137 Cs and 90 Sr Concentration Changes in Seawater and Bottom Sediments Collected from 2011 to 2013 around Japan after the Fukushima Daiichi Nuclear Power Plant Accident

Marine Ecology Research Institute has been conducting monitoring of marine radioactivity in the coastal sea nearby all nuclear power stations around Japan since 1983 and in the waters off Fukushima Daiichi Nuclear Power Plant (FNPP) in response to the FNPP accident since 2011. Concentration of radionuclides driven from the FNPP showed decline over time after the accident in each monitoring area. Three years after the accident, the concentrations of 137 Cs in seawater and bottom sediments were still maintained at the level one order of magnitude higher than those observed in the waters off Fukushima before the accident. The concentrations of 90 Sr in seawater exceeded the pre-accident values by several folds in the sea off Fukushima and Ibaraki.