Temporal variation of iodine-129 concentrations in kelps (Saccharina) from coastal waters off northern Japan.

Concentrations of 129I and 127I in kelps (Saccharina) collected from coastal waters off northern Japan were monitored from 2007 to 2019. During the 2007-2008 test operation of the Rokkasho nuclear fuel reprocessing plant, 129I discharge from the plant increased, and the 129I concentration and 129I/127I atom ratio in the kelps reached maxima of 42 µBq/g-dry and 264 × 10-11, respectively. By 2009, both had decreased by one order of magnitude. After the Fukushima Dai-ichi Nuclear Power Plant accident in 2011, the 129I concentration and 129I/127I atom ratio in the kelps increased to 2.24 µBq/g-dry and 11.6 × 10-11, respectively. After 2012, the ratio in kelps decreased to (2.1-8.9) × 10-11, which is almost the same as the seawater value off Aomori Prefecture before the test operation. The 129I/127I atom ratio in kelps thus represents the ambient seawater ratio during the growth period of the kelps.

Spatiotemporal distribution of 137Cs in the sea surrounding Japanese Islands in the decades before the disaster at the Fukushima Daiichi Nuclear Power Plant in 2011.

The historic spatiotemporal distribution of 137Cs in the seawaters and sea-floor sediments adjacent to nuclear power plants in Japan are summarized, using data obtained over a period of time more than 20 years prior to the disaster at the Fukushima Daiichi Nuclear Power Plant in 2011. Relatively uniform distributions of 137Cs were observed both in the surface seawaters (1 m in depth) and in deeper seawaters (10 to 30 m above the seabed and ranging from tens to hundreds of meters in depth) independent of the geographical position, although lower concentrations were observed in significantly deeper bottom seawaters. Conversely, there were wide variations in 137Cs levels between sediments, such that higher 137Cs concentrations were observed in the deeper sampling locations. A mathematical model describing the successive transfer of 137Cs from surface waters through deeper waters to sediments suggested that the transfer rate of 137Cs from deep water to the sediments, and the loss rate from bottom sediments, were both greater than the transfer rate from surface water to deeper water. It was found that the calculated regression lines for 137Cs depletion rates over time for surface waters, deeper waters, and sediments were approximately parallel when plotted on a semi-logarithmic coordinate system, regardless of the sampling location. A radionuclide depletion half-life was calculated to be 4 months to 16 years with the geometric mean of 2.22 y for the sediments in the Fukushima region, suggesting that nuclear contamination will be remediated over time through sediment redistribution processes such as remobilization, bioturbation, and migration due to sea currents.

Spatial variations of low levels of ¹³4Cs and ¹³7Cs in seawaters within the Sea of Japan after the Fukushima Dai-ichi Nuclear Power Plant accident.

Our method based on low background γ-spectrometry enabled the measurement of low radiocesium concentrations in only 20 L of seawater. In May 2011 after deposition of radiocesium, (134)Cs concentration in surface water within the Sea of Japan was confirmed to be significantly small (<0.1-1 mBq/L) by the method. The concentration was not detected (<0.1 mBq/L) below 50 m depth. The Fukushima-derived radiocesium migrated from the surface water of the Sea of Japan without advection to below the thermocline.

Plutonium isotopes concentration in seawater and bottom sediment off the Pacific coast of Aomori sea area during 1991-2005.

A radioactivity survey was launched in 1991 to determine the background levels of ²³9+²4°Pu in the marine environment off a commercial spent nuclear fuel reprocessing plant before full operation of the facility. Particular attention was focused on the ²4°Pu/²³9Pu atom ratio in seawater and bottom sediment to identify the origins of Pu isotopes. The concentration of ²³9+²4°Pu was almost uniform in surface water, decreasing slowly over time. Conversely, the ²³9+²4°Pu concentration varied markedly in the bottom water and was dependent upon the sampling point, with higher concentrations of ²³9+²4°Pu observed in the bottom water sample at sampling points having greater depth. The ²4°Pu/²³9Pu atom ratio in the seawater and sediment samples was higher than that of global fallout Pu, and comparable with the data in the other sea area around Japan which has likely been affected by close-in fallout Pu originating from the Pacific Proving Grounds. The ²4°Pu/²³9Pu atom ratio in bottom sediment samples decreased with sea depth. The land-originated Pu is not considered as the reason of the increasing ²³9+²4°Pu concentration and also decreasing the ²4°Pu/²³9Pu atom ratio with sea depth, and further study is required to clarify it.