Total amounts of the artificial radionuclide Caesium-137 in Baltic Sea sediments
The most significant source of artificial radioactivity in the Baltic Sea is the fallout from the Chernobyl accident. The distribution pattern of Chernobyl-derived Cs-137 in Baltic Sea sediments was very scattered, with the highest values occurring in the Bothnian Sea and the eastern Gulf of Finland. The total inventory of Cs-137 in the Baltic Sea sediments was estimated at 1900-2200 TBq in 1998.
Results and assessment
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Figure 1. Cs-137 in bottom sediments at 9 sampling stations in 1984-2002, Bq/m2. Click the image to enlarge .
Relevance of the indicator for describing developments in the environment
The indicator shows the total amounts of Cs-137 in bottom sediments of the Baltic Sea since 1984. The distribution pattern of Cs-137 at different sampling stations shows that the strongest sedimentation of Chernobyl-derived Cs-137 occurred in the Bothnian Sea and eastern Gulf of Finland. This is in agreement with the distribution pattern of Chernobyl fallout in the drainage area of the Baltic Sea. However, the highest amounts in sediments are probably not caused by highest site-specific deposi-tion values, but are due to particle transport and the concentration of particle-bound caesium in the deep areas of the accumulation basins . In the central Bothnian Sea the maximum was reached in 1992 and the values have decreased after that, but in the eastern Gulf of Finland the accumulation continued through the 1990s, reaching its peak in 1999, and started to decrease in 2000. In the south-ern parts of the Baltic Sea the amounts of Cs-137 were about one tenth of those in the Bothnian Sea and the Gulf of Finland.
The distribution pattern of Chernobyl-derived Cs-137 in the catchment area of the Baltic Sea was very scattered, with the highest deposition values occurring in the areas surrounding the Gulf of Bothnia and the Gulf of Finland. The highest total amounts of Cs-137 (Bq per square metre) in bot-tom sediments also occurred in these gulfs, but the scattered nature has been further emphasized as a consequence of river discharges, sea currents and varying sedimentation rates on hard (erosion) and soft (sedimentation) bottoms.
The most significant source of artificial radioactivity in the Baltic Sea is the fallout from the Chernobyl accident. The total input of Cs-137 from Chernobyl to the Baltic Sea was estimated at 4700 TBq. The total inventory of Cs-137 in the Baltic Sea sediments was estimated at 1900-2200 TBq in 1998 [1,2].
Description of data
The data are obtained from the monitoring programmes carried out by the countries around the Baltic Sea and give the total amounts of Cs-137 in sediments in Bq/m2.
Methodology and frequency of data collection
Dried sediment samples are analysed by gamma-ray spectrometry and the total amounts per square metre are calculated from the dry weight values. The samples are collected annually.
 Ilus E, Mattila J, Kankaanpää H and Laine A, 1999. Caesium-137 in Baltic Sea sediments since the Chernobyl accident. Marine pollution, Proceedings of a symposium held in Monaco, 5-9 October 1998, IAEA-TECDOC-1094, pp. 379-380. IAEA, Vienna.
 Ilus E, Suplinska M and Mattila J, 2003. Radionuclides in sediments. In: Radioactivity in the Bal-tic Sea 1992-1998. Baltic Sea Environment Proceedings No.85, pp. 55-75. Helsinki Commission, Helsinki.
Last update 30 September 2003