Radionuclide Transport in the Unsaturated Zone at Yucca Mountain: Numerical Model and Preliminary Field Observations

The unsaturated zone at Yucca Mountain is one of the primary barriers to radionuclide migration from the potential repository to the accessible environment and, as such, has received great attention in site characterization activities. In this study, a dual-permeability flow and transport model has been developed that captures the disparate behavior of radionuclides, which depends on whether transport occurs in the fractures or the matrix. The model predicts stratigraphically controlled, abrupt changes in flow and transport- from fracture dominated to matrix dominated. A particle-tracking algorithm has been used to simulate radionuclide transport. This method handles a wide range of solute transport velocities due to fracture and matrix flow, as well as transport processes such as retardation due to sorption and matrix diffusion. Using sorption and diffusion data collected for the Yucca Mountain tuffs, the model predicts that the nonwelded vitric Calico Hills unit is the primary barrier to radionuclide migration in the unsaturated zone. This is because of the long percolation times in the matrix and the intimate contact of sorbing radionuclides with the host rock. Matrix diffusion is also shown to provide significant travel-time delay, especially for regions beneath the potential repository in which fracture transport occurs along the entire flow path to the water table. Because of the importance of the vitric Calico Hills to the performance of the unsaturated zone as a barrier to radionuclide migration, a field-scale Unsaturated Zone Transport Test (UZTT) is being conducted at Busted Butte, located 8 km southeast of the potential repository. Initial results indicate that the vitric Calico Hills unit does indeed exhibit matrix-dominated flow and transport under current and expected future hydrologic conditions at the site.