Transport of Polydisperse Colloids in Fractured Media
Dr. Scott James
Sandia National Laboratories
Albuquerque New Mexico
 

Hazardous wastes, especially radioactive materials, are often disposed of in canisters and buried in deep, fractured, low–permeability rock formations (e.g., granites, slates, salts, and clays). Obvious examples include the Waste Isolation Pilot Plant and the Geologic Repository for the Disposal of Spent Nuclear Fuel and High–Level Radioactive Waste at Yucca Mountain. Research activities surrounding the design and construction of these sites have stimulated a great deal of interest in characterizing subsurface colloid and contaminant migration in fractured media, and in investigating the capacity of natural barriers to retard the movement of leaked contaminants. Although the diffusion of contaminants and colloids through rock media is important, fractures, ubiquitous in these formations, have been shown to provide preferential flow paths. Transport of colloids in such formations has increasingly captured the attention of many researchers, because of the potential impact of colloids facilitating the transport of pollutants and toxic elements. Several experimental and field studies indicate that contaminants can migrate adsorbed on the surface of colloid particles thereby assuming transport characteristics of colloids that may vary significantly from their own. The results of these studies suggest that colloids may not only enhance the mobility of contaminants, but may also inhibit the retardation and dilution of contaminant plumes by reducing the extent of sorption onto fracture surfaces and diffusion into the rock matrix. Unfortunately, conceptual models that describe fractured systems usually do not account for the finite size and polydisperse characteristics of natural colloid plumes.

This discussion will present results of analytical, theoretical, and computational investigations that were undertaken to gain a better understanding of the fate and transport of finitely sized, polydisperse colloid plumes in fractured porous media. Initially, analytical solutions to the mathematical model describing the transport of finitely sized colloids in a uniform aperture fracture subject to several different boundary conditions are developed. A novel particle-tracking algorithm is then verified through comparison with the analytical solutions. This particle-tracking algorithm is used to examine general transport characteristics of polydisperse colloid plumes in a uniform aperture fracture. Finally, because natural fractures are rough, the particle-tracking algorithm is extended to examine colloid and contaminant co–transport within a quasi–three–dimensional spatially variable aperture fracture.

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