Aqueous Diffusion in Repository and Backfill Environments
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AQUEOUS DIFFUSION IN REPOSITORY AND BACKFILL ENVIRONMENTS JAMES L. CONCA*, MICK APTED** AND RANDY ARTHUR**
*Washington State University Tn-Cities, 100 Sprout Rd, Richland, WA 99352. **INTERA Sciences, Denver, CO.
ABSTRACT Aqueous diffusion coefficients have been experimentally determined in a variety of porous/fractured geologic and engineered media. For performance assessment applications, the purely diffusive flux must be separated from retardation effects. The simple diffusion
coefficient, D, does not include any transient chemical effects, e.g., sorption, which lower the diffusion coefficient for some finite time period until equilibrium is reached. D is primarily a function of volumetric water content, 0, and not material characteristics. At high water contents,2 D gradually declines as water content decreases, from 10-5 cm 2/sec at 0 - 50% to 10-7 cm /sec at 0 - 5%, followed by a sharp decline to 10-10 cm 2 /sec at 0 - 0.5%. Although surface diffusion has a strong experimental basis in the transport of gases along metal surfaces, experimental evidence for aqueous geologic/backfill/engineered systems strongly indicates that surface diffusion is not important, even in bentonite, because of the extremely poor connectivity among electric double-layers and the extremely low diffusivities and high aC/ax at small area/point contacts which more than negate the increased flux along intragrain surfaces. INTRODUCTION
Modeling the transport of contaminants in vadose zones surrounding nuclear and hazardous waste repositories requires knowledge of the material characteristics under unsaturated conditions, especially the simple diffusion coefficient, D(O), which is a key input
parameter to existing and developing models of contaminant release.1 Knowledge of D(O) as a
function of the volumetric water content, 0,is particularly important in the engineered system and near-field transition zone around waste packages where changes in temperature, water content, compaction of backfill, infiltration of fines, and secondary mineralization will alter the transport character of near-field environments. Little experimental data has existed on these
systems under unsaturated conditions because traditional methods require very long times to attain homogeneous distributions of water. Traditional half-cell methods cannot completely
eliminate all advection, retardation, reaction and osmotic effects over the long experimental durations needed to acquire sufficient data at low water contents, especially in highly retarding materials such as bentonite and zeolitic tuff. Therefore, a new technology (UFATm) was used to investigate transport parameters in unsaturated media. 2 The large driving forces obtainable with centrifugation techniques are combined with precision fluid flow through a rotating seal. Samples subjected to controlled fluxes and driving forces respond by changing their water
content. Hydraulic steady-state is achieved in hours to days depending on the target water content and intrinsic permeability of the material (dow
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