The second most important source of radioactivity in the Baltic Sea derives from the nuclear tests conducted in the atmosphere in the 1950s and 1960s. Nuclear testing accounts for approximately 14% of the caesium found in the Baltic Sea.
In addition to the above sources, artificial radioactivity from nuclear fuel reprocessing plants has entered the Baltic Sea from beyond its borders. This accounts for about 4% of the caesium-137 found in the Baltic Sea.
In terms of radioactive emissions from nuclear facilities in the catchment area of the Baltic Sea basin, the most important radionuclide is tritium. However, due to its low radiation energy, it is of little importance for its radiation dose to humans.
The consequences of Chernobyl were unevenly distributed in the Baltic Sea
The deposition of radioactive fallout from the Chernobyl nuclear accident was very unevenly distributed in the catchment area of the Baltic Sea. The greatest accumulation of radioactive deposition occurred in the areas surrounding the Bothnian Sea and the eastern Gulf of Finland.
Due to its unobstructed water exchange, the waters in the Gulf of Finland have been cleansed of caesium much faster than in the Gulf of Bothnia. The target levels for caesium in the Baltic Sea have been set at pre-Chernobyl concentrations.
Caesium concentrations in seawater have steadily declined since their values in 1986. Nevertheless, they are still higher than before the Chernobyl disaster.
It is estimated that if there are no further emissions, then pre-Chernobyl target levels could be reached by 2020 to 2030.
Radioactive caesium ends up in fish
Radioactive substances are enriched in the food chain. The highest levels of caesium-137 have been measured in predatory fish, such as pike and cod. Caesium concentrations in fish reflect changes in water concentrations, albeit with a time lag.
Radioactive substances are enriched in predatory fish, such as pike.
Thus, it will take decades for the caesium concentrations in fish to fall to pre-Chernobyl levels.
Caesium is buried in the seabed
Most of the caesium entering the Baltic Sea is currently buried in the seabed. At present, the highest concentrations are measured at a depth of about 10 cm in the sediment deposits.
Caesium is only removed from the bottom substrates by radioactive decay. It has a long half-life, i.e. approximately 30 years. As a result, it will take several decades before the concentrations reach a tolerable level.
At present, the amount of caesium-137 in the Gulf of Finland at its peak is about 20 times the target level.