Diffusion of Water in Yucca Mountain Tuffs

One application for Quasielastic Neutron Scattering (QENS) is to study the mobility of water in unsaturated rock samples. The mobility of the water as a function of temperature is one of the pieces of information that can be used to understand the potential for contaminant transport in the vadose (unsaturated) zone, the near-surface region of the earth above the water table. QENS data were taken on two samples of tuff. One of the samples is a Calico Hills tuff from Yucca Mountain, NV. Calico Hills tuff is a devitrified tuff consisting primarily of zeolites (clinoptilolite, mordenite). The temperature range for this experiment was 200~K--400~K and the water saturations were 5% and 10% by weight. The second sample is a Topopah Springs tuff from Yucca Mountain, NV. Topopah Springs tuff is composed primarily of quartz and feldspar. The sample was room-dry, and the temperature range was 200~K--420~K. Data from two different experiments were analyzed using the model of Teixeira et al.~(1985) for bulk water. We find translational diffusion coefficients, activation energies, and the relative amount of rotational motion versus translational motion. We compare the results from the two samples, keeping in mind the different compositions of the horizons from which the samples were taken. We find that the residence time for translational diffusion is noticeably longer than for bulk water for a given temperature. Also, rotational relaxation times decrease as saturation increases. Teixeira, J., M.-C. Bellissent-Funel, S. H. Chen, A. J. Dianoux, Experimental determination of the nature of diffusive motions of water molecules at low temperatures, Phys. Rev. A 31, 1913-1917, 1985.