Residence half-time of 137Cs in the top-soils of Japanese paddy and upland fields

A series of top-soil samples of 14 paddy fields and 10 upland fields in Japan, were annually collected during more than 30 years, to be examined in the contents of 137Cs. The data, which were obtained by the use of a gamma spectrometric system, received some statistical treatments to distinguish the annual decline of 137Cs contents from deviations. Then the authors calculated 'residence half-time of 137Cs' within top-soil, and 'eluviation rate of 137Cs' from top to the sub-layer of the soil. The following nationwide results were obtained irrespective of paddy or upland field: (1) The 'apparent residence half-time' was estimated as 16 - 17 years. This consists of both effects of eluviation and nuclear disintegration. (2) The true residence half-time' was reported as 41 - 42 years. This depends on the eluviation speed of 137Cs exclusively, because the influence of nuclear disintegration has been compensated. (3) The eluviation rate of 137Cs from top-soil down to the sub-soil was 1.6 - 1.7% per year. (4) The ratio of distribution of 137Cs between top-soil and the sub-soil was estimated to be 6:4 as average at the date of 1996. (author)