Behaviour of cs 137 in soils and pastures a long term experiment

Abstract

The movement of Cs 137 in soil and its absorption by ryegrass have been studied over a period of 6 years and compared with that of Sr 90. Carrier-free nuclides were added to the surface of 4 soils which contrasted in their content of exchangeable cations and clay and exposed to natural weather conditions. The downward movement of Cs 137 was greatest in a calcareous soil which contained over 25 m-equiv./100 g exchangeable cations and least in a soil with a high clay content; after 6 years the median depth ranged from 1 to 3 cm. The corresponding values for Sr 137 were 3 and 5 cm respectively. As with Sr 90 the movement of Cs 137 was unaffected by cropping with ryegrass but unlike Sr 90 the annual additions of fertilizer to all four soils were without effect. During the first 3 years of the experiment the absorption of Cs 137 by ryegrass decreased by factors ranging from 5 to 40, the decrease being greatest from the soil with the highest clay content and least from the most sandy soil. It was thought to be caused by the progressive fixation of Cs 137 on clay minerals. This fixation appeared to be largely complete 3 years after contamination since the Cs 137 content of grass decreased thereafter by only about 30 per cent. The absorption of Cs 137 was also influenced by the concentration of exchangeable potassium in the soils. The Cs 137 content of grass grown on the soil with the lowest exchangeable potassium and the lowest clay content was some 10 times greater than that from the soil with the highest potassium content. From all soils the amount of Cs 137 absorbed was much less than that of Sr 90 and the ratio in which the two nuclides were absorbed decreased with time. In the first year the ratio of Cs 137 absorption to that of Sr 90 ranged from 0·04 to 0·2 whilst corresponding values in the fifth year were 0·01 and 0·04.