Evaluation of solute diffusion in unsaturated porous gravel is very important for investigations of contaminant transport and remediation, risk assessment, and waste disposal (e.g., the potential high‐level nuclear waste repository at Yucca Mountain, Nevada). For a porous aggregate medium such as granular tuff, the total water content is comprised of surface water and interior water. The surface water component (water film around grains and pendular water between the grain contacts) could serve as a predominant diffusion pathway. To investigate the extent to which surface water films and contact points affect solute diffusion in unsaturated gravel, we examined the configuration of water using X‐ray computed tomography (CT) in partially saturated gravel and made quantitative measurements of diffusion at multiple water contents using two different techniques. In the first, diffusion coefficients of KCl in 2‐ to 4‐mm granular tuff at multiple water contents were calculated from electrical conductivity (EC) measurements using the Nernst–Einstein equation. In the second, we used laser ablation with inductively coupled plasma–mass spectrometry (LA/ICP‐MS) to perform microscale mapping, allowing the measurement of diffusion coefficients for a mixture of chemical tracers for tuff cubes and tetrahedrons having two contact geometries (cube–cube and cube–tetrahedron). The X‐ray computed tomography images show limited contact between grains, and this could hinder the pathways for diffusive transport. Experimental results show the critical role of surface water in controlling transport pathways and hence the magnitude of diffusion. Even with a bulk volumetric water content of 1.5%, the measured solute diffusion coefficient is as low as 1.5 × 10−14 m2 s−1 for tuff gravel. Currently used diffusion models relating diffusion coefficients to total volumetric water content inadequately describe unsaturated diffusion behavior in porous gravel at very low water contents.

Methane hydrate was formed in moist sand under a confining stress in a long, x-ray transparent pressure vessel. Three initial water saturations were used to form three different methane hydrate saturations. X-ray computed tomography (CT) was used to observe location-specific density changes caused by hydrate formation and flowing water. Gas permeability was measured in each test for the dry sand, moist sand, frozen sand, and hydrate-bearing sand, and results of these measurements are presented. Water was flowed through the hydrate-bearing sand, and the changes in water saturation were observed using CT scanning. Inverse modeling will be performed using these data to extend the relative permeability measurements.

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This report summarizes the work performed and findings obtained during the Phase 1 (feasibility study) of the Engineered Barrier System (EBS) in-drift diffusion evaluation. The objective of this work is to characterize and reduce uncertainties associated with measurements of diffusion coefficients and modeling of diffusion processes. Phase 1 of the study evaluates measurement and modeling uncertainties as well as scoping alternative measurement and modeling approaches.Phase 2 of the study consists of rigorous diffusion testing of invert materials (employing approaches developed from Phase 1) and the development of a calibrated invert diffusion model (with separate surface and internal water components if needed to interpret measured diffusion data) for use in total system performance assessment (TSPA).The invert between the waste package/drip shield and the tuff host rock is integral to both the EBS and the Unsaturated Zone (UZ) performance, and is important to the TSPA dose calculation.