Moisture characteristics of Hanford gravels: Bulk, grain-surface, and intragranular components Page: 4 of 18
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saturation-potential relations under these conditions have been presented by Dane et al. (1992),
Liu and Dane (1995), Schroth et al. (1996), Jalbert and Dane (2001), and Tokunaga et al. (2002).
The suction plate approach described in Tokunaga et al. (2002) was used in this present work,
with gravel packed to a height of 30 mm in large (82.6 mm ID), modified Tempe cells. Boundary
matric potentials were controlled to within 15 Pa in this region of the drainage curve.
Measurements at lower (more negative) matric potentials were obtained using the pressure plate
method (Dane and Hopmans, 2002), with 2-stage pressure regulation (final regulated potentials
controlled to within 100 Pa), and with a 0.5 bar, high-flow plate (Soilmoisture Equipment
Corp., Goleta, CA).
External grain surface properties
In order to evaluate the role of external grains surfaces on water retention, measurements
were made to characterize grain surface topography, and to quantify the relation between average
water film thickness and matric potential. Grain surface topography was characterized using a
laser profilometer (UMB, Sunnyvale, CA) and an atomic force microscope (AFM, Park
Scientific Autoprobe M5, Veeco Instruments, Woodbury, NY) as previously described
(Tokunaga et al., 2000; Wan and Tokunaga, 2000). A scanning electron microscope was used to
obtain images of gravel grain surfaces.
Grain surface moisture characteristics could in principle be obtained through very
accurately determine adsorption isotherms and moisture characteristic measurements on bulk
sediments, combined with information (or assumptions) on moisture within intragranular pores
and in pendular rings. Instead, we chose to obtain average film thickness measurements directly
at various points on a single grain surface. Microscopic inspection of external surfaces of 1 to 30
mm grain-sizes of Hanford gravels did not reveal any size-dependent topography, thus
measurements on a single grain are expected to be representative of other grain-sizes. Using a
presaturated single grain provided the advantage of effectively constant grain matrix saturation,
and avoiding the complicating influence of pendular rings. The method used to determine area-
averaged film thickness is based on synchrotron x-ray fluorescence of a nonreactive tracer
(selenium(VI) as Se04, which does not undergo reduction because the system is kept
equilibrated with atmospheric oxygen) dissolved in the aqueous phase, with the matric potential
regulated by a modified suction plate device (Tokunaga et al., 2000). This method, previously
tested on roughened quartz glass surfaces, was used on a 13 mm Hanford gravel specimen with
its bottom surface cut flat to provide good hydraulic contact with the underlying porous plate.
The x-ray microprobe beamline X26A at the National Synchrotron Light Source was used to
obtain these measurements.
RESULTS AND DISCUSSION
The 2 and 6 mm Hanford gravels had similar water vapor sorption-desorption isotherms,
with both grain-sizes retaining substantial mass fractions of water (Fig. 2). In contrast, the quartz
gravels had low water contents, barely at the limit of quantification (~ 0.0002 g g"). These
results provide evidence for significant intragranular porosity and intragranular surface area in
Hanford gravels for 2 general reasons. Firstly, the water contents are far too large to have been
associated solely with films on external surfaces of solid (nonporous) grains and pendular rings
in equilibrium with very low water potentials. A uniform water film coating a solid (nonporous)
spherical grain of diameter D has a thicknessf of
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Tokunaga, Tetsu K.; Olson, Keith R. & Wan, Jiamin. Moisture characteristics of Hanford gravels: Bulk, grain-surface, and intragranular components, article, May 2, 2003; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc737752/m1/4/: accessed November 19, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.