Effective-porosity and dual-porosity approaches to solute transport in the saturated zone at Yucca Mountain: Implications for repository performance assessment

Abstract

The effective-porosity approach and the dual-porosity approach are examined as two alternative conceptual models of radionuclide migration in fractured media of the saturated zone at Yucca Mountain. Numerical simulations of one-dimensional radionuclide transport are performed for the domain relevant to repository performance assessment using the two alternative conceptual approaches. Dual-porosity solute transport modeling produces similar results to effective-porosity modeling for fracture spacing of less than approximately 1 m and greater than about 200 m, which corresponds to values of effective porosity equal to the matrix porosity and the fracture porosity, respectively. For intermediate values of fracture spacing, the dual-porosity approach results in concentration breakthrough curves that differ significantly from the effective-porosity approach and are characterized by earlier first arrival, greater apparent dispersion, and lower concentrations at later times. The effective-porosity approach, as implemented in recent performance assessment analyses of saturated zone transport at Yucca Mountain, is conservative compared to the dual-porosity approach in terms of both radionuclide concentrations and, generally, travel times.