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 as sessment models such as FARMLAND, the foodchain model used by NRPB. Ge nerally, these models assume that, following ingestion, the availabili ty of soil-associated radionuclides for uptake would be identical to t hat of biologically incorporated activity, whereas data from field exp eriments and feeding trials suggest that the former would be lower tha n the latter. However, until recently insufficient data were available on which to base recommendations for broad assessment purposes. A sim ple 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 usin g published data and dedicated in vivo experiments. The radionuclides of principal interest were Cs-137, Pu-239/240 and Am-241, although lim ited studies on Sr-90 were also carried out. The interpretation of the data was facilitated by the introduction of a new parameter, the avai lability factor; this represents the fraction of the soil-associated a ctivity that is taken into solution in the rumen. This fraction of the ingested activity may then be considered to be as available for uptak e as that which is incorporated into foliage. Taking the data from in vivo and in vitro experiments together, default values for the UK woul d be: Cs-137, 0.2; Sr-90, 0.3; actinides, 0.1. This parameter can be s imply incorporated into a model of the soil-grass-animal pathway, but an extrapolation to dosimetric models of man would require more care.