MODELING REGIONAL GROUNDWATER FLOW AT YUCCA MOUNTAIN

One of the complicating factors in modeling regional groundwater flow at the Yucca Mountain site is that hydrogeologic boundaries are far away, up to 500 km in the northern direction. Inclusion of hydrogeologic boundaries is essential if one is interested in the effect on the flow field of changing hydrologic conditions. In this presentation it will be shown how an analytic element model may be used to include hydrogeologic boundaries efficiently, while allowing for the inclusion of local detail near the Yucca Mountain site. The Great Basin aquifer that underlies Yucca Mountain is modeled as one aquifer; it will be shown that this is an accurate conceptual model on the regional scale. In the model, rivers and streams are modeled with line-sinks; recharge and evaporation areas are modeled with area-sinks; abrupt changes in aquifer properties are modeled with line-doublets; mountain ranges of negligible permeability are modeled as impermeable walls; and shear zones of low hydraulic conductivity are modeled as leaky walls. The model is calibrated to head measurements and independent estimates of discharge and evaporation. The match between simulated and measured heads is similar to that of previous modeling efforts that are not solely based on hydrogeologic boundary conditions.