Numerical modeling of perched water under Yucca Mountain, Nevada

The presence of perched water near the potential high-level nuclear waste r epository area at Yucca Mountain, Nevada, has important implications for wa ste isolation. Perched water occurs because of sharp contrasts in rock prop erties, in particular between the strongly fractured repository host rock ( the Topopah Spring welded tuff) and the immediately underlying vitrophyric (glassy) subunit, in which fractures are sealed by clays that were formed b y alteration of the volcanic glass. The vitrophyre acts as a vertical barri er to unsaturated flow throughout much of the potential repository area. Ge ochemical analyses (Yang et al, 1996) indicate that perched water is relati vely young, perhaps younger than 10,000 years. Given the low permeability o f the rock matric, fractures and perhaps fault zones must play a crucial ro le in unsaturated flow. The geologic setting of the major perched water bod ies under Yucca Mountain suggests that faults commonly form barriers to lat eral flow at the level of the repository horizon, but may also form importa nt pathways for vertical infiltration from the repository horizon down to t he water table, Using the numerical code UNSAT2, two factors believed to in fluence the perched water system at Yucca Mountain, climate and fault-zone permeability; are explored. The two-dimensional model predicts that the vol ume of water held within the perched water system may greatly increase unde r wetter climatic conditions, and that perched water bodies may drain to th e water table along fault zones. Modeling results also show fault flow to b e significantly attenuated in the Paintbrush Tuff non-welded hydrogeologic unit.