Linking pore structure, pore fluid flow, colloid deposition, and solute transport behavior over
multiple scales
Aaron I. Packman, Cheng Chen*, and Jean-François Gaillard
Department of Civil and Environmental Engineering, Northwestern University
*now at Department of Civil and Environmental Engineering, University of Southern California
Deposition of colloidal particles is one of many processes that lead to the evolution of the
structure of groundwater aquifers and sediment beds. We used x-ray difference
microtomography (XDMT) in combination with lattice-Boltzmann (LB) simulations to obtain
unique information on the way in which changes in pore structure resulting from colloid
deposition affect solute transport through granular porous media. While colloid deposition is
conventionally considered to be homogenous and irreversible, we found that patterns of colloid
accumulation were highly heterogeneous and dependent on local pore structure, and that local
detachment occurred even under conditions of net accumulation. LB simulations were used to
investigate the effects of colloid deposition on both pore fluid flow and solute transport. Particle
accumulation decreased the bulk permeability of the porous medium, increased the tortuosity,
and led to increased variability in pore water velocities. The extent of tailing in solute
breakthrough curves greatly increased following colloid deposition because of the development
of extensive no-flow regions in the porous medium. We also investigated multi-scale patterns of
colloid deposition in streambeds. In this case, spatial variability in particle influx to the
subsurface leads to differential accumulation even in initially homogenous porous media. The
pore-scale analysis described previously was used to represent micro-scale feedbacks between
particle deposition, bed structure, and pore fluid flow. Changes in streambed permeability
associated with particle deposition lead to increased spatial variability in surface-subsurface
exchange and broadening of the subsurface residence time distribution. This approach illustrates
how feedbacks between fluid flow, particle transport, and subsurface heterogeneity can be
assessed over a wide range of scales in environmental systems.