To get a comprehensive and correct answer to this question, we contacted STUK – Radiation and Nuclear Safety Authority of Finland, who answers as follows:
It is true that radioactive substances are present in sediments on the seabed. Most of the radioactivity in the sediments of the Baltic Sea region comes from long-lived radionuclides of natural origin such as K-40, Ra-226 and Th-232.
So-called artificial radioactive substances from human activities have also been released into the environment. The most important of these are the relatively long-lived Sr-90 (half-life 28.8 years) and Cs-137 (half-life 30.1 years). The main source of Sr-90 is mainly atmospheric nuclear weapons testing in the Northern Hemisphere in the 1950s and 1960s and, for Cs-137, the Chernobyl nuclear power plant accident in 1986. The most important of these in terms of sediment radioactivity is Cs-137, which tends to bind to clay and mud particles and settle with them in seabed sediments. Sr-90 remains in the marine environment in a more soluble form and is not readily bound to various particles.
The exact concentrations in the area in question would be determined by taking sediment samples from the area and testing them for activity levels in the laboratory. However, on the basis of the available data, a relatively good estimate of the concentrations and health effects of sediments in the area can be made. Since the 1980s, the activity concentrations of sediments in the Baltic Sea have been continuously monitored in cooperation with HELCOM MORS EG at five different sites in the Finnish maritime area.
Radioactive substances from the Chernobyl accident spread unevenly in the Baltic Sea. The radioactive substances released into the air by the accident were carried by air currents and most of them were washed down by rain. The amount and intensity of the rainfall varied in different areas, so the amount of radioactive material deposited on the land and sea surface varied considerably. According to deposition maps based on measurements made by the Radiation and Nuclear Safety Authority after the accident, the area of the planned project was not included in the area of maximum deposition.
The Cs-137 fallout from the Chernobyl accident peaked at 80 kBq/m2 in 1987.According to a 2013 study by STUK, the Cancer Registry and the National Institute for Health and Welfare (THL), the fallout from the Chernobyl nuclear power plant accident has not noticeably increased the number of cancers in Finland:
(Chernobyl fallout and cancer incidence in Finland 1988–2007 (2013) (Julkari.fi)).
The highest concentrations in sediments are found on so-called soft substrates (clay and mud), while on so-called hard substrates (e.g. sand) the concentrations are estimated to be 10-20% of those on soft substrates. According to the latest estimate, the maximum Cs-137 activity cap in the top 30 cm layer of the bottom sediment in the Finnish sea area was about 35 kBq/m2 (point SR5 in the Bothnian Sea), for comparison the estimated activity cap for the natural radionuclide K-40 at the same point was about 100 kBq/m2. The activity coverage in marine sediments is therefore estimated to be half of the maximum deposition measured in the terrestrial environment in 1987, and probably even lower in the area under consideration than at the point with the highest activity coverage in the Bothnian Sea.
In bottom sediments, Cs-137 is relatively tightly attached to particles and does not readily dissolve in the surrounding seawater. During possible seabed excavations, it is not expected that harmful amounts of Cs-137 will be redissolved and thus re-enriched in food chains. In STUK’s assessment, the radioactive substances in the sediments of the Baltic Sea are not expected to cause any health risks to humans or organisms in the region.