Multiscale Modeling of Radioisotope Transfers in Watersheds, Rivers, Reservoirs and Ponds of Fukushima Prefecture

Abstract:

The highest densities of the radioisotopes in fallout from the Fukushima Daiichi NPP in March 2011 were measured at the north eastern part of Fukushima Prefecture. The post-accidental aquatic transfer of cesium -134/137 includes multiscale processes: wash-off from the watersheds in solute and with the eroded soil, long-range transport in the rivers, deposition and resuspension of contaminated sediments in reservoirs and floodplains. The models of EU decision support system RODOS are used for predicting dynamics of ¹³⁷Cs in the Fukushima surface waters and for assessing efficiency of the remediation measures. The transfer of ¹³⁷Cs through the watershed of Niida River was simulated by DHSVM –R model that includes the modified code of the distributed hydrological and sediment transport model DHSVM (Lettenmayer, Wigmosta et al.) and new module of radionuclide transport. DHSMV-R was tested by modelling the wash-off from the USLE experimental plots in Fukushima prefecture. The model helps to quantify the influence of the differentiators of Fukushima and Chernobyl watersheds, - intensity of extreme precipitation and steepness of watershed, on the much higher values of the ratio “particulated cesium /soluted cesium” in Fukushima rivers than in Chernobyl rivers. Two dimensional model COASTOX and three dimensional model THREETOX are used to simulate the fate of ¹³⁷Cs in water and sediments of reservoirs in the Manogawa River, Otagawa River, Mizunashigawa River, which transport ¹³⁷Cs from the heavy contaminated watersheds to the populated areas at the Pacific coast. The modeling of the extreme floods generated by typhoons shows the resuspension of the bottom sediments from the heavy contaminated areas in reservoirs at the mouths of inflowing rivers at the peaks of floods and then re-deposition of ¹³⁷Cs downstream in the deeper areas. The forecasts of ¹³⁷Cs dynamics in bottom sediments of the reservoirs were calculated for the set of the scenarios of the sequences of the high floods of the next years. MOIRA –LAKE model of long term radioisotopes transfer in water, bottom sediment and fish was used for the assessments of the efficiency of the bottom sediment dredging for the remediation of the irrigation ponds at Okuma town.