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 at the Hanford Site. Moisture retention relations were obtained for 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-squared roughnesses (rmsr) in the micrometer range, with sparsely distributed deep (hundreds of micrometers) 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.

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