Hironori Funaki

Numerical study of sediment and 137Cs discharge out of reservoirs during various scale rainfall events

Contamination of reservoirs with radiocesium is one of the main concerns in Fukushima Prefecture, Japan. We performed simulations using the three-dimensional finite volume code FLESCOT to understand sediment and radiocesium transport in generic models of reservoirs with parameters similar to those in Fukushima Prefecture. The simulations model turbulent water flows, transport of sediments with different grain sizes, and radiocesium migration both in dissolved and particulate forms. To demonstrate the validity of the modeling approach for the Fukushima environment, we performed a test simulation of the Ogaki Dam reservoir over Typhoon Man-yi in September 2013 and compared the results with field measurements. We simulated a set of generic model reservoirs systematically varying features such as flood intensity, reservoir volume and the radiocesium distribution coefficient. The results ascertain how these features affect the amount of sediment or 137Cs discharge downstream from the reservoirs, and the forms in which 137Cs is discharged. Silt carries the majority of the radiocesium in the larger flood events, while the clay-sorbed followed by dissolved forms are dominant in smaller events. The results can be used to derive indicative values of discharges from Fukushima reservoirs under arbitrary flood events. For example the generic model simulations indicate that about 30% of radiocesium that entered the Ogaki Dam reservoir over the flood in September 2015 caused by Typhoon Etau discharged downstream. Continued monitoring and numerical predictions are necessary to quantify future radiocesium migration in Fukushima Prefecture and evaluate possible countermeasures since reservoirs can be a sink of radiocesium.

Distribution of Radioactive Cesium in Trees and Effect of Decontamination of Forest Contaminated by the Fukushima Nuclear Accident

In decontamination pilot projects conducted by Japan Atomic Energy Agency (JAEA), many different techniques were tested to determine their applicability to remediate areas evacuated after the Fukushima Daiichi nuclear accident following the Great Tohoku earthquake and tsunami of March 11, 2011. In addition to buildings, roads and farmland, the forest adjacent to living areas was one of the main decontamination targets.The projects evaluated the radioactive contamination of trees and the effectiveness of decontaminating a highly contaminated evergreen forest. This forest was located 1.3 km southwest of the Fukushima Daiichi Nuclear Power Plant and is dominated by Japanese cedar trees and fir trees.As the first step, three Japanese cedar trees and three fir trees were cut down and the distributions of radioactive cesium (Cs) were measured in each. The total concentrations of 134Cs and 137Cs in the leaves and branches were about 1 MBq/kg for both cedar and fir trees, and were appreciably higher than in the bark for cedar. The concentrations in the outer part of the trunks (under the bark) were lower, on the order of 10 kBq/kg, and those in the core of the trunks were lower than 1 kBq/kg for both kinds of trees. The observation that the Cs concentrations are higher in the outer part of trees, is compatible with the assumption that radio-Cs was mostly adsorbed on the surface of trees and partly penetrated into the trunks through the bark.Evolution of air dose rates in a 100 × 60 m pasture adjacent to the forest was monitored during decontamination of the forest and of the pasture itself. The dose rates in the pasture decreased drastically after stripping contaminated topsoil from the pasture and decreased slightly more after stripping contaminated topsoil of the forest floor and pruning the trees. Cutting down and removing 84 trees in the outermost area (10-m width) of the forest also slightly decreased these dose rates. After decontamination, the residual dose rates around the highly contaminated forest were mostly attributed to radioactive Cs existing in or on trees and topsoil in the untouched forest beyond the decontaminated area.Copyright

Evaluation of particulate 137Cs discharge from a mountainous forested catchment using reservoir sediments and sinking particles

The time and size dependencies of particulate 137Cs concentrations in a reservoir were investigated to evaluate the dynamics of 137Cs pollution from a mountainous forested catchment. Sediment and sinking particle samples were collected using a vibracorer and a sediment trap at the Ogaki Dam Reservoir in Fukushima, which is located in the heavily contaminated area that formed as a result of the Fukushima Dai-ichi Nuclear Power Plant accident of 2011. The inventory of 137Cs discharged into the reservoir during the post-accident period (965 days) was estimated to be approximately 3.0 × 1012-3.9 × 1012 Bq, which is equivalent to 1.1%-1.4% of the initial estimated catchment inventory. The particulate 137Cs concentration showed a decline with time, but the exponent value between the specific surface area and the 137Cs concentration for the fine-sized (<63 μm) particle fraction remained almost constant from the immediate aftermath of the accident. These quantitative findings obtained by reconstructing the contamination history of particulate 137Cs in reservoir sediments and sinking particles have important implications for the evaluation of 137Cs dynamics in mountainous forested catchments.

Applicability of K d for modelling dissolved 137 Cs concentrations in Fukushima river water: Case study of the upstream Ota River

A study is presented on the applicability of the distribution coefficient (Kd) absorption/desorption model to simulate dissolved 137Cs concentrations in Fukushima river water. The upstream Ota River basin was simulated using GEneral-purpose Terrestrial Fluid-flow Simulator (GETFLOWS) for the period 1 January 2014 to 31 December 2015. Good agreement was obtained between the simulations and observations on water and suspended sediment fluxes, and on particulate bound 137Cs concentrations under both base and high flow conditions. By contrast the measured concentrations of dissolved 137Cs in the river water were much harder to reproduce with the simulations. By tuning the Kd values for large particles, it was possible to reproduce the mean dissolved 137Cs concentrations during base flow periods (observation: 0.32 Bq/L, simulation: 0.36 Bq/L). However neither the seasonal variability in the base flow dissolved 137Cs concentrations (0.14-0.53 Bq/L), nor the peaks in concentration that occurred during storms (0.18-0.88 Bq/L, mean: 0.55 Bq/L), could be reproduced with realistic simulation parameters. These discrepancies may be explained by microbial action and leaching from organic matter in forest litter providing an additional input of dissolved 137Cs to rivers, particularly over summer, and limitations of the Kd absorption/desorption model. It is recommended that future studies investigate these issues in order to improve simulations of dissolved 137Cs concentrations in Fukushima rivers.