Potential for Lateral Flow Associated with Fractured, Nonwelded, Layered Tuffs in the Unsaturated Zone

The magnitude of lateral flow caused by permeability and capillary barriers associated with dipping, layered, nonwelded tuffs above the proposed repository horizon is an important issue for evaluating the amount and distribution of water potentially contacting high-level radioactive waste proposed for emplacement at Yucca Mountain, Nevada. Predictions of the distance over which lateral flow occurs vary from the meter scale to the kilometer scale. Numerical models using homogeneous properties and sharp stratigraphic boundaries overpredict the amount of lateral flow along stratigraphic contacts. Primary heterogeneity or secondary discontinuities (e.g., fractures and faults), however, could lead to preferential flow paths through the Paintbrush nonwelded tuff (PTn) and into the Topopah Spring welded tuff below. Heterogeneity of matrix properties, including primary depositional textures and vapor-phase alteration combined with later structural overprinting and mineralization, will affect the ability of permeability and capillary barriers to produce lateral flow. The potential for lateral flow to occur is evaluated using observations of field hydrogeologic characteristics, laboratory measurements, and results of numerical models that simulate fluid flow in the unsaturated zone. These include (i) observations of field water content profiles; (ii) observations on the nature of stratigraphic contacts (e.g., unconformity or gradational); (iii) analysis of laboratory measurements of matrix permeability; (iv) analysis of unsaturated zone constitutive relations to assess potential for permeability and capillary barriers occurrence in dual permeability models; (v) analysis of fault and fracture sizes, orientations, and spacings; and (vi) expected matrix deformation in small and large faults. The available geologic and hydrologic information, in conjunction with intuitive knowledge of geologic and hydrologic factors, is used to assess the possible extent of lateral flow along stratigraphic contacts in and near the PTn. This information helps to support the Nuclear Regulatory Commission's reviews of unsaturated zone studies being performed at Yucca Mountain by the U.S. Department of Energy. This abstract is an independent product of the CNWRA and does not necessarily reflect the views or regulatory position of the NRC.