The Availability of Soil-Associated Radionuclides for Uptake by Ruminants: Implications for Radiological Assessment Models

The potential importance of inadvertent ingestion of soils as a route of intake of radionuclides by grazing animals has been recognised for many years. The process has been taken into account in radiological assessment models such as FARMLAND, the foodchain model used by NRPB. Generally, these models assume that, following ingestion, the availability of soil-associated radionuclides for uptake would be identical to that of biologically incorporated activity, whereas data from field experiments and feeding trials suggest that the former would be lower than the latter. However, until recently insufficient data were available on which to base recommendations for broad assessment purposes. A simple in vitro method has been developed to estimate the availability of soil-associated radionuclides for uptake by ruminants. The procedure has been applied to a range of soil types, and has been validated using published data and dedicated in vivo experiments. The radionuclides of principal interest were 137 Cs, 239/240 Pu and 241 Am, although limited studies on 90 Sr were also carried out. The interpretation of the data was facilitated by the introduction of a new parameter, the availability factor; this represents the fraction of the soil-associated activity that is taken into solution in the rumen. This fraction of the ingested activity may then be considered to be as available for uptake as that which is incorporated into foliage. Taking the data from in vivo and in vitro experiments together, default values for the UK would be: 137 Cs, 0.2; 90 Sr, 0.3; actinides, 0.1. This parameter can be simply incorporated into a model of the soil-grass-animal pathway, but an extrapolation to dosimetric models of man would require more care.