Sediments and benthos: Three-dimensional distribution of radiocesium in sea sediment derived from the Fukushima dai-ichi nuclear power plant

Abstract

This section introduces results of an investigation for radiocesium ( 134 Cs and 137 Cs) in sea sediment. The three-dimensional spatial distributions of radiocesium in sea sediment to a 14-cm core depth were surveyed from off the northern part of Ibaraki Prefecture to off Fukushima Prefecture with 5-min horizontal resolution in July 2012, approximately 16 months after the Fukushima Dai-ichi Nuclear Power Plant (FNPP) accident. A high concentration band was observed along the 100-m isobaths where the concentration of the 137 Cs reached 1,240 Bq/kg-dry at the maximum and where vertical profiles of the concentration generally had an exponential- type decline with depth. The concentrations were very low at the area shallower than 100 m of depth north from the FNPP, where vertical concentration peaks often occurred in deeper layers. These horizontal and vertical distribution patterns are suggested to be mainly determined by the supplied amount of radiocesium from the radiocesium-contaminated bottom seawater and the ability of radiocesium adsorption as dependent on the grain size of the sediment.