SUMMARY OF A 5-YEAR LYSIMETER STUDY ON THE TIME-DEPENDENT TRANSFER OF CS-137, SR-90, PU-239, PU-240 AND AM-241 TO CROPS FROM 3 CONTRASTING SOIL TYPES .2. DISTRIBUTION BETWEEN DIFFERENT PLANT-PARTS

Loam, peat and sand soils were artificially contaminated with Cs-137, Sr-90, Pu-239, Pu-240 and Am-241 in the winter 1983/84 as part of a five-year lysimeter study on the time dependent transfer of radionuclides to crops. Carrot, cabbage and barley crops were grown in rotation in subsequent years and after harvesting were separated into edible and unpalatable portions for radiochemical analysis. The study therefore provided an opportunity to investigate variations in the partitioning of Cs-137, Sr-90, Pu-239, Pu-240 and Am-241 between edible and non-palatable parts of crops when soil was the only source of activity. Soil-to-plant concentration ratios were calculated for each crop part once equilibrium conditions had been established in the lysimeters, i.e. 1986-1989 inclusive. Observed concentration ratios were higher in unpalatable portions, particularly for Sr-90, Pu-239, Pu-240 and Am-241. Variations in observed concentration ratios between edible and unpalatable parts were attributed to differences in translocation, soil contamination or a combination of the two processes. Titanium was used as an approximate surrogate for soil contamination of crops. Whilst soil contamination was responsible for all of the additional Pu-239, Pu-240 and Am-241 present in carrot foliage, barley straw and outer leaves of cabbage, their enrichment in carrot peel seemed to reflect a genuine accumulation in the cells of the root cortex. Soil contamination made an important contribution to the Cs-137 content of non-edible parts of crops growing in sand and loam due to the relatively low soil-to-plant concentration ratios in these soils. For peat soils, root uptake and translocation were responsible for most of the Cs-137 in unpalatable crop parts. For Sr-90, differences in the distribution between crop parts were entirely attributable to differences in mobility and translocation due to plant physiological factors. Finally, the distribution of Cs-137 and Sr-90 in different plant parts was compared to that of their stable nutrient analogues K and Ca. In general, the partitioning of Cs-137 and Sr-90 was similar to K and Ca.