Evaluation of the effect of future climate change on the distribution and movement of moisture in the unsaturated zone at Yucca Mountain, NV

The objective of this paper is to predict the effect of changes in future climatic conditions on the distribution and movement of moisture in the unsaturated zone (UZ) at Yucca Mountain. Modeling is conducted as part of a continuing effort (Wittwer et al., 1995; Bodvarsson and Bandurraga, 1996; Bodvarsson et al., 1997) using TOUGH2, a multiphase, integrated finite difference numerical model (Pruess, 1991) calibrated using available saturation, water potential and pneumatic data from six boreholes (Bodvarsson et al., 1997). Modeling results indicate that wetter future climatic conditions may increase lateral diversion above low permeability units of the Calico Hills nonwelded (CHn) hydrogeologic unit, and decrease lateral diversion above the Paintbrush nonwelded (PTn) hydrogeologic unit. The existence of lateral diversion above the CHn hydrogeologic unit is important if this diversion allow moisture to bypass altered zeolitic layers where radionuclides potentially released from the repository could be sorbed. Lateral diversion within the PTn hydrogeologic unit is important because it could redistribute the spatial distribution of net infiltration above the repository horizon. Nevertheless, no field evidence of lateral diversion in the PTn unit has been observed, suggesting that faulting or interface heterogeneities limit lateral flow. Collection and analysis of additional geochemical data may decrease uncertainty in capability of the PTn and altered CHn hydrogeologic units to create barriers to vertical flow, improving confidence in future climatic predictions.