Characterizing unsaturated diffusion in porous tuff gravel

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 x 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.