Characterizing Unsaturated Diffusion in Porous Tuff Gravel

latest developments in instrumentation and techniques. Improved understanding of unsaturated diffusion in Evaluation of solute diffusion in unsaturated porous gravel is very gravel will help in the characterization and remediation important for investigations of contaminant transport and remedia- tion, risk assessment, and waste disposal (e.g., the potential high-level effort in gravel deposits at the Hanford Reservation nuclear waste repository at Yucca Mountain, Nevada). For a porous (Washington). It will also help in the invert diffusion aggregate medium such as granular tuff, the total water content is barrier concept for the potential underground high-level comprised of surface water and interior water. The surface water radioactive waste repository at Yucca Mountain, Ne- component (water film around grains and pendular water between vada, where tuff gravel has been considered as an invert the grain contacts) could serve as a predominant diffusion pathway. material (material filling the bottom of a tunnel having a To investigate the extent to which surface water films and contact circular cross-section) to contain radionuclide transport. points affect solute diffusion in unsaturated gravel, we examined the The invert placed between the waste package or drip configuration of water using X-ray computed tomography (CT) in shield and the tuff host rock at Yucca Mountain is an partially saturated gravel and made quantitative measurements of dif- integral component of the repository's performance. If fusion at multiple water contents using two different techniques. In the first, diffusion coefficients of KCl in 2- to 4-mm granular tuff at effective, an invert diffusion barrier (caused by slow multiple water contents were calculated from electrical conductivity radionuclide diffusion through the invert) can greatly (EC) measurements using the Nernst-Einstein equation. In the sec- enhance waste-isolation capacity. Conca and Wright ond, we used laser ablation with inductively coupled plasma-mass (1992) measured effective diffusion coefficients (De )i n spectrometry (LA/ICP-MS) to perform microscale mapping, allowing unsaturated soil, gravel, bentonite, and whole rock for the measurement of diffusion coefficients for a mixture of chemical a wide range of volumetric water contents (this free tracers for tuff cubes and tetrahedrons having two contact geometries water content does not include interlayer water in clays (cube-cube andcube-tetrahedron). The X-raycomputed tomography or other structural water; Conca and Wright, 2000). images show limited contact between grains, and this could hinder They found that De values in all media were primarily the pathways for diffusive transport. Experimental results show the a function of volumetric water content and not material critical role of surface water in controlling transport pathways and hence the magnitude of diffusion. Even with a bulk volumetric water characteristics. CRWMS M&O (2000b) reported that content of 1.5%, the measured solute diffusion coefficient is as low this diffusion data set was well correlated in terms of a as 1.5 10 14 m 2 s 1 for tuff gravel. Currently used diffusion models power-dependence (Archie's Law type) on the volumet- relating diffusion coefficients to total volumetric water content inade- ric water content, and in this study a resultant "univer- quately describe unsaturated diffusion behavior in porous gravel at sal" power function was used to represent diffusive very low water contents. transport of radionuclides through the invert. However, in waste emplacement drifts, characterized by a humid environment with or without the presence of liquid