Moisture characteristics of Hanford gravels: Bulk, grain-surface, and intragranular components Page: 2 of 18
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microporous networks of boundaries between microcrystals, and because of low porosity.
Diffusive transport times within grains scale with the square of the diameter, such that
equilibration times within 4 mm gravel will be 400 times longer than in 200 pm fine sand of
otherwise equal nature. Potassium ion diffusion coefficients ranging from 2.2 x1014 to 1.7 x10-11
m2 s~1 were measured in a variety of gravels by Conca and Wright (1990). Thus, porous gravels
can behave as sources and sinks of solutes, including contaminants, with slow exchange times.
Water retained in intragranular pores will likewise exchange with the surrounding environment
only over long times, thus behave hydraulically as "residual moisture". The Kd values measured
by Kaplan et al. (2000) for Sr2+ equilibration provided strong evidence for significant amounts of
intragranular surface in Hanford gravels. The Kd values in their gravels were about 50% that of
the < 2 mm size fraction, hence still quite significant, and much greater than expected from
external surface area alone.
The external surfaces of gravels can also be important since their unsaturated hydraulic
properties can be strongly influenced by water films coating grain surfaces. Grain surface
wettability and microtopography are then important in controlling water film hydraulic relations.
The various possible configurations illustrated in Fig. 1 suggest that the unsaturated hydraulic
properties of gravels can be quite diverse. Measurements of intragranular and surface
characteristics will help identify which of the characteristics illustrated in Fig. 1 apply to specific
types of gravels.
In a recent study on Hanford formation gravels, large saturation changes occurring within
the near-zero matric potential region were reported (Tokunaga et al., 2002). Since that study was
restricted to a narrow energy range, an understanding of the nature of "residual" saturation
remained to be developed. This was especially warranted because the levels of residual
saturation were relatively high, being in the range of 0.1 to 0.2. Thus, the present work addresses
a much broader range of potentials and saturations in Hanford gravels, utilizing suction plate,
pressure plate, and vapor pressure methods to characterize samples. Water vapor adsorption-
desorption measurements were obtained in order to test the hypothesis that the intragranular
porosity of these gravels is substantial.
In addition to these bulk measurements, external grain surface properties were
characterized microscopically (laser profilometry, atomic force microscopy, and scanning
electron microscopy) and hydraulically (surface water film measurements at controlled matric
potentials). We define the gravel grain surface moisture characteristic as the relation between the
average (external surface) water film thickness and the matric potential (Tokunaga and Wan,
1997). The grain surface moisture characteristic is the surface analog of the familiar volumetric
moisture characteristic relation. This relation is important to quantify because it can strongly
influence unsaturated flow and transport in open framework (well-sorted, with minor fine
MATERIALS AND METHODS
Gravel samples were obtained from the Hanford Site in south-central Washington State.
The Hanford formation gravels and sandy gravels were deposited during cataclysmic floods
released from Pleistocene ice-dammed lakes. Our samples were excavated from Site 218-E-12B,
in the north-east corner of the 200-East Area. In the present study, we report primarily on the 2.0
to 2.4 mm and the 5.6 to 6.3 mm size fractions, referred to here as 2 and 6 mm Hanford gravels,
respectively. As shown later, the Hanford gravels have significant intragranular porosity and
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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/m1/2/: accessed January 16, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.