An ability to predict radionuclide activity concentrations in biota is a requirement of any method assessing the exposure of biota to ionising radiation. Within the ERICA-Tool fresh weight whole-body activity concentrations in organisms are estimated using concentration ratios (the ratio of the activity concentration in the organism to the activity concentration in an environmental media). This paper describes the methodology used to derive the default terrestrial ecosystem concentration ratio database available within the ERICA-Tool and provides details of the provenance of each value for terrestrial reference organisms. As the ERICA-Tool considers 13 terrestrial reference organisms and the radioisotopes of 31 elements, a total of 403 concentration ratios were required for terrestrial reference organisms. Of these, 129 could be derived from literature review. The approaches taken to selecting the remaining values are described. These included, for example, assuming values for similar reference organisms and/or biogeochemically similar elements, and various simple modeling approaches.

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same period as the Norwegian study 1 . Our results for vegetation and water contamination (examples in Fig. 1) show the same two-component exponential decline (Q t ǃQ 1 e ǁk,t +Q 2 e ǁk,t ) observed for immature fish 1 . The decline in 137 Cs in mature fish was influenced by slower biological uptake rates during the initial period after Chernobyl 1,3 , so only the second component of the decline is shown for our fish data (Fig. 1).Our results show that the effective ecological half-life (T eff , the time for the 137 Cs concentration to reduce by 50%) in young fish, water and terrestrial vegetation has increased from 1-4 years during the first five years after Chernobyl 1,4 to 6-30 years in recent years. The common rate of decline in 137

The transfer of radioactivity to the milk and meat of farm animals is likely be a major exposure pathway of human populations, following an environmental release of radioactivity. The importance of source-dependent bioavailability in determining absorption from the ruminant gastrointestinal tract of the radiologically significant radionuclides (radiocesium, radiostrontium, radioiodine, and plutonium) is reviewed. The requirements for and suitability of in vitro methods of determining bioavailability for absorption of these radionuclides is also assessed. Radiocesium absorption varies over a 50-fold range, depending upon dietary source. Source-dependent bioavailability is therefore an important factor in determining the radiocesium contamination of ruminant-derived food products, and reliable in vitro techniques have been developed to rapidly determine its bioavailability. In contrast, under conditions of adequate calcium intake, the absorption of radiostrontium will not be greatly influenced by the dietary source. Results of in vitro extractions of radiostrontium could be misleading, as they indicate differences in bioavailability that are not observed in the animal. Absorption of radioiodine is complete and independent of source. There is currently a lack of data on plutonium absorption in ruminants. However, on the basis of the limited data available, in vitro incubations provide a measure of the relative bioavailabilities of plutonium from different sources. The applicability of the techniques reviewed to the study of other radionuclides and heavy metals is discussed.