Tracing Sediment Transport at River Mouths in Tokyo Bay using Cesium Originated from Fukushima Daiichi Power Plant

The impact of sediment transport on the total amount of fine-grained cohesive sediment has not been elucidated at estuaries. Cesium134 and cesium137 were spread from the Fukushima Daiichi Nuclear Power Plant (FDNPP) after the earthquake of March 11 of 2011, and attached to the fine-grained sand on the land. The contaminated sand flowed into the river mouths through the rivers possibly due to the complex physical processes in estuarine areas. To evaluate the fine-grained sediment transport around Tokyo Bay, field observations were carried out utilizing the sediment core of radionuclides originated from FDNPP as an effective tracer. The cohesive sediment transport at three different river mouths around Tokyo was successfully quantified every 3 months. The cohesive sediment transport deposited in the estuary was found to be dependent on the land use, geometry, river discharge and salinity. As a result, most of a sediment transport occurred in 2011. This sediment accumulated near the river mouth depending on the river discharge at rain fall events. Each flux of sediment was shown by this observation results including run-off from water shed, flux of rivers and stocks in rivers and the bay. Spatial distributions of radionuclide in Tokyo Bay

Time series of surface radio-nuclide in Arakawa-river.