Dealing with Uncertainty in the Chemical Environment in Bentonite Backfill
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DEALING WITH UNCERTAINTY IN THE CHEMICAL ENVIRONMENT IN BENTONITE BACKFILL R.C. ARTHUR*, M. J. APTED*, AND J. L. CONCA** * Intera Information Technologies, Environmental Division, 3609 South Wadsworth Blvd., Denver, CO 80235 ** Washington State University Tri-Cities, 100 Sprout Rd., Richland, WA 99352 ABSTRACT Analytical and conceptual deficiencies in understanding compositional variability in the smectite clays are expected to generate uncertainty in models used to simulate the chemical environment in bentonite backfill. Equilibrium models accounting for nonstoichiometry in smectite can nevertheless bound ranges in aqueous solution compositions that are an explicit function of the uncertainty in smectite compositions. In one approach, we quantify uncertainty in terms of ranges in concentrations of octahedral and tetrahedral Al, and exchange-site cations and vacancies. Heterogeneous mass transfer in bentonite-water systems is modeled using conventional mass-action relations and standard Gibbs energies for stoichiometric minerals, and the site-occupancy constraints combined with site-mixing relations for smectite. The resultant bounding conditions in groundwater compositions may be large or small depending on which aqueous species are of interest in a given situation, but they are valid irrespective of whether equilibrium in smectite-water reactions is attained or is inhibited by slow intracrystalline reaction rates. INTRODUCTION It has been suggested that massive amounts of bentonite backfill in geologic repositories for nuclear waste could control near-field groundwater compositions for very long periods of time [1]. An essentially static chemical environment possibly resulting from bentonite-water interactions would provide an invaluable simplifying constraint in assessments of repository performance. Consequently, there is considerable interest at present in evaluating the scientific credibility and accuracy of thermodynamic models for bentonite-water systems at near-field temperatures and pressures. Unfortunately, limitations in such models are suggested by data from recent studies of natural and experimental systems containing the smectite clays' [3,4]. Those studies indicate that because their compositions are significantly variable, the smectites should be regarded as true solid solutions rather than stoichiometric minerals or thermodynamically ill-defined substances capable of only simple ion exchange. Moreover, the compositional variations apparently extend to unit-cell dimensions, which seriously complicates efforts to determine compositions unambiguously from quantitative chemical analyses of even single smectite particles. One can infer that the presence in backfill of many such particles engenders an extraordinary level of virtually irreducible complexity, and therefore uncertainty, in models of bentonite-water interactions. Our purpose is to demonstrate that thermodynamic models accounting for smectite's solidsolution behavior can nevertheless: 1) clarify stability relations in bentonite at a given te
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