Diffuse degassing

Investigations aimed to the estimation of gas fluxes, especially CO2, released to the atmosphere through diffuse emission from the soil are of fundamental importance for the evaluation of the gas budget in volcanic and hydrothermal systems. At Campi Flegrei (southern Italy), for example, diffuse degassing represents the main gas release and significantly contributes to the heat transfer from the hydrothermal system (e.g. Chiodini et al. 2005, 2012).

Indirect methods for the determination of soil gas fluxes are based on the analysis of gas concentrations at different depths in the soil. However, this approach assumes steady state diffusive fluxes and a correct estimation of soil porosity. Hence, direct methods, generally consisting of dynamic and static procedures, are to be preferred. The accumulation chamber (AC) method (e.g. Sorey et al. 1998; Chiodini et al. 1996, 1998, 2001; Gerlach et al. 2001) is a dynamic procedure commonly used to measure fCO2. The AC apparatus consists of: 1) a metal cylindrical vase (the chamber) placed on the ground, 2) a CO2 detector, mostly consisting of a Infra-Red (IR) spectrophotometer, 3) an analog-digital (AD) converter, and 4) a palmtop computer. A low-flux pump (in the order of 20 mL s-1) continuously transfers the soil gas from the chamber to the IR for the continuous measurement of the CO2 concentrations. To minimize the disturbance effects due to changes of barometric conditions, the soil gas continuously circulated from the chamber to the detector.

The CO2 flux (fCO2) values are computed on the basis of the measured CO2 concentration inside the chamber over time (dCCO2/dt), according to the following equation:

 fCO2 = cf × dCCO2/dt.    (1)            

The proportionality factor (cf) between dCCO2/dt and the fCO2 is theoretically (e.g., Chiodini et al. 1998) proportional to height of the chamber, but has to be determined for any specific measuring apparatus by laboratory tests. The laboratory tests consist in repeated measurements of different known gas fluxes (in a proper range of values), that allow to compute the cf of a specific apparatus as the slope of the best-fit line to the imposed CO2 flux versus measured dCCO2/dt.

A different approach (Lewicki et al. 2005) has been used to calculated fCO2 (g m−2 day−1) considering the theoretical relation:

 fCO2 = k × V/A × T0/T × P/P0 × dCCO2/dt.    (2)

 where k is a constant (1,558,656 g s m−3 day−1), T and T0 are measured and standard temperature (K), respectively, P and P0 are measured and standard pressure (kPa), respectively,V is the system volume (m3), A is the accumulation chamber footprint area (m2), and dCCO2/dt is the initial rate of change of CCO2 in the chamber after the chamber is placed on the soil surface (vol % s−1).

The cf determined theoretically using Eq. (2) (cf = k × V/A × T0/T × P/P0) resulted higher for specific measuring apparatus when compared to that determined by experimental laboratory test (Lewicki et al. 2005), suggesting that real measurement deviates from the theoretical one.

Other gas compounds (e.g. CH4, Hg0) have been successfully measured using this method (Etiope 1997; Cardellini et al. 2003a; Bagnato et al. 2014).

Alternatively, diffuse fluxes from the soil of hydrothermal gas, including fCH4, fC6H6, Hg0, can be measured using a static approach, the so-called closed-chamber (CC) method (Hutchinson and Mosier 1981; Rolston 1986; Livingstone and Hutchinson 1995), which was developed in agricultural sciences to determine soil respiration (e.g. Parkinson 1981; Raich et al. 1990; Smith et al. 1995; Dueñas et al. 1996; Tsuyuzaki et al. 2001; Huttunen et al. 2003; Hirota et al. 2004), whereas it has been less frequently used in geothermal and volcanic environments (Klusman and LeRoy 1996, Etiope 1999; Klusman et al. 2000; Castaldi and Tedesco 2005; D'Alessandro et al. 2009; Tassi et al. 2011, 2013).

The CC equipment consists of a cylindrical chamber equipped with a three-way valve on its top. Once a chamber was positioned on the ground, an aliquot (5 to 10 cc) of gas is collected from the chamber at fixed time intervals using a syringe, and transferred into sampling vials equipped with a rubber septum for subsequent analyses. The variation in time of the CH4 and C6H6 concentrations in the chamber is proportional to fCH4 and fC6H6, respectively, as described by the following equation:

 f(X) = dCx/dt × V/A     (3)

where dCx/dt is the rate of concentration change of the X gas compound within the chamber positioned on the ground, whereas V and A are the volume and the basal area of the chamber, respectively. The dCx/dt value is calculated from the linear regression of the concentrations of the X compound in samples collected from the chamber starting from zero time. 

Gas fluxes from soil can be effectively shown using contour maps constructed by using interpolation algorithms, generally kriging (e.g. Bergfeld et al. 2001; Chiodini et al. 2001; Rogie et al. 2001), which provide the estimate of a variable without specific regard to the resulting spatial statistics of the all estimates taken together (Deutsch and Journel 1998). The kriging interpolation is considered to smooth out the extreme extrapolated values, with small values being overestimated, whereas large values, i.e. those that define degassing structures, are underestimated.

The total soil gas emission can be calculated either by multiplying the arithmetic mean value of the gas fluxes by the investigated areas, or by applying volume or by a Graphical Statistical Approach (GSA; Chiodini et al. 1998). Using the kriging interpolation, the estimation of local accuracy is incomplete, except if a Gaussian model for errors is assumed, the GSA approach allows the definition of a confidence interval for the estimation, but it does not take into account the spatial correlation of the data, thus causing an overestimation of the uncertainty.

For mapping purposes and to estimate the total CO2 release from an investigated area is, currently, largely used an approach based on stochastic simulation algorithms, and in particular on Sequential Gaussian Simulation (SGS, Deutsch and Journel 1998; Cardellini et al. 2003b). SGS allows generate a set of equi-probable representations of the spatial distribution of the gas flux values, able to reproduce reasonably their global statistic and spatial features. The uncertainty is thus related to the differences among many simulated maps (Goovaerts 2001). We suggest that the sequential Gaussian simulation method yields the most realistic representation of the spatial distribution of CO2 fluxes and allows to define a reliable uncertainty of the estimation of the total CO2 release from an investigated area.   



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