Abstract
We investigated radiocesium concentrations in sediments and in Manila clams Ruditapes philippinarum. The samples were taken from three points in Tokyo Bay on September 2014. The ranges of radiocesium concentration in sediments were <0.35～0.59 Bq/kg-dry of 134Cs and 1.2～1.9 Bq/kg-dry of 137Cs. Each highest value was obtained from the most inside point of the bay. The ranges of radiocesium concentration in Manila clams Ruditapes philippinarum were <0.091 Bq/kg-wet of 134Cs and 0.13～0.16 Bq/kg-wet of 137Cs. The highest value of 137Cs in Manila clam also was obtained from the most inside point of the bay. Radiocesium concentrations in Manila clam living in Tokyo Bay are in safe level as a food. These data suggested that the level of radiocesium in the sediment affected that of Manila clam.
In order to assess the risk of radiocesium in ecosystem of Tokyo Bay after the Fukushima Daiichi Nuclear Power Station (FDNPS) accident, we need to know accurate concentration of radiocesium (134Cs, 137Cs) in Manila clam. We estimated 134Cs concentrations of Manila clam by using 137Cs concentrations of that and 134Cs/137Cs activity ratio of FDNPS in that time. The estimated concentrations of 134Cs in Manila clam were in 0.042～0.051 Bq/kg-wet. There was no discrepancy between the measured concentration value and the estimated one. It is considered that this estimation method is useful in case of low level radiocesium.
