Resuspension: Decadal monitoring time series of the anthropogenic radioactivity deposition in Japan

Monthly atmospheric depositions of Sr-90 and Cs-137 have been observed at the Meteorological Research Institute (MRI), Tsukuba, Japan. This study reports temporal trends and levels of Sr-90 and Cs-137 depositions in the 1990s. Although the current Sr-90 and Cs-137 Concentrations declined dramatically, they have been found continuously in the deposition samples throughout the 1990s. During this period, the annual Sr-90 (Cs-137) deposits at MRI ranged from 70-180 (140-350) mBq/m(2)/year. With a sufficiently long time series, the decreasing trend of the deposition evidently differs from the past stratospheric fallout; it is far slower. Thus, reservoirs other than the stratosphere provide small amounts of Sr-90 and Cs-137 to the atmosphere. A simple calculation clearly refutes the significance of the ocean as a potential source of airborne anthropogenic radioactivity. We will demonstrate that these radionuclides in the deposited materials originate from resuspension processes (soil dust suspension processes). The temporal trends of the time series monitoring reveal differences from those in the UNSCEAR Report 2000, which were predicted by a model that disregarded resuspension. The specific activity of Sr-90 (Cs-137) in the annual depositions exhibited a 10-year (20-year) half-life. Those data were comparable with values reported in the literature for the half-residence time (HRT) of Sr-90 and Cs-137 in Japanese surface soils. They were also comparable to those calculated from nationwide data of Sr-90 and Cs-137 concentrations in the surface soil (0-10 cm) obtained from the Ministry of Education, Culture, Sports, Science and Technology Environmental Radiation Database (the MEXT Database). Regarding the activity ratio of Cs-137/Sr-90, the Japanese nationwide surface soil data collected during the 1990s in the MEXT Database (median: 5.3, n = 584) did not accord with that in the deposition samples (average: 2.1, n = 82) at MRI. This supports our previous hypothesis that Asian dust may transport a large fraction of anthropogenic radioactivity into the Japanese atmosphere. We need to study the fate of long-lived anthropogenic radioactivity dispersed in the environment over greater spatial and temporal scales.