Radioactive waste, accumulated during Pu production, has leaked into the subsurface from underground storage tanks at the U.S. Department of Energy's Hanford site. The leaking solutions contained 137Cs and were of high ionic strength. Such a tank leak was simulated experimentally in steady-state flow experiments with packed Hanford sediments. The initial leak was simulated by a 1 M NaNO3 solution, followed by a decrease of ionic strength to 1 mM NaNO3. Cesium breakthrough curves were determined in both 1 M and 1 mM NaNO3 background. Colloidal particles were mobilized during the change of ionic strength. Mobilized colloids consisted mainly of quartz, mica, illite, kaolinite, and chlorite. Electrophoretic mobilities of colloids in the eluent solution were -3(microm/s)(V/cm) and increased to less negative values during later stages of mobilization. Mobilized colloids carried a fraction of the cesium along. While transport of cesium in 1 M NaNO3 background was much faster than in 1 mM NaNO3, cesium attached to colloids moved almost unretarded through the sediments. Cesium attached to mobilized colloids was likely associated with high affinity sorption sites on micas and illites.

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ards, 1948, 1965 Richards and Ogata, 1961), has been the method of choice for determining water-retention Pressure plates are used routinely to measure water-retention charcharacteristics for literally thousands of soil samples acteristics of soils. Plates of varying porosity are used, depending on the pressure range of interest. For applied pressures up to 1.5 MPa, during the past 50 yr (Clapp and Hornberger, 1978; 15-bar porous ceramic plates with fine porosity are used because of Rawls et al., 1982). Specific details for use of pressure their high bubbling pressure (Ͼ1.5 MPa), which limits airflow through plates for matric suction measurements are provided by the plate. The typical saturated hydraulic conductivity of the 15-bar Richards (1965), Klute (1986), Topp et al. (1993), and plate is Ͻ3 ϫ 10 Ϫ11 m s Ϫ1 . Low plate conductance coupled with decreas- Townend et al. (2001). Typically, soils are tested over ing soil hydraulic conductivities at high pressures strongly influence the pressure ranges from 0 to 0.1 MPa, from 0.1 to 0.5 equilibrium times, which theoretically may extend to months or years.MPa, and from 0.5 to 1.5 MPa. The soils are tested onWe measured the soil water pressures (suctions) for three soils, a plates that have bubbling pressures (air-entry values) sand, a silt loam, and a clay, placed on 15-bar pressure plates for 10 Ͼ0.1 MPa, Ͼ0.5 Mpa, and Ͼ1.5 MPa, respectively. Table d or longer, with and without static loads and with and without using 1 lists characteristics for plates that cover the pressure a kaolinite slurry to improve plate contact. Total matric suctions, inferred from peltier psychrometry data, were always Ͻ1.0 MPa. range from 0 to 1.5 MPa.

Pressure plates are used routinely to measure water‐retention characteristics of soils. Plates of varying porosity are used, depending on the pressure range of interest. For applied pressures up to 1.5 MPa, 15‐bar porous ceramic plates with fine porosity are used because of their high bubbling pressure (>1.5 MPa), which limits airflow through the plate. The typical saturated hydraulic conductivity of the 15‐bar plate is <3 × 10−11 m s−1 Low plate conductance coupled with decreasing soil hydraulic conductivities at high pressures strongly influence equilibrium times, which theoretically may extend to months or years. We measured the soil water pressures (suctions) for three soils, a sand, a silt loam, and a clay, placed on 15‐bar pressure plates for 10 d or longer, with and without static loads and with and without using a kaolinite slurry to improve plate contact. Total matric suctions, inferred from peltier psychrometry data, were always <1.0 MPa. When sample height was increased from 1.5 to 3 cm, the water contents increased and total suctions decreased to 0.15 MPa for sand, 0.3 MPa for silt loam, and 0.55 MPa for clay. These data suggest that alternative methods other than pressure plates may be required to measure equilibrium water suctions of soils in reasonable times in the 1.5‐MPa (15‐bar) pressure range and that loading of the samples and use of kaolinite slurry appear to be ineffective in speeding equilibrium.

Accurate prediction of evaporative water loss from dryland agricultural soils requires knowledge of diffusive resistance. Our objective was to experimentally determine the effective vapor diffusion coefficients and the diffusive resistances for water vapor through wheat (Triticum aestivum L.) residue layers. A laboratory diffusion chamber was designed to investigate the effects of wheat residue type (straw vs. chaff), residue amount (2, 4, 8, and 12 Mg ha−1), and residue orientation (horizontal vs. vertical straw) on vapor diffusion. The diffusion chamber consisted of two well‐stirred chambers separated by a residue layer. In one chamber, a constant saturated water vapor pressure was maintained, while in the other chamber, vapor pressure increased with time due to the diffusion of vapor through the residue layer. Vapor pressure was measured with a humidity sensor. The diffusion equation was used to obtain the effective diffusion coefficient from the measured data by inverse modeling. Results showed that for both straw and chaff residue, increasing the quantity of residue did not necessarily decrease the effective diffusion coefficient but did increase the diffusive resistance. For the same quantity of residue, chaff had a lower effective diffusion coefficient than straw, but the diffusive resistances were similar. The diffusive resistances were in the order of 800 to 3700 s m−1 for 2 to 12 Mg ha−1 residue treatments. The factor affecting the diffusive resistance the most was the thickness of the residue layer: the thicker the residue layer, the larger the diffusive resistance.