Impact of surface heterogeneity on colloid transport over a natural fractured rock
Résumé
The transport of colloids in fractured media has long been identified as a potential source for enhanced pollutant migration. While extensive research has been carried out for systems at different length scales, the understanding of fundamental mechanisms controlling the enhancement or retardation of colloids in fractured media is still incomplete. In particular, the direct observation of colloid transport over natural fractured rock surfaces, and the relation between transport behavior to the heterogeneity of the surface, has rarely been investigated. Here we used a custom designed flow cell to perform colloid transport experiments with natural rock samples taken from a chalk formation in the Negev desert of Israel. We used samples containing a natural fracture surface with varying degrees of heterogeneity, and a sample cut and polished from non-fractured rock cores. We used a transparent glass top cover to allow direct visualization of the synthetic fluorescent colloids. We mounted the flow cell under a fluorescence microscope, and passed a suspension containing the colloids through the cell. We took images of the rock surface periodically to assess the dynamics of colloid flow over the surface, and the deposition of colloids on the surface itself. Images of the advancing colloid front revealed that their breakthrough was strongly influenced by the topography of the rock surface samples, which resulted in a strongly preferential transport pattern for the more heterogeneous rock surface. Natural fracture samples also exhibited slightly earlier arrival of the colloids as measured from their breakthrough curves. The natural fracture surfaces also exhibited higher residual concentration of deposited colloids, after the pulse was washed from the cell. In addition, areas with no colloid deposition on the natural fracture surfaces were related to the deposition of various metal oxides on the rock surface, as indicated by scanning electron microscope analysis. Our results show how both physical and chemical heterogeneity of natural rock surfaces can impact colloid transport processes, and indicate their importance for understanding colloid transport on larger scales.