Inequilibrium between Fallout 137Cs and Stable Cs in Cultivated Soils.
Hirofumi Tsukada, Akira Takeda, Shun'ichi Hisamatsu, and Jiro Inaba. Institute for Environmental Sciences, 1-7 Ienomae, Obuchi, Rokkasho, Aomori, 039-3212, Japan
Physical-chemical form of radionuclides is an essential factor to decide the fate of radionuclides in the environment, and also play an important role in their migration in soils. Cesium-137 is an important radionuclide for the assessment of radiation exposure to the public around the nuclear facilities because of its high fission yield and relatively long half-life. In order to investigate the physical-chemical forms of fallout 137Cs (mainly derived from nuclear weapons tests from 1940s to 1980s) in cultivated soils, 137Cs and stable Cs (133Cs) extracted with two extractants were determined. Residue after the stronger extraction was separated on the particle size, and then analyzed for 137Cs and stable Cs. Soil samples were collected in 1991-1994 from agricultural fields throughout Aomori Prefecture, Japan, where the first commercial nuclear fuel reprocessing plant and a fission reactor are located. Two fractions were individually extracted with 1M CH3COONH4 solution (exchangeable fraction, E2), and 0.8M CH3COONH4 in 5% HNO3 solution after H2O2 oxidization (E2+organic bound fraction, E4). The residues after extracting the E4 fraction were divided into four size fractions; clay (<0.002 mm), silt (0.002-0.02 mm), fine sand (0.02-0.2 mm) and coarse sand (0.2-2 mm). The silt fraction was not recovered because of incomplete separation of silt from clay. The concentration of 137Cs in the fractions and total content (E5) was determined using a low background 2p gas flow proportional counter after radiochemical separation of Cs or a well-type Ge gamma-ray detector. The concentration of stable Cs in the samples was determined by ICP-MS after adequate pretreatment. The distribution pattern of 137Cs contents among the fractions were different for the soil samples, and has no correlation with soil properties including pH, a cation exchange capacity (CEC) and a carbon content. The content of 137Cs in the exchangeable fraction was less than 20% (4-18%), and that in the organic bound (E4-E2) and strongly bound factions (E5-E4) were 0-57% and 25-96%, respectively. The concentration of 137Cs in the clay fraction was several folds higher than that in other size fractions. The specific activity of 137Cs (the ratio of the concentration of 137Cs to that of stable Cs) decreased in the order of the exchangeable, organic bound and strongly bound fractions. Equilibrium between fallout 137Cs and stable Cs has not reached among those fractions, even though most of the 137Cs had been deposited on the soils more than several decades ago. This work was supported by a grant from the Government of Aomori Prefecture, Japan.