We present an analysis of fault hydraulic architecture, based on Ͼ700 spatially distributed ground and geothermal spring temperature measurements taken in an active fault zone. Geostatistical simulations were used to extrapolate the measured data over an 800 ؋ 100 m area and develop a high-resolution image of temperatures in the fault. On the basis of the modeled temperatures, a simple analytical model of convective heat transport was used to infer a probability distribution function for hydraulic conductivities in a twodimensional plane parallel to the land surface, and the partitioning of flow between flow paths of different conductivities was calculated as a fraction of the total flux. The analysis demonstrates the existence of spatially discrete, high-permeability flow paths within the predominantly lower-permeability fault materials. Although the existence of fast-flow paths in faults has been hypothesized for Ͼ10 yr, their prevalence and contribution to the total flow of fluid in a fault zone are debated. On the basis of our findings, we conclude that the flux transmitted by an individual fast-flow path is significantly greater than that of an average flow path, but the total flux transported in fast-flow paths is a negligible fraction of the total flux transmitted by the fault.

[1] This study presents a description of permeability in an active fault zone located in the Great Basin extensional province. The fault hydraulic structure is inferred from geostatistical analysis of temperatures in 143 geothermal springs, located along a fault trace in the Alvord Basin of southeast Oregon. Based on this analysis, we conclude that the fault zone is predominately low permeability, interspersed with relatively few, spatially-discrete, highpermeability channels. The conceptual model presented is in agreement with the findings of other investigators, but extends their work by offering a representation of fault properties at the tens to hundreds of meters scale.

[1] This work presents a series of numerical modeling studies that investigate the hydrogeologic conditions required to form capillary barriers and the effect that capillary barriers have on fluid flow and tracer transport processes in the unsaturated fractured rock of Yucca Mountain, Nevada, a potential site for storing high-level radioactive waste. The modeling approach is based on a dual-continuum formulation of coupled multiphase fluid and tracer transport through fractured porous rock. The numerical modeling results showed that effective capillary barriers can develop where both matrix and fracture capillary gradients tend to move water upward. Under the current hydrogeologic conceptualization of Yucca Mountain, strong capillary barrier effects exist for diverting a significant amount of moisture flow through the relatively shallow Paintbrush nonwelded unit, with major faults observed at the site serving as major downward pathways for laterally diverted percolation fluxes. In addition, we used observed field liquid saturation and goechemical isotopic data to check model results and found consistent agreement.

A wide array of field observations, in situ testing, and rock and water sampling (and subsequent analyses) within the unsaturated zone of Yucca Mountain demonstrate the importance of fractures to flow and transport in the welded tuffs. The abundance of fractures and the spatial variability in their hydraulic properties, along with the heterogeneity within lithologic formations, make evaluation of unsaturated processes occurring within the potential repository horizon complex. Fracture mapping and field testing show that fractures are well connected, yet considerable variation is seen within and between units comprising the potential repository horizon with regard to fracture trace length, spacing, permeability, and capillarity. These variations have important implications for the distribution and movement of water and solutes through the unsaturated zone. Numerical models designed to assess such phenomena as unsaturated flow, transport, and coupled thermal-hydrological processes each require their own conceptual model for fracture networks, in order to identify the subset of all fractures that is relevant to the particular study. We evaluate several process-dependent conceptual models for fractures and identify the relevant fracture subsets related to these processes.

The presence of perched water near the potential high‐level nuclear waste repository area at Yucca Mountain, Nevada, has important implications for waste isolation. Perched water occurs because of sharp contrasts in rock properties, in particular between the strongly fractured repository host rock (the Topopah Spring welded tuff) and the immediately underlying vitrophyric (glassy) subunit, in which fractures are sealed by clays that were formed by alteration of the volcanic glass. The vitrophyre acts as a vertical barrier to unsaturated flow throughout much of the potential repository area. Geochemical analyses (Yang et al. 1996) indicate that perched water is relatively young, perhaps younger than 10,000 years. Given the low permeability of the rock matrix, fractures and perhaps fault zones must play a crucial role in unsaturated flow. The geologic setting of the major perched water bodies under Yucca Mountain suggests that faults commonly form barriers to lateral flow at the level of the repository horizon, but may also form important pathways for vertical infiltration from the repository horizon down to the water table. Using the numerical code UNSAT2, two factors believed to influence the perched water system at Yucca Mountain, climate and fault‐zone permeability, are explored. The two‐dimensional model predicts that the volume of water held within the perched water system may greatly increase under wetter climatic conditions, and that perched water bodies may drain to the water table along fault zones. Modeling results also show fault flow to be significantly attenuated in the Paintbrush Tuff non‐welded hydrogeologic unit.