GNGTS 2015 - Atti del 34° Convegno Nazionale

GNGTS 2015 S essione 1.3 159 maxima from yellow to red colors) shows large variability at small spatial scale. Zones with high values are evidenced not only in the SE sector near Bocca Grande and Bocca Nuova fumarolic vents (BG and BN), but also in the central and western investigated sector. Short- scale variations are commonly associated to micro-fracturing systems of the soil that occur even in apparently homogeneous ground. Also, the spatial variation of the flux is enhanced by the contiguous presence of hydrothermally altered and sealed terrain, low-permeability or less fractured zones (Granieri et al., 2010). But short-scale variations can also depend on fluid interacting with very shallow soil levels, as we will discuss further. The total soil flux estimated for this first H 2 S survey sums up to 0.4 tonnes/day from an area of 0.2 km 2 . In-soil gas assessment. Soil gas were sampled and T were measured at 0.5 m depth over the same 50 point grid of soil flux surveys. Furthermore, we sampled soil gas at different depths (���� ���� ���� � 0.3, 0.5, 0.8, 1 m� �� ����� ����� ������ ������� ) at seven sites chosen amongst the previous 50, in order to identify processes governing gas reactions at very shallow levels. ��� ��� Dry gas concentrations were measured with Agilent 490 Micro GC Analyzer. Data acquired at 0.5mdepth over the fixed grid, point out that CO 2 is the predominant species, with percentages spanning 73 to 99 vol. % depending on air contamination, while H 2 , CH 4 and H 2 S soil concentrations often are even higher than in H-T fumaroles (up to the 50 % more). H 2 spans 2 to 2900 ppm, and it shows a clear positive correlation with soil temperature. H 2 S maxima and minima are anticorrelated with those of the other gases. The latter species (CO 2 , CH 4 , H 2 , He) do not show a perfect match in maxima and minima distribution (Fig. 2): as uprising gases, a mixture of magmatic fluids flashing hydrothermal liquid to vapour (Caliro et al. , ������ �������� ���� ������������� ������ 2007), interact with air-saturated waters in shallow low T (< 100 C) levels, and diffuse through rock volumes of different permeability, explaining the differences in the observed soil gas concentrations. Nevertheless, also other phenomena need to be involved in generating short-scale variations in soil gases: in fact, H 2 , CH 4 and H 2 S are enriched in some samples compared to fumarolic composition. We, therefore, sampled soil gases at different depths in 7 of the previous 50 points, in order to investigate very shallow phenomena governing gas reactions. Figs. 3a and 3b clearly show Fig. 1 – a) Hillshade DEM of Campi Flegrei caldera and location of Solfatara crater (mod. after Caliro et al. 2007). b) H 2 S soil flux map at Solfatara of this work.