INFLUENCE OF TRANSITIONAL VOLCANIC STRATA ON LATERAL DIVERSION AT YUCCA MOUNTAIN, NEVADA

Natural hydraulic barriers at Yucca Mountain, Nevada, a potential high-level nuclear waste repository, have been identified as possible contributors to lateral diversion of water through the unsaturated, sloping volcanic strata at the site, which reduces the probability of deep percolation of infiltrated precipitation and surface water through the waste storage area. Historical development of the conceptual model of lateral diversion has been limited by the sparsity of measured state variables, such as in-situ matric potential. However, numerical flow investigations indicate the possibility of significant lateral diversion above the potential repository horizon due to an overlying thin, porous tuff, the Paintbrush nonwelded unit. In this study, analytical calculations of the influence of transitional changes in the hydraulic properties of this nonwelded tuff suggest that minimal lateral diversion is likely at Yucca Mountain. Numerical models, to this point, have not accounted for the gradual transition of properties or the existence of multiple layers that have different properties, either of which could influence the simulation of lateral diversion as an artifact of numerical model development. Field data at various spatial scales also were evaluated for evidence of lateral diversion. These include subsurface matric potentials, obtained from boreholes installed in an underground access tunnel, to evaluate ground water flow directions in the Paintbrush nonwelded unit and moisture conditions in the potential repository horizon. The subsurface moisture conditions were compared to surface infiltration estimates and percolation flux estimates in the repository horizon using chloride-mass-balance calculations and simulations of measured temperature profiles. Results suggest that insignificant lateral diversion of ground water has occurred above the repository horizon, and flow is primarily vertical through the Paintbrush nonwelded unit.