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Permeation grouting with colloidal silica gel is a potentially effective means for creating hydraulic barriers to prevent the advective migration of radioactive contaminants in shallow permeable sediments. However, the effectiveness of silica gel grouted barriers in controlling transport of fission product radionuclides through sorption and diffusion is unknown. To resolve this question, static-batch sorption measurements with Cs + and Sr 2+ were conducted at ambient temperature (23 • C) and for times up to 96 days on silica gel and on two kaolinitic sediments from the Savannah River Site in South Carolina. Sorption experiments on the latter were conducted both in the presence and in the absence of silica gel. Cesium and strontium concentrations were varied between 10 −3 to 10 −8 M (mol/L) and 10 −4 to 10 −8 M, respectively. The pH values after one day varied for each experiment and ranged from 3.6 to 6.6. The pH of the solutions were, however, buffered by the sediments and/or silica gel, and changed little over the duration of the experiments. The fraction of both cesium and strontium retained on the solids increased with decreasing radioelement concentration. The sorption data were fitted to the Freundlich isotherm model.

The rate at which elements can be transported in groundwater systems is governed in part by the solubility of the element in the groundwater. This report documents plutonium solubility experiments, conducted over the past two years at the Lawrence Berkeley Laboratory, in a brine simulant relevant to the Waste Isolation Pilot Plant. Approximately 1 to 2.5 mL of five stock solutions containing single oxidation states of plutonium (Pu(IV)-polymer, Pu 3+ , Pu 4+ , Pu0 2 + , and PuOD were added to -75 mL of synthetic H-17 Brine in five reaction vessels. Initial plutonium concentrations ranged from 1.3 X10 4 to 5.1 Χ10" 4 Μ (moles per liter) total plutonium. Because these initial concentrations were far above the plutonium solubility limit in H-17 Brine, plutonium-containing solids precipitated. Aqueous plutonium concentrations were measured over time until steadystate was reached, requiring over 300 days in H-17 Brine. Steady-state plutonium concentrations ranged from 3.0 X 10~8 to 7.6 X 10~7 M, and the predominant plutonium oxidation state in solution was Pu(VI). The solid phase from the initially Pu(IV)polymer experiment remained polymeric-Pu(IV). The solids that formed in the initially Pu 3+ and Pu 4+ experiments were crystalline yet remain unidentified. The solids that formed in the initially Pu0 2 + and PuOi + experiments appear to be sodium plutonyl(V) carbonates.

The rate at which elements can be transported in groundwater systems is governed in part by the solubility of the element in the groundwater This report documents plutonium solubility experiments, conducted over the past two years at the Lawrence Berkeley Laboratory, in a brine simulant relevant to the Waste Isolation Pilot Plant Approximately 1 to 25 mL of five stock solutions containing single oxidation states of plutonium