Tracer diffusion in compacted, water-saturated bentonite

Ian Charles Bourg, Garrison Sposito, Alain C.M. Bourg

Research output: Contribution to journalArticlepeer-review

125 Scopus citations


Compacted Na-bentonite clay barriers, widely used in the isolation of solid-waste landfills and other contaminated sites, have been proposed for a similar use in the disposal of high-level radioactive waste. Molecular diffusion through the pore space in these barriers plays a key role in their performance, thus motivating recent measurements of the apparent diffusion coefficient tensor of water tracers in compacted, water-saturated Na-bentonites. In the present study, we introduce a conceptual model in which the pore space of water-saturated bentonite is divided into 'macropore' and 'interlayer nanopore' compartments. With this model we determine quantitatively the relative contributions of pore-network geometry (expressed as a geometric factor) and of the diffusive behavior of water molecules near montmorillonite basal surfaces (expressed as a constrictivity factor) to the apparent diffusion coefficient tensor. Our model predicts, in agreement with experiment, that the mean principal value of the apparent diffusion coefficient tensor follows a single relationship when plotted against the partial montmorillonite dry density (mass of montmorillonite per combined volume of montmorillonite and pore space). Using a single fitted parameter, the mean principal geometric factor, our model successfully describes this relationship for a broad range of bentonite-water systems, from dilute gel to highly-compacted bentonite with 80% of its pore water in interlayer nanopores.

Original languageEnglish (US)
Pages (from-to)363-374
Number of pages12
JournalClays and Clay Minerals
Issue number3
StatePublished - Jun 2006

All Science Journal Classification (ASJC) codes

  • Water Science and Technology
  • Soil Science
  • Geochemistry and Petrology
  • Earth and Planetary Sciences (miscellaneous)


  • Bentonite
  • Diffusion
  • Geometric factor
  • Interlayer
  • Montmorillonite
  • Nanopore
  • Smectite
  • Waste containment


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