Numerical Modeling of Seepage into Underground Openings in a Heterogeneous Continuum: A Sensitivity Study of Media Parameters

The amount of water seeping into the waste emplacement underground openings is an important issue for the long-term performance of any underground waste repository. Seepage controls the corrosion rates of waste packages, the waste mobilization rates and transport rates of contaminant leaving the underground openings. Predicting the seepage rate is difficult under any circumstances, and is particularly complicated for repository sites located in unsaturated, heterogeneous media. In this study, a 3-D numerical model is used to study unsaturated flow and seepage through a heterogeneous medium into the underground openings with strongly variable parameters and boundary conditions. The model has been compared with homogeneous model. The seepage rate into the underground openings increases with an increase in percolation flux introduced into the system. Four parameters, (1) fracture continuum permeability, (2) Van Genuchten parameter α, (3) spatial correlation length and (4) standard deviation σ, are chosen to measure permeability-field heterogeneity. Based on a stochastically realized permeability distribution, the authors scale the mean permeability to study its effect on seepage into underground openings. For a chosen percolation flux, the seepage rate into the underground openings was reduced as the Van Genuchten parameter α decreased. Simulation results show that the seepage into the underground openings is sensitive to the mean permeability, with seepage rate decreasing as the mean permeability increased. Simulation results indicate that the seepage is also very sensitive to the correlation length and standard deviation of the heterogeneous field. Either a high correlation value or a high standard deviation σ value increases seepage into the underground openings for a substantially heterogeneous continuum.