The distributions of radiocesium in seawaters and sediments collected off the Niida River estuary, Fukushima Prefecture

Lateral and vertical distribution of radiocesium, grain size and organic matter content off the Niida River estuary, Fukushima Prefecture were investigated in mid-October 2013. The dissolved ¹³⁷Cs activities in surface water samples collected at NR1, NR3 and NR5 stations, which are located within 5.0-10 km from the Niida River estuary, were higher than those of other stations outside the 10 km zone. In bottom water samples, the activities had a small variation with distance from the estuary. The activities in surface water at most stations were higher than those in bottom water. There were a negative correlation between dissolved ¹³⁷Cs activities and salinity in surface water. These results suggested that the dissolved radiocesium in seawater have been transported to north-south direction in surface layer along the continent. High fractions for coarse to very fine sand particles in the sediments were observed at NR1 and NR2 stations, which are located in a northern direction from the Niida River estuary. At NR4, there were high fractions for granule and very coarse sand particles. The grain size fraction became granular with increasing core depth. The ¹³⁷Cs activities in sediments at NR1 were higher compared to those of other stations. It seems that lateral and vertical distributions of radiocesium in sediments were affected by the particle composition. It is necessary to get more station data because the vertical profiles of ¹³⁷Cs activity were not especially similar to those of grain size and contents in sediments. Thus, it is necessary to continue the monitoring in the estuary area in order to elucidate the effects of the grain size and contents on cesium in the sediments found off the Fukushima Prefecture coast. (author)