Colloid transport in fractured rock formations is an important process
impacting the fate of pollutants in the subsurface. Despite intensive
and outstanding research on their transport phenomena, the impact of
small-scale surface heterogeneity on colloid behavior at the fracture
scale remains difficult to assess. In particular, there is relatively
little direct experimental evidence on the impact of natural fracture
surface heterogeneity on colloid transport. To investigate this, we
developed an experimental setup allowing the direct visualization of
fluorescent colloid transport, in a flow cell containing a chalk rock
sample. We used samples containing both a natural fracture surface and
an artificially made smooth surface from the same chalk core. We
characterized the roughness and chemical composition of both surface
types. From the experiments, we obtained direct images of colloid
transport over the surfaces, the colloid breakthrough curves at the
outlet of the flow cell, and the residual deposition of colloids on the
rock surface. The natural fracture surface exhibited larger physical and
chemical heterogeneity than the smooth surface. The aperture variability
across the natural fracture surface led to preferential flow and colloid
transport, as well as their earlier breakthrough from the flow cell,
compared to the artificially made surface. Our experimental setup can be
used to further investigate the link between surface heterogeneity, both
chemical and physical, on colloid transport and deposition in natural
rock fractures.