Characterizing unsaturated diffusion in porous tuff gravel Page: 4 of 47
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Hu et al. Characterizing unsaturated diffusion ...
1 this diffusion data set was well correlated in terms of a power-dependence (Archie's law type) on
2 the volumetric water content, and used a resultant "universal" power function to represent
3 diffusive transport of radionuclides through the invert. However, in waste emplacement drifts,
4 characterized by a humid environment with or without the presence of liquid water, crushed
5 porous rock may provide unique characteristics that vary greatly from this generic power function
6 (Wang et al., 2001; Hu and Wang, 2003). For example, Conca (1990) placed four different size
7 fractions of tuff gravel samples (2-4 mm, 4-6.3 mm, 6.3-9.5 mm, and 15-25.4 mm) for
8 equilibrium inside a chamber with a nearly 100% humidity atmosphere. After equilibrating about
9 70 days, all samples of individual grains were observed to be dry, despite the 2.7% intragranular
10 water content. No electrical conductivity could be measured on these samples, resulting in an
11 inferred diffusion coefficient below 10"15 m2/s, which is the detection limit reported by Conca
12 (1990) using electrical conductivity (EC) for estimating the diffusion coefficient. The inferred low
13 diffusion value of 10"15 m2/s at this water content deviates significantly from the "universal"
14 power function with a diffusion coefficient (about 2.8 x 1012 m2/s), which is obtained with
15 continuous fluid introduction. In other words, at the same water content, diffusion in samples
16 prepared using high-humidity (without fluid source contact) is almost three orders of magnitude
17 lower than in the samples prepared with liquid-water introduction.
18 This diffusion behavior is caused by the water distribution exhibited by porous-rock gravel
19 (Figure 1). Rock surfaces in partially saturated environments will generally be wetted by liquid
20 films that can be held either by adsorptive forces on mineral surfaces or by capillary forces in
21 surface depressions. For a porous-rock-gravel system, the total water content is comprised of
22 surface (intergranular) water (surface film water around grains and pendular water between rock
23 grain contacts) and interior (intragranular) water (water contained within the rock matrix pores).
24 Small pores and the relatively large porosity of tuff gravel could play an important role in
4
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Hu, Qinhong; Kneafsey, Timothy J.; Roberts, Jeffery J.; Tomutsa, Liviu & Wang, Joseph, S.Y. Characterizing unsaturated diffusion in porous tuff gravel, article, November 12, 2003; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc781662/m1/4/: accessed April 23, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.