In assessments of the safety of final repositories for nuclear waste situated in crystalline basement rock it is usually postulated that the transfer of radionuclides to the biosphere can take place only through transport by water. However, in order for such an assumption to be valid, it must be verified that any geogas that is present will not affect the transport - at least not to any significant degree. (The word geogas refers to the occurrence in the crystalline basement rock of substances which become gaseous at normal pressures and temperatures.)
Geogas in crystalline rock consists of species such as nitrogen, argon, helium, hydrogen, methane, carbon monoxide, carbon dioxide, hydrogen sulfide, and oxygen. The gas originates from the atmosphere, chemical reactions in the rock, the decay of radioactive elements in the rock, and degassing from the mantle of the earth. In most observed cases, geogas is dissolved in the groundwater. However, at elevated pressures and at low temperatures, methane may combine with water to form a solid phase commonly called methane-ice.
The transfer of geogas through the rock and to the surface takes place through flow in fractures. Firstly, dissolved geogas migrates due to the flow of the groundwater, and secondly, pockets of gas may form and eventually be released in the form of bursts. In the second case, the gas might act as a carrier for heavy elements through four different mechanisms: 1) formation of volatile compounds, 2) formation of surface active complexes, 3) flotation, and 4) formation of aerosols.
When a potential site for waste disposal is being evaluated, studies of geogas should form part of such a characterization programme. Favourable conditions for the formation of free gas may develop as a result of the heating of the rock by radioactive decay in the waste. It is also conceivable that methane-ice might form in the backfill of a repository in connection with a glaciation. The decomposition behaviour of such methane-ice appears to be largely unknown. Positive aspects may include the possibility of utilizing geogas flow for the non-destructive monitoring of a site after closure of the repository.