Italy, Amiata volcano, Hydrothermal gases, CO2, Environmental impact. tures and structural lows (e.g. sinkholes), have caused fatal accidents, which have involved animals and human beings [e.g. Carapezza et al. 2003, Vaselli et al. 2006a, Tassi et al. 2009]. Locally, CO2 accumulation in air may be favored since this gas is relatively heavy (gas density of CO2 and air being of 0.001977 and 0.001280 g mL -1, respectively). As a consequence, when the climatic con- ditions are characterized by low wind, cloudy weather and cold air temperature, the morphologically de- pressed areas hosting these gas vents may become deadly traps. Such CO2-rich gas discharges are com- monly accompanied by the presence of other toxic, dangerous and inflammable gases such as H2S and/or CH4, whose contents can be up to a few percentages [e.g. Minissale et al. 1997a, Minissale 2004]. The Mt. Amiata volcanic edifice is located in south- ern Tuscany (Figure 1). It represents the biggest and youngest volcano (~200,000 years old) of this region and hosts the second largest geothermal field of Italy after Larderello. Its south-westernmost (e.g. Selvena, Banditella and Zancona; Minissale et al. [1997b], Fron- dini et al. [2009]) and east-northernmost (e.g. Bagni San Filippo and Pietrineri; Minissale et al. [1997b], Frondini et al. [2009], Tassi et al. [2009]) flanks are characterized by the presence of several CO2-rich (>95% by volume) gas discharges, mostly recognized during mercury (cinnabar) exploitation in the eastern part of this vol- canic complex. In 1938 and 1959 two wells, named Acqua Passante (1,048 a.s.l.) and Ermeta (1,079 m a.s.l.), were drilled close to the town of Abbadia San Salvatore (Figure 2) in the volcanic cover at the depth of 116 and 298 m, respectively. It is worth to point out that, despite the large number of mining galleries, none of them was intercepted during the drilling activity. Currently, gases from these wells are discharged at the height of about 6 m above the ground, as two chimneys were built to minimize the disagreeable effects for the people ap- proaching these areas due to the presence of CO2 and H2S, the latter having the typical smell of rotten eggs. Once the Hg mining activity was terminated, the owner of the mining concession (E.N.I.: National Agency for Hydrocarbons, AGIP Division) produced a series of documentation for the reclamation of the Hg extraction and processing areas. Finally, in 2008, an agreement between E.N.I. and the municipality of Ab- badia San Salvatore was signed and, consequently, the ownership of the reclamation passed to the public agency [e.g. Vaselli et al. 2013]. In the agreement the Ermeta and Acqua Passante wells were also mentioned, since the local municipality was in charge to: i) period- ically monitor the discharged gases; ii) efficiently main- tain the chimneys and iii) propose alternative strategies to reduce the impact of the discharged gases released to the atmosphere. The Acqua Passante well is located close to a picnic area along the main road that leads to the top of Mt. NISI ET AL. 2 Figure 1. Geological sketch map of Mt. Amiata and location of the study area. Legend: (1) town; (2) fault; (3) third volcanic complex: the Ermeta olivine-latite lava flow: 209±0.29 ka; (4) second volcanic complex: porphyritic rhyodacite dome and lava flow, from 300± 0.04 ka to 190± 0.23 ka; (5) first volcanic complex, upper unit: trachydacite: from 318± 0.08 ka to 217± 0.01 ka; (6) first volcanic complex, lower unit: trachydacite: from 305± 0.005 ka to 241± 0.07 ka; (7) clay and sand: Early Pliocene; (8) Ligurian Units: Jurassic-Middle Eocene; (9) Tuscan Nappe: late Triassic-Oligocene (modified after Brogi et al. [2010]). 3 Amiata, where skiing and trekking facilities are present. That of Ermeta is close the homonymous church (built in 1296) that is an important tourist and religious site. In this paper we present the results of a geochem- ical and isotopic survey (from March 2009 to January 2014) carried out on the gases discharged from these two chimneys. The CO2 flux from both the two vents and those from the soil carried out in July 2011 in an area of 653,550 m2 located SE of the Ermeta chimney (Figure 2), was measured by using the hot-wire anemometer and accumulation chamber method [Baubron et al. 1990, 1991], respectively. The chemical composition of the gas discharges located close to the study area [Tassi et al. 2009] was reported for comparison. The main goals of this study were those to: i) in- vestigate the genetic relationships between the well gases and those naturally discharging nearby and ii) ver- ify the possible implications related to the closure of the Ermeta and Acqua Passante wells by the local mu- nicipality to reduce the impact of the discharged gases on the local population and tourists. 2. Geological setting and mining activity The Mt. Amiata area (southern Tuscany, central Italy; Figure 1) is characterized by a volcanic activity that lasted ca. 100,000 years, the related deposits, dom- inated by a trachydacitic composition, being emplaced between 300 and 190 ka [Ferrari et al. 1996, Cadoux and Pinti 2009]. The “residual” volcanic activity of this edi- fice related to the emplacement, at 6-7 km below the sea level, of a magmatic body in the Pliocene, produced two geothermal reservoirs, which are feeding a num- ber of CO2-rich gas manifestations and Ca-SO4-rich thermo-mineral waters, mainly located in NE and SW sector of the volcanic edifice [Minissale et al. 1997b, Frondini et al. 2009, Tassi et al. 2009]. Presently, two ge- othermal plants are operating at Bagnore and Pian- castagnaio (Figure 1). The geodynamic setting of the Mt. Amiata geot- hermal area is dominated by extensional structures de- veloped from the Middle Miocene to the Quaternary, whilst collisional structures occurred from the Creta- ceous to the Early Miocene [e.g. Brogi et al. 2010 and references therein]. The anomalous heat flux related to the Pliocene magmatic body, likely caused the em- placement of Hg ore deposits [Tanelli 1983, Klemm and Neumann 1984]. In the recent past, Mt. Amiata was one of the biggest Hg producers in the world [e.g. Ri- mondi et al. 2011, Vaselli et al. 2013 and references therein]. The Mt. Amiata Hg district is mainly distrib- uted in the central-eastern and south-western sectors of this volcanic complex, although the principal min- ing activity was located in the municipality of Abbadia San Salvatore (Figure 1). The extraction and metallurgy of cinnabar (HgS) was aimed to produce metallic mer- cury from historic times until the middle seventies, when most of the Hg mines in the world were shut down due to environmental concerns. In the second half of the last century several wells, including those of Ermeta and Acqua Passante, were drilled for mining ex- ploration close to Abbadia San Salvatore (Figure 2). It is worthwhile to mention that some of the mining gal- THE CO2-RICH GASES FROM THE ACQUA PASSANTE AND ERMETA WELLS (MT. AMIATA, ITALY) Figure 2. Location of the i) Ermeta and Acqua Passante wells and ii) soil diffuse ϕCO2 measurement sites. leries were affected by dangerous CO2 emissions, occa- sionally causing fatalities, and CO2 is still abundantly present in some of them [Vaselli et al. 2006a, 2011]. Both the Ermeta and Acqua Passante wells are located in the volcanic cover and that of Ermeta is situated few hun- dreds meters from the Ermeta olivine-latite lava flow that marked the latest event of the volcanic history of Mt. Amiata [e.g. Ferrari et al. 1996]. 3. Material and methods 3.1. Sampling and analysis of gases The Ermeta well is located to the east of Abbadia San Salvatore whilst that of Acqua Passante is about 1 km NE on a fly of bird from Ermeta (Figure 2). From the two wells, 18 gas samples were collected from March 2009 to January 2014. The well gases were collected using a 40 cm long Tygon® tube inserted into an artificial opening (25 mm in diameter) located at the base of the two ~6 m high chimneys (whose diameters are 300 mm) and con- nected to a pre-evacuated 60 mL glass thorion-tapped flask filled with 20 mL of a 4N NaOH and 0.15 M Cd(OH)2 suspension [Vaselli et al. 2006b and references therein]. The analysis of the inorganic gases stored in the headspace of the sampling flasks (N2, O2, H2, He, CH4, Ar and Ne) was carried out with a Shimadzu 15A gas-chromatograph equipped with a 10 m long 5A mo- lecular sieve column and a Thermal Conductivity De- tector (TCD). The alkaline solution, separated from the solid precipitate by centrifugation, was used for the de- termination of CO2, as CO3 2-, by titration (Metrohm Basic Titrino) with a 0.5N HCl solution. The solid CdS was oxidized to SO4 2- with H2O2 and analyzed for the determination of H2S by ion-chromatography [Mon- tegrossi et al. 2001]. Analytical error was <5%. The 13C/12C isotopic ratio of CO2 (expressed as d 13C-CO2 ‰ V-PDB) was measured after the purification and ex- traction of CO2 by using the two cryogenic traps method [Vaselli et al. 2006b]. The analytical error and the reproducibility were ±0.05‰ and ±0.1‰, respec- tively. The determination of the 3He/4He ratios (ex- pressed as R/Ra, where R is the 3He/4He measured ratio and Ra is the 3He/4He ratio in the air: 1.39*10-6; Mamyrin and Tolstikhin [1984]) was carried out at the University of Rochester Rare Gas Facility using a VG 5400 Rare Gas Mass Spectrometer by using the method described in Poreda and Farley [1992]. The measured 3He/4He ratios were corrected for the addition of air on the basis of the 4He/20Ne ratios measured by mass spectrometry, assuming that 20Ne had an atmospheric origin [Craig and Lupton 1976, Sano and Wakita 1988]. Analytical error for R/Ra determination was ≤0.3 %. 3.2. Measurements of the CO2 flux from the chimneys and diffuse soil The CO2 flux of the two chimneys was deter- mined by inserting into the chimneys (whose section had an area of 0.07 m2) a Delta Ohm model HD2103.2 hot-wire anemometer probe. CO2 flux records were set to acquire the data from August 5 to September 5, 2011, during which the CO2 flux was measured every 5 min- utes and was calculated on the basis of the discharged gas velocity and temperature inside the pipelines. The CO2 flux values were normalized at 20°C. The tem- perature of the Ermeta chimney was ranging between 26.8 and 29°C, while that of Acqua Passante was be- tween 16.9 and 22.5°C. The survey of diffuse soil CO2 flux ( July 2011) cov- ered an area of 653,550 m2 with 301 evenly distributed measuring sites and was carried out in the SE sector of the Ermeta chimney (Figure 2). All the measurements were carried out where soils developed on the Mt. Ami- ata volcanic products, avoiding those areas where the volcanic rocks were cropping out. To measure the CO2 flux from the soil, a direct and dynamic method that follows the principle of the so- called accumulation chamber [Baubron et al. 1990, 1991] was used. This method, initially used to determine soil respiration [i.e. Kanemasu et al. 1974, Parkinson 1981], has been widely applied for measuring diffuse soil CO2 flux in volcanic and geothermal [e.g. Tonani and Miele 1991, Chiodini et al. 1996, 1998, 2000, 2003, 2004a, Pan- cioli et al. 2009, Tassi et al. 2009] and solid waste disposal landfills [e.g. Raco et al. 2006, 2008, Moretti et al. 2007]. The instrument consisted of: i) an inverted chamber equipped with a system to mix the air in the chamber headspace (volume = 3.06 × 10-3 m3 and area height = 100 mm), ii) an Infra-Red (IR) spectrophotometer with LICOR Li-820 sensor (measuring range of 0-20,000 ppm, accuracy of 4% of reading), iii) an analogue-digital (AD) converter and iv) a palmtop computer (PC). In order to prevent damages to both the pump and the CO2 detec- tor, a magnesium perchlorate desiccant trap was placed between the outlet fitting of the accumulation chamber and the inlet of the PTFE Filter to adsorb soil humidity. Once the accumulation chamber was positioned on the ground a low-flux pump (20 mL s−1) continu- ously convoyed the soil gas from the chamber to the IR sensor. To minimize the disturbance effects due to changes of barometric conditions, the soil gas was re- injected into the chamber. The dCCO2/dt values were computed in the field with the PC connected with the IR through the AD. The soil {CO2 was calculated on the basis of the measured CO2 concentration increment inside the chamber over time (dCCO2/dt), according to the following equation: NISI ET AL. 4 5 THE CO2-RICH GASES FROM THE ACQUA PASSANTE AND ERMETA WELLS (MT. AMIATA, ITALY) Table 1. Chemical (in mmol/mol) and isotopic (carbon: as d13C-CO2 vs. V-PDB ‰; and helium as R/Ra) composition of the Ermeta and Acqua Passante gases and of the bubbling pools and dry vents (PN, SP HO, PO, PG and MA). (*): data from Tassi et al. [2009]. Name Date CO2 H2S N2 CH4 Ar O2 H2 He d 13C-CO2 R/Ra Ermeta Mar-09 986 0.30 2.8 10 0.014 0.16 0.074 0.00064 n.d. 0.33 Ermeta Jul-09 982 0.77 14 2.8 0.048 0.057 0.078 0.00097 -3.39 Ermeta Oct-09 978 0.62 15 5.7 0.025 0.004 0.68 0.0013 -3.39 Ermeta Dec-09 986 0.56 4.2 8.9 0.038 0.23 0.12 0.0012 -3.33 Ermeta Apr-09 986 0.36 5.1 7.7 0.056 0.16 0.21 0.0014 -3.35 Ermeta Jul-10 987 0.33 5.5 6.9 0.053 0.19 0.12 0.00089 -3.48 0.32 Ermeta Oct-10 987 0.40 6.2 5.8 0.045 0.16 0.24 0.00055 -3.13 Ermeta Nov-10 987 0.33 6.3 5.6 0.049 0.16 0.21 0.00069 -3.27 Ermeta Apr-11 989 0.58 4.6 5.5 0.032 0.14 0.54 0.00091 -3.27 Ermeta Jun-11 987 0.42 5.6 6.7 0.037 0.15 0.55 0.00075 -3.41 Ermeta Oct-11 983 0.77 6.1 9.5 0.045 0.14 0.44 0.00074 -3.45 Ermeta Dec-11 987 0.34 5.6 6.6 0.056 0.19 0.25 0.00066 -3.09 Ermeta Mar-12 988 0.55 5.3 5.1 0.052 0.21 0.34 0.00039 -3.33 Ermeta Apr-12 989 0.45 5.2 5.0 0.046 0.21 0.28 0.00031 n.d. Ermeta Jul-12 988 0.52 5.6 5.0 0.056 0.15 0.30 0.00028 -3.36 Ermeta Jan-13 989 0.72 5.1 4.9 0.052 0.19 0.26 0.00018 -3.27 Ermeta Jul-13 989 0.81 5.0 4.6 0.061 0.22 0.27 0.00011 -3.32 Ermeta Jan-14 989 0.66 5.5 4.8 0.036 0.16 0.22 0.00016 Acqua Passante Mar-09 987 0.34 6.4 5.9 0.029 0.11 0.080 0.00069 -3.31 0.29 Acqua Passante Jul-09 977 0.33 12 11 0.043 0.73 0.069 0.0022 -2.92 Acqua Passante Oct-09 988 0.30 7.5 4.0 0.020 0.01 0.12 0.00063 -3.38 Acqua Passante Dec-09 988 0.39 6.1 5.0 0.032 0.16 0.085 0.00087 -3.33 Acqua Passante Apr-09 988 0.22 6.7 4.5 0.063 0.22 0.076 0.00061 -3.31 Acqua Passante Jul-10 988 0.22 6.4 5.2 0.060 0.20 0.087 0.00055 -3.47 0.40 Acqua Passante Oct-10 989 0.32 5.9 4.4 0.053 0.13 0.18 0.00087 -3.26 Acqua Passante Nov-10 988 0.36 6.1 4.9 0.056 0.11 0.22 0.00065 -3.13 Acqua Passante Apr-11 989 0.59 5.2 5.0 0.036 0.14 0.39 0.00086 -3.13 Acqua Passante Jun-11 987 0.37 5.7 6.0 0.041 0.16 0.34 0.00061 -3.11 Acqua Passante Oct-11 985 0.66 6.4 7.8 0.030 0.12 0.33 0.00066 -3.39 Acqua Passante Dec-11 986 0.30 6.3 6.6 0.061 0.27 0.14 0.00044 -3.25 Acqua Passante Mar-12 989 0.40 5.5 5.0 0.061 0.24 0.26 0.00035 -2.84 Acqua Passante Apr-12 989 0.41 5.4 5.1 0.053 0.19 0.23 0.00025 n.d. Acqua Passante Jul-12 989 0.45 5.3 4.9 0.055 0.18 0.25 0.00024 -3.33 Acqua Passante Jan-13 989 0.56 5.1 4.6 0.056 0.22 0.25 0.00015 -3.21 Acqua Passante Jul-13 989 0.63 5.1 4.8 0.050 0.19 0.31 0.00013 -3.35 Acqua Passante Jan-14 989 0.58 5.3 4.9 0.054 0.18 0.26 0.00019 Name ID CO2 H2S N2 CH4 Ar O2 H2 He d 13C-CO2 R/Ra Polla nera* PN 980 0.54 11 8.7 0.025 0.0047 0.0020 0.0031 Spuntone* SP 972 0.55 16 11 0.032 0.0027 0.00003 0.0045 -2.87 Hole* HO 982 0.61 11 6.0 0.021 0.0020 0.0014 0.0033 -2.61 Poggio all'Olivo* PO 983 0.95 8.6 7.2 0.018 0.0038 0.00043 0.0026 -3.24 Polla Grigia* PG 976 0.79 13 11 0.029 0.0028 0.0034 0.0039 Mammellone* MA 978 0.81 14 7.5 0.022 0.0021 0.00057 0.00086 -2.63 0.13 {CO2 = cf × (dCCO2/dt) (1) where the proportionality factor (cf ) between dCCO2/dt and {CO2 was determined by laboratory tests through an artificial soil, during which an interval of controlled {CO2 values (from 10 to 10,000 g m -2 day-1) was meas- ured (at least 6 measurements were repeated for each {CO2 value). The cf factor was then computed as the slope of the linear best-fit line of {CO2 vs. dCCO2/dt. 4. Results 4.1. Chemical and isotopic (d13C-CO2 and R/Ra ) com- positions of gases from wells, bubbling pools and dry vents The chemical and isotopic (d13C-CO2 and R/Ra values) compositions of the Ermeta and Acqua Passante gases and, for comparison, those of six bubbling pools and dry gas vents (namely PN, SP HO, PO, PG and MA; see Tassi et al. [2009] for their location), located at about 5 km NE of Abbadia San Salvatore, are reported in Table 1. The Ermeta and Acqua Passante wells discharged gases having a similar composition, being both domi- nated by CO2 (up to 989 mmol mol -1), with significant concentrations of N2 (up to 14 and 12 mmol mol -1, re- spectively), CH4 (up to 10 and 11 mmol mol -1, respec- tively) and H2S (up to 0.81 and 0.66 mmol mol -1, respectively). Hydrogen concentrations ranged from 0.069 (Acqua Passante, July 2009) to 0.55 mmol mol-1 (Er- meta, June 2011), while those of noble gases, i.e. Ar, He and Ne, were up to 0.061, 0.00022, and 0.00003, mmol mol-1, respectively. The chemical composition of the bub- bling pools was basically identical to that of the two wells, with the only exception of H2, whose concentration in the well gases was up to 3 orders of magnitude lower than in the wells. Helium (R/Ra) and carbon (d13C-CO2) isotopic compositions in the wells were in a narrow range, from 0.32 to 0.40 and from −3.48 to −2.84‰ vs. V-PDB, respectively. The helium isotopic ratio for the nat- ural gas discharges was only available for MA, whose value was moderately lower (R/Ra=0.13) that those of the Ermeta and Acqua Passante gases, while those of car- bon in CO2 were, on average, slightly more positive (be- tween −3.24 and −2.61 ‰ vs. V-PDB). 4.2. CO2 flux from the chimneys and diffuse soil The database of the CO2 flux measurements from the Ermeta and Acqua Passante chimneys consisted of more than 9,000 measurements (not reported) and ranged from 8.86 to 11.06 (mean value: 10.05±0.45) mol sec-1 and from 0.89 to 2.51 (mean value: 1.32±0.17) mol sec-1, respectively. The calculated CO2 output from Ermeta was ≈13,000 ton y-1, whereas that of Acqua Pas- sante clustered around 1,800 ton y-1. The spatial distribution of the soil diffuse CO2 flux measurements was partly dictated by the rough and in- tensely vegetated territory. The measured {CO2 values (301 data), ranging from 0.07 to 3.42 mol m-2 day-1 with an average value of 0.38 mol m-2 day-1 (Table 2), showed a normal logarithmic behavior with a 0.05 significance level (Figure 3), indicating that the ln({CO2) values had a modal density distribution. Mean, median, minimum, maximum {CO2 values and standard deviation are re- ported as ln-values in Table 3. 5. Discussion 5.1. Origin of gases The chemical composition of the Ermeta and Acqua Passante gases is basically consistent with that shown by the gas discharges located NE of Mt. Amiata, which are likely fed by a hydrothermal system consist- ing of two aquifers at distinct depth (500-1,000 m and >3,000 m, respectively) and temperature (up to 230 and 350°C, respectively) [Calamai et al. 1970, Gianelli et al. 1988]. The d13C-CO2 values (Table 1) are slightly heavier than the typical range of mantle CO2 [Rollinson 1993, Hoefs 1997, Ohmoto and Goldhaber 1997], suggesting that CO2 has a twofold origin, i.e. mantle degassing and thermometamorphic reactions acting on limestone [Mi- nissale et al. 1997a, Frondini et al. 2009]. This feature is common to the great majority of the gas emissions from the peri-Tyrrhenian central Italy (Tuscan-Roman Degassing System, TRDS; Chiodini et al. [2004a]). In this region, a significant mantle contamination by crustal fluids released from the subducted Adriatic plate occurs [Martelli et al. 2004, Frondini et al. 2009], as tes- tified by the relatively low R/Ra values (<0.4; Table 1). Thermogenic processes involving organic material buried in sedimentary formation are the most likely source for the high N2/Ar ratios and CH4 and H2S con- centrations measured in gases from both the two wells and the nearby gas discharges. The relatively high H2 concentrations characterizing the Ermeta and Acqua Passante gases, significantly higher than those of the Mt. Amiata gas discharges (Figure 4), are likely resulting by the corrosion of the internal iron tubing of the wells, which is favored by the presence of CO2- and H2S-rich gases. It is likely that this alteration process allows air to enter the chimneys. This would also explain the Ar en- richment shown by the wells gases with respect to the naturally discharges ones (Figure 5). 5.2. Spatial variation and output of soil diffuse {CO2 In order to better constrain the total {CO2 and vi- sualize its spatial distribution, the CO2 iso-flux map is reported in Figure 6, which was drawn by means of the NISI ET AL. 6 7 THE CO2-RICH GASES FROM THE ACQUA PASSANTE AND ERMETA WELLS (MT. AMIATA, ITALY) D at e U T M -W E ST (W G S8 4) Z on e 32 U T M -N O R T H (W G S8 4) Z on e 32 ϕ C O 2 D at e U T M -W E ST (W G S8 4) Z on e 32 U T M -N O R T H (W G S8 4) Z on e 32 ϕ C O 2 D at e U T M -W E ST (W G S8 4) Z on e 32 U T M -N O R T H (W G S8 4) Z on e 32 ϕ C O 2 D at e U T M -W E ST (W G S8 4) Z on e 32 U T M -N O R T H (W G S8 4) Z on e 32 ϕ C O 2 Ju l-1 1 71 59 23 47 51 61 0 0. 38 Ju l-1 1 71 70 23 47 51 41 0 0. 74 Ju l-1 1 71 65 23 47 51 21 0 0. 64 Ju l-1 1 71 70 23 47 50 86 0 0. 15 Ju l-1 1 71 59 73 47 51 61 0 0. 21 Ju l-1 1 71 70 73 47 51 41 0 0. 34 Ju l-1 1 71 65 73 47 51 21 0 0. 45 Ju l-1 1 71 62 23 47 51 01 0 0. 49 Ju l-1 1 71 60 23 47 51 61 0 0. 22 Ju l-1 1 71 71 23 47 51 41 0 0. 76 Ju l-1 1 71 66 23 47 51 21 0 0. 27 Ju l-1 1 71 62 73 47 51 01 0 0. 41 Ju l-1 1 71 60 73 47 51 61 0 0. 59 Ju l-1 1 71 59 73 47 51 36 0 0. 50 Ju l-1 1 71 66 73 47 51 21 0 0. 35 Ju l-1 1 71 63 23 47 51 01 0 0. 46 Ju l-1 1 71 61 23 47 51 61 0 0. 23 Ju l-1 1 71 60 23 47 51 36 0 0. 77 Ju l-1 1 71 67 23 47 51 21 0 0. 40 Ju l-1 1 71 63 73 47 51 01 0 0. 33 Ju l-1 1 71 61 73 47 51 61 0 0. 28 Ju l-1 1 71 60 73 47 51 36 0 0. 27 Ju l-1 1 71 67 73 47 51 21 0 0. 17 Ju l-1 1 71 64 23 47 51 01 0 0. 47 Ju l-1 1 71 62 23 47 51 61 0 0. 39 Ju l-1 1 71 61 23 47 51 36 0 0. 38 Ju l-1 1 71 68 23 47 51 21 0 0. 29 Ju l-1 1 71 64 73 47 51 01 0 0. 28 Ju l-1 1 71 59 23 47 51 56 0 0. 26 Ju l-1 1 71 61 73 47 51 36 0 0. 24 Ju l-1 1 71 68 73 47 51 21 0 0. 71 Ju l-1 1 71 65 23 47 51 01 0 0. 57 Ju l-1 1 71 59 73 47 51 56 0 0. 32 Ju l-1 1 71 62 23 47 51 36 0 0. 18 Ju l-1 1 71 69 23 47 51 21 0 0. 91 Ju l-1 1 71 65 73 47 51 01 0 0. 65 Ju l-1 1 71 60 23 47 51 56 0 0. 25 Ju l-1 1 71 62 73 47 51 36 0 0. 54 Ju l-1 1 71 69 73 47 51 21 0 0. 40 Ju l-1 1 71 66 23 47 51 01 0 0. 77 Ju l-1 1 71 60 73 47 51 56 0 0. 46 Ju l-1 1 71 63 23 47 51 36 0 0. 87 Ju l-1 1 71 70 23 47 51 21 0 0. 45 Ju l-1 1 71 66 73 47 51 01 0 0. 14 Ju l-1 1 71 61 23 47 51 56 0 0. 55 Ju l-1 1 71 63 73 47 51 36 0 0. 52 Ju l-1 1 71 70 73 47 51 21 0 0. 75 Ju l-1 1 71 67 23 47 51 01 0 0. 71 Ju l-1 1 71 61 73 47 51 56 0 0. 42 Ju l-1 1 71 64 23 47 51 36 0 0. 26 Ju l-1 1 71 71 23 47 51 21 0 1. 65 Ju l-1 1 71 67 73 47 51 01 0 0. 35 Ju l-1 1 71 62 23 47 51 56 0 0. 84 Ju l-1 1 71 64 73 47 51 36 0 0. 44 Ju l-1 1 71 61 73 47 51 16 0 0. 18 Ju l-1 1 71 70 73 47 50 91 0 0. 45 Ju l-1 1 71 59 73 47 51 51 0 0. 44 Ju l-1 1 71 65 23 47 51 36 0 0. 27 Ju l-1 1 71 62 23 47 51 16 0 0. 12 Ju l-1 1 71 71 23 47 50 91 0 0. 35 Ju l-1 1 71 60 23 47 51 51 0 0. 54 Ju l-1 1 71 65 73 47 51 36 0 0. 35 Ju l-1 1 71 62 73 47 51 16 0 0. 32 Ju l-1 1 71 61 73 47 50 86 0 0. 60 Ju l-1 1 71 60 73 47 51 51 0 0. 24 Ju l-1 1 71 66 23 47 51 36 0 0. 26 Ju l-1 1 71 63 23 47 51 16 0 0. 22 Ju l-1 1 71 62 23 47 50 86 0 0. 49 Ju l-1 1 71 61 23 47 51 51 0 0. 46 Ju l-1 1 71 66 73 47 51 36 0 0. 15 Ju l-1 1 71 63 73 47 51 16 0 0. 23 Ju l-1 1 71 62 73 47 50 86 0 0. 86 Ju l-1 1 71 61 73 47 51 51 0 0. 38 Ju l-1 1 71 67 23 47 51 36 0 0. 25 Ju l-1 1 71 64 23 47 51 16 0 0. 53 Ju l-1 1 71 63 23 47 50 86 0 0. 21 Ju l-1 1 71 62 23 47 51 51 0 0. 64 Ju l-1 1 71 67 73 47 51 36 0 0. 20 Ju l-1 1 71 64 73 47 51 16 0 0. 29 Ju l-1 1 71 63 73 47 50 86 0 0. 57 Ju l-1 1 71 62 73 47 51 51 0 0. 23 Ju l-1 1 71 68 23 47 51 36 0 0. 41 Ju l-1 1 71 65 23 47 51 16 0 0. 66 Ju l-1 1 71 64 23 47 50 86 0 0. 22 Ju l-1 1 71 63 23 47 51 51 0 0. 22 Ju l-1 1 71 68 73 47 51 36 0 0. 89 Ju l-1 1 71 65 73 47 51 16 0 0. 39 Ju l-1 1 71 64 73 47 50 86 0 0. 27 Ju l-1 1 71 63 73 47 51 51 0 0. 65 Ju l-1 1 71 69 23 47 51 36 0 0. 31 Ju l-1 1 71 66 23 47 51 16 0 0. 30 Ju l-1 1 71 65 23 47 50 86 0 0. 45 Ju l-1 1 71 64 23 47 51 51 0 0. 31 Ju l-1 1 71 69 73 47 51 36 0 1. 04 Ju l-1 1 71 66 73 47 51 16 0 0. 47 Ju l-1 1 71 65 73 47 50 86 0 0. 36 Ju l-1 1 71 64 73 47 51 51 0 0. 26 Ju l-1 1 71 70 23 47 51 36 0 0. 72 Ju l-1 1 71 67 23 47 51 16 0 0. 41 Ju l-1 1 71 66 23 47 50 86 0 0. 43 T ab le 2 ( co nt in u es o n pa ge s 8 an d 9) . S am pl in g da te , g eo gr ap hi ca l c oo rd in at es ( in U T M ) an d ϕ C O 2 va lu es ( in m ol m -2 da y- 1 ) . NISI ET AL. 8 D at e U T M -W E ST (W G S8 4) Z on e 32 U T M -N O R T H (W G S8 4) Z on e 32 ϕ C O 2 D at e U T M -W E ST (W G S8 4) Z on e 32 U T M -N O R T H (W G S8 4) Z on e 32 ϕ C O 2 D at e U T M -W E ST (W G S8 4) Z on e 32 U T M -N O R T H (W G S8 4) Z on e 32 ϕ C O 2 D at e U T M -W E ST (W G S8 4) Z on e 32 U T M -N O R T H (W G S8 4) Z on e 32 ϕ C O 2 Ju l-1 1 71 65 23 47 51 51 0 0. 43 Ju l-1 1 71 70 73 47 51 36 0 0. 29 Ju l-1 1 71 67 73 47 51 16 0 0. 38 Ju l-1 1 71 66 73 47 50 86 0 0. 57 Ju l-1 1 71 65 73 47 51 51 0 0. 35 Ju l-1 1 71 71 23 47 51 36 0 0. 39 Ju l-1 1 71 68 23 47 51 16 0 0. 53 Ju l-1 1 71 71 23 47 50 86 0 0. 57 Ju l-1 1 71 66 23 47 51 51 0 0. 38 Ju l-1 1 71 61 73 47 51 31 0 0. 31 Ju l-1 1 71 68 73 47 51 16 0 1. 93 Ju l-1 1 71 69 73 47 50 86 0 0. 32 Ju l-1 1 71 66 73 47 51 51 0 0. 26 Ju l-1 1 71 62 23 47 51 31 0 0. 47 Ju l-1 1 71 69 23 47 51 16 0 1. 01 Ju l-1 1 71 68 23 47 51 01 0 0. 45 Ju l-1 1 71 67 23 47 51 51 0 0. 24 Ju l-1 1 71 62 73 47 51 31 0 0. 36 Ju l-1 1 71 69 73 47 51 16 0 0. 45 Ju l-1 1 71 68 73 47 51 01 0 1. 25 Ju l-1 1 71 67 73 47 51 51 0 1. 12 Ju l-1 1 71 63 23 47 51 31 0 1. 23 Ju l-1 1 71 70 23 47 51 16 0 0. 46 Ju l-1 1 71 69 23 47 51 01 0 0. 27 Ju l-1 1 71 68 23 47 51 51 0 0. 11 Ju l-1 1 71 63 73 47 51 31 0 0. 92 Ju l-1 1 71 70 73 47 51 16 0 0. 88 Ju l-1 1 71 69 73 47 51 01 0 0. 23 Ju l-1 1 71 68 73 47 51 51 0 0. 40 Ju l-1 1 71 64 23 47 51 31 0 0. 33 Ju l-1 1 71 71 23 47 51 16 0 1. 76 Ju l-1 1 71 70 23 47 51 01 0 0. 36 Ju l-1 1 71 69 23 47 51 51 0 0. 63 Ju l-1 1 71 64 73 47 51 31 0 0. 71 Ju l-1 1 71 61 73 47 51 11 0 0. 27 Ju l-1 1 71 70 73 47 51 01 0 0. 64 Ju l-1 1 71 59 73 47 51 46 0 0. 34 Ju l-1 1 71 65 23 47 51 31 0 0. 59 Ju l-1 1 71 62 23 47 51 11 0 0. 26 Ju l-1 1 71 71 23 47 51 01 0 0. 24 Ju l-1 1 71 60 23 47 51 46 0 0. 26 Ju l-1 1 71 65 73 47 51 31 0 0. 11 Ju l-1 1 71 62 73 47 51 11 0 0. 27 Ju l-1 1 71 61 73 47 50 96 0 0. 24 Ju l-1 1 71 60 73 47 51 46 0 0. 32 Ju l-1 1 71 66 23 47 51 31 0 0. 56 Ju l-1 1 71 63 23 47 51 11 0 0. 18 Ju l-1 1 71 62 23 47 50 96 0 0. 13 Ju l-1 1 71 61 23 47 51 46 0 0. 26 Ju l-1 1 71 66 73 47 51 31 0 0. 53 Ju l-1 1 71 63 73 47 51 11 0 0. 61 Ju l-1 1 71 62 73 47 50 96 0 0. 41 Ju l-1 1 71 61 73 47 51 46 0 0. 44 Ju l-1 1 71 67 23 47 51 31 0 0. 46 Ju l-1 1 71 64 23 47 51 11 0 0. 17 Ju l-1 1 71 63 23 47 50 96 0 0. 37 Ju l-1 1 71 62 23 47 51 46 0 0. 58 Ju l-1 1 71 67 73 47 51 31 0 1. 12 Ju l-1 1 71 64 73 47 51 11 0 0. 38 Ju l-1 1 71 63 73 47 50 96 0 0. 42 Ju l-1 1 71 63 23 47 51 46 0 0. 46 Ju l-1 1 71 68 23 47 51 31 0 0. 59 Ju l-1 1 71 65 23 47 51 11 0 0. 17 Ju l-1 1 71 64 23 47 50 91 0 1. 21 Ju l-1 1 71 63 73 47 51 46 0 0. 24 Ju l-1 1 71 68 73 47 51 31 0 0. 51 Ju l-1 1 71 65 73 47 51 11 0 0. 07 Ju l-1 1 71 64 73 47 50 91 0 0. 58 Ju l-1 1 71 64 23 47 51 46 0 0. 29 Ju l-1 1 71 69 23 47 51 31 0 0. 27 Ju l-1 1 71 66 23 47 51 11 0 0. 18 Ju l-1 1 71 65 23 47 50 91 0 0. 49 Ju l-1 1 71 64 73 47 51 46 0 0. 20 Ju l-1 1 71 69 73 47 51 31 0 1. 07 Ju l-1 1 71 66 73 47 51 11 0 0. 47 Ju l-1 1 71 65 73 47 50 91 0 0. 45 Ju l-1 1 71 65 23 47 51 46 0 0. 52 Ju l-1 1 71 70 23 47 51 31 0 0. 30 Ju l-1 1 71 67 23 47 51 11 0 0. 83 Ju l-1 1 71 66 23 47 50 91 0 0. 27 Ju l-1 1 71 65 73 47 51 46 0 0. 34 Ju l-1 1 71 70 73 47 51 31 0 0. 39 Ju l-1 1 71 67 73 47 51 11 0 0. 79 Ju l-1 1 71 66 73 47 50 91 0 0. 40 Ju l-1 1 71 66 23 47 51 46 0 0. 24 Ju l-1 1 71 71 23 47 51 31 0 0. 13 Ju l-1 1 71 68 23 47 51 11 0 0. 32 Ju l-1 1 71 67 23 47 50 91 0 0. 39 Ju l-1 1 71 66 73 47 51 46 0 0. 47 Ju l-1 1 71 61 73 47 51 26 0 0. 35 Ju l-1 1 71 68 73 47 51 11 0 0. 52 Ju l-1 1 71 67 73 47 50 91 0 0. 43 Ju l-1 1 71 67 23 47 51 46 0 0. 38 Ju l-1 1 71 62 23 47 51 26 0 0. 63 Ju l-1 1 71 69 23 47 51 11 0 0. 20 Ju l-1 1 71 68 23 47 50 91 0 0. 34 Ju l-1 1 71 67 73 47 51 46 0 0. 53 Ju l-1 1 71 62 73 47 51 26 0 0. 20 Ju l-1 1 71 69 73 47 51 11 0 0. 27 Ju l-1 1 71 68 73 47 50 91 0 0. 20 T ab le 2 ( co nt in u ed fr om p re vi ou s pa ge ). S am pl in g da te , g eo gr ap hi ca l c oo rd in at es ( in U T M ) an d ϕ C O 2 va lu es ( in m ol m -2 da y- 1 ) . 9 THE CO2-RICH GASES FROM THE ACQUA PASSANTE AND ERMETA WELLS (MT. AMIATA, ITALY) T ab le 2 ( co nt in u ed fr om p re vi ou s pa ge ). S am pl in g da te , g eo gr ap hi ca l c oo rd in at es ( in U T M ) an d ϕ C O 2 va lu es ( in m ol m -2 da y- 1 ) . D at e U T M -W E ST (W G S8 4) Z on e 32 U T M -N O R T H (W G S8 4) Z on e 32 ϕ C O 2 D at e U T M -W E ST (W G S8 4) Z on e 32 U T M -N O R T H (W G S8 4) Z on e 32 ϕ C O 2 D at e U T M -W E ST (W G S8 4) Z on e 32 U T M -N O R T H (W G S8 4) Z on e 32 ϕ C O 2 D at e U T M -W E ST (W G S8 4) Z on e 32 U T M -N O R T H (W G S8 4) Z on e 32 ϕ C O 2 Ju l-1 1 71 68 23 47 51 46 0 0. 29 Ju l-1 1 71 63 23 47 51 26 0 1. 44 Ju l-1 1 71 70 23 47 51 11 0 0. 23 Ju l-1 1 71 69 23 47 50 91 0 0. 32 Ju l-1 1 71 68 73 47 51 46 0 0. 18 Ju l-1 1 71 63 73 47 51 26 0 0. 41 Ju l-1 1 71 70 73 47 51 11 0 0. 20 Ju l-1 1 71 69 73 47 50 91 0 0. 72 Ju l-1 1 71 69 23 47 51 46 0 0. 22 Ju l-1 1 71 64 23 47 51 26 0 0. 63 Ju l-1 1 71 71 23 47 51 11 0 1. 05 Ju l-1 1 71 70 23 47 50 91 0 0. 17 Ju l-1 1 71 59 73 47 51 41 0 1. 01 Ju l-1 1 71 64 73 47 51 26 0 0. 27 Ju l-1 1 71 61 73 47 51 06 0 0. 71 Ju l-1 1 71 69 23 47 50 86 0 0. 30 Ju l-1 1 71 60 23 47 51 41 0 0. 18 Ju l-1 1 71 65 23 47 51 26 0 0. 26 Ju l-1 1 71 62 23 47 51 06 0 0. 26 Ju l-1 1 71 64 23 47 50 96 0 0. 70 Ju l-1 1 71 60 73 47 51 41 0 0. 30 Ju l-1 1 71 65 73 47 51 26 0 0. 42 Ju l-1 1 71 62 73 47 51 06 0 0. 64 Ju l-1 1 71 64 73 47 50 96 0 0. 59 Ju l-1 1 71 61 23 47 51 41 0 0. 24 Ju l-1 1 71 66 23 47 51 26 0 0. 66 Ju l-1 1 71 63 23 47 51 06 0 0. 86 Ju l-1 1 71 65 23 47 50 96 0 0. 54 Ju l-1 1 71 61 73 47 51 41 0 0. 54 Ju l-1 1 71 66 73 47 51 26 0 0. 51 Ju l-1 1 71 63 73 47 51 06 0 0. 44 Ju l-1 1 71 65 73 47 50 96 0 0. 23 Ju l-1 1 71 62 23 47 51 41 0 0. 27 Ju l-1 1 71 67 23 47 51 26 0 0. 35 Ju l-1 1 71 64 23 47 51 06 0 0. 45 Ju l-1 1 71 66 23 47 50 96 0 0. 29 Ju l-1 1 71 62 73 47 51 41 0 0. 28 Ju l-1 1 71 67 73 47 51 26 0 0. 31 Ju l-1 1 71 64 73 47 51 06 0 0. 36 Ju l-1 1 71 66 73 47 50 96 0 0. 19 Ju l-1 1 71 63 23 47 51 41 0 0. 15 Ju l-1 1 71 68 23 47 51 26 0 0. 53 Ju l-1 1 71 65 23 47 51 06 0 0. 19 Ju l-1 1 71 67 23 47 50 96 0 0. 21 Ju l-1 1 71 63 73 47 51 41 0 0. 27 Ju l-1 1 71 68 73 47 51 26 0 0. 35 Ju l-1 1 71 65 73 47 51 06 0 0. 26 Ju l-1 1 71 67 73 47 50 96 0 0. 11 Ju l-1 1 71 64 23 47 51 41 0 0. 15 Ju l-1 1 71 69 23 47 51 26 0 0. 32 Ju l-1 1 71 66 23 47 51 06 0 0. 10 Ju l-1 1 71 68 23 47 50 96 0 0. 24 Ju l-1 1 71 64 73 47 51 41 0 0. 27 Ju l-1 1 71 69 73 47 51 26 0 0. 30 Ju l-1 1 71 66 73 47 51 06 0 0. 52 Ju l-1 1 71 68 73 47 50 96 0 0. 37 Ju l-1 1 71 65 23 47 51 41 0 0. 15 Ju l-1 1 71 70 23 47 51 26 0 0. 14 Ju l-1 1 71 67 23 47 51 06 0 0. 60 Ju l-1 1 71 69 23 47 50 96 0 0. 55 Ju l-1 1 71 65 73 47 51 41 0 0. 39 Ju l-1 1 71 70 73 47 51 26 0 0. 61 Ju l-1 1 71 67 73 47 51 06 0 0. 36 Ju l-1 1 71 69 73 47 50 96 0 0. 20 Ju l-1 1 71 66 23 47 51 41 0 0. 23 Ju l-1 1 71 71 23 47 51 26 0 0. 58 Ju l-1 1 71 68 23 47 51 06 0 0. 65 Ju l-1 1 71 70 23 47 50 96 0 3. 42 Ju l-1 1 71 66 73 47 51 41 0 0. 82 Ju l-1 1 71 61 73 47 51 21 0 0. 20 Ju l-1 1 71 68 73 47 51 06 0 0. 36 Ju l-1 1 71 70 73 47 50 96 0 0. 38 Ju l-1 1 71 67 23 47 51 41 0 0. 31 Ju l-1 1 71 62 23 47 51 21 0 0. 29 Ju l-1 1 71 69 23 47 51 06 0 0. 90 Ju l-1 1 71 71 23 47 50 96 0 0. 50 Ju l-1 1 71 67 73 47 51 41 0 0. 52 Ju l-1 1 71 62 73 47 51 21 0 0. 43 Ju l-1 1 71 69 73 47 51 06 0 0. 31 Ju l-1 1 71 61 73 47 50 91 0 0. 71 Ju l-1 1 71 68 23 47 51 41 0 0. 77 Ju l-1 1 71 63 23 47 51 21 0 0. 19 Ju l-1 1 71 70 23 47 51 06 0 0. 14 Ju l-1 1 71 62 23 47 50 91 0 0. 39 Ju l-1 1 71 68 73 47 51 41 0 0. 48 Ju l-1 1 71 63 73 47 51 21 0 0. 35 Ju l-1 1 71 70 73 47 51 06 0 0. 43 Ju l-1 1 71 62 73 47 50 91 0 0. 32 Ju l-1 1 71 69 23 47 51 41 0 0. 64 Ju l-1 1 71 64 23 47 51 21 0 1. 32 Ju l-1 1 71 71 23 47 51 06 0 0. 26 Ju l-1 1 71 63 23 47 50 91 0 0. 30 Ju l-1 1 71 69 73 47 51 41 0 0. 97 Ju l-1 1 71 64 73 47 51 21 0 0. 67 Ju l-1 1 71 61 73 47 51 01 0 0. 28 Ju l-1 1 71 63 73 47 50 91 0 0. 43 Ju l-1 1 71 70 73 47 50 86 0 0. 30 Ju l-1 1 71 68 23 47 50 86 0 0. 30 Ju l-1 1 71 68 73 47 50 86 0 0. 30 Ju l-1 1 71 67 23 47 50 86 0 0. 37 Ju l-1 1 71 67 73 47 50 86 0 0. 23 log-normal Kriging method [e.g. Krige 1951, Matheron 1970] using the ISATIS© software package of Geovari- ances. The processing of the geostatistical data indi- cates that the spherical model with a nugget effect is the best model to describe the spatial variability of {CO2. The variogram parameters have a range of 115 m, a sill of 0.12 and a nugget of 0.2 {CO2 mol m -2 day-1. The iso-flux map shows that the diffuse {CO2 is generally low (average value of 0.38 mol m-2 day-1), although val- ues >0.6 and up to 3.42 mol m-2 day-1 were occasionally measured at NW and SW and E, respectively, of the studied area (Figure 6). Since the CO2 flux values tended to be log-normally distributed, the whole fre- quency distribution was analyzed by using a Gaussian (normal) distribution in the complete data set (Figure 3), and the ln({CO2) data were processed according to the method proposed by Sinclair [1974]. The Sinclair procedure allows to choose threshold values between anomalous and background data using probability graphs. On a log-probability plot, a single log-normal population results as a straight line, whereas a curve with an inflection point describes the theoreti- cal distribution of two overlapping log-normal popula- tions and n overlapping log-normal populations result on a curve characterized by n-1 inflection points. Gen- erally speaking, the log-probability plot shows a straight line for the CO2 flux data related to the Ermeta area (Figure 3). NISI ET AL. 10 Figure 3. Probability plots of ϕCO2 values obtained by applying the partitioning method of Sinclair [1974, 1991]. Circles show the meas- ured soil diffuse ϕCO2 (in mol m -2 day-1). Surveyed surface (m2) Number of measurements Mean Median Minimum Maximum Std. Dev. 653,550 301 -0.95 -0.97 -2.63 1.23 0.56 Table 3. Surveyed surface (in m2), number of measurements and statistical parameters (mean, median, minimum, maximum and standard deviation) of the ϕCO2, expressed as ln(ϕCO2) (mol m -2 day-1). Figure 4. H2S–CH4–H2 ternary diagram for the Ermeta and Acqua Passante gases and the Mt. Amiata gas discharges (by Tassi et. al. [2009]). Figure 5. N2–He–Ar ternary diagram for the Ermeta and Acqua Passante gases and the Mt. Amiata gas discharges (by Tassi et. al. [2009]). ASW: air-saturated-water (ASW). 11 On the basis of this reasonable consideration, the data set can be modeled with a population that can be considered as the “local CO2 flux” (average value of 0.38 mol m-2 day-1) and comparable with the mean bi- ological CO2 flux (~0.11 mol m -2 day-1; e.g. Raich and Schleisinger [1992]). The respective statistical parameters of the considered population are reported in Table 4. The total CO2 output for the Ermeta area was calculated by applying the Sichel’s t-estimator (Mi) [David 1977], whose value was derived by multiply- ing Mi times the area covered by the population (Table 4). In the same way, the central 95% confidence in- tervals of the total CO2 output were used to calculate the uncertainty of the population. According to the Sichel’s t-estimator (Mi), the total amount of CO2 re- leased through diffuse degassing is estimated to be 11.81 ton day-1. 5.3. Computation of the amount of dangerous gases re- leased from the Ermeta and Acqua Passante wells since their construction and their relationships with the soil diffuse gases The impact of the hydrothermal gases released to the atmosphere from the two chimneys is of critical im- portance for the municipality of Abbadia San Salvatore since disagreeable effects for people approaching these areas, such as odor of rotten eggs, are clearly felt. Fur- thermore, locally lethal gas accumulation in air may occur as testified by the presence, though rare, of dead animals found in proximity of the wells. Our data show that the chemical composition of the well gases in the last 5 years had no significant vari- ations and the CO2 flow rate from the two chimneys has remained fairly constant during the period of ob- servation. Consequently, we may reasonably assume that the amount of CO2 has not significantly changed since the construction of the Ermeta and Acqua Pas- sante wells, i.e. 54 and 75 years ago, respectively. This al- lows us to compute that since then the two wells have emitted up to 850,000 tons of CO2 in the atmosphere. This calculation can be extended to H2S and CH4 on the basis of the measured mean values of the H2S/CO2 (Ermeta = 0.0005 and Acquapassante = 0.0004) and CH4/CO2 (Ermeta =0.0063 and Acquapassante = 0.0056) ratios, obtained by the 5 years of periodic sam- pling collection of the well gases. Thus, the total amount of H2S and CH4 emitted from the two chim- THE CO2-RICH GASES FROM THE ACQUA PASSANTE AND ERMETA WELLS (MT. AMIATA, ITALY) Figure 6. Map of soil diffuse ϕCO2 (in mol m -2 day-1) for the area close to the Ermeta well. Red dashed lines refer to preferential alignments (faults or fractures) where highest CO2 flux values were measured. Population Number of measuraments CO2 flux population mean ln (ϕCO2) flux value (Mi ) total diffuse CO2 output (ton/day) 95% confidence interval (ton/day) 1 301 local background -1.076 11.81 13.07-10.84 Table 4. Estimated parameters of partitioned populations and derived total CO2 output calculated according to Sinclair [1974] and Sichel’s t-estimator (Mi ) [David 1977]. neys is of about 430 and 5,150 tons, respectively, the Er- meta well discharging slightly less than one order of magnitude the amount of H2S and CH4 released by that of Acqua Passante. It is worth noting that the sum of CO2 released on a year basis from Ermeta and Acqua Passante wells was up to twice the amount of CO2 emitted from bubbling pools and dry vents computed in the surrounding areas of Mt. Amiata, such as: Acqua Borra at Castelnuovo Be- rardenga (Siena) (5 ton y-1; Vaselli et al., unpublished data), Lo Spuntone, Il Palazzo and Buca alle Colline at Campiglia d’Orcia (Siena) (≈ 370 ton y-1 at each site; Vaselli et al. [2006a], Tassi et al. [2009]), Bossoleto at Ra- polano (Siena) (4,380 ton y-1; Van Gardingen et al. [1995], Vaselli et al. unpublished data), Pienza (5,475 ton y-1; Giuli et al. [1997]) and Ambra River (7,300 ton/year; Cuccoli et al. [2006]). A similar comparison was not computed for H2S and CH4 since in most cases the CO2 flux was determined by open-path IR laser measure- ments [Belotti et al. 2003], which were related to the gas discharges from various vents, contributing to the calculated amount of CO2. No specific investigations were performed to highlight possible differences in the H2S/CH4 ratios from each emission discharging from a certain site. If present, the calculated amount of H2S and CH4 might be affected by a large error. The generally low CO2 soil flux values measured in SE the Ermeta well seem to indicate that the diffuse gas emission from the soil is mainly related to soil res- piration. Plant roots and soil microbial activities are ex- pected to produce a relatively homogeneous distribution of the diffuse soil CO2 when a soil cover is present, likely modulated by seasonal patterns of moisture availability, land use and biological cycles [e.g. Alma- gro et al. 2009, Elío et al. 2013 and references therein]. As previously mentioned, the studied area is generally characterized by a well-developed vegetation and no measurements were performed where the volcanic rocks were outcropping. Nevertheless, the spatial dis- tribution of the CO2 fluxes (Figure 6) shows the occur- rence of preferential NW-SE and NNE-SSW oriented alignments characterized by relatively high CO2 flux values, which might be related to weakness zones, such as fractures and/or faults. As a first approximation, these sites may suggest the presence of gases fed by the deep-seated reservoir. In this respect, an isotopic survey of the CO2 soil gases is required to assess whether these zones can be regarded as the preferential pathways for the uprising of deep-originated CO2. The carbon iso- topic signature of the Ermeta gas (−3.5 V-PDB ‰), as well as that of Acqua Passante, is indeed significantly different with respect to that associated with a biogenic source (~ −20 V-PDB ‰; Schoell [1980]). To further support this hypothesis a geostructural survey is re- quired to verify the correspondence between the geo- chemical and geological signals. 5.4. Is the closure of the Ermeta and Acqua Passante wells feasible and safe? Since 2008, the municipality of Abbadia San Sal- vatore has become the owner of the mining concession from where cinnabar was extracted and in agreement with the authorities of the Tuscany Region it was ap- pointed to i) monitor the gases discharged from the Er- meta and Acqua Passante wells and ii) actuate prevention measurements to minimize such emissions to the at- mosphere. As previously mentioned, the two wells are located in the proximity of tourist areas and the reduc- tion of their gas discharges is expected to provide ben- efits in terms of safety and healthy conditions. Presently, ~15,000 tons of CO2, along with 8 and 92 tons of H2S and CH4, respectively, are emitted on a yearly basis from the Ermeta and Acqua Passante chim- neys and they are related to the discharge of a natural subsurface CO2 reservoir intercepted during Hg prospec- tion campaigns few kms away from Abbadia San Salva- tore. The Ermeta well contributes for >85% of the total CO2 discharge. The amount of CO2 is more than three times higher than that released by diffuse soil degassing released from an area of 653,550 m2 located SE of the Ermeta chimney, this value being of 4,310 ton y-1. According to the aforementioned agreement, the municipality of Abbadia San Salvatore would intend to proceed with the closure of one of the wells and that of Ermeta was thought to be considered as the most suit- able since it is i) discharging a larger quantity of gas, ii) located more far away from the main roads and iii) sur- rounded by a vegetation (and soil) cover (Figure 2), sim- ilar to that where {CO2 fluxes were directly measured suggesting that the diffuse soil CO2 can be assumed to be only affected by soil respiration, e.g. no gas dis- charges are present at the surface. Nevertheless, one of the most critical points is to understand and recognize which are those sites that might possibly be affected by potential leakage or micro-seepage of CO2 [e.g. Klusman 2013, Elío et al. 2013, Nisi et al. 2013 and references therein] as gases in deep reservoirs are buoyant and tend to vertically or lat- erally migrate following the most permeable pathways in response to pressure. Several authors have proposed buoyancy of gas (micro)bubbles [e.g. MacElvain 1969, Davidson 1982, Klusman 1993, Klusman and Saeed 1996] as a potential process of vertical migrations of gases and gas overpressuring underground, as that ex- pected once the closure of the Ermeta well will occur, plays an important role [Klusman 1997]. In this respect, NISI ET AL. 12 13 faults and fractures, may then vehiculate the CO2-rich gases upwards [e.g. Etiope 1999, Brown 2000 and refer- ences therein]. As pinpointed by Klusman [2013], the rapidity of transport is another important parameter to be taken into account. It appears to be realistic the fact that the weakness zones, individuated by the direct measurements of the CO2 soil fluxes (Figure 6), can likely be affected by an increase in the CO2 flux, being these areas character- ized by a higher permeability. This would theoretically imply the possibility to recognize those areas from where a higher probability of CO2 leakage is expected in the case of a gas overpressure by applying a relatively simple, though effective, method such as that of the ac- cumulation chamber. Owing to the lack of specific in- vestigations addressed to a detailed geostructural study in the surrounding areas of the Ermeta well, to tenta- tively reduce the discharge of CO4, CH4 and H2S we highly recommend the municipality of Abbadia San Salvatore to solely temporarily close the Ermeta well by using N2-filled air-bags before a definitive cementa- tion. Although it is difficult to predict the side effects deriving by such an action, we may speculate that an increasing gas flow rate could be expected. Further- more, no indications are also presently available about the timing before a modification of the soil diffuse CO2 is to be recorded. This implies that a geochemical and isotopic monitoring program has to be designed and mainly focused on the diffuse soil CO2 with particularly reference to the recognized weakness zones. In the case of increasing activity in terms of release of soil CO2, the tube closing being temporary, previous conditions could easily be restored by re-opening the Ermeta well. Thus, currently there are no sufficient clues to defini- tively shut down the Ermeta well. A similar approach should be considered if the closure of the Acqua Pas- sante well is to be considered. 6. Conclusions The municipality of Abbadia San Salvatore is in- tending to reduce the impact related to the emission in the atmosphere of CO2(H2S,CH4)-rich gases by two wells: Acqua Passante and Ermeta, built in 1938 and 1959, respectively, during Hg prospection campaigns and located in the eastern part of the Mt. Amiata vol- canic system. The easiest solution to avoid the gas dis- charge is to close down the wells, the Ermeta drilling being the one to be tested first. In this work we have determined that the Ermeta and Acqua Passante drillings are emitting up to 15,000 ton y-1 of CO2 and 92 and 8 ton y -1 of CH4 and H2S, re- spectively. These gases can be regarded as the discharge of a natural subsurface CO2-rich reservoir. The origin of these gases is similar to those naturally released from dry vents and bubbling pools located few kilometers away from the two wells, with the exception of H2, whose higher content is apparently related to corrosion processes of the iron tubing of the chimneys as the deep-seated gases flow through them. We have also es- timated the CO2 output from soil (4,310 ton y -1 in an area of about 653,500 m2) close to the Ermeta well and determined by the accumulation chamber method. Rel- atively high CO2 fluxes were recognized along prefer- ential alignments, possibly associated with the presence of faults or fractures (Figure 6). Standing the absence of detailed geostructural in- vestigations nearby Ermeta, we strongly recommend a temporarily closure of the Ermeta well by means of N2-filled air-bags before proceeding with a definitive ce- mentation. 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