Vertical distribution and temporal changes of 137Cs in soil profiles under various land uses after the Fukushima Dai-ichi Nuclear Power Plant accident

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Abstract

We monitored the vertical distribution of (137)Cs in soil profiles under eight different land uses for the 2y after the Fukushima Dai-ichi Nuclear Power Plant accident, and discussed the temporal changes in the early-stage of the migration and the determinants of the initial distribution. The soil samples were collected for four surveys using a scraper plate at each study site, which consisted of three forests (mixed forest, mature cedar, and young cedar), two grasslands (pasture and meadow) and three abandoned agricultural fields (farm land, tobacco field, and paddy field). The land use patterns have a large influence on some soil properties and the migration processes of (137)Cs above ground, resulting in different distribution of (137)Cs in those soil profiles. Specifically, the secondary deposition of (137)Cs from the coniferous canopy, retention of (137)Cs by litter layer, and the homogenization of (137)Cs concentrations in surface soil by natural soil mixing such as the disturbance by cattle grazing, roots growing and the formation of needle ice were important to cause redistribution of the deposited (137)Cs. Only in the paddy field, the (137)Cs inventory in subsurface soils (5-10cm) gradually increased and comprised 26% of the total (137)Cs in 2y, showing the downward migration of (137)Cs to subsurface soil. In the other sites, it was considered that (137)Cs were strongly adsorbed by soil particles and rarely migrated downward as soluble form. Vertical distributions during the first survey were able to be used as the initial distributions and were well fitted to the exponential equation. The distribution parameters α (relaxation depth) and β (relaxation mass depth), calculated by the exponential equation were correlated with RIP (r=-0.806, p<0.05), macro pore (r=0.651, p=0.11), and dispersible fine particle content (r=0.856, p<0.05). It indicated that the initial distribution would be influenced by the Cs fixation ability of soil, and the penetration process of water and particles in soils.

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