GNGTS 2015 - Atti del 34° Convegno Nazionale

158 GNGTS 2015 S essione 1.3 Feasibility of high-frequency micro-gaschromatography soil gas monitoring revealed at la Solfatara (Campi Flegrei, Italy) S. Sicola 1 , M.L. Carapezza 2 , M. Ranaldi 1 , F. Sortino 3 , L. Tarchini 1 1 Dipartimento di Scienze, Università Roma Tre, Roma, Italy 2 Istituto Nazionale di Geofisica e Vulcanologia, Sez. di Roma, Italy 3 Istituto Nazionale di Geofisica e Vulcanologia, Sez. di Palermo, Italy Solfatara crater is a tuff cone located in the central part of Campi Flegrei caldera (Campania, Southern Italy) (Fig. 1). The most recent unrests (1970-1972; 1982-1984) were centered in the caldera inner part, exactly below Solfatara zone (Barberi et al., 1984; 1989; Orsi et al. , 1996). After the last bradyseismic event (1982-1984) upwards ground movement slightly decreased, but since 2000 a new increase was recorded, likely linked with new magmatic fluids input into the hydrothermal system, which caused both fluid-pressure increment in the system and chemical-physical macroscopic changes in fumarolic activity (Chiodini et al. , 2008, 2010; 2012). Moreover, Orsi et al. (2004) include Solfatara crater in the most prone area for new vent opening. In our work we studied soil gas reactions in near-surface soil, aiming at assessing the feasibility of a novel geochemical monitoring methodology. We performed two CO 2 and H 2 S soil diffuse degassing surveys with the accumulation chamber and soil concentration measurements of CO 2 , H 2 , H 2 S, CH 4 , O 2 , N 2 , He at 0.5 m depth over a target area of 50 points. Soil gas samples were also collected at different depths (0.3, 0.5, 0.8, 1 m) at 7 points out of the previous 50. The presented study permitted to highlight the main gas reactions taking place in near-surface soil, and showed up the feasibility of a new monitoring technique, high-frequency micro-gaschromatography, which could be settled in more easily accessible sites and under less aggressive environmental conditions than those found close to Solfatara fumaroles. Diffuse soil degassing assessment. During February and April 2014, soil flux surveys were performed at Solfatara with an accumulation chamber equipped with a Li-820 infrared detector (0-2 vol. %) for CO 2 and with TOX-05 detector for H 2 S (0-20 ppm) (Carapezza et al. , 2011). Measurements were acquired over a fixed grid of 50 points 5 m spaced. Both maps were processed using ordinary kriging in ArcGis. CO 2 flux survey. The decadal monitoring of CO 2 , in active and quiescent volcanoes, and at Campi Flegrei as well, plays an important role in volcanic surveillance. Besides fumarolic degassing, widespread gas diffusion from soil is affected by the presence of faults and fracture systems. CO 2 anomalies-trend therefore outlines the presence of faults and fractures, which represent the most prone way for magmatic/hydrothermal fluids to reach the surface (Granieri et al. , 2010). Soil CO 2 flux map shows that the main faults within Solfatara crater have a NW- SE direction and an associated NE-SW trend, and that the principal CO 2 anomalies are located in the inner area and in the zone of high-temperature fumaroles (e.g. Bocca Nuova T 160 C) in agreement with those indicated by Todesco et al. (2003), Chiodini et al. (2010) and Granieri et al. (2010). We estimated the total diffuse CO 2 soil flux rate to 101.2 tonnes/day. A value very similar to the one estimated by Tassi et al. (2013), 79 tons/day, during the nearest in time published survey. H 2 S flux survey. According toGiggenbach (1980), H 2 S fugacity in hydrothermal environments is controlled by pyrite coexisting with an unspecified aluminium-silicate. H 2 S dominates in low- temperature fumaroles and solfataras, where discharges arise from deep hydrothermal systems (Giggenbach, 1980). Since decades, fumarolic gas emissions were sampled and analyzed in order to monitor the hydrothermal system activity, and H 2 S concentration in Solfatara fumaroles spanned 0.5 to 1.5 vol.% (Caliro, et al. , 2007; Chiodini et al. , 2008; Tassi et al. , 2013). In this work we surveyed the H 2 S diffuse soil flux for the first time at Solfatara, using the same technique for CO 2 (Fig. 1). The H 2 S flux map (minima from green to yellow colors;