Evolution of the conceptual model of unsaturated zone hydrology at Yucca Mountain, Nevada

Yucca Mountain is an arid site proposed for consideration as the United Sta tes' first underground high-level radioactive waste repository. Low rainfal l (approximately 170 mm/yr) and a thick unsaturated zone (500-1000 m) are i mportant physical attributes of the site because the quantity of water like ly to reach the waste and the paths and rates of movement of the water to t he saturated zone under future climates would be major factors in controlli ng the concentrations and times of arrival of radionuclides at the surround ing accessible environment. The framework for understanding the hydrologic processes that occur at this site and that control how quickly water will p enetrate through the unsaturated zone to the water table has evolved during the past 15 yr. Early conceptual models assumed that very small volumes of water infiltrated into the bedrock (0.5-4.5 mm/yr, or 2-3 percent of rainf all), that much of the infiltrated water flowed laterally within the upper nonwelded units because of capillary barrier effects, and that the remainin g water flowed down faults with a small amount flowing through the matrix o f the lower welded, fractured rocks. It was believed that the matrix had to be saturated for fractures to how. However, accumulating evidence indicate d that infiltration rates were higher than initially estimated, such as inf iltration modeling based on neutron borehole data, bomb-pulse isotopes deep in the mountain, perched water analyses and thermal analyses. Mechanisms s upporting lateral diversion did not apply at these higher fluxes, and the f lux calculated in the lower welded unit exceeded the conductivity of the ma trix, implying vertical flow of water in the high permeability fractures of the potential repository host rock, and disequilibrium between matrix and fracture water potentials. The development of numerical modeling methods an d parameter values evolved concurrently with the conceptual model in order to account for the observed field data, particularly fracture flow deep in the unsaturated zone. This paper presents the history of the evolution of c onceptual models of hydrology and numerical models of unsaturated zone flow at Yucca Mountain, Nevada (Flint, A.L., Flint, L.E., Kwicklis, E.M., Bodva rsson, G.S., Fabryka-Martin, J.M., 2001. Hydrology of Yucca Mountain. Revie ws of Geophysics in press). This retrospective is the basis for recommendat ions for optimizing the efficiency with which a viable and robust conceptua l model can be developed for a complex site. (C) 2001 Elsevier Science B.V. All rights reserved.