Assessing the influence of matrix diffusion with finite block size on particle transport in fractured media

Modeling particle transport within subsurface fractures is critical to national security issues, including nuclear waste disposal and extraction of hydrothermal energy. The shape of the breakthrough curve produced in a fractured media is influenced by mass transfer between the fracture network and the surrounding rock matrix. Previous work studied this interplay assuming infinite matrix block size, thereby neglecting the potential of a particle transferring into nearby fractures, which in turn leads to the over estimation of total transport time. In this study, we assess the impact of finite matrix block size on breakthrough curve profiles at different spatiotemporal scales. Breakthrough curves are generated using particle tracking simulations within the three-dimensional discrete fracture network simulator dfnWorks, where matrix diffusion is integrated using a time-domain random walk. We find that transport is generally dominated by advection near the inlet, with the influence of matrix diffusion increasing with distance. For very small fracture spacings, however, advective effects intensify with distance from the source.