S11-4

PICES 2012 Annual Meeting

Distribution of Radioactive Cesiumin Sea Sediment and Bottom Boundary Layer
after the Fuku
kushima Daiichi Nuclear PoPower Plant AcAccident
Daisuke Ambe1, Hideki Kaeriyama1 Y
Yuya Shigenobu1 K
Ken Fujimoto1 Hajime Saito2 Hideki Sawada2 Ts
Tsuneo Ono1, Takashi Setou
tou1 and Tomowo Watanabe1
1 National Research Institute of Fisheries Science, Fisheries Research Agency, Japan
2 National Research Institute of Fisheries Engineering, Fisheries Research Agency, Japan

Objective and Materials
the western North Pacific Ocean by the Fukushima Daiichi
Nuclear Power Plant (FDNPP) disaster following the Great
East Japan Earthquake and tsunami on 11 March 2011. One of
the emerging fisheries issues is to comprehend the effects of
the radioactive materials on demersal fishes. For thispurpose, we conducted a basic study on radioactive cesium
concentrations in sea sediment off Fukushima in February of
2012. Undisturbed sea sediment columns of 14-cm-high werecollected with bottom water at 40 locations which werearrayed every 5 minutes.

Fig 1. Survey area and locations. The evacuation
zone was set within a 20 km circuit of theFDNPP (indicated by +).

bottom water
sediment

Results and Discussions

Layer 0-1 cm-100 m

Vertical profiles of 137Cs
Almost concentration peaks
were found in upper twolayers (0-1 and 1-2 cm).

Main spatial variation scale is 101
kmorder.
However,theconcentrations between the adjacentgrids often have a large difference,suggesting that sub-grid scalevariations are also predominant.

The concentration values
generally
exponentiallydecayed with the sedimentlayer depth.

The concentrations were often lower
in northhern area from the FDNPP
than those in southern area.

Most of the sampledradioactive cesium (morethan 90 %) distributed 0-10cm layers.

Higherconcentrations
wereespecially along the 100 m isobath.

Amount of total 137Cs in thissurveyed area (Fig. 2) was
estimated about 0.3 % of thetotal discharge from theFDNPP (3.6 × 1015 Bq)

These horizontal characteristics
were commonly observed in alllayers.
Fig. 2. Concentrations of 137Cs in each sliced sediment(0-1, 1-+2, 2-4, 4-6, 6-10 and 10-14 cm)

Layer 1-2 cm

Grain size of the sedimentLayer 2-4 cm

Layer 4-6 cm

Layer 6-10 cm

27.4 %
20.5 %
23.9 %
12.3 %
10.4 %
Estimated total 137Cs
1.03 × 1013 Bq
5.5 %Median Value
Fig. 3. Vertical profiles of the 137Cs concentration in the
all surveyd positions (black lines). Gray bold line
indicates median value in each layer.

Layer 10-14 cm

137
Layer 1-2 cm

Cs of suspended particles in the bottom boundary
layer (0-15 cm from sea-bottom)

Layer 0-1 cm(Median Value)Layer 1-2 cm

Fig. 5. Median grain size of the sea sediment in the 1-2
cm layer. Red dotted ellipse indicates the area wherehigh 137Cs concentrations were found (Fig. 2).
137

The highCs concentrations did notappeared in lager grain areas which locatedin the northern near-shore region.
Sediment composition associated with thegrain size is suggested as one of thefactors in forming the distribution ofradioactive cesium at the sea bottom.
Fig. 4. Scatter plot between the 137Cs concentrationand the median grain size of the sediment in the0-1 cm and 1-2 cm layers.

Smaller grains were found parallel to thehigh 137Cs concentrations in the southern
region with some spatial distance (blueellipse in Fig. 5).

Fig. 6. Scatter plot of the 137Cs concentrations inthe sediment of the 0-1 cm layer and in thesuspended particles .

Fig. 7. 137Cs concentrations in the suspended particles.

Correlation between the consentrations of sediment and suspended
particles was not significant (Fig. 6).
Relative high values were often detected around the 100 m isobath (Fig. 7).
But high values were also sometimes observed at offshore or nothern areas
where the concentrations were not high in the sea sediment (Fig. 7).
The 137Cs might be frequently transported in the bottom boundary layer.
If so, the formation of the radioactive cesium field on sea-bottom is stillin transitional state at this time. Consecutive monitoring forcomprehending the temporal variation is absolutely needed.

Acknowledgement - We wish to thank Captain Y. Terada and his crew members of the R/V Soyo Maru for supporting sample collection. We also thank Dr. T. Morita of Fisheries Agency for useful comments and advice. This study was financially supported by Fisheries Agency of Japan.