Sediment and ¹³⁷ Cs transport and accumulation in the Ogaki Dam of eastern Fukushima

Abstract

The Ogaki Dam Reservoir is one of the principal irrigation dam reservoirs in the Fukushima Prefecture and its upstream river basin was heavily contaminated by radioactivity from the Fukushima Daiichi Nuclear Power Plant accident. For the purpose of environmental assessment, it is important to determine the present condition of the water in the reservoir and to understand the behavior of sedimentsorbed radioactive cesium under different modes of operation of the dam, as these factors affect further contamination of arable farmlands downstream of the reservoir through sediment migration. This paper addresses this issue with numerical simulations of fluvial processes in the reservoir using the twodimensional Nays2D code. We distinguish three grades of sediment (clay, silt, and sand), as cesium adherence depends on sediment grain size and surface area. Boundary conditions for the simulations were informed by monitoring data of the upstream catchment and by the results from a separate watershed simulation for sediment transport into the reservoir. The performance of the simulation method was checked by comparing the results for a typhoon flood in September 2013 against field monitoring data. We present results for sediment deposition on the reservoir bed and the discharge via the dam under typical yearly flood conditions, for which the bulk of annual sediment migration from the reservoir occurs. The simulations show that almost all the sand and silt that enter into the reservoir deposit onto the reservoir bed. However, the locations where they tend to deposit differ, with sand tending to deposit close to the entrance of the reservoir, whereas silt deposits throughout the reservoir. Both sand and silt settle within a few hours of entering the reservoir. In contrast, clay remains suspended in the reservoir water for a period as long as several days, thus increasing the amount that is discharged downstream from the reservoir. Under the current operating mode of the dam, about three-quarters of clay that enters the reservoir during the flood is discharged downstream during and in the days following the flood. By raising the height of the dam exit, the amount of clay exiting the reservoir can be reduced by a factor of three. The results indicate that the dam can be operated to buffer radioactive cesium and limit the contamination spreading into lowland areas of the Ukedo River basin. These results should be a factor in considerations for the future operation of the Ogaki Dam, and will be of interest for other operators of dam reservoirs in areas contaminated by radioactive fallout.

Sediment-associated organic carbon and nitrogen inputs from erosion and irrigation to rice fields in a mountainous watershed in Northwest Vietnam

“…The OC content of erosion sediments was around onethird of the OC content in irrigation water, and this difference in nutrient content was therefore caused by biogeochemical processes in the reservoir: breakdown of organic matter and nutrient mobilization in an aquatic environment contribute to the increased nutrient content of the irrigation water. Furthermore, sedimentation of the coarse, sandy material resulted in irrigation water containing fine-textured, fertile sediments higher in OC and N (Lick 1982;Yamada et al 2015). Results from Slaets et al (2015) on texture analysis for sediments in the same study site also demonstrated that irrigation water sediments were characterized by a finer texture (average 33 % sand, 33 % silt, 34 % clay) than those in overland flow (average 50 % sand, 30 % silt and 20 % clay).…”

Radiocesium transfer from hillslopes to the Pacific Ocean after the Fukushima Nuclear Power Plant accident: A review

“…Importantly, the fine particles, to which radiocesium is preferentially bound to, are the particles that are preferentially eroded and likely to be transported the farthest during erosive rainfall events (Govers, 1985;Malam Issa et al, 2006;Motha et al, 2002). Fundamentally, soil erosion and riverine transport processes are the primary factors governing the long-distance transfer of radiocesium from hillslopes to the Pacific Ocean Yamada et al, 2015).…”

Horizontal and vertical distributions of 137Cs in seabed sediments around the river mouth near Fukushima Daiichi Nuclear Power Plant

“…1). Because the upper part of the Ukedo River Basin has high levels of radiological contamination, several studies and analyses have been conducted in this basin to examine the features of 137 Cs transport from upstream to downstream (e.g., Kitamura et al 2014;Kurikami et al 2014;Yamada et al 2015). The annual 137 Cs discharge from the Ukedo River into the Pacific Ocean in the first few years after the FDNPP accident was approximately 2.0 TBq, the highest among rivers in the region (Oota River: 0.27vTBq, Odaka River: 0.13 TBq, Maeda River: 0.4 TBq, Kuma River: 0.28 TBq, and Tomioka River: 0.11 TBq) .…”

“…Understanding the features of 137 Cs transport from contaminated mountain forests to coastal sinks is key to the revitalization of marine industries. Therefore, evaluations of forest ecosystems and fluvial and lacustrine environments have been conducted (e.g., Funaki et al 2014;Yamaguchi et al 2014;Kitamura et al 2014;Kurikami et al 2014;Yamada et al 2015;Dohi et al 2015;Niizato et al 2016). 137 Cs distribution in shallow seas, which are major settlement areas for particle-sorbed 137 Cs, is a very important factor when studying 137 Cs transport from contaminated mountain forests to coastal sinks.…”

“…The numbers of data locations of 137 Cs distribution in seabed sediments in regions shallower than 50 m, as described in Kusakabe et al (2013), Otosaka and Kato (2014), and Black and Buesseler (2014), are 4, 3, and 4, respectively. Moreover, these sampling locations were situated in river mouths that had highly contaminated river basins (e.g., Ukedo River Basin; Kitamura et al 2014;Kurikami et al 2014;Yamada et al 2015). Thornton et al (2013) and NRA (2016) used a towed gamma ray spectrometer along with a depth sensor and a bathymetric survey, which indicated a relationship between the 137 Cs concentrations in seabed sediments and the features of seafloor topography.…”