Influence of ionic strength and cation charge on transport of colloidal particles in fractured shale saprolite.

The role of solution chemistry (cation charge and concentration) and particle size on colloid transport was examined in an intact monolith of fractured shale saprolite (highly weathered rock). Recovery of the microsphere tracers consistently decreased with increasing ionic strength of either mono- (Na+) or divalent- (Ca2+) dominated solutions, but a much greater concentration of Na+ in the influent solution was required to result in a similar reduction in colloid recovery as compared to Ca2+. However, composition of the solution along the flow path, and hence the degree of microsphere retention, was also strongly influenced by cation exchange and diffusive exchange between pore water in the fractures and in the fine-grained, Ca- and Mg-rich matrix. The influence of "matrix diffusion" on solute transport is also evident in the 5-fold difference between the arrival of the center-of-mass of microspheres as compared to the much later arrival of a bromide tracer. Particle size affected the extent of microsphere transport, but the solution chemistry appears to be a more dominant control. While confirming the importance of ionic strength, counterion charge, and particle size on colloid migration, this study emphasizes the profound effect that pore structure and geochemical processes such as cation exchange have on solution chemistry and thus on colloid transport.