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Fukushima Ocean Releases Study, Now In English

A rough translation to English of the IRSN French study on Fukushima ocean discharges. The original document in French can be found here.

Date summary of knowledge on the impact on the marine environment of radioactive discharges from nuclear site rugged Fukushima Dai-ichi
October 26, 2011
This information note presents and discusses the latest information collected
by IRSN, since the previous briefing note of 11 July on the same subject.

A strong radioactive contamination of the marine environment occurred after the accident in the plant
Nuclear Fukushima Dai-ichi March 11, 2011. It had the main source direct discharge
contaminated water from the plant, which lasted until about April 8, and to a lesser extent,
impact in the ocean part of radionuclides discharged into the atmosphere between 12 and 22 March. A
immediate vicinity of the plant, the concentrations in seawater reached the end of March and early April
up to several tens of thousands of becquerels per liter (Bq / L) for cesium-134 and 137 and even
exceeded 100 000 Bq / L for iodine-131. Iodine 131 has declined rapidly because of its half-life
short (8 days) and the measurement results came under the detection limit in late May. The concentrations of
cesium-137 and 134 began to decline in this area from April 11 and since mid-July, are
placed below the detection limits (5 Bq / L) measurement techniques used for monitoring.
In interpreting the results of measurement of cesium-137 in seawater, IRSN has updated its estimate of
the total amount of cesium-137 released directly into the sea from 21 March until mid-July. The value thus
obtained was 27.10
15
Bq, the majority (82%) was rejected by 8 April. The radioactive release at sea
is the largest one-time contribution of artificial radionuclides to the marine environment ever
observed. However, the location of the site of Fukushima has a dispersion of radionuclides
exceptional, with one of the most important currents of the world that far to the contaminated water
wide in the Pacific Ocean. Thus, the measurement results obtained in the seawater and coastal sediments
suggest that the consequences of the accident, in terms of radiation protection, to become weak
pelagic species from the fall of 2011 (low concentrations in seawater and storage
sediment limited).
However, significant pollution of sea water on the coast near the central hilly
may persist over time because of continuous intake of radioactive substances transported to the
sea ​​by runoff of surface waters of contaminated soils. Also, some coastal areas, not
yet identified, could show the conditions of dilution or sedimentation less favorable than
those observed so far. Finally, the possible presence of other radionuclides persistent, as
strontium 90, plutonium, has not been sufficiently characterized by the measurements.
The measurement results show the persistence of recent contamination of marine species (fish
mostly) caught off the coast of Fukushima Prefecture. Benthic organisms and filter feeders
and fish at the top of the food chain are, over time, the most sensitive to pollution
cesium. It is therefore justified to continue monitoring of marine species harvested in the waters
coast of Fukushima.

 

1. EVOLUTION OF RADIOACTIVE POLLUTION IN SEA WATER
1.1. The main radionuclides found in seawater from the 21
March 2011
As of March 21 and in the days that followed, a strong radioactive pollution was observed
in the marine environment near the nuclear plant in Fukushima Dai-ichi. Characterization of
this pollution was mainly provided by measurements on samples of water
sea ​​sediments and species in the marine environment, the results are published by the
MEXT
a
and TEPCO. IRSN, which is not in a position to carry out measurements in Japan, collects and
regularly analyze these results to monitor radioactive pollution of the environment
sailor.
The measurement results published in Japan focus on-emitting radionuclides
gamma, listed in Table 1.

Table 1 – Main radionuclides measured in sea water
Radionuclides detected mainly
Radionuclide                                                      half-life
Iodine 131 (131I)                                               8 days
Cesium 137 (137Cs)                                       30.15 years
Cesium 134 (134Cs)                                       2.1 years
Cesium 136 (136Cs)                                      13.1 days
Tellurium 132/Iode 132 (132Te-132I)          78 hours

 

Other artificial radionuclides, most with a short half-life, were also detected occasionally, with lower concentrations. Measurements for Radionuclides pure beta emitters are less numerous: 9 results for strontium-90 in seawater at concentrations between 1 and 10 Bq / L, representing between 1 and 20% of the activity of Cs-137 measured in the same samples, that is to say, a proportion higher than that observed in atmospheric deposition of the accident on the land part of Japan, which is of about 0.1%.Iodine 131 (131I) and Cesium 137 (137Cs) are the main radionuclides were monitored by Following in the marine environment. Although dominant at the time of the accident, I-131 has sharply in the weeks that followed, because of its rapid decay, thepoint no longer be detectable since late May

The radioactive pollution had two main sources: radioactive liquid discharges from Site terrain and atmospheric deposition on the surface of the sea at the time of
dispersion of emissions released into the air. Changes in concentrations of radionuclides in sea ​​water (see next paragraph) indicates that these two major sources of pollution do not
are detected by monitoring. However, there is always a risk Regular intake of radionuclides in the marine coastal land by leaching contaminated and transport of radioactive pollution in the rivers.

a
Ministry of Education, Culture, Sports, Science and Technology, Japan

 

1.2. Dispersion of radioactive pollutants in sea water Measurements made near the center showed a strong environmental contamination Marine from 21 March, a consequence of the flow to the sea part of the water very units present in the contaminated terrain. The changes in time and space concentrations 131 I and 137 Cs are representative those of all the radionuclides measured at sea The results are summarized by two figures below:
- Concentrations measured near the outfall, which are representative of flow
radionuclides released (Figure 1);
- Cards isovalues ​​of cesium-137 concentrations, which show the distribution of
radioactive pollution in sea water at different times (Figure 2).

 

 

 

 

 

 

 

 

 

 

Figure 1 - concentrations 137 Cs in seawater and compared IR (131 I /137 Cs) in less than 500 m from the center of Fukushima Dai-ichi. The value of the ratio IR has been corrected
radioactive decay and conventionally reported in March 11 to allow a comparison of results between them.

Measurements made near the facility provide a report IR (131 I /137 Cs) for homogeneous, at about 20, with a low tendency to decrease over time until April 19. This decrease, which does not result from the radioactive decay of iodine, which is already taken into account in calculating the ratio, suggests a process of elimination regular I-131 measured in sea water, with a period of decay (half-life) apparent 35 days. A similar trend is observed at 10 and 20 km to the south between March 27 and April 16, but is not noticeable at a distance from the coast. The causes of this phenomenon to phase out iodine in seawater are not currently identified, this could be a specific behavior of iodine, either in the damaged installations, prior to discharge or in the marine environment.

After April 19, the ratio IR (131 I / 137 Cs) is highly variable, probably because of the imprecision increasing the measurement results of iodine 131 in seawater, as and as its activity decreases as a result of radioactive decay. It is also possible that these variations resulting from fluctuations in the composition of residual waste from the site accident, even if they had to be much lower from April 11, when begins a significant decrease of radioactivity in sea water near the Central.

The maps in Figure 2 show the spatial distribution of average concentrations of cesium 137 for successive periods between April 11 and July 11, the duration (7 days from April 11
May 2 and 14 days for subsequent periods) was selected by IRSN in order to have a sufficient number of measurements spread over the area of ​​study to achieve an interpolation
representative.

These maps show a similar distribution of pollution from the vicinity of the nuclear power plant to sea. Concentrations decreased significantly over time and the extent of colored areas, corresponding to the measurements above the detection limit (about 5 Bq / L) decreases accordingly. After July 11, the concentrations measured at sea are mostly below the limits
detection measurement techniques used for monitoring, it is no longer possible to an inventory represents the residual pollution at sea

1.3. Updating the estimate by the IRSN quantities of cesium-137 discarded at sea From the maps in Figure 2, the IRSN has updated its estimate of amounts of 137 Cs present at sea, in the area in the blue box shown on these maps. This estimate takes into account the bathymetry and the thickness of the mixed layer derived from the salinity profiles, temperature achieved by the MEXT, when the thickness is less than the height of sea water The results are presented in Table 2 and the evolution over time of these quantities estimated is shown in Figure 3.

Table 2 - Quantities of 137 Cs in the coastal marine area close to the central Fukushima Dai-ichi, estimated from the interpolation of the individual measurements in water sea​​, over different periods from April 11.

Inventory period
Start               End             Middle                        number of steps                     amount of  137Cs (TBq)
11/04/11       18/04/11    14/04/11                     92                                              11600
18/04/11       25/04/11     21/04/11                    77                                                4750
25/04/11       02/05/11     28/04/11                 118                                                 3380
02/05/11       16/05/11     09/05/11                 293                                                   667
16/05/11       30/05/11     23/05/11                 233                                                   261
30/05/11       13/06/11     06/06/11                 227                                                   163
13/06/11        27/06/11    20/06/11                 250                                                     42
27/06/11       12/07/11     04/07/11                 202                                                      2.4

Figure 3 - Changes in amounts of 137 Cs in seawater of the coastal area close to the Central Fukushima Dai-ichi, between April 11 and July 11, 2011.

This development follows an exponential decay with a period of T1 / 2 of 6.9 days (confidence interval 95%: 5.7 to 8.6 days). This means that the quantities of cesium-137
present in the seawater inside the calculation area was halved every 6.9 days. This decrease results from the dilution of seawater polluted by marine currents that provide regular clean water in the area. This rate of Renewal is particularly fast and it results from the intensity of the currents and Kuroshio Oyashio which occur in this area and their general approach to sea. The regularity of the dilution is also remarkable, taking into account the variability of vortex flow observed in this mixing zone.

This dilution actively supports the reduction of the impact of the accident in coastal waters. Contaminated water are transported rapidly eastward, toward the center of  Pacific, where they continue to be diluted as a result of the dispersal of marine waters. In the longer term, the pace of this dilution could be altered by two phenomena:
- Seasonal changes in ocean circulation (Kuroshio and Oyashio);
- Return to the area of ​​marine waters previously polluted by discharges of the Fukushima Dai-ichi accident, due to the recirculation of water masses in the area Pacific Northwest. This may prevent or delay the return to levels of cesium-137 concentrations comparable to those that existed before the accident (between 0.001 and 0.004 Bq / L)2.

Thus, unlike the terrestrial environment in which a residual deposit will persist for several years, the period of acute contamination of the marine environment is circumscribed about six months. This period is not a general feature of accidental pollution in urban sailor. It is the result of particularly favorable hydrodynamic conditions related to current dynamics, orientation towards the sea and the size of the receiving environment (the ocean Pacific). If this event had taken place in an enclosed sea (west of Japan, for example) or in a bay, the consequences could have been increased tenfold in the short and long term. As a comparison, the half-life of the waters of Norman-Breton gulf where is located the central Flamanville in the Manche is about three months, that is to say twelve times longer than observed in the region of Fukushima, although tidal currents will be particularly intense.

Extrapolation of the regression line as of April 8 to estimate the total amount of 137 Cs released at the end of main period of release (March 26-April 8). The amount estimated by extrapolation is October 22 15  Bq (22 million billion becquerels), the confidence interval 95% being from 20.8 to 23.1 10 15  Bq. The major error associated with this calculation is related to the estimation of
the depth of mixing, this uncertainty is estimated at about 50%. This revaluation release of cesium-137 at sea leads to a result about two times higher than that estimated by IRSN in July (see note of 11 July IRSN on the impact on the marine discharges radioactive nuclear site accident Fukushima Dai-ichi), and 20 times greater than the estimate made by TEPCO, published in June
IRSN has been able to establish an empirical correlation between the total amount of cesium-137 estimated for the period from March 26 to April 8, and the average concentrations of cesium 137 measured in sea water close to the central hilly, during the same period, considering that the phenomenon of dilution are stable and homogeneous at this scale. In Applying this correlation to measurements made until July 18, the date beyond which the

2
Nakanishi, T., Zheng, J., Aono, T., Yamada, M., Kusakabe, M., 2011. Vertical distributions of 99Tc and the
99Tc/137Cs activity ratio in the coastal water off Aomori, Japan. J. Approx. Radioactivity V.102, 8, 774-779.
Povinec, PP, Hirose, K., Honda, T., Ito, T., Scott, EM, Togawa, O., 2004. Spatial distribution of 3H, 90Sr,
137Cs and 239,240 Pu in Surface Waters of the Pacific and Indian Oceans – GLOMARD database. J.Approx.
Radioactivity V.76, Issues 1-2, 113-137

number of significant actions becomes too weak to make a correct estimate of the flow rejected, the IRSN has been able to determine the total amount of cesium-137 released into the seawater until mid-July. The resulting value is October 27 15   Bq. As expected, the bulk of the discharge occurred prior to April 8, the estimated releases after this date represents only 18% of rejection global. It is the largest one-time contribution of artificial radionuclides in the environment sailor ever observed.

For information, the overall contribution of 27.10 15   Bq of cesium-137, diluted across the Pacific assuming that there is between 0 and 100 meters deep, would lead to concentrations
associated (0.004 Bq / L) that would double the background noise in the residual seawater due to fallout from atmospheric nuclear tests (0.002 Bq / L). Although measurable with
current techniques, these concentrations represent only the 3000 th  concentrations natural potassium-40 in sea water (12 Bq / L). Cesium is mainly dissolved in sea ​​water and remain measurable for decades, so the resulting cesium-137 testing atmospheric nuclear-1960s is still clearly recognizable in the world. During the accident Fukushima, the activity of cesium-134 released at sea was the same level as that of cesium-137, but the half-lives of these two radionuclides are different, respectively 2 and 30 years, the activity report 134 Cs / 137 Cs will decrease over time and can be used for many years to identify and date the water bodies contaminated by Fukushima discharges across the surface waters of the North Pacific

1.4. Atmospheric deposition on the surface of the sea Mainly between 12 and 22 March, atmospheric radioactive releases caused by explosions and depressurization of the containment of the reactor core Fukushima Dai-ichi were dispersed, especially over the sea Part of radionuclides in the plume was falling to the sea surface, causing diffuse pollution of surface water to tens of kilometers from the accident site. This diffuse pollution is difficult to identify at sea as surface waters receiving atmospheric deposition are rapidly mixed with the rest of seawater through phenomena of advection and dispersion. Only measures of sea water made a few days after the deposit can provide information on the impact of radioactive fallout. Before March 24, when the direct liquid discharges were still relatively low concentrations measured in sea water to over 10 km of the plant can be attributed to deposits atmospheric. They range from 9 to 13 Bq / L for 137
Cs compared with IR ( 131 I / 137 Cs) from 5 to 12, comparable to what has been observed on land in Japan (see Note 27 of the IRSN September 2011 on the radioactive contamination of the terrestrial Japanese caused by the accident at Fukushima Dai-ichi). During this same period, the measures published in Japan revealed the presence of another polluted area along the coast to over 10 km south of the installation with values ​​from 20 to 100 Bq / L 137 Cs, with a ratio IR ( 131 I / 137 Cs) from 35 to 110. This pollution can be attributed Atmospheric deposition to a different, or direct liquid discharges prior to those identified from March 21. Atmospheric deposition on marine surfaces were re-evaluated by the IRSN, thanks to the updating of atmospheric dispersion modeling of emissions from the central Fukushima Dai-ichi. Under the new assessment, the cumulative deposition of cesium-137 in sea radius of 80 km would be 76.10 12  Bq (76,000 billion becquerels), a value about 10 times lower than that estimated in July. This contribution of contamination in the sea would be only

0.3% of the overall activity of cesium-137 released directly into the sea by the Central Fukushima Dai-ichi, estimated by IRSN in paragraph 1.3. The map in Figure 4 shows the spatial distribution of the deposit on the sea area of Japan. This estimate of the deposit air over the sea is based on an estimated total atmospheric release 11,5.10 15   Bq of 137 Cs. In addition to the deposit formed on the Japanese land, much of this rejection had to deposit diffusely over the oceans and continents of the Northern Hemisphere, over large distances

Figure 4 - Distribution of total atmospheric deposition 137 Cs on Wed March 23, estimated by IRSN by modeling the atmospheric dispersion of releases from the accident Fukushima Dai-ichi.

1.5. Simulation of the dispersion of cesium-137 in seawater off Japan IFREMER has been asked by IRSN to carry out simulations of the dispersion of releases of Fukushima Dai-ichi. The model is the model in March 3d, the boundary conditions

hydrodynamic come from Mercator-Ocean, the meteorological forcing is given by ECMWF European model.

The rejection of cesium-137 taken into account in this simulation is derived from calculations in paragraph 1.3, it was hypothesized that cesium-137 dispersed in a soluble form.

Concentrations measured and simulated within a kilometer of the plant show concordant results, the source term considered reproduces the small-scale dispersion.

Figure 5 shows the simulation results of the dispersion of releases of Fukushima-wide Pacific Northwest. It illustrates the complexity and variability of currents resulting from the confrontation between the Kuroshio from the south and Oyashio from the north. Structures dispersion are comparable to those simulated by the model Sirocco in Toulouse.

From a st   July, the simulated concentrations are generally below the limits of applied to detection monitoring. they will be identifiable with the techniques t conventionally used in oceanography (LD <0.001 Bq.L-1 ).

Figure 5 - Concentrations 137 Cs in seawater simulated 3D March between April 15 and July 26, 2011 in the Pacific Northwest.

2. RADIONUCLIDES IN SEDIMENTS
The suspended matter in seawater tend to set some of the radionuclides dissolved in the water column, depending on activity levels in the surrounding water. These materials eventually settle to the bottom of the sea, causing a contaminated surface deposit. Sediment samples were collected up to 186 km from Fukushima Dai-ichi and 70 km off the coast at a depth ranging from 20 to 200 m. The depth and conditions sampling, the nature and size of the samples were not provided during publication of results. As these parameters can strongly influence sediment mass concentrations measured to interpret the measurement results published in Japan with caution. The results are given in kg of dry weight when the accuracy is made (one in ten). Of the 184 samples measured since April 29, the main radionuclides measured in the sediments are
- Cesium 137 (183 significant results);
- Cesium 134 (178 significant results), with a report 134 Cs /137 Cs reported in March 11 of 0.95;
- Cesium 136 (6 significant results);
- I-131 (17 significant results from late April, no detection after June 9) with a report 131 I / 137 Cs reported at March 11, 23;
- The tellurides 129 and 129m (28 and 36 significant results ranging from 10 to 16 000 Bq.kg -1 )
- The Strontium 89 and 90 (2 significant results, 10 to 140 Bq.kg
-1 );
- The plutonium 239 and 240 (6 significant results from 0.09 to 0.49 Bq.kg-1);
- Barium 140 (1 significant result of 2900 Bq.kg -1 ).

Figure 6 shows the evolution of the concentrations  137 Cs in all samples inventoried and the respective distance to the outfall except near the center of Fukushima Dai-ichi,
concentrations typically range from 1 to 10 000 Bq.kg -1 With an average trend increase over time. This change may result from the transfer of kinetic Cs to sediment particles, and deposition processes of particles over fine. Given the decrease in concentrations in sea water, this should stabilize in the coming months. The highest concentrations are found near the the outfall (100 000 150 000 Bq.kg -1 ). Four values ​​greater than 10 000 Bq.kg -1  were identified more than 38 km distance before April 7, but have not been confirmed by the measures later.

Figure 6 – Graphic representation of the concentrations 137 Cs in sediments with time, indicating the distance of the sampling point to the central Fukushima Dai-ichi

Figure 7 shows a map of the distribution of concentrations 137 Cs in sediments. It was established by IRSN excluding the six measures close to the site with concentrations exceeded 10 000 Bq.kg
-1 . Concentrations achieved are generally lower than 1000 Bq.kg -1  And are given the relatively low coefficient of distribution at steady cesium from sea water and sediments, which is usually greater than 1000. Thus, with concentrations above 100 Bq / L measured in coastal seawater (see § 1.2), we would expect to find concentrations of 100 000 Bq.kg -1  in sediments. The
transient pollution of sea water by cesium-137 may not have allowed an up- equilibrium with the stock sediment sampled. Only recently deposited particles have contributed to the marking of surficial sediments, and they represent only a fraction of the volume sampled during the sampling.

Eventually, some of the radionuclides that are attached to sediment particles is likely to be remobilized in the marine water column. The sediment will behave as secondary sources of contamination, delayed, distant and diffuse. In coastal areas, sediment contamination is mainly the result of direct contact between seawater polluted surface sediments and could be spread through transport and mixing with deeper sediments. In pelagic realm, we should expect to find traces of radionuclides to the sediment-water interface as a result of their transport to the bottom by processes related to biological activity in the open ocean (primary production of solid material by phytoplankton, zooplankton grazing, production of feces, direct transport to the bottom).

Figure 7 - Mapping of concentrations 137 Cs in the sediments (mean values measured until 14 October.

According to the map in Figure 7, the stock of cesium-137 attached to the sediment is relatively low, it should not result in labeling of the seawater in the very high release the future. The period of release of cesium found in the sediments of the Irish Sea is two years 3 . Under these conditions, the top ten centimeters of sediment contaminated 1000 Bq.kg -1   bring in two years 500 Bq.dm
-2   with 20 to 200 m of water above them. account Given the kinetics of water renewal observed (50% renewal every 6.9 days), concentrations are induced by an average of about 5 to 50 mBq / L in seawater These concentrations should not have any impact in terms of radiation protection for pelagic organisms.

These concentrations may be higher in areas shallower or increased lower renewal. Benthic organisms living in direct contact with the bottom or filter feeders could then be directly affected by pollution residual sediment.

3
Finegan, P., Vintró, LL, Mitchell, PI, Boust, D., Gouzy, A., Kershaw, PJ, Lucey, JA, 2009.
Accumulation, solid partitioning and remobilization of intertidal and subtidal 99Tc in sediments in the Irish
Sea. Continental Shelf Research Volume 29, Issue 16, Pages 1995-2010.
Jones, DG, Kershaw, PJ, McMahon, CA, Milodowski, AE, Murray, M., Hunt, GJ, 2007. Changing
patterns of radionuclide distribution in Irish Sea subtidal sediments. Journal of Environmental Radioactivity
Volume 96, Issues 1-3, Pages 63-74

3. RADIONUCLIDES IN MARINE SPECIES AND RIVER

3.1. Measurement results obtained on fish caught at sea or in
some rivers

Samples of seafood (mostly fish) were carried out in Japan
since the end of March in various locations, mostly south of the central Fukushima Dai-ichi
(Figure 8)

Figure 8 - Map of sampling sites of fishery products. The black dots show samples of species with an excess of cesium contamination maximum permitted levels for drinking (> 500 Bq / kg fresh). The white dots samples indicate that contamination is below these levels (<500 Bq / kg free).

The list of species for which concentrations exceed the levels of cesium maximum permitted for food consumption (500 Bq / kg for the sum of cesium-134 and 137) was extended by the Japanese authorities during the summer (Table 3). As a Overall, the new species included in this list are:

- Is the species that were very promptly taken before the month of June and whose Surveillance has been intensified during the summer;
- Is the species that have been taken until this summer.

It is not possible, with the only information, to attribute this increase in the number of species with high levels of contamination to a change in the contamination of marine environment. It is important to stress that all these bodies come from the Prefecture of Fukushima and no body fished out of the area near the center or off does exceeds the maximum permitted levels (Figure 8 B and C).

Table 3 - List of animals and plants have shown concentrations cesium 134 137 higher than the maximum allowable levels for consumption.

List of June 7, 2011                                                            List August 21, 2011
Aramis (algae)                                                                     Aramis (algae)
Hijiki (seaweed)                                                                  Hijiki (seaweed)
Wakame (seaweed)                                                           Wakame (seaweed)
urchins                                                                                   urchins
clams                                                                                     clams
Sand eels (Japanese launch)                                          Sand eels (Japanese launch)
Smelts Japanese                                                                Smelts Japanese
fries                                                                                        fries
mussels médirerranéenne                                               mussels médirerranéenne
Ayus (freshwater fishing)                                                   Ayus (freshwater fishing)
Masou salmon (freshwater fishing)                                Masou salmon (freshwater fishing)
Tribolodons (freshwater)                                                   Tribolodons (freshwater)

Ainames
Flounder (flatfish)
Halibut (flatfish)
Hakelings
rockfish
rays
crabs
Char (freshwater)
Cyrpinidés (freshwater rearing)

In Figure 9, are reported the results of measurements on marine species for which data are obtained fairly regularly. In addition to the exclusively marine animals, this figure also shows data for two diadromous species (species migrate between freshwater and seawater), which were taken from Start May lake or river (Ayu and masou salmon).

Figure 9 - Temporal evolution of the concentrations 137 Cs + 134 Cs (Bq / kg) in some seafood marks correspond to the Roses diadromous species that have been mainly fished in the river.

3.2. Concentrations observed in marine animals Among the marine products, the contamination levels higher, detected early in the monitoring of fishery products, concerned the Japanese launch. The cesium-137 and 134 have were detected in all samples of this species collected in the prefectures of Fukushima and Ibaraki. These radionuclides have not been detected in two samples taken off. The eel sand eel sand or Japanese (Ammodytes personatus) is caught and consumed by the Japanese in the larval and juvenile stages are pelagic (living in water column) in the period from January to April. Adults for their live buried in the sediment May to December and are not caught, which explains the quasidisparition data on this species from the end of April.

In addition to the concentrations in the sand eels, Figure 10 shows the evolution of concentrations for the two isotopes of cesium in other species that are the subject of regular sampling. It is difficult to distinguish a temporal evolution of the contamination fish, given the wide dispersion of results. However, the halibut, the rays and to a lesser extent the gurnard, all characterized by a strong lifestyle relationship with the sediment, are rather high values ​​of the concentration range cesium observed in Japan. In addition to fish, it should be noted that the samples of sea urchins, abalone and clams collected in the prefecture of Fukushima can also reach high levels. As for 131, it is no longer detected in the bodies from mid-June (Figure 10), in accordance with changes in the environment (see paragraph 1.2



Figure 10 - Temporal evolution of the concentrations of iodine-131 (Bq / kg) in some seafood marks correspond to the Roses diadromous species that have been mainly fished in the river.

3.3. Concentrations observed in fish caught in water amphihaline fresh The acquired data on fish amphihaline concern Ayu-sweetfish (Plecoglossus altivelis), double-ended case 4
and salmon, mostly masou salmon (Onchorynchus masou), anadromous species 5 . The strongest samples were all collected in rivers Fukushima Prefecture, which is to be directly related to levels of significant contamination of these environments.

3.4. Expected trend for marine species In general, these are fish that are medium and long term best indicators of contamination of cesium in the marine field. Indeed, cesium has higher concentration factors in fish and shows an upward trend for species highest in the food chain. Therefore, if the short term, The highest concentrations are found in more species located at the beginning of the string Food, in the longer term, once the transfer to the various links of the networks trophic be effective, it will be the top predators of the food chain should present at higher levels. These levels should be even higher for species with living in close relationship with sediments and having their habitat close to the area contaminated. So even if the contamination of cesium in sea water has fallen sharply in the vicinity the central Fukushima Dai-ichi, it is justified to maintain a monitoring species Marine caught in coastal waters of the northeast coast of Japan.

4
Double-ended a species moves between seawater and freshwater, but not for the needs of reproduction
5
An anadromous species live in saltwater and spawn in freshwater

 

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