Monitoring ^90Sr and ^137Cs in Rice, Wheat, and Soil in Japan from 1959 to 2000

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  • わが国の米、小麦および土壌における^90Srと^137Cs濃度の1959年から2000年にわたるモニタリング調査

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We monitored the radioactive contamination of rice (unpolished and polished), wheat (wheat grain and wheat flour), and paddy and upland soils collected annually from 17 agricultural fields in Japan over 42 years from 1959 to 2000. In 1963, when the largest annual precipitation of radioactive fallout was observed, the ^90Sr and ^137Cs concentrations in rice and wheat reached their maximum, 0.27 Bq/kg for ^90Sr and 4.2 Bq/kg for ^137Cs in polished rice and 12 Bq/kg for ^90Sr and 44 Bq/kg for ^137Cs in wheat grain. The concentrations of ^90Sr and ^137Cs in the plowed layer of paddy and upland soils reached their maximum from 1963 to 1966. After 1966, the concentrations of ^90Sr and ^137Cs in rice, wheat, and soils gradually declined. Due to the Chernobyl accident in 1986, an increased concentration of ^137Cs was found in some wheat grains harvested in areas that had been contaminated by fallout at the heading time of wheat. On the other hand, the contamination of rice and soil was not obvious. The ^90Sr and ^137Cs concentrations in rice, wheat, and soils tended to be higher on the Japan Sea side than on the Pacific Ocean side, corresponding to the trends in the level of radioactive fallout. The ^90Sr ratios for polished rice/unpolished rice and wheat flour/wheat grain were 0.2 and 0.3, and the ^137Cs ratios for polished rice/unpolished rice and wheat flour/wheat grain were 0.4 and 0.5, respectively. The uptake of ^90Sr by wheat was higher than that by rice, whereas the uptake of ^137Cs by wheat was lower than that by rice.

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