Radionuclide transport in the unsaturated zone at Yucca Mountain, Nevada

This paper presents a comprehensive treatment of large-scale unsaturated zone (UZ) transport below the repository at the Yucca Mountain, Nevada, reporting on studies investigating the UZ as a component of the Lower Natural Barrier to radionuclide transport from the site. Regardless of the disposition of Yucca Mountain as the site for a nuclear waste repository, these studies and models of radionuclide transport through the UZ have relevance to the study of contaminant transport through deep vadose zones in general. For many radionuclides, including most strongly sorbing species, the Yucca Mountain UZ was found to prevent or substantially reduce the rate of movement of radionuclides to the accessible environment. The UZ radionuclide transport models developed for Yucca Mountain simulate the features and processes that influence the capability of the UZ below the repository to reduce the movement of radionuclides. Transport of radionuclides away from the repository depends on the rate and pathways of flow within the UZ, the retardation of radionuclides, and the rate of transport by colloids. Several basic processes affect radionuclide transport in the UZ, including the quantity of recharge and deep percolation of water, flow partitioning between fractures and rock matrix, diffusion of radionuclides from water flowing in fractures into the pores of the rock matrix, sorption of radionuclides onto rock or mineral surfaces, and colloid filtration. This paper summarizes the treatment of these and other processes, describing the experimental and observational basis for the models, conceptualizations, and predictions, and then presents some representative results.