Finland has been a pioneer in monitoring radioactivity in the Baltic Sea. Research into radioactivity began towards the end of the 1950s. There was an impetus to start research due to atmospheric nuclear weapons testing being carried out at Novaya Zemlya in the 1950s and 1960s. Over the years, the monitoring of radioactivity has expanded in all the Baltic States. Fallout from the nuclear weapons testing spread evenly across the whole of the Baltic Sea drainage area. The fallout consisted mainly of cesium-137 and strontium-90.

The radioactive cloud from the Chernobyl accident in 1986 travelled along airstreams straight towards the Baltic Sea, depositing fallout rather unevenly in the drainage area of the Baltic Sea. More fallout was deposited in the Baltic Sea than in other sea areas such as the Black Sea, the Mediterranean Sea, the North Sea, or the North-East Atlantic Ocean. As a result of the fallout, the Baltic Sea can be classified as one of the most contaminated sea areas in the world.

Distribution of cesium-137 fallout caused by the Chernobyl incident in the Baltic Sea area.

In the twenty years since the accident, the Baltic Sea’s waters have been cleaned up of cesium relatively quickly as cesium has deposited into the sediments of the Baltic sea. Immediately after the fallout, the highest concentration levels of cesium-137, measured at coastal stations in the Gulf of Finland and the Bothnian Sea, were about 5000 becquerels per cubic metre (Bq/m3) of water. In the late 2000, levels had dropped to one hundredth of the previous figures. The Gulf of Finland has cleared more quickly than the Bothnian Sea due to the different sedimentation rate and the fact that the exchange of water between itself and the Baltic Sea is more efficient than that between the Bothnian Sea and the Baltic Proper.

Concentration of cesium-137 in seawater in various parts of the Baltic Sea during 1974-2008.

Currently most of cesium in the Baltic Sea is accumulated in bottom sediments. Areal distribution of cesium in sediment follows the distribution pattern of the Chernobyl deposition. Fish from the Baltic Sea contain less than 50 Bq/kg  cesium-137, clearly lower levels than those found in freshwater fish. The radiation doses received by Finnish people due to Baltic Sea fish have remained at a very low level, just a few thousandths of a millisievert per year. 

Total amounts of  Cs-137 at different sampling station in the Baltic Sea in the 2000s.

Annual report STUK-B103 contains summary of environmental monitoring results in 2008. Results from the Baltic Sea are also found from this report. STUK is involved in the international Monitoring of Radioactivity in the Baltic Sea (MORS) programme of the Helsinki Commission (HELCOM).

Further information

Radioactivity in the Baltic Sea, 1999-2006. HELCOM thematic assessment. Baltic Sea Environment Proceedings No. 117 (pdf)