Inverse modeling of tracer experiments in FEBEX compacted Ca-bentonite Javier Samper a, * , Zhenxue Dai a,1 , Jorge Molinero b , M. Garcı ´a-Gutie ´rrez c , T. Missana c , M. Mingarro c a Escuela Te ´ cnica Superior de Ingenieros de Caminos, Canales y Puertos, Campus de Elvin ˜a s/n, 15192 La Corun ˜a, Spain b Escola Polite ´cnica Superior, Universidad de Santiago de Compostela, 27002 Lugo, Spain c Dpto. de Impacto Ambiental de la Energı ´a, Caracterizacio ´ n Hidrogeoquı ´mica de Emplazamientos, CIEMAT, Edificio 20-A, Avda. Complutense, 22, 28040 Madrid, Spain Received 18 September 2005; received in revised form 28 January 2006 Available online 22 June 2006 Abstract Solute transport parameters of compacted Ca-bentonite used in the FEBEX Project were derived by Garcı ´a-Gutie ´rrez et al. (2001) 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-CORE 2DÓ , a finite element code which accounts for both dissolved and sorbed concentra- tion 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 coef- ficients. Diffusion and permeation experiments performed on FEBEX compacted bentonite using tritium, cesium, selenium, and stron- tium have been effectively interpreted by inverse modeling. Estimated parameters are within the range of reported values for these tracers in bentonites. It has been found that failing to account for the role of sinters may lead to erroneous diffusion coefficients by a factor of 1.4. Possible ways to improve the design of in-diffusion and permeation experiments have been identified. The interpretation of the tritium permeation experiment requires the use of a double-porosity model with mobile porosity of 0.14 for a dry density of 1.18 g/cm 3 . Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Inverse modeling; Solute transport parameters; FEBEX; Bentonite; Diffusion experiments; Sorption 1. Introduction Predictions of reactive solute transport through com- pacted bentonites are needed for the performance assess- ment of the disposal of hazardous wastes such as high- level nuclear waste (HLW). The FEBEX bentonite was extracted from the Cortijo de Archidona deposit, exploited by Minas de Ga ´dor, S.A. in Serrata de Nı ´jar (Almerı ´a, Spain). This bentonite was selected by ENRESA (Empresa Nacional de Residuos Radioactivos, S.A.) prior to the FEBEX project (Huertas et al., 2000) as a suitable material for backfilling and sealing of a HLW repository because it has a very large montmorillonite content, large swelling pressure and sorption capacity, extremely low permeabil- ity, acceptable thermal conductivity and ease of compac- tion for the fabrication of blocks. Bentonite used in the FEBEX project was homogenized to reduce the uncertain- ties in parameter variability (Huertas et al., 2000). The solution of the inverse problem provides a way whereby measurements of state are used to determine unknown flow and transport parameters by fitting model 1474-7065/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.pce.2006.04.013 * Corresponding author. Address: Escuela de Caminos, Campus de Elvin ˜ a s/n, Universidad de La Corun ˜ a, 15192 La Corun ˜ a, Spain. Tel.: +34 981 16 70 00x1433; fax: +34 981 16 71 70. E-mail address: jsamper@udc.es (J. Samper). 1 Present address: Earth and Environmental Sciences Division, Los Alamos National Laboratory, New Mexico, USA. www.elsevier.com/locate/pce Physics and Chemistry of the Earth 31 (2006) 640–648