Investigation of conceptual and numerical approaches for evaluating moisture, gas, chemical, and heat transport in fractured unsaturated rock

We investigate the utility and appropriateness of various conceptual and nu merical approaches for modeling several flow and transport processes in the unsaturated zone (UZ) at Yucca Mountain, NV, using a one-dimensional (I-D) column extracted from a three-dimensional (3-D) UZ site-scale model. Simul ations of steady-state and transient moisture flow, transient gas now, trac er transport, and thermal loading scenarios are performed, using a variety of numerical approaches to treat fracture-matrix (F-M) interactions, includ ing an effective continuum model (ECM) and several varieties of dual-contin ua models. For the dual-continua models, we investigate the effect of varyi ng the number of matrix,gridblocks per fracture gridblock, the formulation applied for calculating F-M interface area, and whether or not global matri x-to-matrix flow occurs (dual-permeability vs. dual-porosity models). The k ey findings of the work based on a 1-D column are as follows. For steady-st ate moisture flow, most of the numerical and conceptual models provide simi lar results for saturation and fracture flow profiles. The ECM adequately m odels the steady-state processes because the system is not too far away fro m F-M equilibrium. For both transient moisture flaw and transient transport in a steady flow field, the ECM is not adequate in general. Within the dua l-continua models, the formulation for F-M interface area can have a major effect on the hydrodynamic response to an infiltration pulse and tracer arr ival at various horizons, with fracture responses becoming earlier as F-M i nterface area decreases. The number of matrix blocks also has a significant effect on response time, with the more accurate multi-matrix-gridblock mod els yielding slower fracture response times. For transient gas flow arising from barometric pressure variations, the ECM adequately models the process , because F-M gas flow occurs rapidly compared to the time scale of the bar ometric pressure variations. For thermal loading, preliminary studies indic ate that the ECM does not capture all the significant physical processes, b ecause rapid fluid and heat flow can occur in the fractures before the matr ix has a chance to equilibrate. (C) 1999 Elsevier Science B.V. All rights r eserved.