Identifying the Retention Mechanisms of (Bio)Colloids in Single, Saturated, Variable-Aperture Fractures

Abstract

<p>Owing to the lack of knowledge pertaining to the fate and transport of microorganisms in fractured aquifers, the research presented in this thesis was designed to improve the mechanistic understanding of particulate transport in fractures by conducting tracer experiments in natural and epoxy replica fractures. This research demonstrated that particulate retention within fractures is heavily dependent on the equivalent mass balance aperture, followed by the coefficient of variation of the aperture field, and then by the flow conditions. It was also shown that the fracture aperture field alone, not the flow rate or the matrix properties, determines the number of fracture volume flushes required to achieve a 2-3 log decrease in effluent concentration. Moreover, a statistical model was developed that identifies the most important factors affecting particulate retention as the ratio of the ionic strength of solution to the charge of the collector, the ratio of the particle to collector charges, and the Peclet number. The model is able to reasonably predict particulate retention. Finally, tracer experiments conducted in a natural fracture and an epoxy replica of that fracture isolated the effects of matrix properties on attachment, and hence, retention. The transparent nature of the replica fracture was exploited to capture images of <em>E. coli</em> RS2-GFP transport. These images reveal preferential transport within the fracture, and also show that the preferential pathway broadens slightly under increasing flow conditions. This broadening is likely due to higher fluid pressures associated with larger specific discharges. In the groundwater field, there is so little fracture-specific information available that coupling the understanding of a critical environmental setting (fractures) with high-quality particulate tracer experiments and associated modeling represents a significant contribution to the body of science.</p>