  We model flow and transport in three-dimensional fracture networks with
varying degrees of fracture-to-fracture aperture/permeability heterogeneity and
network density to show how changes in these properties can cause the emergence
of anomalous flow and transport behavior. If fracture-to-fracture aperture
heterogeneity is increased in sparse networks, velocity fluctuations can
inhibit high flow rates and solute transport can be delayed, even in cases
where hydraulic aperture is monotonically increased. As the density of the
networks is increased, more connected pathways allow for particles to bypass
these effects. We discover transition behavior where with relatively few
connected pathways in a network from inflow to outflow boundaries, the first
arrival times of particles are not heavily affected by fracture-to-fracture
aperture heterogeneity, but the scaling behavior of the tails is strongly
influenced due to the particles being forced to sample some of the
heterogeneity in the velocity field caused by aperture differences. These
results reinforce the importance of considering multi-scale effects in
fractured systems and can inform flow and transport processes in both natural
and engineered fracture systems, especially the latter where high aperture
fractures are often stimulated and connect to existing fracture networks with
smaller apertures.
