Characterization of Arid Land Water-Balance Processes at Yucca Mountain, Nevada

Abstract

Water-balance processes were characterized to estimate net infiltration at Yucca Mountain, Nevada, to help determine the suitability of this site as a potential high-level radioactive waste repository. Detailed water-content data were collected from 98 boreholes located in four topographic positions (ridgetops, sideslopes, alluvial terraces, and active channels) representing four infiltration zones. These data include monthly volumetric water-content readings with depth for 1984 through 1995 and water potential measurements made at a soil-bedrock contact in 1995. These data, combined with measured evapotranspiration and precipitation data, provide the seasonal and areal distribution of changes in volumetric water content needed to assess hydrologic processes contributing to net infiltration. The conceptual model of infiltration at Yucca Mountain describes the processes of precipitation, runoff, evapotranspiration, and vertical redistribution of water in the shallow unsaturated zone. Field observations and measurements and data analysis indicate that, in order for net infiltration to occur, water must reach and nearly saturate the soil-bedrock contact to initiate flow in the underlying fractured bedrock, and water must penetrate deep enough to escape the influences of evapotranspiration. The amount of net infiltration is a function of how long or how frequently the contact is saturated. Water must penetrate deep enough to escape the influences of evapotranspiration. The penetration of water through the soil is influenced primarily by the seasonal timing and areal distribution of precipitation, the storage capacity of soil, and the properties of the underlying bedrock.