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A simple in-vitro method has been developed to estimate the availability of soil-associated radionuclides for uptake by grazing ruminants. The radionuclides of principal interest were 137Cs, 239240Pu and 241Am. A range of soils was incubated with freshly-collected rumen liquor over a 24 h period under simulated in-vivo conditions. The mixtures were then separated into liquid and solid phases, and the distribution of each radionuclide between the two phases was determined. The percentage of the activity found in the liquid phase was generally low and for the radionuclides of principal interest was never more than 20%. All of the radionuclides were found to be most readily extracted from poorly developed organic soils. The effect of pH on radionuclide extractability was evaluated for two soils, an alluvial gley and a peaty ranker. For both soils, radiocaesium extractability was constant across the range of pH likely to be encountered in the rumen, and was in good agreement with values derived from in-vivo studies in the literature. In contrast, actinides were found to be more readily extracted at pH 6.5 than 5.5 from the peaty ranker. For all of the soils studied, estimates of actinide availability based on this technique were low and comparable with those derived from the limited number of in-vivo studies that have been published. However, a comparison with published values of gut uptake factors implied that not all of the activity taken into solution in the rumen would be taken up by the animal. This was not an unexpected result, and may be due to the association of actinides in the solution phase with species that are not amenable to transfer across the gut, such as organic complexes of high molecular weight or colloids. The results so far suggest that this technique can be used to estimate the availability of soil-associated radionuclides for uptake by ruminants, at least for use in radiological assessments; such estimates are more appropriate for this purpose than values inferred from sequential extraction techniques. |
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