Radionuclide transfer to plants in the human food chain is often quantified using concentration ratios for different groups of plants and soil types. A recent IAEA handbook provides compiled concentration ratio values for the human food chain.27,28 There is an assumption that equilibrium exists between the plant and soil, which is not valid if there are large temporal spikes in releases.

Similarly, for estimation of exposure of plants themselves in environmental assessment models, the most common approach is the whole organism con­centration ratio (CRwo), where:

In human food chain models, CR is most usually defined on the basis of plant dry matter activity concentrations. The CR values, categorised by soil type, for human food chain assessments have been collated for many radionuclides in the IAEA handbook.28 More mechanistically based approaches enabling pre­dictions of radionuclides, such as radiocaesium and radiostrontium, which vary with soil properties are also available in some assessment tools for the human food chain. In contrast, current CR values for estimation of plant exposure generally do not distinguish between different soil types.

Whilst root uptake is a key pathway of plant contamination, for radio­nuclides which have a low root uptake, such as plutonium and americium, resuspension and adherence of contaminated soil on plant surfaces can con­stitute a significant proportion of the radionuclide content of plants as sampled in the environment.

As examples, the CR values for the selected radionuclide-wildlife group combinations as anticipated to be reported in the new IAEA handbook on transfer to wildlife are given in Figure 1.