Monte Carlo analyses of unsaturated flow in thick vadose zones of layered, fractured rocks

We conducted Monte Carlo simulations of flow in unsaturated fractured rocks using a two-phase, non-isothermal, flow simulator. In this simulator the fractured rock is idealized as a dual-continuum porous media, in which the matrix and fracture constitute two distinct continua represented by two overlapping, interacting numerical grids. Darcy's law and the area of the matrix-fracture interface open to flow govern the exchange of fluids between the two continua. To investigate the applicability of the dual-continuum approach for modeling unsaturated flow in a thick vadose zone of fractured rocks, we applied the model to site data collected from Yucca Mountain. We use grid blocks with dimensions of 1m that is commensurate with the support volume of fracture permeabilities estimated from single-hole pneumatic injection tests. We investigated the consequences of simplifying fracture permeability on unsaturated flow by comparing the model results using uniform formation properties to a stochastic model that represents spatial variability of the fracture permeability within the layers as a multivariate lognormal random field. In both models, the water flux boundary condition was varied to simulate the effects of variable recharge rates. We found that the variability in fracture permeability causes the development of preferential flow paths in the fracture continuum for the welded tuff units and in the matrix continuum for the nonwelded unit. The magnitude of variance in fracture permeability correlates well with the degree of flow focusing. Water flow rates in preferential flow pathways have been found to be locally very high (more than ten times the input flow rate). Flow focusing due to the development of preferential pathways increases saturation locally. This local increase in saturation causes an increase in relative hydraulic conductivity along the pathway and may reduce the wetted surface area for fracture-matrix interaction. Comparison of results obtained from the homogeneous and heterogeneous models of unsaturated flow through thick vadose zones shows that deep percolation can take place rapidly through persistent, preferential flow paths. These pathways are hard to detect and may carry large volumes of water. Simplification of site hydrogeology may lead to erroneous conclusions on the spatial and temporal distribution of unsaturated flow through thick, fractured vadose zones. This work, funded by the U.S. Nuclear Regulatory Commission (NRC), is an independent product of the Center for Nuclear Waste Regulatory Analyses and does not necessarily reflect the views or regulatory position of the NRC.