Interpretation of solute transport experiments in compacted Ca-bentonites using inverse modeling

Full text of publication follows: Solute transport parameters of compacted Ca-bentonite used in the FEBEX Project were derived in [1] from through- and in-diffusion experiments using analytical solutions for their interpretation. Here we expand their work and present the numerical interpretation of diffusion and permeation experiments by solving the inverse transport problem which is formulated as the minimization of a weighted least squares criterion measuring the differences between computed and measured concentration values. The inverse problem is solved with INVERSE-CORE2DC, a finite element code which accounts for both dissolved and sorbed concentration data, uses either the Golden section search or Gauss-Newton-Marquardt methods for minimizing the objective function and allows the estimation of transport and retardation parameters such as diffusion coefficient, total and kinematic porosity and distribution coefficients. Numerical inverse methods overcome the limitations of analytical solutions for interpretation of transport experiments because they can account for: 1) Measurement errors; 2) The role of the sinters; 3) Time-varying boundary conditions in permeation experiments; and 4) Transient concentrations in the cell and total concentration data at the end of in-diffusion experiments. Moreover, the inverse method provides not only optimum parameter estimates but also approximate confidence intervals. All these issues are highlighted with the numerical interpretation of through-diffusion, in-diffusion and permeation experiments performed on FEBEX Ca-bentonite using tritium, strontium, cesium and selenium. [1] Garcia-Gutierrez, M., T. Missana, M. Mingarro, J. Samper, Z. Dai and J. Molinero, Solute transport properties of compacted Ca-bentonite used in FEBEX Project, J. Contaminant Hydrology, 47(3), 127-137, 2001. (authors)