Moisture characteristics of Hanford gravels: Bulk, grain-surface, and intragranular components Page: 1 of 18
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Moisture Characteristics of Hanford Gravels:
Bulk, Grain-surface, and Intragranular Components
Tetsu K. Tokunaga, Keith R. Olson, and Jiamin Wan
Although gravels comprise large portions of some vadose zones, their unsaturated hydraulic
properties have received relatively little attention. This study examines moisture retention
relations in the 2 and 6 mm size fractions of gravels from the Hanford formation vadose zone
(Washington State). Understanding flow and transport within this formation is important because
parts of it have become contaminated by leakage of radioactive wastes. Moisture retention
relations were obtained over a very wide energy range, with attention to water retained in
intragranular pores and along grain surfaces. External surfaces of these gravels have root mean-
square roughnesses in the pm range, with sparsely distributed deep (hundreds of pm) pits. Water
films on these external surfaces are volumetrically insignificant at matric potentials less than
about -2 kPa. "Residual" water in these gravels occurs in intragranular pores, accounts for about
10% of the total porosity, and is effectively hydraulically immobile. The intragranular domain in
Hanford gravels also has a large specific surface area of about 11 m2 g"1. Thus, exchanges of
solutes (including contaminants) between the intragranular domain of Hanford gravels and their
immediate surrounding are significant and diffusion-limited.
Gravels can make up large fractions of the subsurface, including vadose zones. The
extensiveness of some gravel deposits is evident from the fact that they serve as major aquifers
that supply groundwater for agricultural, industrial, and municipal use. Some gravel deposits are
mined for use in construction. Because some gravel deposits do occur in the vadose zone,
understanding unsaturated flow and transport in such settings requires knowledge of their
hydraulic properties. Some gravel deposits occur in heavily contaminated vadose zones such as
the Hanford Site (Washington State) where radioactive wastes have leaked. Gravels are also an
important component in engineered capillary barriers for subsurface waste isolation (Ross, 1990;
Selker, 1997). However, relatively little information is available on the unsaturated hydraulic
properties of gravels. Measurements of moisture characteristics on gravels have been reported by
Miyazuki (1988), Khaleel and Relyea (1997), Conca et al., (1998), and Tokunaga et al. (2002).
Unsaturated hydraulic conductivities of gravels have been reported by Miller and Bunger (1963),
Conca et al., (1998), and Khaleel and Relyea (2001).
Because of their large size, water retention in unsaturated gravels must be mainly
associated with grains rather than with the macroscopic pore network. Thus, water retention
depends on intragranular and grain surface morphology. We suggest several possible basic grain
morphologies depicted in Fig. 1 for cross-sections through rounded grains. Analogous varieties
exist for angular grains. The simplest representation is that of a smooth-surface ellipsoid of
revolution, with negligible intragranular porosity. Other possibilities include rough-surface
grains, and varying importance of internal porosity, as well as possible additional weathering-
induced near-surface alterations.
Whether crystalline or amorphous, the larger grain-size of gravels offers the potential for
significant intragranular porosity and intragranular surface area. Diffusion coefficients within
this intragranular domain can be very low because of constrictive and tortuous pathways 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/m1/1/: accessed November 15, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.