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

https://doi.org/10.1016/j.jenvrad.2016.07.004Get rights and content

Highlights

  • Systematic analysis of sediment and 137Cs discharge from generic models of reservoirs.

  • Parameters employed (flood intensity, reservoir volume and Kd) are similar to those occurring in Fukushima.

  • Simulations determine the effect of these parameters on radiocesium discharge.

  • 137Cs mainly discharges in silt-sorbed form in larg floods, while clay-sorbed and dissolved forms dominate in small events.

  • Results can be used to estimate 137Cs discharges from reservoirs in arbitrary flood events.

Abstract

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.

Introduction

Although most of the radiocesium within Fukushima Prefecture remains adsorbed to soils on the ground surface, accumulations can be found within reservoirs across the region. Identifying practical countermeasures against radiocesium migration within the Prefecture is an important issue, particularly as there are ∼3700 reservoirs within the region used for irrigation, surface water management and drinking water supply.

For example, discussions are ongoing at the Prefectural level for implementing countermeasures against contamination in the Ogaki Dam reservoir, which is located in one of the highest radioactive fallout regions of Fukushima. This is in prospect of residents returning and restarting agriculture downstream of the Ogaki Dam. There is particular concern about outflow of contaminants from Fukushima’s reservoirs during typhoon floods and the long term contamination of the reservoir and river ecosystems with radiocesium. It is essential to understand the behavior of radiocesium in reservoirs to evaluate potential countermeasure options.

Various investigators have studied aquatic systems affected by fallout from atmospheric nuclear weapons testing and the Chernobyl nuclear accident. Based on field investigations and modeling studies, Smith et al. (2002) classified lakes as either closed or open depending on water residence times; closed lakes have long water residence times, while open lakes have a more rapid turnover of the reservoir water. In closed lakes, resuspension and remobilization from the bed sediments dominate long term migration of radioactivity in the lake (Smith et al., 2002). In the vicinity of Chernobyl, closed lakes tended to have higher activity concentrations in the water and aquatic biota than typical open lakes and rivers (Bulgakov et al., 2002, IAEA, 2006). In open lakes, the input of radioactivity is dominated by inflow from the upstream catchment (Smith et al., 2002). Spezzano et al. (1993) reported that lakes in catchments containing soils poor in clay minerals were likely to receive significant radiocesium input from the catchment, resulting in a high concentration of radiocesium in such lakes.

After the Fukushima accident, there have been many reports of open lakes containing high accumulations of radiocesium (e.g. Ochiai et al., 2013, Chartin et al., 2013, Mouri et al., 2014). Evrard et al., 2013, Evrard et al., 2014 found that dam releases are a major factor controlling dispersion of contaminated sediment in Fukushima Prefecture. In our previous numerical studies, we identified that reservoirs play an important role in delaying and buffering the movement of radiocesium in heavy rainfall events (Kurikami et al., 2014, Yamada et al., 2015). Buffering of radiocesium in a reservoir depends strongly on the reservoir water level and migration behavior of different sediment grades. In a review of the literature related to the accident at the Fukushima Dai-ichi Nuclear Power Plant, Evrard et al. (2015) concluded that the majority of radiocesium is transported from hillslopes to the ocean in the particulate fraction, attached to fine sediments during major runoff events. The importance of the particulate fraction is higher than seen in Ukraine after the Chernobyl accident, explained by the relatively high distribution coefficient of radiocesium to Fukushima soils.

This paper describes simulations using the FLESCOT code to understand the discharge of radiocesium during flood events. An application of FLECOT is presented for the Ogaki Dam reservoir over Typhoon Man-yi in 2013. The results of this simulation were validated against results from field investigations. A set of simulations of generic models for reservoirs are reported, where the parameters affecting radiocesium discharge were varied systematically. The results determine the effect of flood intensity and duration, reservoir volume and the radiocesium distribution coefficient on radiocesium discharges from the reservoirs. Future discharges from reservoirs in Fukushima Prefecture can be gauged using the results.

Section snippets

Model description

The FLESCOT (Flow, Energy, Salinity, Sediment Contaminant Transport) code used in this study was developed by the Pacific Northwest National Laboratory (Onishi et al., 1993). FLESCOT is a 3D finite-volume code that calculates distributions of flow, turbulent kinetic energy and its dissipation, water temperature, salinity, sediment concentrations of suspended sand, silt and clay, dissolved and particulate radionuclide concentrations. It also simulates changes in fractions of sand, silt and clay

Simulation of the Ogaki Dam reservoir

To validate the FLESCOT code for simulating Fukushima reservoirs, we simulated the concentrations of suspended sediment and 137Cs at the outlet (monitoring station Exit marked on Fig. 1) of the Ogaki Dam reservoir during a large typhoon in September 2013. Fig. 8, Fig. 9 show comparisons of the simulation results and measurements of sediment and 137Cs outflow from the reservoir, respectively. The simulation results were in good agreement with the measurements on the total concentrations of both

Conclusions

We performed a three-dimensional simulation of sediment and 137Cs migration in the Ogaki Dam reservoir in Fukushima Prefecture over Typhoon Man-yi. The simulation results were consistent with various monitoring data, demonstrating the applicability of FLESCOT for simulating Fukushima reservoirs.

We performed a study of generic model reservoirs to determine how radiocesium discharges from reservoirs are affected by the reservoir volume, and flood parameters such as inflow rate and flood duration.

Acknowledgments

The authors would like to thank the reviewers and editors for their comments and suggestions on the manuscript. We thank to the Tohoku Regional Agricultural Administration Office of MAFF for sharing the data. We appreciate Dr. Loren Eyler, Dr. Satoru T. Yokuda, Dr. Jie Bao and Dr. Kevin A. Glass of the Pacific Northwest National Laboratory, and Dr. Masahiko Machida, Dr. Mitsuhiro Itakura and Dr. Toshiyuki Nemoto of JAEA for improving the simulation code. We are grateful to Dr. Masahiko Okumura,

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