2014 Volume 19 Issue 2 Pages 13-25
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.
