Quantifying 12/13CH4 migration and fate following sub-surface release to an agricultural soil

Following gas generation in a Geological Disposal Facility (GDF), 14C-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 12/13CH4 (as a surrogate for 14CH4) in a typical agricultural soil in the UK. Rates of CH4 oxidation were generally low in the field and undisturbed soil columns, though a re-packed column of homogenised topsoil oxidised ambient atmospheric CH4 20× faster than an undisturbed soil column. In contrast to low observed rates of CH4 oxidation, the effective diffusion of CH4 through the soil was rapid. Isotopically labelled CH4 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 CH4 diffusion through the soil was increased by the presence of ryegrass roots which increased soil porosity and decreased water content. δ13C values for laboratory column soils after labelled CH4 injection experiments showed no sign of residual 13C, despite the extremely high δ13C values of the injected 12/13CH4. If laboratory observations are confirmed by measurements in field samples it can be concluded that the majority of 14CH4 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.