The results of models for predicting the migration of toxic substances
through the environment are, generally, affected with large uncertain
ties that are due to the complexity of the modelled environmental syst
ems and to the difficulty of obtaining reliable values of the model pa
rameters. Surprisingly, some models for predicting the migration of ra
dioactive substances in water bodies, despite the complexity of the in
volved processes, are characterised by small levels of uncertainty. In
the present paper, it was demonstrated that some of the parameters us
ed in such sort of models show small variability, which is due to the
statistical or 'collective' effects caused by the mutual compensation
of some processes in the environment. As a consequence, the values of
the 'collective' parameters vary within small ranges despite the wide
variability of the environmental circumstances. This occurrence implie
s that collective models show relatively small levels of uncertainty.