![]() Baltic Sea Environment Proceedings No.117, Helsinki Commission, Helsinki. Radioactivity in the Baltic Sea 1999-2006. Total amounts of Cs-137 (Bq m -2) in sediments of different sampling stations in the Baltic Sea. Distribution of Cs-137, as percent of total inventory in sediment, between different subregions of the Baltic Sea. While there are still considerable amounts of artificial radioactivity in the Baltic Sea sediments due to the radionuclides with long half-lives, the radioactivity is not expected to cause harmful effects to the Baltic Sea wildlife.įigure 2. The inventory of K-40 is estimated to be roughly 8 500 TBq, four times higher than the total inventory of Cs-137 given above. However, most of the radioactivity in the sediments of the Baltic Sea originate from naturally occurring radionuclides, such as K-40. Nowdays the inventory of Cs-137 is 8 - 9 times higher compared to the pre-Chernobyl level indicating that the concentrations of artificial radionuclides are still higher than the target of the HELCOM ecological objective “radioactivity at pre-Chernobyl level”. Chernobyl-derived caesium has continued to deposit into the seabed, but due to the slower accumulation rate and radioactive decay of Cs-137 (half-life is 30.1 years), the total inventories of this radionuclide have stopped increasing (figure 3). The Bothnian Sea, the main accumulation basin for Chernobyl caesium, contains 73% of the total inventory as illustrated in figure 2. The total inventory of artificial Cs-137 in the seabed of the Baltic Sea was estimated to be about 2 100 - 2 400 TBq in the beginning of the 2000s. Total amounts of Cs-137 activities (Bq/m 2) in the Baltic Sea sediments during 2002-2006 in different sampling stations. In addition to uneven deposition, the total amounts have also been affected by the type of the bottom sediments, bottom topographies, sediment accumulation rates in different sea areas, unstable sedimentation processes and river discharges. Also, the variability inside the sea areas is large. In the Baltic Proper, Belt Sea, Kattegat and Sound the amounts are considerably lower than in the northern parts of the Baltic Sea (figure 1). The highest deposition values occured in the areas surrounding the Gulf of Bothnia and the eastern Gulf of Finland and also the highest total amounts of Cs-137 activities (Bq per square metre) are observed in the bottom sediments of these areas (figure 1). The distribution pattern of Chernobyl-derived Cs-137 in the drainage area and in the sediments of the Baltic Sea was very scattered. In the course of time a significant share of caesium has sunk to the bottom and accumulated into the sediments. The total input of Cs-137 activity from Chernobyl to the Baltic Sea was estimated at 4 700 TBq while that from the global fallout was 900 TBq. The main sources of Cs-137 have been the fallout from the Chernobyl accident in 1986 and the global fallout caused by nyclear weapon tests in the 1950s and 1960s. The continuous monitoring work and observations of time trends of the radioactive substances are the bases for knowing and understanding the state of the radioactivity in the Baltic Sea. At present the radioactivity in the sediments is not expected to cause harmful effects to the Baltic Sea wildlife. Nonetheless, most of the radioactivity in the sediments of the Baltic Sea originate from naturally occurring radionuclides. The total inventory of Cs-137 in the Baltic Sea sediments was estimated to be about 2 100 - 2 400 TBq in the beginning of the 2000s. The highest amounts occur in the sediments of the Bothnian Sea and the eastern Gulf of Finland. The distribution pattern of Cs-137, the major radionuclide in the Chernobyl deposition, in the Baltic Sea sediments is scattered due the uneven deposition and the sediment accumulation to the bottoms. The most significant source of artificial radioactivity in the Baltic Sea sediments is the fallout from the Chernobyl accident. Corresponding author: Iisa Outola, STUK - Radiation and Nuclear Safety Authority, Finland
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