Saturation‐matric potential relations in gravel

Abstract

[1] Some environmentally sensitive unsaturated zone sediments, such as those underlying radioactive waste tanks in Hanford (Washington State), contain large fractions of gravels and coarse sands. Coarse, granular media are also included in designs of engineered capillary barriers for subsurface waste isolation. Thus knowledge of the unsaturated hydraulic properties of gravels is needed to understand flow and transport in these critical settings. When standard methods for measuring moisture characteristics or water retention relations are used for gravels, corrections are needed in the near-zero region of matric (pressure) potentials. The need for correction results from gravity stratification of saturation profiles within even short sample columns. Such a correction method was developed and used to determine drainage curves for Hanford gravels having characteristic grain sizes of 8.0-9.5, 4.8-5.3, and 2.0-2.4 mm. In 30 mm tall sample columns, gravity corrections were essential for the 9 and 5 mm gravels and less significant for the 2 mm gravel. Validity of the correction method was demonstrated through accurately reconstructing average column saturation-potential relations from their predicted local saturation-potential relations. The method and results presented here are part of an ongoing study on Hanford gravels and on limits to classical unsaturated hydraulic scaling encountered at large grain sizes.

Moisture Characteristics of Hanford Gravels: Bulk, Grain‐Surface, and Intragranular Components

“…5 The near‐zero matric potential region (Region c in Fig. 5) within which large decreases in saturation accompanied small energy decreases is gravity‐corrected for variable saturation within individual equilibrium states (Tokunaga et al, 2002). The intermediate potential range results obtained with a pressure plate are shown in Region b of Fig.…”

“…At matric potentials close to zero, the upper portion of an equilibrium gravel column can be significantly less saturated than the bottom portion. Methods to obtain correct 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 here, with gravel packed to a height of 30 mm in large (82.6‐mm i.d. ), modified Tempe cells.…”

“…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)…”

“…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 remains to be developed.…”

Conceptual and Parametric Representation of Soil Hydraulic Properties: A Review

“…Evidence suggests that soil water storage and water flow characteristics in soils containing appreciable amounts of stones or gravel layers is significantly modified (Fiès et al, 2002; Tokunaga et al, 2002; Unger, 1971; Verbist et al, 2013). For mixed stones in a background of a finer soil matrix, the effect on the hydraulic behavior may involve hydraulic decoupling of the embedded stones due to large pore size contrast, thereby reducing effective water availability for plants (Cousin et al, 2003) or reducing average hydraulic conductivity due to lower‐conducting obstacles (Fiès et al, 2002; Cousin et al, 2003; Coppola et al, 2013).…”