THE SATURATED ZONE A PART OF THE NATURAL BARRIER BELOW THE REPOSITORY AT YUCCA MOUNTAIN

The U.S. Department of Energy is preparing a License Application for development for geologic disposal of high-level radioactive waste. Engineered and natural barriers are expected to act as obstructions to radionuclides release. Groundwater beneath Yucca Mountain is the primary medium through which most radionuclides might move away from the geologic repository. Although groundwater flow is a transport mechanism, the entire saturated zone system is expected to act as a barrier to radionuclide movement by delaying radionuclides transport and reducing their concentration at the accessible environment. The Yucca Mountain SZ flow system is contained within the Alkali Flat-Furnace Creek Groundwater Basin, a part of the Death Valley Flow System. In this basin, water generally moves from recharge areas at higher elevations north of Yucca Mountain to southern areas of discharge at Alkali Flat (Franklin Lake Playa) and Death Valley. Information obtained from Yucca Mountain Site Characterization Project (YMP) activities is used to estimate flow rates through the site-scale model area and for constraining general conceptual models of flow in the site-scale area. As groundwater in the Death Valley system moves from recharge to discharge areas, flow rates and paths depend largely on the hydraulic properties of the rocks along the flow paths. The potentiometric surface in the Yucca Mountain area can be divided into three general areas: a large, moderate and small hydraulic gradient. Along the flow path, the water table transitions from fractured volcanic tuffs to alluvium. The site-scale flow and transport model is a synthesis of what is known about flow and transport processes at the scale required for Performance Assessment calculations. This knowledge builds on and is consistent with knowledge that has accumulated at the regional scale, but is more detailed because more data are available at the site-scale level. Transport processes include advection dominated by fracture flow, sorption, dispersion. Matrix diffusion in the tuffs and sorption in the alluvium are two important retardation mechanisms that are hypothesized to contribute to the SZ performance. Confidence in the results of the model was obtained by comparing Field and lab data to model predictions. Alternative models and new data were investigated to identify their impact on performance.