PARTICLE-TRANSPORT THROUGH 2-DIMENSIONAL, SATURATED POROUS-MEDIA - INFLUENCE OF PHYSICAL STRUCTURE OF THE MEDIUM

Recent work in the transport of particulate matter (e.g. bacteria, viruses, colloids, mineral grains, etc.) through saturated porous media has indicated that the physical structure of the porous medium plays an important role in both the ability of the particles to be transported and the distribution of the particles deposited within the medium. The present paper addresses the influence on particle transport within saturated porous media of high-permeability pathways and contacts between different grain size distributions. The experiments discussed in the present paper were conducted in two dimensions. The porous media were constructed from glass beads and the tracer utilized consisted of latex spheres in the size range 2-90 mu m (2-30 mu m in most experiments). Among the media designs investigated were simple layers in which flow was parallel to layers, simple layers in which flow was at an angle to layers, a simple form of heterogeneity including non-parallel layers and disjoint inclusions, and a complex heterogeneity involving three interconnected permeabilities. Results from these experiments demonstrated that significantly greater numbers of particles were deposited at media contacts where water moved from larger to smaller diameter glass beads. Further, it was observed that outflow concentrations of particles for media containing continuous flow paths within the coarse glass beads were substantially higher than outflow concentrations of particles for media containing a number of interfaces between different-sized glass beads. Specifically, a comparison of the results for a medium in which flow was parallel to layers with results from a medium in which flow was at an angle to the layers led to the observation of higher concentrations of latex particles in the outflow from the former medium and high concentrations of particles remaining in the region of layer contacts within the latter medium. Similar observations were made in a comparison of the simple and complex heterogeneities. Based on these results and prediction of straining based on the filtration literature, it is argued that the enhanced deposition observed near contacts results from sorption leading to increased straining in regions of lower uniformity in the glass bead distribution. These results demonstrate the importance of particle transport along high-permeability pathways and the enhanced deposition which may occur at contact surfaces within heterogeneous porous media.