Unsaturated moisture and radionuclide transport: laboratory analysis and modeling

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Abstract
This report describes several laboratory procedures and computer model simulations used to evaluate the transport of water and radionuclides through unsaturated Hanford soils. The unsaturated hydraulic conductivity was measured using the steady state methods of Klute and the transient state method of Rose. These experimental data were compared to other conductivity models. Good agreement was found between all methods in the wet range; however, disagreement was found in the dry range. None of the conductivity models explicity addresses the water vapor component of the conductivity. This may explain the under prediction of the hydraulic conductivity in the dry range where vapor transport is important. Radionuclide transport through unsaturated media was investigated by using two solute transport models to describe the transport of tritium and strontium-85 in laboratory columns. A two parameter convective-dispersive model was compared with a four parameter mobile-immobile water model. Both models adequately described the movement of tritium and strontium through small (5 cm x 27.5 cm) columns and the movement of tritium and strontium through a large (0.5 m x 1.7) column. The dispersion coefficient was found to be sensitive to changes in both velocity and column length. The mobile-immoble water equations were not as sensitive tomore » changes in experimental scales as the convective-dispersive equation. Both models were relatively successful in describing the rapid flush of strontium-85 from a column initially leached with a low salt solution followed by a high salt solution, a phenomona called the snow plow effect. The four parameter mobile-immobile water model predicted the initial release of the strontium more accurately than the two parameter convective-dispersive model. Both models confirm enhanced mobility of strontium-85 with leaching solutions of increased salt concentration.« less

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