Contributions of Thermodynamic and Mass Transport Modeling to Evaluation of Groundwater Flow and Groundwater Travel Time at Yucca Mountain, Nevada

Constraints on groundwater flow and groundwater travel time (GWTT) at the proposed HLW repository site at Yucca Mountain, Nevada, U.S.A., can be provided through thermodynamic modeling of relations between gas, liquid, and solid phases, and mass transport modeling. In the unsaturated zone at Yucca Mountain the observed distribution of 14C can be reasonably represented by a mass transport and mass transfer model representing diffusion of 14C02 in the gas phase from the ground surface and equilibrium between aqueous and gas phases, in the absence of net advective flow. This mechanism could also account for aqueous phase 14C activities in the unsaturated zone. Thermodynamic interpretations indicate that groundwaters extracted from boreholes in the water saturated tuffaceous aquifer at Yucca Mountain are undersaturated with respect to calcite, providing evidence that groundwater flow bypasses zones of rock containing calcite and therefore must be channelized. This conclusion is supported by published isotopic data for calcites and groundwaters.