Water and contaminant migration in the vadose, or unsaturated, zone are fast becoming critical to water resource development, site restoration and waste disposal strategies, especially in arid regions where low precipitation results in vadose zones that are tens to hundreds of meters thick. Because liquid transport is always much slower in the unsaturated zone relative to the saturated zone, transport through the vadose zone is usually the rate limiting step for contaminant release. Imminent vadose zone regulations from the United States Environmental Protection Agency will govern most activities in the unsaturated zone and will require the measurement of the unsaturated transport parameters for each geologic unit, soil horizon and engineered component for modelling and performance assessment needs. However, these parameters are all strong, non-linear functions of the volumetric water content, . These functional relationships are determined by the pore-size distribution which, in turn, is determined by the grain-size distribution, density and the pore connectivity factor.

The primary transport parameters include hydraulic conductivity and intrinsic permeability, diffusion coefficient, retardation factor, vapor diffusivity and matric potential, . All of these properties are required to effectively describe subsurface transport and behavior, especially with respect to migrating fluids and contaminants. However, by far the two most important parameters are the hydraulic conductivity and the matric potential. For many performance assessments and licensing requirements by the EPA and NRC, contaminant release rates and groundwater travel times must be determined. For the vadose zone, the hydraulic conductivity and the matric potential can be used in predictive modelling to determine the travel times.

Alternatively, the matric potential can be determined using closed centrifugation where no fluid enters the sample during rotation (Hassler, G. L. and E. Brunner. 1945. Trans. Am. Inst. Min. Metall. Eng. 160:114-123.; Hoffman, R. N. A. 1963. Soc. Pet. Eng. Jour., 3:227-235.). The UFA can be used to apply an adjustable acceleration to a core sample (Conca, J. L. and J. V. Wright. 1992. Applied Hydrogeology 1:5-24.). The sample is first saturated using an appropriate method, e.g., by performing a falling head experiment. The sample is then placed in the UFA and the speed is stepwise increased from 300 rpm to 10,000 rpm. At each speed the sample drains until the matric potential is equal and opposite to the equivalent pressure of the acceleration. The water exiting the sample is monitored using a strobe light, or the sample weight is measured at each step, and the run is continued at the next speed. The equivalent pressure at each speed can be determined by

P is the equivalent pressure (cm H₂O)
g is the acceleration due to gravity (981 cm/s²)
r₁ is radial distance to the sample top (cm)
r₂ is radial distance to the sample bottom (cm)
w is the rotation speed (radians/s)
r is the fluid density (g/cm³)

Table 1 lists the equivalent pressures for the UFA using the 50 cm3 sized sample.

The equivalent pressure at each point is plotted against the volumetric water content to obtain the () relationship.The equivalent pressure can be plotted in any convenient unit, e.g., centimeters of H₂O, bars, atmospheres, MPa, etc. Figure 1 shows () relationships obtained using the UFA for splits of the Warden Silt Loam from the McGee Ranch site in Eastern Washington State. Also shown are results on the same soil using traditional methods (Fayer, M. J., M. L. Rockhold, and D. J. Holford. 1992. Model Assessment of Protective Barriers: Part III, Status of FY 1990 Work. Technical Report PNL-7975, Pacific Northwest Laboratory, Richland, Washington.). The three curves compare well, although slight differences occur from the different packing densities of each sample.

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