Environmental Geology (1996) 27: 226-232 9 Springer-Verlag 1996 G. Etiope 9 S. Lombardi Laboratory simulation of geogas microbubble flow Received: 11 May 1995 / Accepted: 10 July 1995 Abstract Preliminary laboratory tests provided first data on the behavior of gas microbubbles through porous me- dia in the framework of the geogas theory. Under experi- mented conditions with laboratory equipment arranged for pressure controlled gas-tracer injection and sampling, gas microbubbles moved up to ten times faster than single- phase flow in dry media under the same injection pressure. Microbubbles were determined to be very sensitive to changes in injection pressure and their terminal velocity seems to be described with good approximation by the Stokes formula. The capability of microbubbles to pick up and transport upward for short distances solid ultra-small particles (metallic and radionuclide compounds) has been proved. Results are consistent with a time-dependent pro- cess linked to the transport properties of microbubbles (e.g., flotation), such as that reported by some authors. Key words Geogas 9 Microbubbles - Radionuclide transport- Gas advection Introduction The mechanisms of gas-phase migration in the ground include diffusion, due to concentration gradients, and advection (or mass transport), due to pressure gradients. The relative importance of the two transport mechanisms in geological environments was discussed by Lerman (1979), Tanner (1980), Wilkening (1980), with references to other papers in this field. A third way for gas to move through geologic media is by groundwater transport. For a long time most authors considered diffusion and ground- G. Etiope ([~) Research Doctorate c/o Earth SciencesDepartment,"La Sapienza" Rome University, Rome, Italy S. Lombardi Earth SciencesDepartment,"La Sapienza"Rome University, P.le A. Moro, 5 00185 Rome(I), Italy water transport as the main mechanisms for gas migration in the geosphere. Nevertheless, since Gingrich and Fisher (1976) and Mogro-Campero and Fleischer (1977) reported some evidence of long-distance (> 100 m) Rn transport in the ground and then King (1978) reported surface Rn increases in connection with earthquakes, the impor- tance of diffusional and groundwater transport has been reappraised. In the 1980s, Kristiansson and Malmqvist (1982) proposed a new hypothesis of radon transport: they considered that radon movement may be linked to the existence of a naturally occurring microflux of gas (geogas), which is mainly enhanced in faults in the crust. Such a geogas flow is advective and may take form of "micro- bubbles" when carrier gas crosses groundwater bodies. The model also gives a straightforward explanation for the occurrence of anomalies of endogenetic gas at the surface. After an extensive literature survey, it was evident that the geogas idea fitted much experimental (laboratory and field) evidence; thus, several hypotheses on gas migration in the geosphere could be grouped into a single unified approach termed the "geogas theory." This theory includes the following features: (1) Significant Earth outgassing processes are accomplished by the ascent of enhanced microflow of endogenetic gases through faults and frac- tures in the crust (Dikun and others 1975; Gold and Soter 1985; Kropotkin 1985; Torgersen and O'Donnell 1991) not only in active areas but also within sedimentary basins, shields, and forelands (Malmqvist and Kristiansson 1984; Hermansson and others 1991a, b). (2) This microflow includes advective movement of a mixture of naturally occurring gases (geogas), formed by carrier gases (e.g., CO 2, N2) which transport rare gases (e.g., He, Rn) (Malmqvist and Kristiansson 1984; Durrance and Gregory 1990; Etiope and Lombardi 1995). (3) When geogas microflow crosses groundwater, or when groundwater becomes oversatu- rated, a microbubble stream can form; fault-linked bubble flows can take place in various geological environments (MacElvain 1969; Gold 1979; Malmqvist and Kristiansson 1985; Sugisaki 1987; and references therein); the bubble movement through porous media has been theoretically and experimentally recognized as a fast gas migration