Noble gas diffusion in tuff: Effects of temperature and pressure investigated with a modified time-lag method

Noble gas transport through geologic media has important applications in the characterization of underground nuclear explosions. However, transport of noble gases from the source to the surface is a complex process relying on the advective and diffusive processes in a fractured porous medium to bring detectable levels to the surface. To better understand the diffusion process in relevant porous lithofacies, laboratory experiments are necessary to characterize the diffusion of noble gas from rock fractures to the surrounding rock matrix and through porous lithofacies. In this study, a time-lag method using a chromatographic column based on Fick's law was developed and used to measure krypton and xenon diffusion through crushed (fragmented) tuff in the presence of a background gas (N₂) under different temperatures and pressures. The results show that the noble gas diffusivity in tuff material increases with temperature and decreases with pressure. The noble gases with lower elemental mass and radius demonstrated greater diffusivity. These findings indicate that predictions of noble gas transport in the post-explosion subsurface geologic media should account for the effects of pressure and temperature on noble gas diffusivity.