Six models that may be used to describe the interaction of radioactive waste solids with aqueous solutions are as follows:

1. Simple linear mass transfer;
   
2. Simple parabolic mass transfer;
   
3. Parabolic mass transfer with the formation of a diffusion-limiting surface layer at an arbitrary time;
   
4. Initial parabolic mass transfer followed by linear mass transfer at an arbitrary time;
   
5. Parabolic (or linear) mass transfer and concomitant surface sorption; and
   
6. Parabolic (or linear) mass transfer and concomitant chemical precipitation.
   

Some of these models lead to either illogical or unrealistic predictions when published data are extrapolated to long times. These predictions result because most data result from short-term experimentation. Probably for longer times, processes will occur that have not been observed in the shorter experiments. This hypothesis has been verified by mass-transfer data from laboratory experiments using natural volcanic glass to predict the composition of groundwater. That such rate-limiting mechanisms do occur is reassuring, although now it is not possible to deduce a single mass-transfer limiting mechanism that could control the solution concentration of all components of all waste forms being investigated. Probably the most reasonable mechanisms are surface sorption and chemical precipitation of the species of interest. Another is limiting of mass transfer by chemical precipitation on the waste form surface of a substance not containing the species of interest, that is, presence of a diffusion-limiting layer. The presence of sorption and chemical precipitation as factors limiting mass transfer has been verified in natural groundwater systems, whereas the diffusion-limiting mechanism has not been verified yet.