Reproduction of sediment deposition and prediction of 137Cs concentration in the major urban rivers of Tokyo

Goro Mouri

doi:10.1038/s41598-020-65700-y

Abstract

Radioactive caesium- 137 (137Cs) can be used as a tracer to infer sediment dynamics due not only to its long radioactive half-life but also its affinity for fine sediment. A novel advanced interpolation assessment was conducted to examine radionuclide activity in terraced land covered with volcanic ash soil in Tokyo, Japan, which had a time-dependent input function and incorporated the effects of mixed-sediment particle dynamic behaviour on radioactive decay. In addition, transport parameters derived from Chernobyl measurements were applied as predictors of the long-term contamination of the cardinal urban rivers by the fallout from the Tokyo Electric Power Fukushima Daiichi Nuclear Power Plant (FDNPP) accident in 2011. The behaviour of suspended sediment substances, incorporating the effects of deposition and pickup, was assessed using a mixed-sediment particle dynamics model. The concentrations of 137Cs adsorbed on fine sediment particles of each size fraction were determined. Removal of 137Cs from the cardinal urban river channel had significant effects on both long-term decline, including extreme flash flood events, and the dynamic and time-dependent behaviours of interspersed 137Cs and sediment activity. A novel advanced interpolation assessment method was used to examine radionuclide activity in terraced land covered with volcanic ash soil in Tokyo, Japan. The assessment procedure has a time-dependent input function and incorporates the effects of mixed-sediment particle dynamics on this time dependence. The results indicated that sediment and 137Cs concentrations could decline more rapidly than observed in the Fukushima and Chernobyl regions. This rate of decrease depended on terraces covered with volcanic ash soil, which incorporated the effects of fine sediment behaviour for particle adsorption. In addition, comparatively large impacts were observed during extreme flash flooding events, which were associated with the land cover of the major urban river catchments in Tokyo. This work provides a new perspective for understanding 137Cs behaviour associated with reproduction of sediment deposition and prediction of 137Cs concentration in the major urban rivers of Tokyo, incorporating the effects of baseline 137Cs behaviour with the impact of sediment particle adsorption in a volcanic ash soil-covered terrace.

Highlights

Radioactive caesium- 137 (137Cs) can be used as a tracer to infer sediment dynamics due to its long radioactive half-life and its affinity for fine sediment
The study was performed to gain insight into novel aspects of sedimentary geology, including the effects of the behaviour of river sediment and radionuclides through the accident of Fukushima Daiichi Nuclear Power Plant (FDNPP) in terraced land covered with volcanic ash soil area close to the Tokyo Metropolitan Area
We assessed the characteristics of terraced land covered with volcanic ash soil, incorporating the amphibious effects of sediment particle size distributions and their time-dependent behaviours, to examine the effects of human activity and a major developed urban area on sediment dynamics and 137Cs behaviour in the riverine environment

Summary

Introduction

Radioactive caesium- 137 (137Cs) can be used as a tracer to infer sediment dynamics due to its long radioactive half-life and its affinity for fine sediment. The results indicated that sediment and 137Cs concentrations could decline more rapidly than observed in the Fukushima and Chernobyl regions This rate of decrease depended on terraces covered with volcanic ash soil, which incorporated the effects of fine sediment behaviour for particle adsorption. Distributed 137Cs redistribution models, which employ algorithms for soil loss calculation such as RUSLE, have been applied to uncultivated and cultivated sites in southeast Australia (Martinez et al, 2009; Teng et al, (2016)[55]) To this end, we modified a novel method for rural areas proposed by Mouri et al (2014a)[1] to Fukushima-derived 137Cs in the urban region of Tokyo, Japan, to compare the mobilities of sediment and 137Cs components between selected urban rivers to provide novel insights that can be applied to different urban regions around the world. Sedimentary geochemistry and related phenomena were incorporated into the assessment; in particular, sediment characteristics have a number of important effects in the context of the environment and climate systems, with both environmental and social impacts (Shiozawa et al, (2013)[56]; Grygar et al, (2018)[57]; Liu et al, (2018)[58]; Apler et al, (2019)[59]; Guo et al, (2019)[60]; Abdou et al, (2019)[61])

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