To be able to estimate radionuclide activity concentrations in exposed organ­isms we need to quantify and model the transfer processes. The approach used varies depending on the objective and the need for detailed information. Some models mathematically describe the transfer processes through the application of steady state compartment models which assume that there is an equilibrium established in the environment between the source and the receptor. However, in some cases there may be a need to describe transfer in more detail, either to take account of some of the many environmental factors which affect the extent of transfer in the environment, or to quantify changes in transfer with time after radionuclides have been received by ecosystems.

The pathways leading to exposure of organisms in terrestrial ecosystems can be subdivided into external (see section 4) and internal components. Internal irradiation occurs from radionuclides which are absorbed and distributed within the organism. Ingestion of plants, animals, soil/sediment and detritus also leads to direct irradiation of the digestive tract.

Since the dosimetric calculation to estimate absorbed doses are derived for a defined shape of a whole organism (see section 4), there is a requirement to estimate the whole organism activity concentrations. This contrasts with the human food chain where the focus is on the part of the organism that is ingested by humans.

The processes involved in environmental terrestrial pathways are briefly summarised here before we describe the current methods by which exposure of organisms to radionuclides are currently quantified and evaluated.