Earth Sciences Division Berkeley Lab
Nuclear Waste Program
Annual Report 2000–2001
CAPILLARY-BARRIER EFFECTS IN UNSATURATED FRACTURED ROCKS OF YUCCA MOUNTAIN Yu-Shu Wu, Lehua Pan, Jennifer Hinds, and Gudmundur S. Bodvarsson Contact: Yu-Shu Wu, 510/486-7291, yswu@lbl.gov
RESEARCH OBJECTIVES Naturally occurring capillary barriers have been of considerable interest for many subsurface flow and contiminant transport problems. The objective of this study is to investigate the effects of capillary barriers on water-flow processes in the unsaturated zone (UZ) of Yucca Mountain. We identify factors controlling the formation of capillary barriers and estimate their effects on the extent of possible large-scale lateral flow within the unsaturated, fractured rocks. This study indicates that the UZ of Yucca Mountain is capable of producing strong capillary-barrier effects, which may laterally divert significant amounts of flow.
APPROACH This modeling study uses five different 2D vertical crosssectional models to explore capillary-barrier formation and lateral diversion under current ambient flow conditions. The conceptual hydrogeological framework is based on the one used to develop the current UZ flow model of Yucca Mountain (Wu et al., 2000). In this framework, the UZ is represented by a stack of three-dimensional layers, each with its own set of fracture and matrix properties. In addition, these layers are intersected by several major faults. Numerical simulations in this study are performed with the TOUGH2 code. Hydrogeological layers and faults along the cross-sectional models are captured by refined numerical grids. A dual-permeability approach is used to handle fracture-matrix interactions. The bottom model boundary is located at either the water table or at the interface between the Paintbrush (PTn) and
Topopah Spring (TSw) units of Yucca Mountain, while the top model boundary coincides with the bedrock surface of the mountain. Surface net infiltration of water is applied to the top boundary to approximate present-day mean infiltration rates.
ACCOMPLISHMENTS Examination of simulated percolation fluxes at the PTn–TSw interface using the five cross-sectional models reveals that significant lateral flow occurs within the PTn unit, with a large amount of water being diverted to down-slope faults. Faults serve as major downward flow pathways crossing the PTn under the current conceptual model. Figure 1 shows two simulated layers within the PTn with high fluxes. These layers control lateral flow within the PTn unit as a result of the contrast in rock properties between these layers and adjacent layers. The figure also shows that major faults provide the main flow pathways for vertical percolation flux.
SIGNIFICANCE OF FINDINGS This study indicates that with the current conceptualization, significant capillary-barrier effects may exist within the PTn unit and, as a result, large-scale lateral flow could occur. Faults serve as major downward pathways for laterally diverted water. Capillary barriers within the PTn unit are mainly the result of contrasts in matrix hydraulic parameters, while fracture properties have a less significant impact on water flow. Compared with detailed, spatially varying distributions, the magnitude of the average net-infiltration rate has a larger impact on flow patterns through the PTn unit. The model results also show that capillary-barrier effects are strongly correlated with surface infiltration rates, with lower infiltration leading to larger lateral flow.
RELATED PUBLICATION Wu, Y.S., J. Liu, T. Xu, C. Haukwa, W. Zhang, H.H. Liu, and C.F. Ahlers, UZ flow models and submodels, Report MDL-NBSHS-000006, CRWMS M&O, 2000.
ACKNOWLEDGMENTS
Figure 1. Simulated mass flux (kg/s/m2) along an east-west cross section, showing lateral flow along naturally occurring capillary boundaries and vertical flow along faults
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This work was supported by the Director, Office of Civilian Radioactive Waste Management, U.S. Department of Energy, through Memorandum Purchase Order EA9013MC5X between Bechtel SAIC Company, LLC, and Berkeley Lab. The support is provided to Berkeley Lab through U.S. Department of Energy Contract No. DE-AC03-76SF00098.
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