Testing conceptual unsaturated zone flow models for Yucca Mountain

An important component of site characterization and suitability assessment of the proposed nuclear waste repository at Yucca Mountain, Nevada is determination of the most appropriate conceptual model of the hydrologic mechanisms governing saturated and unsaturated flow for the site. As observers in the ITNRAVAL Unsaturated Zone Working Group, L. Lehman ampersand Associates conducted a modeling exercise which numerically examined alternative conceptual flow models. Information was provided to the Working Group by the U.S. Geological Survey. Additional published data were utilized to fill in data gaps and to provide additional confidence in results. Data were modeled utilizing one and two dimensional matrix and fracture numerical models. Good agreement was obtained using a 2-dimensional dual porosity fracture flow model. Additional measures are needed to constrain the field conditions enough to validate conceptual models using numerical models. Geochemical data on tritium, chlorine-36, or carbon-14 concentrations or temperature profiles which can give estimates of time since recharge for water in the unsaturated zone, are needed to eliminate the non-uniqueness of various model solutions