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Accidental leakage of radionuclides from the Fukushima Nuclear Power Plant (FNPP1) took place in the aftermath of the catastrophic tsunamis associated with the Great East Japan Earthquake that occurred on March 11, 2011. Significant amount of radionuclides released into the atmosphere were reportedly transported and deposited on land located near FNPP1. The Niida River, Fukushima, Japan, has been recognized as a terrestrial source of highly contaminated suspended radiocesium adhering to sediment particles in the ocean through the river mouth as a result of hydrological processes. Remaining scientific questions include the oceanic dispersal and inventories of the sediments and suspended radiocesium in the ocean floor derived from the Niida River. Complementing limited in situ data, we developed a quadruple nested 3D ocean circulation and sediment transport model in an extremely high-resolution configuration to quantify the transport processes of the suspended radiocesium. Particularly, we investigated the storm and subsequent floods associated with Typhoon 201326 (Wipha) that passed off the Fukushima coast in October 2013, and subsequently promoted precipitation to a considerable extent and associated riverine freshwater discharge along with sediment outfluxes to the ocean. Using in situ bed sediment core data obtained from regions near the river mouth, we conducted a quantitative assessment of the accumulation and erosion of the sediments and explored the resultant suspended radiocesium distribution around the river mouth and nearshore areas along the Fukushima coast. We identified three major accumulative areas, near the river mouth within an area 1 km, around the breakwaters in the north of the river mouth, and along the southern coastal area, while offshore and northward transports were minor. The present study clearly exhibits substantial retention of the land-derived radiocesium adsorbed to the sediments in the coastal areas, leading to possible long-term influences on the surrounding marine environment.

Accidental leakage of radionuclides from the Fukushima Nuclear Power Plant (FNPP1) took place in the aftermath of the catastrophic tsunamis associated with the Great East Japan Earthquake that occurred on March 11, 2011. Significant amount of radionuclides released into the atmosphere were reportedly transported and deposited on land located near FNPP1. The Niida River, Fukushima, Japan, has been recognized as a terrestrial source of highly contaminated suspended radiocesium adhering to sediment particles in the ocean through the river mouth as a result of hydrological processes. Remaining scientific questions include the oceanic dispersal and inventories of the sediments and suspended radiocesium in the ocean floor derived from the Niida River. Complementing limited in situ data, we developed a quadruple nested 3D ocean circulation and sediment transport model in an extremely high-resolution configuration to quantify the transport processes of the suspended radiocesium. Particularly, we investigated the storm and subsequent floods associated with Typhoon 201326 (Wipha) that passed off the Fukushima coast in October 2013, and subsequently promoted precipitation to a considerable extent and associated riverine freshwater discharge along with sediment outfluxes to the ocean. Using in situ bed sediment core data obtained from regions near the river mouth, we conducted a quantitative assessment of the accumulation and erosion of the sediments and explored the resultant suspended radiocesium distribution around the river mouth and nearshore areas along the Fukushima coast. We identified three major accumulative areas, near the river mouth within an area

The Fukushima Nuclear Power Plant (FNPP) accidentally leaked substantial amount of radiocesium to the environment in March 2011. The radiocesium is known to have three input pathways to the marine environment: atmospheric deposition,direct discharge from the FNPP, and river discharges. The last process has lagged behind the accident by occasional floods that have delivered terrestrial deposition through hydrological processes, while the other two processes occurred immediately after the accident. Therefore, the local coastal seas have suffered from continuous contamination due to riverine suspended radiocesium inputs that must be assessed carefully. Niida River,Fukushima,has been a source of riverine suspended radiocesium even after the FNPP accident, as its watershed was prominently covered with the atmospheric deposition at the accident. In this study, we examined the oceanic dispersal and inventories of the sediments and suspended radiocesium deposited in the ocean floor derived from Niida River byusing a quadruple nested JCOPE2-ROMS 3D oceanic circulation model in a high-resolution configuration at the lateral grid spacing of 50 m.This model is coupled with a 3D multi-class sediment transport model along with a two-layer stratigraphy model of the marine bed,the iRIC-Nays 2DH river sediment transport model,the SWAN spectral wave model,and a static radiocesium absorption model. A particular attention was paid to the storm and subsequent flood event associated with Typhoon 201326 (Wipha)passed off the Fukushima Coast in October 2013,which provoked an enormous amount of precipitation,subsequent increase of the riverine freshwater discharge, and associated sediment and suspended radiocesium fluxes to the ocean. The model results were diagnosed with a guide of several in situ observed data collected in the Fukushima Coast after the storm. We found that the modeled and observed lateral and vertical distributions of sediment grain sizes and attached radiocesium concentrations were reasonably well correlated and consistent each other. We then investigated the accumulation and erosion of the sediments and resultant suspended radiocesium distribution around the river mouth and nearshore areas along the Fukushima Coast., Annual Meeting Asia Oceania Geoscience Society 2019

Niida River, Fukushima, Japan, is well known to be a source of highly contaminated suspended radiocesium to the ocean, which is originated from the Fukushima Dai-ichi Nuclear Power Plant during the accidental leakage occurred in March 2011. We examined the oceanic dispersal and inventories of the sediments and suspended radiocesium in the ocean floor derived from Niida River, using a quadruple nested JCOPE2-ROMS 3D circulation model in a very high resolution configuration, coupled with a 3D multi-class sediment transport model, the iRIC-Nays 2DH river sediment model, the SWAN spectral wave model, and a static radiocesium absorption model. A particular attention was paid to the storm and subsequent flood event associated with Typhoon 201326 (Wipha) passed off the Fukushima Coast in October 2013, which provoked an enormous amount of precipitation, and subsequent riverine freshwater discharge and sediment fluxes to the ocean. With a guide of several in situ observed data, we carried out a quantitative assessment of the accumulation and erosion of the sediments and resultant suspended radiocesium distribution around the river mouth and nearshore areas along the Fukushima Coast.