[Translated from French] This graph shows the evolution of sampled cesium-137 concentration and the respective distance from the Fukushima Daiichi nuclear plant. Concentrations vary typically from 1 to 10,000 Bq/kg, with an increasing trend over time. This evolution may be the result of the kinetics of cesium transfer to sedimentary particles 
and particle deposition processes the most fine. In view of the decrease in the concentrations in seawater, this should to stabilize in the coming months. The highest concentrations are found near the nuclear plant (100,000 to 150,000 Bq/kg). Before 7 April 2011, four values greater than 10,000 Bq/kg had been identified at a distance of more than 38 km but have not been confirmed by later measurements. Some of the dissolved radionuclides in the water column remain suspended and determine the level of radioactivity in the surrounding water. These materials eventually settle at the bottom of the sea, causing a superficial deposit contaminated sediment. Sediment samples were collected up to 186 km from Fukushima Daiichi and 70 km off the coast at a depth ranging from 20 to 200 m. The figure to the right presents a map of the distribution of cesium-137 concentration in sediments. IRSN established that six measurements close to the site had concentrations exceeding 10,000 Bq/kg. Most concentrations are generally less than 1000 Bq/kg; they are relatively low in view of the equilibrium distribution coefficient of cesium between sea water and sediment, which is usually higher than 1000. Thus, with concentrations above 100 Bq/L measured in coastal sea water (cf. § 1.2), we would have expected to find 100,000 Bq/kg concentrations in sediments. The transient pollution of seawater by cesium-137 has without doubt not allowed one to the balance with the sampled sedimentary stock. Only recently deposited particles contributed to the marking of the surficial sediments and they represent only a fraction of sample in the samples volume.
Accident nucléaire de Fukushima-Daiichi: l’IRSN publie une mise à jour de sa note d’information sur l’impact sur le milieu marin des rejets radioactifs consécutifs à l’accident [pdf in French]
