Calibrating hydrogeologic parameters for the 3-D site-scale unsaturated zone model of Yucca Mountain, Nevada

An important issue in the evaluation of the unsaturated zone (UZ) at Yucca Mountain, Nevada is the calibration of the parameters used in the 3-D site-scale numerical flow model. The hydrogeologic parameters are calibrated using an inversion code (ITOUGH2) to fit measured core sample and in situ data according to statistical criteria. The available data include saturations, water potentials, pneumatic pressure records, and pore water age estimates based on geochemical analyses. Five 1-D columns are extracted from the 3-D site-scale model and used in an inversion that matches data from the five boreholes simultaneously. This results in layer-averaged fracture and matrix permeabilities and van Genuchten fitting parameters. The columns are then refined and individual inversions are performed with each 1-D model to develop a spatially varying parameter set. A number of sensitivity studies are performed to evaluate the effects of modeling assumptions on the calibrated parameters. Several fracture-matrix interaction formulations are also investigated during the study. The results show that the calibrated parameter set concluded to be most consistent with the conceptual model provides a good fit to the ambient data. The study also shows that the choice of the fracture-matrix interaction formulation has a significant effect on the calibrated parameter set.