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Characterizing Unsaturated Diffusion in Porous Tuff Gravel

^a 7000 East Ave., MS L-231, Lawrence Livermore National Laboratory, Livermore, CA 94550
^b 1 Cyclotron Road, MS 90-1116, Lawrence Berkeley National Laboratory, Berkeley, CA 94720

View larger version (93K): [in a new window]   Fig. 1. Schematic of different components of water present in an aggregate medium and their effects on diffusion. DInterior and DSurface denote interior and surface diffusion, respectively. (Modified from Conca and Wright, 1992.)

View larger version (24K): [in a new window]   Fig. 2. Electrical conductivity measurement cell.

View larger version (55K): [in a new window]   Fig. 3. Half-element diffusion experimental setup and schematics of microscale mapping. Face number designation is in the center of each face; Face 1 and Face 6 are the interface and far-side faces, respectively.

View larger version (125K): [in a new window]   Fig. 4. Cross-sectional computed tomography images of two sizes of tuff gravel packed inside a column with an inner diameter of 6.22 cm: (a) 16- to 25-mm tuff gravel; (b) 6.3- to 9.5-mm tuff gravel. Spatial resolution in the viewed plane is approximately 0.2 by 0.2 mm. Voxel depth is 1 mm.

View larger version (129K): [in a new window]   Fig. 5. One-millimeter cross-sectional computed tomography images for 6.3- to 9.5-mm tuff gravel (a) under a partially saturated condition and (b) showing water distribution (wet scan–dry scan).

View larger version (19K): [in a new window]   Fig. 6. Diffusion coefficients versus volumetric water content for this and other work. Legend "15 mm tuff, this work (LA-ICP/MS)" includes the coefficients of surface diffusion for Br– in both cubic and tetrahedral sinks (Table 2). Diffusion coefficients reported in Conca and Wright (1992) were for 86 soil and gravel samples, various bentonites, and rock core of tuff basalt and mudstone. The size of crushed tuff and silica sand was not reported (CRWMS M&O, 2000a).

View larger version (26K): [in a new window]   Fig. 7. Tracer distributions from surface mapping using LA/ICP-MS (100-µm spot size and 10 laser pulses) for cube–cube configuration after diffusion inside nearly 100% RH chamber after 149.7 d. y axis: intensity ratio (dimensionless) denotes the signal of each tracer (solid circle: bromide; solid square: perrhenate) divided by the signal of aluminum.

View larger version (22K): [in a new window]   Fig. 8. Tracer distributions (200-µm spot size and 20 laser pulses) on the surface of Face 3 for both source (filled symbols) and sink (open symbols) cubes inside (a) 98% RH chamber after 0.79 d and (b) 43% RH chamber after 16.3 d. x axis: 0 indicates the interface, and increasing positive number indicates increasing distance from the interface in the sink cube.

View larger version (19K): [in a new window]   Fig. 9. Relative Br– concentrations in the sink elements from 98% RH experiments. Lines are the fitted analytical diffusion solutions (Eq. [1]) with the effective diffusion coefficients shown in the legend.

View larger version (12K): [in a new window]   Fig. 10. Adsorption isotherm of water on (a) fully hydroxylated quartz with contact angle = 0° (modified from Gee et al., 1990, Fig. 1) and (b) heat-dehydroxylated quartz with contact angle = 43° (modified from Gee et al., 1990, Fig. 2).