[en] The design and assessment of underground waste disposal options requires modelling the dispersal of contaminants within aquifers. The logical structure of the development and application of disposal models is discussed. In particular we examine the validity and interpretation of the gradient diffusion model. The effective dispersion parameters in such a model seem to depend upon the scale on which they are measured. This phenomenon is analysed and methods for modelling scale dependent parameters are reviewed. Specific recommendations regarding the modelling of contaminant dispersal are provided. (author)