Introduction

Natural degassing phenomena can be studied as "natural analogues" in the frame of geological storage and sequestration of anthropogenic CO2 emissions, especially when the risk of possible leakage at surface is taken into account with potential consequences in the biosphere.

Active and quiescent volcanic areas release high amounts of CO2 to the atmosphere (Chiodini et al., 1998) from both active craters, as plumes and fumaroles, and along the flanks of volcanic edifices as diffusive soil emanations (Allard et al., 1991a; Baubron et al., 1990).

The two basic processes which are recognised to drive soil gas movement through rocks and sediments are diffusion and advection.

If transport through a stationary medium take place by diffusion, the steady state diffusive flux Od is proportional to the concentration gradient, dC/dA, as expressed by Fick's first law:

where v and D represent soil porosity (i.e., the fraction of pore volume to total soil volume) and the diffusion coefficient, respectively, and the minus sign indicates that gas moves from points of high concentration towards points of low concentration (or partial pressure). On the contrary, advection involves the movement of matter due to the action of a force, i.e., a pressure gradient dP/dA.. Advective flow Oa is described by the well-known Darcy's law:

where k is the specific permeability and ^ is the viscosity of the fluid.

Although equation 2 was experimentally derived for the steady flow of liquids in porous media, it has also been extensively used to describe the advective flow of compressible fluids in porous media (Gurrieri and Valenza, 1988). For the sake of correctness, the steady advective flow of compressible fluids is described by comparatively complex, well-known equations (e.g. Scheidegger, 1974). Because of mathematical complexities many workers have decided to study the steady flow of gases assuming that they are incompressible. Although this assumption might seem unreasonable, it is justified when the pressure gradient is comparatively small.

Exhalation is the process that transfers gas from the soil to the atmosphere via the already-described mechanisms of diffusion and advection. Diffusive gas exhalation takes place due to the different

concentrations of that gas in the soil and atmosphere. Advective exhalation exists when the gas in the soil has a higher total pressure than that in the atmospheric (eg. barometric pumping). Exhalation can be measured in terms of flux from the soil, i.e. the quantity of gas per unit area per unit time ( Omax = Kg m-2s-1; ®volume = m3m-2s-1). Generally, mean values for CO2 exhalation are 3.7 x 10-7 m3m-2s-1 (de Jong and Schappert, 1972) or 0.4 - 4 x 10-7 m3m-2s-1 (Kanemasu et al., 1974).

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