Carbon dioxide diffuse degassing and estimation of heat release from volcanic and hydrothermal systems

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 110,, doi: /2004jb003542, 2005 Carbon dioxide diffuse degassing and estimation of heat release from volcanic and hydrothermal systems G. Chiodini, D. Granieri, R. Avino, S. Caliro, and A. Costa Osservatorio Vesuviano, Istituto Nazionale di Geofisica e Vulcanologia, Naples, Italy C. Werner Institute of Geological and Nuclear Sciences, Taupo, New Zealand Received 16 November 2004; revised 19 April 2005; accepted 3 May 2005; published 16 August [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 CO 2 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 (K c ). For each area, we computed the values of K c which range from 0.4 to 2.3 W m 1 C 1. Using the CO 2 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. Citation: Chiodini, G., D. Granieri, R. Avino, S. Caliro, A. Costa, and C. Werner (2005), Carbon dioxide diffuse degassing and estimation of heat release from volcanic and hydrothermal systems, J. Geophys. Res., 110,, doi: /2004jb Introduction [2] The energy released through hydrothermal and volcanic gaseous emissions is an important component of the energy balance of quiescent volcanoes. Typically energy balances of volcanoes are derived from mass balance calculations at crater lakes. In previous studies at 14 crater lakes, energy outputs ranged from to W [Brown et al., 1989; Sheperd and Sigurdsson, 1978; Hurst et al., 1991; Pasternack and Varekamp, 1997]. In four volcanic systems without crater lakes (Vulcano, Solfatara di Pozzuoli, Nisyros, Ischia), the thermal energy released was estimated to range from to W[Caliro et al., 2004; Chiodini et al., 2004, 2001a, 1996] based on measurements of volcanic-hydrothermal CO 2 released through soil diffusion emission. For instance, the energy dissipated daily by the diffuse degassing structure of Solfatara di Pozzuoli (0.5 km 2 ) is the main source of energy in the entire Campi Flegrei caldera in the current period of quiescence. According to Chiodini et al. [2001a], the thermal energy released daily is one order of magnitude greater than the elastic energy released during seismic crisis, greater than the energy associated with ground deformation, and is about 10 times grater than the conductive heat flux over the Copyright 2005 by the American Geophysical Union /05/2004JB003542$09.00 caldera (90 km 2 ). These data suggest that the monitoring of energy fluxes associated with diffuse degassing may be an important tool in the surveillance of volcanic activity, and continued development of quick and reliable methods to measure the thermal energy released by the hydrothermal activity will encourage the systematic use of this technique. [3] In this work we focus our attention on the energy released by subaerial diffuse degassing, i.e., where the gas is not directly emitted by fumaroles. A recent study has shown that diffuse degassing does not occur across entire volcanoes, but rather in restricted areas named diffuse degassing structures (DDSs [Chiodini et al., 2001a]) commonly associated with regions of high permeability (faults, fractures). [4] Thermal Infrared images of DDSs present an intriguing visualization of the heat flux associated with these structures (Figure 1). It is worth noting both the large extent of the hot soil areas and the generally low magnitude of the thermal anomaly, i.e., during the survey periods, maximum soil temperatures rarely exceeded C. At these temperatures steam condenses below the surface and most of the heat should be transmitted by conduction. [5] In a purely conductive regime, the heat flux is a linear function of the soil thermal gradient with a slope proportional to the thermal conductivity of the medium. On the basis of this consideration, previous studies have estimated the heat released from hot soils measuring soil thermal 1of17

2 gradients and assuming thermal conductivities of the soils [Chiodini et al., 2001a; Brombach et al., 2001; Lardy and Tabbagh, 1999; Severne and Hochstein, 1994]. Chiodini et al. [2001a] and Brombach et al. [2001] assumed the thermal conductivity to be 1 W m 1 C 1 following Clauser and Huenges [1995], while Severne and Hochstein [1994, p. 212] used appropriate thermal conductivity values to assess the surface heat transfer. Lardy and Tabbagh [1999] directly measured the ground thermal conductivity using active methods in superficial glassy ash, and obtained a mean of 0.51 ± 0.16 W m 1 C 1 with a large distribution of the data ( W m 1 C 1 ) depending on the variability of the soil composition. However, the long time required for the direct measurements of the thermal conductivity limits the use of this technique. [6] An alternative way to estimate the heat flux from DDSs is based on CO 2 flux measurements. The degassing of CO 2 from a DDS can be computed and mapped with adequate precision applying the sequential Gaussian simulation (sgs) method to the measurement points [Cardellini et al., 2003]. Because CO 2 is the second most abundant gas species (after H 2 O) in volcanic and hydrothermal systems considered here and because it is a noncondensable gas at T-P conditions of the DDSs, it serves as a tracer of the diffuse degassing process. In particular, CO 2 fluxes were used to estimate the heat released by DDSs [Brombach et al., 2001; Chiodini et al., 2001a, 2004; Frondini et al., 2004]. These studies calculated the total amount of steam released at depth based on the assumption that the fluids which supply the diffuse degassing process have, at depth, the same composition as those emitted by the fumarolic vents of highest temperature and flow rate. The thermal energy was then computed by multiplying the steam flux by the enthalpy of the steam minus the enthalpy of the liquid at ambient temperature. [7] In this paper, we discuss the interrelation of CO 2 fluxes, thermal gradients, and fumarolic compositions at 15 DDSs from eight different volcanic systems. The aim is to test the reliability of the above method for heat flux estimation by using the measured soil thermal gradients, CO 2 fluxes and fumarole compositions. We will describe also a new method for the estimation of the thermal conductivity of the DDSs soil based on the combination of the different measured parameters. [8] In addition, we present the results of a long period of continuous monitoring of the soil temperature at different depths in the crater of Solfatara di Pozzuoli. This data set is used to determine the influence of diurnal and seasonal temperature variations on the thermal heat released from the soil. 2. Study Sites [9] The DDSs investigated in this study are located in 8 different volcanic systems in the world: Stefanos, Kaminakia, Polybotes Micros, and SU area in the Nisyros volcanic system (Greece), Solfatara in the Campi Flegrei district (Italy), Donna Rachele in Ischia Island (Italy), NE rim and Bottom Crater in the Vesuvio system (Italy), Comalito cinder cone in the Masaya Caldera Complex (Nicaragua), Cerro Negro in the Cerro Negro volcanic system (Nicaragua), Vulcano Porto di Levante Beach and Vulcano Crater in Vulcano island (Italy), Favara Grande and Favara Piccola in Pantelleria island (Italy). Typically, the DDSs are not exclusively contained within crater areas but rather are often related to structural features such as faults/ fracture zones. [10] Nisyros volcanic island, built up during the last 100 ka, lies at the eastern end of the Aegean active volcanic arc. Hydrothermal eruptions were frequent in historical times. The oldest hydrothermal craters are located at Kaminakia, while the last eruption, which occurred in September 1887, originated in Polybotes Micros crater. At present, Stefanos hydrothermal crater is the most active fumarolic site. According to the geochemical model by Chiodini et al. [1993a], the hydrothermal system of Nisyros is made up of a deep reservoir (>1000 m) with a temperature of 300 C, and of a shallower system, with temperatures of C. [11] Ischia is the westernmost volcanic complex of the Campania area and the last eruption took place in 1301 A.D. [Vezzoli, 1988]. Our study is focused on the so-called Donna Rachele fumarolic area, which is located on the western flank of Mount Epomeo. It is characterized by hydrothermally altered terrain, steaming ground and focused vents. Chiodini et al. [2004] proposed two distinct hydrothermal reservoirs feeding the fumarolic area: a shallower reservoir characterized by a temperature of 250 C and a pressure of 40 bars and a deeper reservoir characterized by a temperature of 300 C and a pressure of 90 bars. [12] Solfatara is a tuff cone affected by intense diffuse degassing. It is located inside Campi Flegrei caldera complex. A conceptual geochemical model of the hydrothermal system suggests one or multiple aquifers are located over the magma chamber [Cioni et al., 1984; Chiodini and Marini, 1998]. Gas equilibrates in a superheated vapor zone at temperature of C in the shallower part of the hydrothermal system [Chiodini et al., 2001a]. [13] Somma-Vesuvio is a central composite volcano located in the southern sector of the Campanian Plain. It was formed by an ancient stratovolcano, Mount Somma, and by a more recent cone, Vesuvio [Santacroce, 1987]. At present, Vesuvio volcano is characterized by widespread fumarolic activity on the inner slopes and bottom of the crater, as well as by diffuse soil CO 2 degassing. Geochemical studies of crater fumarolic fluids reveal the presence of a hydrothermal system located inside the Somma caldera [Chiodini et al., 2001b]. On the basis of C-H-O gas equilibria of crater bottom fumarolic fluids, the temperature of this system was estimated to be C. Hydrostatic models suggest a hydrothermal reservoir exists at depths of 2 4 km within the carbonate sequence, which is present at depths >2.5 km underneath the volcano [Chiodini et al., 2001b]. [14] Masaya is an active basaltic volcano on the Central American volcanic front, situated about 25 km southeast of Managua, Nicaragua. It is an unusual subduction zone volcano because of its shield-like form, consistent tholeiitic basaltic composition (Masaya is one of the few volcanoes known to have hosted basaltic Plinian activity [Williams, 1983]), frequent eruptive activity, and a 25-year cycle of major noneruptive degassing crises [Stoiber et al., 1986]. The flux of SO 2 and HCl from Masaya represents the largest reported noneruptive volcanic release of these species in the world [Stoiber et al., 1986]. Comalito cinder cone is located approximately 3.5 km NE of the Masaya crater, along a 2of17

3 Figure 1 3of17

4 northeast trending lineament. The site investigated was located adjacent to the Comalito cinder cone, and was characterized by steam and diffuse CO 2 emissions, minimal vegetation, and soil temperatures up to 80 C [Lewicki et al., 2003]. [15] Cerro Negro has been interpreted to be an old cinder cone [Mooser et al., 1956; Walker and Carr, 1986; Simkin and Siebert, 1994] or a young composite volcano [Wood, 1978; McKnight and Williams, 1997] and is located in northwestern Nicaragua on the flank of the El Hoyo Las Pilas volcanic complex. It erupted 22 times since its formation in 1850, with the last eruption in May August Thermal features are limited to superheated fumaroles in the crater (350 C), and low-temperature (<100 C) fumaroles located in arcuate fractures on the crater rim and near phreatic pit craters between Cerro Negro and the cinder cone Cerro La Mula [Connor et al., 1996]. At the summit of Cerro Negro volcanic edifice, where we investigated a small area (300 m 2 ), the soil was affected by intense CO 2 diffuse degassing (up to 35,000 g m 2 d 1 ) and high temperatures [Salazar et al., 2001]. [16] Vulcano is one of the currently active volcanoes of the Aeolian island arc, north of Sicily, Italy, and is characterized mainly by explosive eruptions. The last eruption in the La Fossa crater occurred in [Mercalli and Silvestri, 1891]. Since that time the crater has hosted hightemperature fumaroles (up to 700 C in 1993) with chemical composition typical of volcanic emissions [Chiodini et al., 1995; Nuccio et al., 1999], i.e., high content of acidic gases like HCl, HF, and SO 2. Fumarolic activity is also present at Porto di Levante Beach (PL Beach). The beach fumaroles have temperatures of 100 C and chemical compositions typical of the hydrothermal environment [Chiodini et al., 1995; Tedesco et al., 1995], i.e., absence of acidic gases and presence of reduced gases species, like CH 4 and H 2 S. [17] Pantelleria island is a quiescent volcano located in the Sicily Channel, 110 km south of Sicily and 70 km north of Tunisia. Most of the exposed volcanics on the island are alkaline or peralkaline products with high silica contents (pantellerites). The most recent volcanic activity in the area took place during the 19th century with two submarine eruptions (1831 and 1891) in the vicinity of the island [Civetta et al., 1988]. At present, the volcanic activity at Pantelleria is represented by steam emissions in the areas of Favare (Favara Grande and Favara Piccola), Passo del Vento, and Cuddia di Mida and gas manifestations in the Lake of Venus and the Gadir gulf. A thermal aquifer is present at depth, with a maximum estimated temperature of 260 C [D Alessandro et al., 1994; Squarci et al., 1994]. 3. Methods 3.1. Field and Laboratory Methods [18] The following procedures were used in each DDS: [19] 1. CO 2 soil flux measurements were made within the diffuse degassing zones using the accumulation chamber method. This is a direct, passive method based on the measurement of the initial CO 2 concentration increase inside a chamber of known volume that is placed on the soil. The increase in CO 2 concentration is directly proportional to the CO 2 flux [Parkinson, 1981; Tonani and Miele, 1991; Chiodini et al., 1996]. The method has been tested by several authors under controlled laboratory conditions. Chiodini et al. [1998] measured values within 15% of the imposed fluxes, whereas Evans et al. [2001] recognized a systematic underestimation of the flux (average of 12.5%), over an imposed flux range of ,000 g m 2 d 1. In a field reproducibility test of the method, carried out at two points with high and low CO 2 flux, Carapezza and Granieri [2004] found an uncertainty of 12% for high fluxes (mean of 997 g m 2 d 1 ) and 24% for low fluxes (mean of 5.7 g m 2 d 1 ). [20] 2. Temperatures were measured along vertical profiles at 0.01 to 0.05 m spacing between 0.02 and 0.40 m depth in the soil by means of a digital thermocouple type K (the measurement accuracy is ±0.05% of reading). Soil temperature profiles were measured in the sites corresponding with the CO 2 flux locations. [21] 3. Fumaroles were sampled within the degassing areas. We selected fumaroles at each site with the highest flow rate and temperature to obtain samples that were most representative of the fluids at depth. Complete chemical analyses were generally conducted at the Osservatorio Vesuviano laboratories following standard methods [Giggenbach, 1975, 1991; Giggenbach and Goguel, 1989; Chiodini et al., 1993b]. Fumaroles from Cerro Negro and Masaya (Nicaragua) were analyzed at the INGV-Palermo Laboratories. [22] In addition, soil temperatures were continuously measured during two years (October 2001 to October 2003) within the crater of Solfatara using an automatic station. The station was equipped with a Lastem BABUC ABC data logger that acquires, at 1-hour time intervals, the soil temperature at 0.1, 0.2, and 0.3 m depth. CO 2 soil flux and various parameters that can potentially influence the CO 2 soil flux, i.e., barometric pressure, air and soil temperature, air and soil humidity, wind speed, and rainfall were also measured at the same location [Granieri et al., 2003]. The station was used as a base station to investigate the thermal gradient and its temporal variability in the shallower part of a typical volcaniclastic soil Modeling Procedures [23] Physical modeling was applied to simulate heat and fluid flow through the uppermost few meters of each hydrothermal system, including the surface, where the thermal gradients and CO 2 fluxes were measured. The used geothermal simulator (TOUGH2 [Pruess, 1991; Pruess et al., 1999]) accounts for the coupled transport of heat and a multiphase (gas and liquid) and a multicomponent (water and carbon dioxide) fluid, and was recently used to model large-scale diffuse degassing at Solfatara crater [Chiodini et al., 2003; Todesco et al., 2003]. These studies were based on a two-dimensional, axisymmetric domain that was 2500 m wide and 1500 m deep. The model successfully reproduced measured changes in both the gas composition and discharge Figure 1. Thermal infrared images of (a) Vulcano crater, (b) Vulcano PL Beach, (c) Vesuvio crater, (d) Solfatara crater, (e) Lakki plain of Nisyros and (f) Stefanos crater. Images were taken in the IR wavelengths 8 14 mm using a Thermo Tracer Nec TH7102MV. 4of17

5 Table 1. Composition of Sampled Fumaroles a DDS Date T, C H 2 O CO 2 S tot Ar O 2 N 2 CH 4 H 2 He HCl HF Campi Flegrei Volcanic System Solfatara Mar E-05 b 1.86E E E E E-04 nd nd Apr E-05 bdl 5.41E E E E-04 nd nd May E E E E E E-04 nd nd Jun E E E E E E-04 nd nd Jul E-05 bdl 4.53E E E E-04 nd nd Mar E-05 bdl 4.24E E E E-04 nd nd Nisyros Volcanic System Stefanos Sep E-05 bdl 2.99E E E E-05 nd nd May E-05 bdl 2.81E E E E-05 nd nd Sep E-05 bdl 3.77E E E E-05 nd nd Feb E E E E E E-05 nd nd May E-05 bdl 2.18E E E E-05 nd nd Sep E-05 bdl 1.54E E E E-05 nd nd Feb E-06 bdl 1.42E E E E-05 nd nd Jul E-05 bdl 1.43E E E E-05 nd nd May E E E E E E-05 nd nd SU area May E E E E E E-05 nd nd Polybotes Micros May E-05 bdl 1.77E E E E-05 nd nd Kaminakia Sep E-04 bdl 6.63E E E E-05 nd nd Masaya Volcanic System Comalito Mar nd nd 9.94E E E E E-04 nd nd Cerro Negro Volcanic System Cerro Negro Mar nd 1.95E E+00 bdl 4.10E E E E-05 Pantelleria Volcanic System Favara Grande Jul E E E E E E-05 nd nd Favara Piccola Jul E E E E E E-05 nd nd Ischia Volcanic System Donna Rachele Oct E E E E E E-06 nd nd May E E E E E E-06 nd nd Vulcano Volcanic System Crater Mar E E E-02 bdl 1.26E E E E-02 PL Beach Mar E-04 bdl 7.57E E E E-05 nd nd PL Beach Mar E-05 bdl 3.83E E E E-06 nd nd Vesuvio Volcanic System NE Rim Feb nd 6.06E E E E-04 nd nd nd nd Bottom Crater Mar E E E E E E-05 nd nd a Diffuse degassing structures (DDSs); analytical data are expressed in vol %; nd, not determined; bdl, below detection limit; PL, Porto di Levante. b Read 4.89E-05 as rate with time, and modeling results were correlated to recent bradyseismic crises at Campi Flegrei. In the present paper we used the same physical approach to model the heat and the fluid transfer in the uppermost part of the Solfatara hydrothermal system and test the applicability of using shallow soil thermal gradients to estimate total energy transfer by soil diffuse degassing. In order to investigate the effect of the input of fumarolic fluids into the soil, a two-dimensional (xz), axisymmetric domain, 10 m wide (x) and 2 m deep (z) was considered. The domain was discretized into 1000 elements (25 40 cells) of variable size, with finer elements located near the axis of symmetry and at the top and bottom boundaries. The medium is assumed homogeneous, with constant permeability (10 12 m 2 ), density (2000 kg m 3 ), thermal conductivity (0.5 W m 1 C 1 ) and specific heat (1000 J kg 1 C 1 ). At the initial condition the uniform porous (n = 0.4) medium is filled with gas at atmospheric pressure (i.e., CO 2 at 1 bar) and a temperature of 30 C. A fluid source is located along the bottom boundary, from the symmetry axis to a maximum radial distance of 1.2 m and it supplies a gas phase composed of water vapor and CO 2 at a temperature of 100 C, with a H 2 O/CO 2 ratio consistent with the composition of fumarolic gas of the modeled system (e.g., H 2 O/CO 2 = 2.2 by weight at Solfatara). At Solfatara the rate of the gas injection was assumed kg s 1. This value was chosen to reproduce the highest measured thermal gradients in the area (see section 4). The bottom boundary was impermeable to the fluid and heat fluxes while the side and the upper boundaries are open with respect to heat and fluid flux, with temperature and pressure values fixed at the initial conditions (30 C and atmospheric pressure). After days of injection the simulated temperature and the pressure values reach constant values. 4. Results and Discussion 4.1. Chemical Composition of the Fumaroles [24] The main component of the fumaroles within the different DDSs is generally H 2 O followed by CO 2 and 5of17

6 Table 2. Summary of Measured and Estimated Parameters in 15 DDSs in the World a DDS Date Number of Measurements rt, j CO2, j GAS, j H2O, K c, C m 1 mol m 2 s 1 x H2O /x GAS mol m 2 s 1 mol m 2 s 1 Wm 1 C 1 Campi Flegrei Volcanic System Solfatara Mar E-04 b E E Apr E E E May E E E May E E E Jun E E E Jul E E E Mar E E E Nisyros Volcanic System Stefanos Sep E E E May E E E Sep E E E Feb E E E May E E E Sep E E E Feb E E E Jul E E E May E E E SU Area May E E E Polybotes Micros May E E E Kaminakia Sep E E E Masaya Volcanic System c Comalito Mar E E E Cerro Negro Volcanic System c Cerro Negro Mar E E E Pantelleria Volcanic System Favara Grande Jul E E E Favara Piccola Jul E E E Ischia Volcanic System Donna Rachele Oct E E E May E E E Vulcano Volcanic System Crater Mar E E E PL Beach Mar E E E PL Beach Mar E E E HF, Wm 2 Vesuvio Volcanic System NE Rim Feb E E E Bottom Mar E E E a Measured parameters are the number of measurements, thermal gradient rt, soil CO 2 flux j CO2, and fumarolic molar ratio x H2O /x GAS and the estimated parameters are soil gas flux j GAS, steam flux j H2O, soil thermal conductivity K c, and heat flux (HF). b Read 4.79E-04 as c Data collected during the 8th Field Workshop on Volcanic Gases (IAVCEI-CCVG), Nicaragua and Costa Rica, March certain other gases. The absence of acidic gases (SO 2, HCl, and HF) and relatively high CH 4 contents indicate hydrothermal origin of fluids at all DDSs, apart from Cerro Negro and Vulcano crater DDSs which are characterized by hightemperature fumaroles rich in acidic gases indicating a more typical volcanic environment (Table 1) CO 2 Fluxes and Temperatures in the Soils [25] The data of CO 2 soil fluxes and thermal gradients in the soil were collected in 15 DDSs during the last 5 years. The number of measurements varies from 10 to 200, depending on the different extensions of the studied DDSs. Campaigns comprising a different number of measurements, distributed in the same area, implicate different sampling densities (e.g., Solfatara, Stefanos and Donna Rachele DDSs). A summary of measured and estimated parameters is shown in Table 2. [26] The measured parameters are thermal gradients (rt), soil CO 2 fluxes (jco 2 ) and the ratio between H 2 O and the content of noncondensable gases of the fumaroles (x H2O /x GAS where x GAS is the sum of noncondensable gases molar fractions). Thermal gradients range from 34 C m 1 at Favara Grande DDS to 405 C m 1 at Polybotes Micros. Soil CO 2 fluxes range from mol m 2 s 1 to mol m 2 s 1. The molar ratio x H2O /x GAS varies in a large range from 0.5, at Vesuvio NE Rim, to 410 at Donna Rachele, Ischia. [27] The estimated parameters are the flux of gas (j GAS ), the flux of the steam (j H2O ), the thermal conductivity of the soil (K c ) and the soil heat flux (HF). The flux of gas from soil is computed by multiplying soil CO 2 flux (j CO2 ) times the ratio x GAS /x CO2. The computed j GAS values are close to the original j CO2 values in all the DDSs with the exception of Vesuvio NE Rim and Comalito DDSs (Table 2), where 6of17

7 Figure 2. Vertical soil temperature profiles measured in Solfatara crater. Profiles are grouped into four classes based on their different shapes and ranges of values. Therm. Grad. and jco 2 are the mean thermal gradient and the mean CO 2 soil flux for all the n samples of the class. Selected profiles of classes a, b, and c are reported. The grey (with triangles) profile in the class d is relative to the 84 (see text and Figure 7). the main component of the fumaroles is air (essentially N 2 and O 2 ) followed by H 2 O and CO 2 (Table 1). Similarly the flux of the steam (j H2O ) is computed by multiplying the measured j CO2 values by the fumarolic ratio x H2O /x CO2. The derivation of K c and thermal heat release is described in detail in subsection 4.4. [28] The data of the campaign performed at Solfatara crater in July 2000 (Table 2) were used as a base case to investigate the possible shapes of the thermal gradients. This campaign consisted of the measurement of 94 vertical temperature profiles ( m depth) and soil j CO2 (Table 2). The temperature profiles were subdivided into four classes (Figure 2) according to different shapes and ranges of thermal gradients: class a (comprising 18% of the profiles) was characterized by low thermal gradients and, in this case, the effect of the diurnal solar radiation was relevant, resulting in a decrease in soil temperature with depth in the uppermost part of the soil ( m); class b(18% of the profiles) includes the profiles with a break in the slope of the soil temperatures with increased depth, due to presence of two thermally different layers; class c (53% of the profiles) was characterized by medium thermal gradients, constant slope with depth and temperatures at m depth up to C; and class d (10% of the profiles) included profiles with the highest thermal gradient and included curves which asymptotically tend to a common isotherm of 95 C. [29] We use soil temperatures measured continuously at Solfatara at 0.1, 0.2, and 0.3 m depths to investigate the influence of the daily and seasonal thermal variations on the thermal gradient of the upper part of the soil. Daily and seasonal thermal variations could potentially modify the heat flux measured at the surface arising from the input of the deep hydrothermal system. The soil temperatures at 0.2 and 0.3 m depths were measured from October 2001 to October 2003, while the 0.1 m depth was measured from October 2001 to July Soil temperatures increase with depth due to the presence of the deep thermal source (Figure 3). The low mean value of m thermal gradient (59 C m 1 ) indicate that this point belongs at the class a, in which the effects of daily and seasonal thermal excursions are most evident. The high frequency variations in soil temperature are due to the rainfall episodes which also produce positive peaks in the soil humidity (grey line in Figure 3). The amplitude of these temperature fluctuations is damped with depth. This suggests that measurement of the vertical temperature gradient should be avoided in the rainy (transient) periods, the effects of which 7of17

8 Figure 3. Soil temperature at three depths (0.1, 0.2, and 0.3 m) and relative humidity of the soil at 0.15 m depth in the same site (grey line). The uppermost sensor (0.1 m) was in operation until the end of July The positive peaks in the soil humidity coincide with rainfall episodes. generally tend to last 4 5 days. The seasonal (or diurnal) variations of the temperature can be described by a sinusoidal law: T ¼ T a þ A o sinðwt þ fþ ð1þ where A 0 and T a are the amplitude of the seasonal (or diurnal) temperature wave and the average annual (or daily) soil temperature at z = 0, w is 2p/t, where t is the period of the cycle (1 year or 1 day), t is the time and f is the initial phase of the temperature wave. Using relation (1) we fitted the seasonal temperature variations of the Figure 3 which are evident at both investigated depths (the shallower sensor was in operation until July 2002). The amplitude of the seasonal temperature fluctuations (A o =9.6 C) does not change significantly between 0.2 and 0.3 m depth (Figure 4). This means that temperatures at 0.2 m and 0.3 m depth reflect the periodic surface variations in the same way and that the thermal gradient, at least in the m depth, has a comparable mean value for different seasons. Moreover, using continuous data from Solfatara, it is evident that the daily temperature excursions of the soil (SDTE) are minimal (few percentage points) compared to the average daily temperature (SDAT) at those same depths (Table 3 and Figure 4 insert). In other words, at depths higher than 0.2 m in the hot soil of volcanic-hydrothermal areas, the stationary component of temperature, reflecting the endogenous effects, is dominant relative to the external temperature changes (due to solar heating, wind, etc.) Modeling Results and Comparison to Measured Gradients [30] The results of the TOUGH2 modeling at the steady state (9000 days) are presented in Figures 5a 5c and 6a 6c. Figures 5a, 5b, and 5c, which represent the mass flux of steam, liquid and CO 2, respectively, describe a convective plume of hot carbon dioxide-steam mixture extending from the source toward the surface. The composition of the Figure 4. Soil temperature at 0.2 and 0.3 m depths and relative sinusoidal fitting. The correlation coefficient between measured and estimated values is for both recordings. Insert shows daily fluctuations in 1-week period. 8of17

9 Table 3. Summary of Results of the Continuous Soil Temperature Monitoring at Solfatara Crater Depth, m SDAT a SDTE b C c 4.13 C (14.1%) c C 2.32 C (6.6%) C 0.98 C (2.4%) a SDAT, soil daily average temperature. b SDTE, soil daily thermal excursion (average value). The values in parentheses are the percentage value of the ratio SDTE/SDAT. c Data from October 2001 to July gaseous mixture changes during its ascent: the upflowing steam (Figure 5a) condenses and produces a liquid phase that flows from the border of the plume toward the bottom of the domain (Figure 5b), whereas CO 2, which is noncondensable gas, is discharged almost entirely at the soil surface, extending to a maximum radial distance of 4 m (Figure 5c). [31] Figures 6a 6c show the modes of the heat transfer associated with the rising fluid. The heat is transferred at the surface by advective and conductive processes (Figure 6a). At depth the distribution of temperature is almost uniform and the conductive heat transfer rate can be neglected. Twophase advection is therefore the primary mode of heat transfer in this region. Some energy is transported laterally by the liquid phase produced by steam condensation (Figure 6b). Near the surface, where the vapor phase is negligible, the heat is primarily transferred by conduction (Figure 6c). Numerical simulations show that the original heat of the deep source (6348 W) is almost totally transferred at the surface (5940 W corresponding at 93.6% of the original heat). Of this amount about 98.4% is released by conduction and causes linear thermal gradients in the upper part of the domain. A larger fraction of the heat transported by conduction is computed when the amount of energy injected in the system is reduced. This finding suggests that measurements of vertical thermal gradients in the upper part of the soil ( m), provide a good estimate of the total energy transferred by soil diffuse degassing at Solfatara. [32] The injection rate of the steam-gas mixture for the simulation (Figures 5 and 6) was chosen to reproduce, in correspondence with the symmetrical axis of the domain, one of the highest thermal gradients measured at Solfatara crater (i.e., sample n.84 rt = 562 C m 1, see Figure 2). The correlation between simulated and measured temper- Figure 5. Simulation results showing (a) steam, (b) liquid and (c) CO 2 fluxes. The initial composition of the carbon dioxide-steam mixture changes during its ascent: (a) most of the steam condenses in the soil and (b) produces a liquid phase that flows toward the bottom of the domain, (c) whereas carbon dioxide is discharged almost entirely at the soil surface. 9of17

10 Figure 6. Simulation results showing (a) total, (b) advective, and (c) conductive heat fluxes. At depths, where two-phase advection dominates heat transport, the distribution of temperatures is almost uniform. In Figure 6a, heat associated with the rising fluid is transferred by both advection and conduction toward the surface. In Figure 6b, some heat is transported laterally by advection by the liquid phase. In Figure 6c, near the surface, the heat is mainly transferred by conduction. atures at depth is very good (Figure 7) and shows that temperatures asymptotically reach T 95 C at a depth of 0.15 m. We reproduced the thermal gradients measured in other areas, for example at Stefanos crater and Comalito (Figure 7), by changing x H2O /x GAS ratio of the injected mixture (Table 2), rock properties (i.e., thermal conductivity computed in the subsection 4.4 and reported in Table 2), and the injection rate of the fluid ( kg s 1 for Stefanos crater and kg s 1 for Comalito). Here we find the shape of the vertical thermal profile strongly depends on the initial content of noncondensable gases. In the case of Stefanos crater, where the noncondensable gas content is smaller (i.e., x H2O /x GAS = 65.7) than in the fumarolic fluid of Solfatara (i.e., x H2O /x GAS = 5.48), the asymptote of the curve is at 99 C and the curve shows a clear change from advective to conductive regime. The conductive component of heat transfer can be observed by the linear change of temperature with depth, whereas the advective component is represented by constant temperature with depth. At Comalito, where fluid emissions are richer in noncondensable gases (i.e., x H2O /x GAS = 0.72), the thermal profile is nonlinear in the entire range of the considered depths and shows an asymptote at 79 C. In this case, the advective heat transfer occurs until the surface. However, even at Comalito, where the contribution of advection to total heat transport near the surface is higher relative to the Solfatara and Stefanos cases, 84% of the total heat is still transferred by conduction. This finding suggests that in the uppermost part of the soil, the heat released through the diffuse degassing process is mainly by conduction in all the modeled cases. Similar conclusions were suggested by the results of other simulations performed where the controlling parameters (i.e., depth of the injection, properties of the rocks) were varied. [33] Our finding contrasts with the results of a recent work in an area of steaming ground at Karapiti, New Zealand [Hochstein and Bromley, 2004]. Hochstein and Bromley [2004, p. 131] concluded that the total thermal flux at the surface of steaming ground consists of a convective and a conductive component, even in the ab- 10 of 17

11 Figure 7. Simulated and measured soil temperatures at Solfatara, Stefanos, and Comalito. sence of any visible steam discharge at the surface. At Karapiti the measurements were conducted using a new type of calorimeter which consists of a cylindrical vessel containing up to 1.5 L water and a very thin and flat stainless steel bottom. The heat flux measurements are made by placing the calorimeter over the hot soils and measuring the temperature increase of the water in the vessel. Heat fluxes between 30 and 2000 W m 2 were measured. At sites where heat fluxes were greater than 200 W m 2, Hochstein and Bromley [2004] found the ground was usually slightly moist at the surface and traces of liquid were visible on the underside of the calorimeter when it was lifted. This was interpreted as the contribution of diffuse steam which condenses at the cool (stainless steel) bottom of the calorimeter. A first observation is that the heat fluxes measured in our study, except in a few cases, were typically lower than 200 W m 2 (Table 2) and, consequently, no diffuse steam emission should occur according to the results of Hochstein and Bromley [2004]. However, in order to understand the origin of the moisture at the soil surface at Karapiti, we modeled a similar case. We selected high input rate of steam (0.046 kg s 1 ) in order to obtain at the surface, in the axial zone, a very high heat flux (1000 W m 2 ). Figure 8a shows the simulated liquid saturation of the soil (S l ) in the axial zone as a function of depth. A high concentration of liquid (condensate) forms at a depth of only 0.1 m, corresponding to the maximum production of condensates. It is worth noting that measured S l profiles at Karapiti [Hochstein and Bromley, 2004] are very similar to that shown in Figure 8a. Simulation results show a significant flux of liquid water from the depth at which condensation occurs toward the surface (Figure 8b), while only a small amount of steam is emitted (Figure 8c). In conclusion, the simulation suggests that the liquid on the underside of the calorimeter of the Karapiti measurements was the liquid condensed at a shallow depth and possibly transferring to the surface by capillarity and evaporating. This result suggests that even at the Karapiti area, the heat is released mainly by conduction in the upper part of the soil Heat Transfer in Diffuse Degassing Structures: Theoretical Considerations and Field Observations [34] Assuming conduction is the main mode of heat transfer in the uppermost portion of a soil affected by diffuse degassing, a simple heat balance between the source zone and the conductive zone gives K c * rt ffi Q Tj ; where the terms on the left represent the heat conducted in the upper part of the soil (K c is thermal conductivity of the soil, rt is average thermal gradient), while Q Tj is the total energy injected at temperature T j. The assumption expressed by the equation (2) is pivotal for the following applications where we assume that the thermal gradients measured in the upper part of the hot soils are directly proportional to the total heat transported by diffuse degassing. To validate this assumption, we first derive a theoretical relationship between the ratio of the thermal gradient to j GAS at the surface and the initial H 2 O/gas ratio (x H2O /x GAS ) Tj of the source of the hydrothermal or volcanic gas emissions, and then demonstrate that the data collected at different volcanic systems validate the theoretical relationship. [35] In the injection zone, the total heat flux (Q Tj ) is equal to the heat transported by the vapor phase which is approximately given by the heat content of the steam H H2O(Tj,g) at the injection temperature (T j ) times the steam mass flux (jh 2 O (Tj) ). Contribution of CO 2 is neglected, as verified in the numerical simulation. Expressing jh 2 O (Tj) as a function of j GAS (in mol m 2 s 1 ) and of the molar ratio (x H2O /x GAS ) Tj of the injected gas mixture, Q Tj is given by: ð2þ Q Tj ffi j GAS * ðx H2O =x GAS Þ Tj *H H2OðTj;gÞ ð3þ Now we examine the process of energy transfer in a onedimensional domain, i.e., in which the heat and mass flows are constrained to the z direction. Because of the process of continuous condensation, we can assume the presence of 11 of 17

12 Figure 8. (a) Liquid saturation (S l ) versus depth in the axial zone of the simulated domain. A condensate-rich level at 0.1 m depth is formed. (c) Upward movement of the vapor phase and (b) the downward movement of the liquid phase produced by the condensation of the steam in the uppermost part of the soil are evident. Only a small amount of steam is emitted from the surface (Figure 8c). liquid water at depth and compute the pressures of the gaseous species at any temperature T n of the soil. The partial pressure of H 2 O(P H2 O) is a function of the temperature (Steam Tables [Keenan et al., 1969]) and the pressure of the noncondensable gases (P GAS ), primarily CO 2. Assuming that the total pressure (P tot ) of the fluids 1 bar, which is consistent with the shallow depth of the investigated soil layer, this relationship is given as P GAS =P tot P H2 O ffi 1 P H2O. If we assume the j GAS remains constant with depth, the heat transported by the vapor phase (Q H2 O (Tn,g) ) at any temperature T n is given by Q H2O Tn;g ð Þ ¼ j GAS * ðx H2O =x GAS Þ Tn *H H2O ðtn;gþ ffi j GAS * ðp H2O =P GAS Þ Tn *H H2O ðtn;gþ ð4þ where the molar fraction ratio x H2O /x GAS is equal to P H2 O/ P GAS assuming fugacity coefficients equal to 1, and the heat content of the steam H H2 O (Tn,g) is known at any temperature (Steam Table [Keenan et al., 1969]). [36] Dividing equation (4) by equation (3) we obtain the relation Q H2O Tn;g h ð Þ =Q Tj ffi ðp H2O =P GAS h = ðx H2O =x GAS Þ Tn *H H2O ðtn;gþ i Þ Tj *H H2OðTj;gÞ [37] The percentage ratios Q H2 O (Tn,g) /Q Tj computed through equation (5) for temperatures between T j and T min (where T min is the minimum temperature measured in the vertical profiles corresponding to the shallower depth) are reported versus temperature in Figure 9. Figure 9 considers five different volcanic systems (Solfatara, i ð5þ Stefanos, Kaminakia, Vesuvio, and Comalito), which are characterized by different (x H2O /x GAS ) Tj ratios measured at the respective fumaroles. In general, the shape of the Q H2 O (Tn,g) /Q Tj versus T curve largely depends on the ratio (x H2O /x GAS ) Tj. For relatively high (x H2O /x GAS ) Tj ratios (i.e., in the cases of Stefanos, Kaminakia, and Solfatara) the contribution of the heat transported by the steam to the total energy quickly decreases with the temperature becoming negligible (Q H2 O (Tn,g) /Q Tj < 0.10) at T < 70 C. Below this temperature, the heat is transported almost exclusively by conduction and linear trends in the soil thermal profiles are expected. In fact, all measured thermal gradients were linear at the DDSs characterized by fumarolic fluids with x H2O / x GAS ratios greater than 5.4 (i.e., Solfatara, Kaminakia and Stefanos) at temperatures <70 C. At Comalito and Vesuvio NE Rim, characterized by fumarolic x H2O /x GAS molar ratios of and 0.503, respectively, our calculations suggest that significant amount of heat is transported by advection at relatively low temperatures (Figure 9). In agreement with these calculations, linear thermal profiles at NE Vesuvio Rim and at Comalito were measured only at temperatures below C. [38] Results of the two-dimensional simulations and the one-dimensional theoretical model reported above suggest that almost all of the energy transferred by advection in the deeper part of the soil is transferred primarily by conduction in the upper part of the soil, with heat transfer capacity depending on the initial content of noncondensable gases. Combining equations (2) and (3) in a logarithmic form, the following relation is obtained: logðx H2O =x GAS Þ Tj ¼ log K c =H H2O g ðþ þ log ð rt=jgas Þ ð6þ 12 of 17

13 Figure 9. Contribution of the heat transported advectively by the steam to the total energy in the range of measured temperature. In the systems with x H2O /x GAS ratios above 5.4 (Solfatara, Kaminakia, and Stefanos) the contribution of the heat transported by the steam to the total energy becomes negligible (Q H2 O (Tn,g) /Q Tj < 0.10) for T < 70 C. At Comalito and Vesuvio, characterized by lower fumarolic x H2O /x GAS ratios, a significant amount of heat is transported by advection process until a few centimeters from the surface. Equation (6) highlights a linear relation between the molar ratios (x H2O /x GAS ) Tj of the fluids before condensation and the ratios rt/j GAS with a proportional factor equal to K c /H H2O(g). Assuming that H H2O(g) does not significantly vary in the temperature range of interest (H H2O(g) ffi J mol 1 ), equation (6) describes a family of rectilinear curves with slope 1 and intercepts proportional to K c on a (x H2O /x GAS ) Tj versus rt/j GAS log diagram (Figure 10). Figure 10 demonstrates that the fumarolic x H2O /x GAS ratios are proportional to the average values of the measured rt/j GAS for 30 campaigns in 15 different DDSs. The measured data plot along the curve K c = 1 and are well correlated (R 2 = 0.954). The K c values, computed for each set of data using the measured values (equation (6)), range from 0.5 to 2.3 W m 1 C 1 (Table 2) with an average value of 1.14 ± 0.4 W m 1 C 1, which are plausible values for hot soils of hydrothermal areas [Brombach et al., 2001; Sorey and Colvard, 1994]. Moreover, thermal conductivities calculated based on data collected during different periods of the year in the same DDS (e.g., Solfatara and Stefanos), are higher in the wet season than the dry season. These results are expected as K c is a function of the soil water content. In order to compare the calculated thermal conductivities shown in Figure 10 with those calculated using an independent method, we applied the relation of Woodside and Messmer [1961] that allows direct derivation of the soil thermal conductivity. They suggest a geometric average law between the conductivities of the soil (K c ), and that of the solid (K s ), water (K w ) and air (K a ) fractions of the soil: K c ¼ K xs s *K xw w *Kxa a ð7þ where x s,x w, and x a are the volumetric content of solid, water and air, respectively. Of the three constituent phases, the solid phase has the highest conductivity. Therefore conductivity is increased as bulk density increases. Air is a poor conductor and in soil it reduces the value of total conductivity. Water content has a marked effect because when water replaces air it provides bridges between particles that greatly increase the conductivity of the soil. Assuming the thermal properties of soil constituents (K s =2.9Wm 1 C 1,K w =0.6Wm 1 C 1 ek a = W m 1 C 1 ) reported by de Vries [1963], varying the porosity n from 0.20 to 0.50, a suitable range for the studied soils (e.g., at Solfatara, n was measured to be 0.46 [Nunziata et al., 1999]), and considering that in temperate climates commonly x w > 0.15, we obtain thermal conductivities K c ranging from 0.4 to 2.1 W m 1 C 1 which practically overlap the range of the values obtained in the Figure 10 (from 0.5 to 2.3 W m 1 C 1 ) for the 30 campaigns. [39] An interesting further consideration can be drawn by the previous results. In previous studies [Chiodini et al., 2001a; Brombach et al., 2001] the soil thermal conductivity was assumed equal to 1 W m 1 C 1 following Clauser and Huenges [1995] or was chosen as appropriate values [Severne and Hochstein, 1994] to assess the surface heat transfer. In this work, the thermal conductivities of the soil, derived from the measured parameters (i.e., soil thermal gradients and diffuse CO 2 flux) and from the composition of the fumarolic fluids in the same area, are directly estimated by the adopted procedure. [40] The proposed method for the estimation of the soil thermal conductivity is affected by the uncertainties of the measured parameters, i.e., of the molar ratio (x H2O /x GAS ) Tj 13 of 17