Table 1-4. Summary of Hypothesis Concerning Flow and Transport Features and Processes
| Type | Current Understanding | Addressed in Section |
| Recharge to the SZ | Recharge to the SZ is localized and focused at higher elevations and along major surface drainage channels. | 3.1.1.1.2, 3.1.1.1.3, 3.2.3.2 |
| Flow Through the Tuff | Groundwater flow in the volcanic tuffs primarily occurs in fractures and in discrete flowing intervals. | 3.2.4.1 |
| Matrix Diffusion in the Tuffs | Matrix diffusion occurs in the volcanic tuffs in which radionuclides can diffuse from the fast flowing groundwater in fractures into the stagnant water in the matrix. | 3.2.4.2, 3.3.2, 3.3.4 |
| Dispersion in the Tuffs | Transverse dispersion in the volcanic tuffs and alluvium | 3.7.2 |
| Large Hydraulic Gradient | The large hydraulic gradient is caused by low-permeability material upgradient from Yucca Mountain. Alternative view: The large hydraulic gradient is a reflection of perched water bodies above the main groundwater system. Regardless of the cause of this hydraulic feature, it is represented in the SZ flow and transport model in such a way that it represents the measured heads. |
3.2.5.1 3.2.2.3, 3.2.5.1 |
| Role of Faults | Some faults are high permeability features that provide a fast flow path. Other faults have low permeability due to rock alteration or formation of gouge. In addition, faults alter the permeability to flow crossing the fault by offsetting or juxtaposing hydrostratigraphic units | 3.2.3.4, 3.2.2.3, 3.2.2.5 |
| Horizontal Anisotropy | Horizontal anisotropy in permeability may be caused by the preferential orientation of faults and fracture zones. | 3.2.5.3 |
| Groundwater Flow in the Alluvium | The alluvium behaves as a porous medium. Consequently, average flow rates are slower than in the tuffs, and more matrix surface area is available for sorption. | 3.7.2 |
| Alluvium in the Flow Path | The flow path from the potential repository transitions from the volcanic tuffs into the alluvium | 3.7.2 |
| Sorption in Alluvium | Laboratory analyses of samples from the alluvium indicate that sorption of some radionuclides (neptunium and iodine) is greater in the alluvium than in tuffs. | 3.1.4.1 |
| Upward Hydraulic Gradient | Increasing hydraulic head with depth, resulting in upward flow from the carbonate aquifer to the overlying volcanic aquifers, was documented in UE-25P#1 (the deep well) in the Yucca Mountain area. Other wells completed in the carbonate aquifer on the Nevada Test Site also have noted the increasing heads with depth. The site-scale SZ flow and transport model simulates an upward hydraulic gradient in this area. The upward flow will keep the flow paths from the potential repository confined to the shallow groundwater at or slightly below the water table. | 3.2.2.3, 3.4.3, 3.1.2.3 |
| Sorption in Volcanic Units | Sorption of radionuclides is likely to occur in the matrix of fractured tuffs. | 3.2.4.3 |
| Climate Change | In the future, wetter, monsoon and glacial transition glacial climate conditions will result in faster groundwater flow in the SZ and higher water table elevations. | 3.2.6 |