Quantifying super(12/13)CH sub(4) migration and fate following sub-surface release to an agricultural soil

Following gas generation in a Geological Disposal Facility (GDF), super(14)C-containing gases could migrate through the geosphere, eventually diffusing into soils at the Earth's surface. This paper reports summary results from laboratory and field experiments to obtain information on the probable rates of a) diffusive transport and b) oxidation of super(12/13)CH sub(4) (as a surrogate for super(14)CH sub(4)) in a typical agricultural soil in the UK. Rates of CH sub(4) oxidation were generally low in the field and undisturbed soil columns, though a re-packed column of homogenised topsoil oxidised ambient atmospheric CH sub(4) 20 faster than an undisturbed soil column. In contrast to low observed rates of CH sub(4) oxidation, the effective diffusion of CH sub(4) through the soil was rapid. Isotopically labelled CH sub(4) injected at a depth of 45 cm in the field diffused to the surface and exited the soil over a time period ranging from 8 to 24 h. The rate of CH sub(4) diffusion through the soil was increased by the presence of ryegrass roots which increased soil porosity and decreased water content. delta super(13)C values for laboratory column soils after labelled CH sub(4) injection experiments showed no sign of residual super(13)C, despite the extremely high delta super(13)C values of the injected super(12/13)CH sub(4). If laboratory observations are confirmed by measurements in field samples it can be concluded that the majority of super(14)CH sub(4) from a GDF which enters a soil with low methanotrophic activity will be lost to the free atmosphere after diffusing rapidly through the soil column.