5:15 PM - 6:45 PM
[ACG43-P05] Suspended Sediment Discharge and 137Cs Discharge from Land Use and Decontamination in rivers in Fukushima Prefecture
Keywords:Suspended Sediment, 137Cs, Discharge, river, Fukushima
Radioactive Cesium (137Cs) released from the Fukushima Daiichi Nuclear Power Plant accident had been deposited in terrestrial area around Fukushima Prefectural, which is estimated approximately 2.7 PBq (Onda et al., 2020). Approximately 67% (1.8 PBq) of this was reported to have been deposited in the forests, and 22.4% (0.6 PBq) in paddy, field, and urban (PFU) (Kato et al., 2019). Although the air dose rates were decreased sufficiently (Ministry of the Environment) owing to the decontamination works in the PFU area from July 2013 to March 2017 and the concentrations of suspended 137Cs in rivers continue decreasing (Fan et al., 2024), it is estimated that the excessive amount of suspended sediment flowing into rivers due to decontamination have a large impact on downstream areas (Feng et al., 2021).Furthermore, it may have an impact on the 137Cs transportationfrom forests to the ocean. It is still unclear that multi-decadal scale effects on suspended sediment loads in downstream areas and suspended 137Cs discharge to the ocean. Continuous observation is necessary.
In this study, we calculated the discharges of suspended sediment (C) and river flow (X) and the amount of suspended 137Cs transferred per month (Lmonth) between October 2012 and December 2020 at 11 monitoring sites in 9 rivers located in the Hamadori area of Fukushima and Miyagi Prefecture based on our long-term monitoring data. The aims were to clarify the relationship between Land Use and these discharges (C, X, Lmonth), and make comparisons of the C and Lmonth at three decontamination periods (before, during, and after).
C, X and Lmonth were calculated using turbidity data, water level data, and the 137Cs concentration of suspended sediment which is collected using a time-integrated suspended sediment sampler, respectively. Decontamination data (decontamination timing, decontaminated area, etc.) was obtained from the Ministry of the Environment's decontamination information site (http://josen.env.go.jp/), and Land Use data for each catchment was obtained from Taniguchi et al (2019).
The Lmonth to the ocean via each river showed a decreasing trend at most sites. While at the Haramachi site (the downstream of the Nitta River), there is no significantly change over about 10 years, which is consistent with the trend of 137Cs flux in the Nitta River before and after decontamination (Feng et al., 2021). C increased at some sites, which may be related to increased human activity following the lifting of evacuation orders. Furthermore, for each location, normalized C by the catchment area associated with the X was drawn as an approximate formula Y= a Xk. The rate constant k is higher in areas intensive land use of PFU (paddy fields and urban) (R2 = 0.65), and k is lower in areas with higher coverage of pastures, bare land, and forest (R2 = 0.64). At the Haramachi site, C during the period of after-decontamination (2016 to 2020) showed a significant increasing compared to the period of before-decontamination (2012 to 2016), which is considered that the C characteristics have contributed to the small Lmonth fluctuation.
In this study, we calculated the discharges of suspended sediment (C) and river flow (X) and the amount of suspended 137Cs transferred per month (Lmonth) between October 2012 and December 2020 at 11 monitoring sites in 9 rivers located in the Hamadori area of Fukushima and Miyagi Prefecture based on our long-term monitoring data. The aims were to clarify the relationship between Land Use and these discharges (C, X, Lmonth), and make comparisons of the C and Lmonth at three decontamination periods (before, during, and after).
C, X and Lmonth were calculated using turbidity data, water level data, and the 137Cs concentration of suspended sediment which is collected using a time-integrated suspended sediment sampler, respectively. Decontamination data (decontamination timing, decontaminated area, etc.) was obtained from the Ministry of the Environment's decontamination information site (http://josen.env.go.jp/), and Land Use data for each catchment was obtained from Taniguchi et al (2019).
The Lmonth to the ocean via each river showed a decreasing trend at most sites. While at the Haramachi site (the downstream of the Nitta River), there is no significantly change over about 10 years, which is consistent with the trend of 137Cs flux in the Nitta River before and after decontamination (Feng et al., 2021). C increased at some sites, which may be related to increased human activity following the lifting of evacuation orders. Furthermore, for each location, normalized C by the catchment area associated with the X was drawn as an approximate formula Y= a Xk. The rate constant k is higher in areas intensive land use of PFU (paddy fields and urban) (R2 = 0.65), and k is lower in areas with higher coverage of pastures, bare land, and forest (R2 = 0.64). At the Haramachi site, C during the period of after-decontamination (2016 to 2020) showed a significant increasing compared to the period of before-decontamination (2012 to 2016), which is considered that the C characteristics have contributed to the small Lmonth fluctuation.