Long-term monitoring and analysis of 90Sr and 137Cs concentrations in rice, wheat and soils in Japan from 1959 to 2000

Atmospheric nuclear tests in the 1950s and thereafter had showered radioactive fallout throughout Japan. Therefore, radioactive contamination of crops cultivated in Japan was concerned. We continued to monitor concentrations of 90Sr and 137Cs in rice, wheat and soils collected annually from sixteen national and prefectural experimental farms for forty-two years, from 1959 to 2000. In 1963, when the largest annual precipitation of radioactive fallout was observed, 90Sr and 137Cs concentration in rice and wheat reached at 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 concentration of 90Sr and 137Cs in the plowed layer of paddy and upland soils reached maximum from 1963 to 1966. After 1966, concentrations of 90Sr and 137Cs in polished rice, wheat grain, and soils were gradually decreased although there were some minor fluctuations. The Chernobyl nuclear power plant accident in 1986 caused contamination of wheat grain by 137Cs up to 6.0 Bq/kg. However, the concentration of 137Cs in wheat grain decreased to its normal level in the following year. There was no evidence for the polished rice contamination in Japan that could be ascribed to the accident at Chernobyl. Based on the analyses of the data above, we made several interesting findings as follows: a) The accumulated amounts of 90Sr and 137Cs in fallout during cultivation period were highly correlated with those concentrations in husked rice, polished rice and wheat grain. The estimate equations derived from the correlations were accurate enough for quick prediction of contamination level of polished rice and wheat grain based on 90Sr and 137Cs contents in fallout in case of contingencies. b) The sensitive response of 90Sr and 137Cs in polished rice and wheat grain to concentrations of fallout suggested that direct absorption of 90Sr and 137Cs from radioactive fallout deposited on plant body played important role. In 1963 the contribution of direct absorption of 90Sr and 137Cs to total absorption was 70-95% for both polished rice and wheat grain. After then, the contribution of direct absorption continued to decrease, and became negligible in 1990. Instead, the indirect absorption through soil became major pathway for the uptake of 90Sr and 137Cs by rice and wheat. c) Concentrations of 90Sr and 137Cs in plowed layer of paddy and upland fields were higher on the Japan Sea side than those on the Pacific Ocean side. d) The residence half-times of 90Sr and 137Cs in plowed layer were estimated as 6-13 years for 90Sr and 9-24 years for 137Cs in paddy fields, and 6-15 years for 90Sr and 8-26 years for 137Cs in upland fields, respectively.