Resuspension: Decadal Monitoring Time Series of the Anthropogenic Radioactivity Deposition in Japan

Monthly atmospheric depositions of ⁹⁰Sr and ¹³⁷Cs have been observed at the Meteorological Research Institute (MRI), Tsukuba, Japan. This study reports temporal trends and levels of ⁹⁰Sr and ¹³⁷Cs depositions in the 1990s. Although the current ⁹⁰Sr and ¹³⁷Cs concentrations declined dramatically, they have been found continuously in the deposition samples throughout the 1990s. During this period, the annual ⁹⁰Sr (¹³⁷Cs) deposits at MRI ranged from 70-180 (140-350) mBq/m²/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 ⁹⁰Sr and ¹³⁷Cs 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 ⁹⁰Sr (¹³⁷Cs) 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 ⁹⁰Sr and ¹³⁷Cs in Japanese surface soils. They were also comparable to those calculated from nationwide data of ⁹⁰Sr and ¹³⁷Cs 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 ¹³⁷Cs/⁹⁰Sr, 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.