Identification of relative conductivity models for water flow and solute transport in unsaturated bentonite

Unsaturated compacted bentonite is the buffer material foreseen for deep geological high-level radioactive waste repositories because it provides mechanical and chemical stability, hydraulic isolation, and radionuclide retardation. FEBEX (Full-scale Engineered Barrier EXperiment) is a demonstration and research project for the engineered barrier of a high-level radioactive waste repository in crystalline rock. An inverse methodology has been applied to estimate FEBEX bentonite hydraulic parameters and identify its relative conductivity function from infiltration tests using transient cumulative water inflow and final water content data. Model identification criteria developed within the context of information theory have been used to select the best relative conductivity function among four candidates, including that of Irmay and three proposed by van Genuchten. The Irmay model provides the best fit in almost all tests and is rated the best by model identification criteria for the test having the largest number of data values. The general van Genuchten model provides the second best fit to all tests and is rated either first or second best model by identification criteria. The relevance of different relative conductivity functions for solute transport has been evaluated by comparing concentrations of two conservative tracers computed with Irmay and van Genuchten models. Concentrations computed with van Genuchten model are steeper than those computed with Irmay model at the solute front. (C) 2008 Elsevier Ltd. All rights reserved.