We investigated radioactivity concentrations of 134Cs and 137Cs in river waters, Tokyo Bay sediments and Japanese whitings (Sillago japonica) lived in Tokyo Bay. Water samples were taken from Arakawa river mouth and Edogawa river mouth. Due to precipitations at inland areas, the river mouth waters had high concentrations of suspended solid (SS) and radiocesium on July 3 2012. The water sample taken from Arakawa river mouth showed higher concentration of radiocesium than that of from Edogawa river mouth. The sediment sample taken from Arakawa river mouth showed the highest concentration of radiocesium in sediments which were sampled on July 3 2012. In the vicinities of river mouth at the sea, sediments showed decreasing of radiocesium concentration (Bg/1,000 cm3 of sediment) with distance from river mouths, but there was no the same distribution pattern with the radiocesium concentrations (Bg/kg of dried sediment) of them. Values of 134Cs/137Cs activity ratio in sediments suggested that the 137Cs, which was generated by nuclear weapon tests or others before the Fukushima Daiichi Nuclear Power Station accident, was included in the river mouth sediments, and these estimated values was 80 Bq/kg-dry in Arakawa river mouth, and 30 Bq/kg-dry in Edogawa river mouth. Effective ecological half-lives of radiocesium in the sediments taken from off the coast of Kisarazu-shi, Chiba, were about one year of 134Cs and about three years of 137Cs. We recognized that the radiocesium activity concentrations in Japanese whitings living in Tokyo Bay are in safe level as a food. Effective ecological half-life of 137Cs in Japanese whiting was estimated for 1.1 ± 0.4 year. Values of 134Cs/137Cs activity ratio of Japanese whitings in 2012 suggested that the radiocesium exposed experience of each fish differed from two year old fish and three year old fish.
To evaluate the accumulation of 137Cs released from the Fukushima Daiichi Nuclear Power Plant in ocean sediment, a sedimentation model was developed and applied to the South Tohoku offshore region. The following results were obtained. (1) Rapid direct adsorption from bottom water explained the time variation of 137Cs in sediment. The inverse result for an adsorption rate of 0.1 d reproduced the observations. (2) The desorption rate is gradual. The inverse of the desorption rate of 25 d for fine sediment and 5 d for coarse sediment reproduced the observations. These results suggest that the differences in the temporal variation of 137Cs in sediment can be attributed to differences in the desorption rate with grain size. (3) The observed vertical migration of 137Cs for coarse sediment is explained by adsorption and desorption between sediment and bottom or interstitial water. (4) The sensitivity analysis suggests that the supply of 137Cs by sinking particles is not a dominant process, except for in the nearshore region.
We estimated the environmental concentration of radiocesium (137Cs and 134Cs) emitted by the 2011 accident at the Fukushima Daiichi Nuclear Power Station in Tokyo Bay in 2012 and 30 years later (2042) using the Risk Assessment Model for Tokyo Bay (AIST-RAMTB). We used measurement results for radiocesium obtained at the estuaries of Arakawa and Edogawa Rivers and the atmospheric deposition flux for the atmospheric and riverine inputs. The spatial trends of the estimation results closely agreed with those of the measurement results, but the estimated values were only approximately 60% of the measurement values obtained for sea water and approximately one-tenth the measurement values obtained for sediment. The reproducibility of AIST-RAMTB was improved when the riverine input was double the estimated value, and when the coefficients for 137Cs and 134Cs were 4.5 and 4.8, respectively; this distributed the ratio of adsorbed radiocesium to dissolved radiocesium. Using the concentration of 137Cs in sediment obtained using the AIST-RAMTB for the estimation in 2042 indicated a decrease in the concentration of 137Cs in sediment to 13% of the 2012 value. In particular, the high concentration of 137Cs at the estuary of Arakawa River in 2012 was predicted to decrease to 10 Bq/kg-dry by 2042.
We have developed a bioaccumulation model to estimate the concentration of hazardous chemicals accumulated in marine organisms. In this study, we applied this bioaccumulation model to estimate the concentration of radioactive materials in coastal fish. Target substances were radiocesium (134Cs, 137Cs) and the target fish were Japanese whiting (Sillago japonica) in Tokyo Bay. The radiocesium concentrations in sediments and fish were examined by field sampling on July 3 and August 9, 2012 in Tokyo Bay. The time course of radiocesium concentrations in Japanese whiting after the Fukushima Daiichi nuclear disaster was simulated using the model. Our model showed that good results were obtained by using the assimilation efficiency parameter of 10–20%, and radiocesium concentrations in Japanese whiting were about same as the levels recorded in the field sampling (517 days after the disaster). In the Edogawa River and the Arakawa River estuary, where high radiocesium concentrations were observed in sediments, Japanese whiting showed accumulations of 134Cs and 137Cs estimated at 5.0 Bq/kg-wet and 6.5 Bq/kg-wet, respectively.