Comparison between different methodologies for

ARTICLE IN PRESS Applied Radiation and Isotopes 67 (2009) 178–185

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Comparison between different methodologies for detecting radon in soil along an active fault: The case of the Pernicana fault system, Mt. Etna (Italy) S. Giammanco a, G. Imme` b, G. Mangano b, D. Morelli b, M. Neri a, a b

Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Catania, Piazza Roma, 2, 95123 Catania, Italy ` degli Studi di Catania, via S.Sofia, 64, 95123 Catania, Italy Dipartimento di Fisica e Astronomia, Universita

a r t i c l e in f o

a b s t r a c t

Article history: Received 11 March 2008 Received in revised form 1 July 2008 Accepted 8 September 2008

Three different methodologies were used to measure Radon (222Rn) in soil, based on both passive and active detection system. The first technique consisted of solid-state nuclear track detectors (SSNTD), CR-39 type, and allowed integrated measurements. The second one consisted of a portable device for short time measurements. The last consisted of a continuous measurement device for extended monitoring, placed in selected sites. Soil 222Rn activity was measured together with soil Thoron (220Rn) and soil carbon dioxide (CO2) efflux, and it was compared with the content of radionuclides in the rocks. Two different soil–gas horizontal transects were investigated across the Pernicana fault system (NE flank of Mount Etna), from November 2006 to April 2007. The results obtained with the three methodologies are in a general agreement with each other and reflect the tectonic settings of the investigated study area. The lowest 222Rn values were recorded just on the fault plane, and relatively higher values were recorded a few tens of meters from the fault axis on both of its sides. This pattern could be explained as a dilution effect resulting from high rates of soil CO2 efflux. Time variations of 222 Rn activity were mostly linked to atmospheric influences, whereas no significant correlation with the volcanic activity was observed. In order to further investigate regional radon distributions, spot measurements were made to identify sites having high Rn emissions that could subsequently be monitored for temporal radon variations. SSNTD measurements allow for extended-duration monitoring of a relatively large number of sites, although with some loss of temporal resolution due to their long integration time. Continuous monitoring probes are optimal for detailed time monitoring, but because of their expense, they can best be used to complement the information acquired with SSNTD in a network of monitored sites. & 2008 Elsevier Ltd. All rights reserved.

Keywords: Soil radon and thoron activity Soil CO2 efflux Pernicana fault system Mount Etna Volcano-tectonic monitoring

1. Introduction There is a wealth of literature on soil–gas radon measurements in faulted areas (King et al., 1996; Mazur et al., 1999; Jonsson et al., 1999; Choubey et al., 1999; Durani, 1999; Vaupoticˇ, 2003) and specifically as regards the recognition of hidden fault systems (Burton et al., 2004). Radon is ubiquitous in the earth’s crust because Uranium and Thorium are present in almost all rock and soil types. Actually, the 238U isotope, upon decay, generates 222Rn, known as radon (half-life of about 3.8 days), whereas 232Th decaying generates 220Rn, known as thoron (half-life of only 55 s). Owing to their different half lives, 222Rn and 220Rn in soil–gas have different sources, i.e., relatively deep soil layers for 222Rn, shallow soil layers for 220Rn.

Corresponding author. Tel.: +39 095 7165858; fax: +39 095 435801.

E-mail address: [email protected] (M. Neri). 0969-8043/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2008.09.007

Recent studies (Atallah et al., 2001; Baubron et al., 2002; Ajari and Adepelumi, 2002; Burton et al., 2004) showed that soil 222Rn concentrations tend to increase in proximity to main fault planes. This behavior was explained as a change in the rock proprieties (i.e., increased soil permeability to gas; higher surface-to-volume ratio in the fracturing rock, that facilitates radon release from the solid matrix because, due to its short half-life, any radon created must be near the free surface of a rock in order to have any probability of escaping into the gas phase) along the faults. In several cases, anomalies of soil–gas 222Rn and other trace gases like helium and hydrogen were correlated with high degassing of carbon dioxide (CO2), thus substantiating a model of advective transport of radon through the soil column by a carrier gas whose flux is controlled by pressure gradients (e.g., Israel and Bjornsson, 1967; Shapiro et al., 1982; Toutain et al., 1992; Baubron et al., 2002; Beaubien et al., 2003). Among the soil gases in volcanic or geothermal environments, CO2 is normally the most important in terms of both abundance and efflux. For this reason, it is essential

ARTICLE IN PRESS S. Giammanco et al. / Applied Radiation and Isotopes 67 (2009) 178–185

to correlate 222Rn and CO2 measurements in soil–gas emissions from such environments. Recently, changes in soil 222Rn activity were observed contemporaneously with volcanic activity at Mount Etna (Alparone et al., 2005; Neri et al., 2006; Giammanco et al., 2007; Imme` et al., 2006a, b; Morelli et al., 2006; La Delfa et al., 2007) and at El Teide Volcano Complex Tenerife, Canary Islands (Soler et al., 2004). Most of the above cited studies were conducted using active detectors. However, other techniques were used to detect soil Radon, as well as solid-state passive detectors both for continuous and for time-integrated measurements. The aim of this work was, therefore, to test the use of different methodologies for measuring soil–gas 222Rn in the same sampling points along an active fault. The significance of our study is also based on the specific location designated for radon monitoring: the Pernicana fault system (PFS) is one of the most active in the world and it cuts a flank affected by spreading phenomena in the most active volcano in Europe (e.g., Neri et al., 2004; Acocella and Neri, 2005).

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fault system (PFS in Fig. 1) (Acocella and Neri, 2005 and references therein) and to the western margin by the N–S-striking transtensive dextral Ragalna fault system (RFS in Fig. 1) (Rust et al., 2005; Neri et al., 2007 and references therein).

2. Investigation area Mt. Etna is an active basaltic volcano located in eastern Sicily, at the front of the Apennine–Maghrebian Chain (Lentini, 1982; Lanzafame et al., 1997 and references therein; Fig. 1). The volcanic edifice is affected by lateral spreading on its eastern to south– western flanks (Borgia et al., 1992). The unstable sector is confined to the North by the E–W-striking sinistral transtensive Pernicana

Fig. 2. Timeline of the November–December 2006 South-East Crater eruptions, distinguishing different eruptive styles and periods of activity (simplified from Behncke et al., 2008). The four periods of soil Radon measurement using SSNTD (CR39 type) during 2006 are highlighted as horizontal alternated light- and darkgray bars in the top part of the plot.

Fig. 1. Map showing the left-lateral Pernicana fault system (PFS). Inset (a) shows the area of flank instability in the southern to eastern sectors of the volcano and the main faults confining three slide blocks; (1) unstable sliding area, (2) front of the anticline that delimits the unstable sector and its possible off-shore continuation, (3) etnean volcanic rocks, (4) undifferentiated sedimentary rocks, (5) faults, (6) direction of movement of the unstable blocks and (7) boundaries of the unstable sector, PFS ¼ Pernicana fault system, RFS ¼ Ragalna fault system (simplified from Neri and Acocella, 2006). Inset (b) illustrates the Digital Elevation Model of the NE part of Mt. Etna, showing the PFS (in black), from the NE Rift to the Ionian coastline. Other faults are in gray. SEC ¼ Southeast Crater. Inset (c) shows the details of the studied area, where the two profiles AB and CD are visible close to the Villaggio Turistico Mareneve. Inset (d) is a photo taken on October 29th, 2002 at the onset of the 2002–2003 eruption, when the PFS accelerated its eastward movement, damaging the road (gray star in inset (c)) close to the Villaggio Turistico Mareneve.

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The PFS develops eastward from the NE Rift (from 1850 m asl) to the coastline, over a length of 18 km (Neri et al., 2004). It is characterized by a near-vertical scarp reaching a maximum height of 70–80 m at altitude between 1000 and 1500 m asl. At lower elevation (from 700 to 800 m asl down to the Ionian Sea), the PFS is characterized by left-lateral faults with a dextral configuration. Shallow (o2–3 km) seismic activity (2oMo3.5; Azzaro et al., 1988; Acocella et al., 2003) accompanies the surface deformation along the central and western portions of the PFS, but aseismic creep characterises this fault as well, especially on its central and eastern portions (Obrizzo et al., 2001). The PFS is kinematically connected, with a feedback mechanism, to eruptions occurring on the NE Rift (Tibaldi and Groppelli, 2002; Acocella and Neri, 2003; Neri et al., 2004, 2005; Walter et al., 2005). In spite of this relationship, the PFS has shown continuous activity between 1947 and 2002 (Neri et al., 2004), a period when no eruptions occurred on the NE Rift, with major surface fracturing and seismic activity in 1984–1988 (Azzaro et al., 1988). In these cases, the PFS movements were interpreted as due to cyclicity in the spreading that affects the unstable eastern sector of Mt. Etna, and the cyclicity was thought to be triggered by volcanic activity (see also Allard et al., 2006). During the second half of 2006, Mount Etna erupted discontinuously from August 31 until December 14 (Behncke et al., 2008). The eruptive activity consisted of mild-to-strong Strombolian explosions at the South–East Crater (SEC in Fig. 1), accompanied by discontinuous lava effusions confined to the uppermost portion of the volcano. On March 29, 2007, eruptive activity resumed at SEC for several hours only, producing lava fountaining and two small lava flows. A comprehensive diagram of the eruptive activity during the period investigated is shown in Fig. 2.

3. Instruments and methods Two different profiles of sampling points, both of them orthogonal to the main fault plane, were investigated in the PFS area (Fig. 1). The first one was located at 1400 m asl (AB profile), the second one at 1370 m asl (CD profile). Each profile consisted of

ten measurement points. It must be noted that in the case of the CD profile the surface expression of the fault is not only a main single plane (as in the case of the AB profile), but is rather a fractured zone several tens of meters wide across the main fault plane. Measurements of 222Rn were performed by means of three different methodologies: passive, spot and continuous (Fig. 3). Passive measurements were performed using track detectors CR-39 type; spot measurements were made using an active portable device. Both of these methods were used in the two profiles. Continuous 222Rn measurements were made very close to some of the points of the AB profile (see Fig. 1c). Preliminary analysis of radionuclide concentrations amounts in rock samples were also undertaken in the survey area. Two rock samples were collected: one (N1 in Table 1) along the downthrown block of the fault near the AB profile at a distance of 10 m from the fault plane; the second sample (N2 in Table 1) was collected in the downthrown block of the fault along the CD profile, 10 m away from the fault plane. Moreover, soil CO2 effluxes were measured at the same sampling points using the accumulation chamber method (Farrar et al., 1995; Chiodini et al., 1998; Fig. 3d). The whole investigation covered the period from November 6th 2006 to April 6th 2007. 3.1. Solid-state nuclear track detectors (SSNTD) measurements The detectors used for passive measurement of 222Rn (SSNTD CR39 Type) have an active area of 25 25 mm2 and were placed inside a diffusion chamber that only allows entry of 222Rn. 222Rn Table 1 Radionuclides activity concentrations of two rock samples collected along the AB (N1) and CD (N2) profiles, along the downthrown side of the fault at a distance of 10 m from the main fault plane Sample code

Uranium (Bq kg–1)

Thorium (Bq kg–1)

N1 N2

116.3674.05 127.1574.30

101.6775.09 128.2976.12

Fig. 3. Sketch diagrams of the different methods used for measuring soil gases in the present work: (a) set-up of Barasol probe for real-continuous Radon measurements; (b) field setup of the RAD7 system for spot 222Rn and 220Rn measurements; (c) setup of the CR39 detectors; and (d) field set-up of the system used for measuring soil CO2 effluxes. The photos in the inset boxes show the appearance of the RAD7, CR39 and soil CO2 efflux measurement devices.


decays inside the diffusion chamber and the emitted alpha particles form ionization tracks in the detector medium that are subsequently ‘‘developed’’ using a chemical etchant. Along both profiles, the passive detectors were deployed inside a closed PVC tube (l ¼ 50 cm, f ¼ 6.3 cm; see Fig. 3c). In order to avoid saturation effects, the CR39 detectors were replaced about at two weeks intervals. Once removed from their sampling point after recovery from the field, the detectors were chemical etched with a 6.25 M NaOH solution at 98 1C for 1 h, in order to enlarge the alpha tracks. Detector reading was performed with a semiautomatic system that had been previously calibrated using detectors with known track densities. This system consists of an optical microscope equipped with a CCD camera connected to a PC. Video acquisition software captured and stored the images ‘‘fields of view’’ (FOV) from the microscope. The stored images were then analyzed by means of ImageJ 1.29 (Image Processing and Analysis in Java) freeware software, in which an appropriate routine was edited. For each detector, more than 200 FOVs were acquired and stored. The software routine opens and processes each FOV, discriminating the tracks according to their minor and major axis and their area, and it gives an output of both the exposure in Bqh/m3 and the 222Rn concentration in Bq/m3, which represents integration over the entire exposure period of the detector. Passive measurements were carried out during two periods: from November 6th, 2006 to December 21st, 2006, with four series of measurements, and from February 2nd, 2007 to April 6th, 2007, with three series of measurements. 3.2. Spot measurements of Radon Spot measurements of soil–gas Radon were performed by means of a commercial portable detector, (model RAD7, Durridge COMPANY Inc., Bedford, USA). The detection system consists of a solid-state ion-implanted planar silicon alpha detector placed in a 0.7 L accumulation chamber (Fig. 3b). The soil–gas is drawn through a 50 cm long soil probe using an internal pump and is then passed through two filters: the first one removes moisture and the second one allows only Radon to enter the detection system chamber by removing atmospheric particulate as well as Radon daughters. The system discriminates 222Rn and 220Rn through a spectroscopy and, hence, only the parent radionuclide (222Rn and 220Rn) are measured. Once in the detection chamber, 222 Rn and 220Rn emit alpha particles that are collected in the silicon detector by an electric field. Spot measurements in the study area were performed about one meter away from the CR39 sampling points (Fig. 3b,c). At each sampling point two consecutive measurements of both 222Rn and 220Rn activities were performed, each one with 5 min integration time. The activities are presented as arithmetic mean values. Spot measurements were performed on November 14th, 2006, December 13th, 2006, March 27th, 2007 and April 4th, 2007 along AB profile, whereas only one survey was done along CD profile, on November 14th, 2006.

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plane, adjacent to three of the sampling points of the AB profile. Each detector is buried at a depth of 1.8 m in a 2.5-m long PVC tube, closed with PVC stoppers on the top. The detection system consists of a cylindrical chamber (l ¼ 570 mm; f ¼ 60 mm) equipped with an implanted silicon junction for alpha particle detection. A microprocessor is used to integrate and store the Radon data, with an integration time of 20 min. The system also allows measurements of meteorological parameters, such as air temperature, barometric pressure and rainfall.

3.4. Soil CO2 measurements Diffuse CO2 effluxes were measured using the accumulation chamber method, which consists of measuring the rate of increase of the CO2 concentration inside a cylindrical chamber opened at its bottom placed on the ground surface. The chamber has an internal fan to achieve an efficient gas mixing and is connected with a portable nondispersive infrared (NDIR) spectrophotometer (PP Systems, UK, mod. EGM4). The change in concentration during the initial measurement is proportional to the efflux of CO2 (Tonani and Miele, 1991; Chiodini et al., 1998). Efflux values are expressed in g m2 d1. This is an absolute method that does not require corrections linked to the physical characteristics of the soil. We tested the method in the laboratory with a series of replicate measurements of known CO2 effluxes. The average error was about 75%, the reproducibility in the range 200–1600 g m2 d1 was about 5%. Soil CO2 effluxes were measured on November 14th, 2006, March 27th, 2007 and April 4th, 2007 along both profiles.

3.5. Uranium concentration in rock samples In order to analyze radionuclide amounts through gamma-ray spectrometry, two rock samples were collected from the study area. The rock samples were first dried at about 80 1C for 4 h in order to remove moisture, then crushed and homogenized to a 250 mm powder and finally dried again at about 80 1C for 24 h in order to eliminate any residual water. The crushed samples were then weighted and put into a 100 cm3 Marinelli beaker that provides a 5 mm sample thickness; hence, gamma self-absorption is negligible. Each sample was sealed for four weeks to reach secular equilibrium (ASTM, 1983, 1986) among the isotopes of the radioactive decay series. Radionuclide concentrations were determined using a high-purity germanium E&G Ortec detector with an efficiency of 30%. The gamma acquisition time was 28,800 s. In order to determine the full-energy peak efficiency in the energy range from 60 to 2000 keV, a calibration source with the same geometry as the samples was prepared by means of a mesh of known activity. Using these settings, we were able to measure the isotopic abundances of the 238U and 232Th decaying series.

3.3. Continuous radon measurements

4. Results

Continuous measurements of soil 222Rn were done using a permanent 222Rn monitoring system placed near the PFS at about 1500 m asl, on the South side of the AB profile (Fig. 1). The system (model Barasol, Algade, France), installed in December 2005 by the Catania Department of the Istituto Nazionale di Geofisica e Vulcanologia, is equipped with three probes connected to a common data-logger. The three probes are placed at 25 m intervals along a direction perpendicular to the main PFS fault

The radionuclide concentrations measured in the two rock samples from the study area, one for each of the two surveyed profiles, are reported in Table 1. Because the uranium and thorium series generate several gamma peaks, the apparent activities of the parent isotopes (238U and 232Th) are obtained by a weighted average of the individual values of activities of the gammaemitting daughters: (i) 234Th, 226Ra, 214Pb and 214Bi for uranium; (ii) 212Pb, 228Ac and 208Tl for 232Th. The two samples, within



Fig. 5. Radon concentration along the CD profile; (a and b) using SSNTD, CR-39 type; (c) using the Durridge RAD 7; (d) thoron concentrations via RAD7 system; and (e) soil CO2 efflux values measured along the CD profile.

Fig. 4. Radon concentration along the AB profile; (a, b) using the SSNTD, CR-39 type; (c) using the Durridge RAD 7; (d) Thoron concentrations via RAD7 system; (e) Radon concentrations via continuous measurements; and (f) soil CO2 efflux values measured along the AB profile.

statistical counting error, have comparable concentrations of parent radionuclides. Soil 222Rn and 220Rn activity, as well as CO2 efflux values obtained at the sample points are shown in Fig. 4 (AB Profile) and 5 (CD profile). Regardless of the period, the pattern of soil 222Rn values measured with the CR39 detectors in the two profiles is clearly similar: higher values were generally measured on the upthrown side of the fault. In detail, 222Rn activities on the upthrown side of the PFS were, on average, twice as high as those measured on the downthrown side along AB profile, and even six times higher on the upthrown side along the CD profile. The lowest soil 222Rn values were generally measured close to the fault. Looking at the time evolution of the data, the highest 222Rn values were observed during the period from 13 to 21 December 2006 in both profiles. Furthermore, in contrast with the general trend, two sampling points in AB profile (respectively, located at 40 and 80 m from the fault on its upthrown side) show

particularly low 222Rn values during the period from November 14th to December 13th, 2006 (Fig. 4a). The 222Rn data obtained with the spot measurements along the two profiles are in agreement with those from the CR39 detectors: higher values were generally recorded on the upthrown side of the fault and the lowest values occurred on the main fault plane (Figs. 4c and 5c). Values of 220Rn were higher on the downthrown side of the fault along the AB profile (Fig. 4d), whereas along CD profile 220Rn shows higher values just North of the fault plane and about 120 m South of it (Fig. 5d). No significant correlation was found between 222Rn and 220Rn data in the spot measurements during each of the surveys performed along the AB profile (correlation coefficients r ¼ 0.14, 0.28, 0.20 and 0.11, respectively, for the November 14th, 2006, December 13th, 2006, March 27th, 2007 and April 4th, 2007 surveys). Conversely, a significant, although not high, correlation was found between 222Rn and 220Rn data in the spot measurements during the November 14th, 2006 survey along the CD profile (r ¼ 0.58). Correlation between passive (CR39) and spot (RAD7) measurements of soil 222Rn shows a good agreement between the two methods in the November 14, 2006 survey (relative to the integrated data from November 6th to 14th, 2006) (r ¼ 0.82), in the March 27th, 2007 survey and in the April 6, 2007 surveys (both relative to the integrated data from March 27th to April 6th,


2007) (r ¼ 0.71 and 0.79, respectively). Conversely, in the December 13, 2006 survey a poor correlation (r ¼ 0.10) was found between the two methods, essentially due to excess 222Rn in some of the spot measurements with respect to the passive ones. However, it must be noted that heavy rainfall occurred just on that day, which caused a temporary decrease in the permeability of the shallowest soil layer that by reducing air exchange with the soil column, led to higher concentrations of 222Rn in the subsoils. The distribution of soil CO2 efflux values along AB and CD profiles is shown in Figs. 4f and 5e, respectively. Along AB profile, high CO2 emissions were always recorded on the fault plane and, at least in the case of the first survey (November 14th, 2006), also about 70 m South of it. Furthermore, in the same survey soil CO2 effluxes were in general higher than those measured in the following surveys. Along the CD profile, the pattern of CO2 emissions was more complex and variable in time (Fig. 5e). During the first survey, higher values were recorded in many points near the fault plane and particularly North of it. During the second survey, high CO2 effluxes were measured both about 90 m South of the fault plane and about 40–60 m North of it. Lastly, during the third survey the pattern was similar to that observed along AB profile, with high effluxes on both the fault plane and about 80 m South of it. Since continuous 222Rn data, recorded by the three barasol sensors, showed a wide range of variation in time, we calculated the average values of 222Rn measured with each of the three sensors during the analyzed periods, except the period from February 16th to March 6th, 2007, when no data were recorded due to power failure of the sensors. The results (Fig. 4e) show

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slight differences with respect to the other methods. Also in this case, the lowest values were recorded close to the main fault plane, 222Rn activity increasing progressively on moving towards the South from the fault.

5. Discussion There are several conditions that affect soil 222Rn activity as measured in this study: (1) the concentration of parent radionuclides present in the shallow subsoil; (2) the surface to volume ratio of the soil and subsoil clasts (a higher surface area to volume ratio will lead to higher efficiencies of escape of 222Rn from the rock matrix); (3) both the average bulk permeability of the subsurface stratigraphy as well as the structure of that permeability; (4) the response of the soil column to moisture content (e.g., swelling soils/clays); and (5) advection driven by gas phase transport from depth. Determining which of these conditions or phenomena are responsible for the differences seen between the upthrown and downthrown sides of the fault, without benefit of detailed stratigraphic data, is clearly impossible. However, we may reasonably suggest that differences in subsurface permeability, due to soil accumulation or ponding of lava flows, on the downthrown side of the fault could contribute to the lower average 222Rn activities. Likewise, differences in the advective gas transport in the upthrown and downthrown sides of the fault may be a contributing factor to the observed differences. The analysis of the gamma-ray amount data shows that the two rock samples collected in the two different profiles have roughly the same level of radionuclides. Assuming this as a

Fig. 6. Model of gas releasing in the studied area, across the PFS. The lowest 222Rn activity was recorded exactly on the main fault plane, while relatively higher 222Rn values were sampled a few tens of meters on both sides of it, as a consequence of the dilution effect played by the soil CO2 efflux. The higher soil 222Rn activity was detected on the upthrown side of the PFS, while higher 220Rn values were collected on the downthrown side, due to the presence of more intense shallow fracturing which affect the mobile block. Here, a secondary peak of 220Rn and CO2 was also recorded, possibly due to the presence of a hidden conjugate fault. On moving further away from the main fault plane, soil–gas activity decreases and becomes more stable. Draft not to scale.



general feature of the rocks in the study area, we would expect that if the soil permeability was constant at all of the sampling sites, both 222Rn and 220Rn activity in the soil should not show significant spatial variations along the profiles surveyed. However, the results obtained show a different picture, with soil 222Rn and 220 Rn values varying up to almost one order of magnitude. These variations can be reasonably ascribed to differences in local soil permeability and/or to different mechanisms of advective soil–gas transport to the surface. The spatial differences sometimes observed between the SSNTD measurements (CR39) and the spot measurements (RAD7), particularly along CD profile, are likely due to a large daily variability of 222Rn activity. This is evident from the continuous measurements thanks to the better time resolution of the Barasol sensors (see error bars in Fig. 4e). Such variability is smoothed in the time-integrated SSNTD method (about two weeks of integration), but can significantly affect the results of the spot measurements (only 15 min of integration). Two prominent Radon peaks straddling the fault zone, with low values just on the main fault plane, are clearly observed along both of the profiles and particularly along AB profile (Figs. 4 and 5). A similar soil radon pattern was described by King et al. (1996) in some faults of California, and it was explained in terms of low permeability of the fault-gauge materials. In that case, soil CO2 concentrations were found spatially well correlated with radon values, thus suggesting outgassing of radon carried by CO2. Conversely, in the case of the PFS, an inverse correlation between radon values and soil CO2 effluxes on the main fault plane is evident. This inverse correlation would, instead, suggest a greater ground fracturing that allows dilution of 222Rn, because the CO2 flux is high enough to overwhelm the source of 222Rn (Giammanco et al., 2007). Furthermore, in the PSF case, ground fracturing due to tectonic activity (Acocella and Neri, 2005) adds up to the already high soil permeability (106–107 cm2) of the Etnean lava flows (Aureli, 1973; Ferrara, 1975). As the CO2 efflux decreases on moving away from the fault plane, the dilution of radon is no longer effective and so the radon activity increases to its maximum. On moving further away, both radon activity and CO2 efflux progressively decreases and becomes more stable, as expected due to the decrease in soil permeability relative to the main fault plane. Higher 222Rn values generally measured on the upthrown (northern) side of the fault could be due to the peculiar mechanical behavior of the PFS. The upthrown side of the fault is actually relatively ‘‘stable’’, while the downthrown side is ‘‘mobile’’ and hence much more fractured (Fig. 6). The mobile side is affected by slow motion toward ESE, part of the block being involved in the spreading process of the East flank of Mt. Etna (Acocella and Neri, 2005, and reference therein). Such a peculiar structural context is also compatible with the pattern of 220Rn values measured along both profiles. Higher 220Rn values generally measured on the downthrown side of the fault would indicate an overall higher permeability in the shallow soil (i.e., down to a few meters depth), possibly due to higher shallow fracturing of the ground (Fig. 6). In particular, the concurrent anomalies of 222Rn and 220Rn activities and of CO2 efflux several tens of meters South of the main fault plane along CD profile would indicate deeper fracturing probably related to a hidden conjugate fault. A similar deduction would apply to the anomalies in 220Rn and soil CO2 efflux measured along AB profile about 70 m south of the main fault plane. Also 222Rn mean values from the continuous monitoring sensors along the AB profile would indicate an anomaly at depth. Finally, no evident correlations were found between the temporal evolution of the collected soil–gas data and the volcanic activity of Mount Etna. It must be noted that the eruptive activity

during the investigated period was confined to the summit area and it involved only the central conduit of the volcano. Furthermore, no acceleration of the spreading affecting the eastern flank of Etna was recorded (A. Bonforte, personal communication).

6. Conclusions Many techniques are, at present, available for determining radon concentration. Accordingly, the choice among the different possibilities can be guided by the particular interest in radon measurements, whether in time-dependent or in space-dependent variations of the concentrations. In particular, spot measurements (RAD7) of soil–gas 222Rn are useful for the quick recognition of high emission sites to be later monitored for 222Rn variations in time. SSNTD (CR39 type) allow for the temporal monitoring of a relatively large number of sites, but cannot distinguish short-term changes due to their long integration times. Continuous monitoring probes are optimal for defining detailed changes in soil–gas radon activities, but are expensive and can thus be used to complete the information acquired with SSNTD in a network of monitored sites. In any case, soil 222Rn activity can be best interpreted if associated with soil 220Rn and soil CO2 efflux data, as well as with the content of radionuclides in the rocks. Concerning the investigated area, some conclusions can be offered: (1) Along the PFS main fault plane, advective transport of deep gases (CO2 and 222Rn) occurs due to the high ground fracturing and permeability. Near the surface, dilution of 222Rn by CO2 prevails, thus producing very low radon values; (2) Peaks of 222Rn activity occur on both sides of the main fault plane, where CO2 advection is high enough to bring 222Rn concentrations that are at equilibrium with the parent radionuclides in the underlying formation, to the ground surface, but not so high as to dilute that concentration with excess CO2; (3) Higher soil 222Rn values were detected on the upthrown side of the PFS than on its downthrown side. This could be due to the greater structural instability of the downthrown side of the fault, and hence to its greater fracturing, which would result in a higher degree of radon dispersion and/or air dilution; (4) The concurrent anomalies of CO2 efflux and 222Rn–220Rn activities identified a hidden conjugate fault in the mobile block; (5) Time variations of 222Rn activity were mostly linked to atmospheric influences and can be best studied with realcontinuous monitoring devices. No significant correlation with the volcanic activity was inferred. These considerations could be applied also to other faults of Mt. Etna with the same characteristics as those of the PFS (i.e., faults characterized by prevalent transtensive movements), and possibly also to other faults in basaltic volcanoes affected by flank instability.

Acknowledgments Work funded by Istututo Nazionale di Geofisica e Vulcanologia and Dipartimento Protezione Civile, Italy. We thank Boris Behncke, Anna Leonardi, Melanie Le Sage and Emilia Neri for their help


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