Relationships between the long-term mobility of Sr-90 in ploughed soil of upland field and soil properties

Though more than 25 years have elapsed since the latest atmospheric nuclear test, long-lived artificial radionuclides, such as 137Cs and 90Sr, still remain in the soil. We evaluated the mobility of 90Sr in ploughed upland soil, which affects the residual amount in the soil and plant uptake on the basis of long-term monitoring data. Soil was taken annually from 1961 to 1995 from 8 agricultural fields, and the concentration of exchangeable 90Sr in soil was determined. The concentration of exchangeable 90Sr in soil decreased exponentially with time. The environmental factor responsible for the decrease of 90Sr, λe, was determined by the exponential fit of the exchangeable 90Sr in ploughed soil vs. year followed by subtraction of the physical decay constant. The main factor controlling λe, the long-term mobility of 90Sr from ploughed soil, was the cation exchange capacity of soil. It was indicated that the entrapment of 90Sr on a cation exchange site retards the downward migration and wheat uptake of 90Sr from ploughed soil. (author)