A rapid and inexpensive incubation and extraction technique for estimating the available low molecular weight and potentially bioavailable fraction of Cs isotopes in vegetation has been developed. Samples of contaminated vegetation (grass, herbage and fungi) and litter were exposed to remen liquid from sheep, by in vitro incubation or by placing nylon bags in the rumen of sheep (in sacco). The results were compared with laboratory extractions using rumen buffer, NH4OAc, CsCl and de-ionized water. The release of low molecular weight 137Cs species after in vitro incubation with rumen liquid for 4–8 h was 75–85% for grass, herbage and fungi, 30% for lichen and 10% for litter. The reproducibility of the method was 5% for digestible matrices and 10–20% for litter where the fraction of Cs isotopes released was small. Extractions with NH4OAc and CsCl released 75 and 80% of the Cs isotopes in grass, respectively. Significantly lower extraction yields were obtained with de-ionized water and buffer. During in sacco incubation, 90–100% of the Cs isotopes in vegetation was released within 1 h; however, potentially available low molecular weight forms and Cs-bearing colloidal material could not be differentiated. The results obtained by incubation with rumen liquid and by extraction with NH4OAc or CsCl were in good agreement with published data from a 3 week feeding trial. For practical reasons extraction with NH4OAc or CsCl rather than incubation with rumen liquid is recommended for estimating the fraction of available low molecular weight Cs species. Results from incubation with rumen liquid showed that the availability of Cs isotopes in grass collected in 1986 was low (15%), but increased with time up to 1989 (75–80%). It is probable that the high fraction of inert 137Cs species reflects surface contamination in 1986. The increasing fraction of available 137Cs in grass with time indicates a higher degree of root uptake and a reduction in resuspension and surface contamination from soil.
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