AN ACTIVE FRACTURE MODEL FOR UNSATURATED FLOW AND TRANSPORT IN FRACTURED ROCKS

The unsaturated zone at Yucca Mountain, a potential repository site of high-level nuclear waste, is a complex hydrologic system in which a v ariety of important flow and transport processes is involved. To quant ify these processes as accurately as possible is a theoretically chall enging and practically important issue. In this study, we propose a ne w formulation for modeling flow and transport in unsaturated fractured rocks. The formulation is mainly based on a hypothesis that only a po rtion of connected fractures are active in conducting water. Analysis of the relevant data with the new formulation suggests that about 18-2 7% of the connected fractures in the Topopah Spring welded (TSw) unit (the potential repository unit) of Yucca Mountain are active under amb ient conditions. The relatively high percentage of active fractures is consistent with field observations from a variety of sources. Sensiti vity analyses are performed to investigate effects of the ''activity'' of connected fractures on flow and transport behavior in unsaturated rocks.