these sources are discussed. The results of testing contaminant release from the waste forms are provided, as well as modeling methodologies to use the release data in performance assessment calculations. At this time, the most sophisticated release model available is for glass waste forms. A kinetic release model has been developed for glass, and a testing methodology has been implemented to provide the necessary rate parameters. Progress is being made on understanding the influence on the dissolution rate of secondary mineral formation on the glass surface. A comprehensive testing methodology also has been developed for residual tank waste; however, the waste material appears to be primarily amorphous and not amenable to common characterization techniques. For the few tanks that have been studied, only empirical release models have been developed because of the difficulty in characterizing the waste and developing more sophisticated mechanistic models. Studies of contaminant release from cementitious waste forms have focused for the past 35 years on a diffusional model in which the contaminants move through the pores of the cement to the surface of the waste form. Advances have been made in understanding the evolution of cement pore water and the chemical alteration of cement solids with time; however, this knowledge has not been incorporated into a more advanced model of contaminant interaction with and release from cementitious waste forms. The release model for contaminants on affected soil near the tanks is also a fairly simple model based on the linear adsorption (Kd) approach. The lack of a more mechanistic model for contaminant release from soil requires a large Kd database to account for differences in soil properties and solution compositions throughout the vadose zone.

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