Abstract
Due to the Fukushima Daiichi Nuclear Power Plant accident in 2011, large amounts of radiocesium were deposited over forest ecosystems in the headwater regions of rivers in Fukushima Prefecture, Japan. There is considerable concern about whether the runoff from these regions will result in recontamination of lower-lying areas further downstream after heavy rainfall events and typhoons. This study examined the sources and levels of cesium-137 (137Cs), the most abundant radioisotope in river sediments, in total suspended solids (TSS) in river water. In addition, changes in the predominant source of TSS associated with changes in river conditions were investigated. The properties of total organic carbon (TOC) in TSS (concentrations and isotopic compositions) were also measured to identify differences among sources. The results showed reductions in 137Cs and TOC properties in TSS when the river conditions changed from base flows to high flows, indicating the existence of a dilution effect through the addition of mineral particles from extraneous sources. Simulation results obtained using a mixing model suggested that forest soils are the primary source of TSS regardless of river conditions, but that the relative contribution of TSS from forest soils was decreased under high-flow conditions. Meanwhile, the relative contributions of both riverbank soil and river sediments were increased under high-flow conditions. Due to the low 137Cs concentrations in riverbank soils and river sediments, it is unlikely that heavy rainfall events will cause serious recontamination of areas downstream in the study area.
Highlights
After the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident in March 2011, the fluvial transportation of radiocesium, especially cesium−137 (137 Cs), through riverine systems in the contaminated areas of Fukushima Prefecture has been investigated extensively [1,2,3,4]
One possible reason for the higher total organic carbon (TOC) concentration in SS under base-flow conditions observed in this study may be the lower specific gravity of organic-rich materials compared to mineral materials, as the former would remain in suspension for a longer period of time [40]
Large amounts of radiocesium have been deposited over extensive areas of forest in the headwater regions of central and coastal Fukushima Prefecture, Japan
Summary
After the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident in March 2011, the fluvial transportation of radiocesium, especially cesium−137 (137 Cs), through riverine systems in the contaminated areas of Fukushima Prefecture has been investigated extensively [1,2,3,4]. Another study concluded that more than 90% of radiocesium was associated with suspended particles during periods of precipitation and flooding [6]. This increase in radiocesium was observed prior to the FDNPP accident in a Japanese river [7]. In the context of the FDNPP accident, this additional TSS will transport much of the 137 Cs that was deposited over terrestrial areas to the ocean It is, important to clarify the dynamics and properties of these additional sources of TSS in order to gain a more comprehensive understanding of the fluvial transportation of 137 Cs through rivers
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