Characteristics of cumulative deposition of fallout Pu in environmental samples collected in South Korea

摘要

This paper describes the cumulative deposition of fallout Pu in soil and lichen at the present time and gives the characteristics of fallout Pu deposits in the soil. In the forest soil the accumulated depositions of ~(239, 240)Pu were estimated to be in the range of 34.0 to 101.2 Bqm~(-2) with an average value of 65.3 + - 21.6 Bq m~(-2). The average inventory of ~(239, 240)Pu in the forest was calculated to be twice that in the hills. Also, the deposited activities of ~(239, 240)Pu in cultivated soil were significantly lower than those in the hills or forest. However, the cumulative depositions of fallout Pu in the volcanic ash soil on Cheju Island were much higher than those in the forest and hill soils. The measured activity concentrations of Pu isotopes in lichens and mosses showed large variations due to characteristics of the species and the life-span of lichen and moss colonies. From depth profiles, it was found that most of the fallout Pu has been accumulated in the upper 10 cm layer of soil. Except for a few cases, the concentration of ~(239, 240)Pu in soil tended decrease exponentially with increasing soil depth. Among parameters affecting the cumulative deposition of fallout Pu, organic substances and rainfall play an important role in the retention and relative mobility of fallout Pu in the soil. However, pH showed a weak correlation with the deposition of fallout Pu in the soil. From sequential leaching experiments, ~(239, 240)Pu was found to be associated predominantly with the "organic" and "oxy-hydroxy" fractions. Both the activity ratios of ~(238)Pu/~(239, 240)Pu and ~(241)Pu/~(239,240)Pu in soils, lichens and mosses and the atomic ratios of ~(240)Pu/~(239)Pu in soils are close to those observed in the cumulative deposits of global fallout from nuclear weapons testings.