In the North Sea, the concentrations of most artificial radionuclides have
decreased in the past few years. In the Baltic Sea, the nuclide Cs-137
(caesium-137) still is found at higher concentrations than before the Chernobyl
nuclear accident in 1986 but meanwhile also shows a clearly decreasing tendency.
Compared to natural radionuclide concentrations in sea water, the concentrations
of artificial radionuclides are extremely low and neither pose a risk to
marine flora or fauna nor a health hazard to the population consuming fish
or other seafood.
The DHI/BSH has carried out measurements of artificial radioactivity in the
marine environment since 1961. Systematic temporal and spatial monitoring
for artificial radionuclides in the North Sea started in 1969, and in the
Baltic Sea in 1977. From time to time, investigations have been carried out
which covered the entire North Sea or Baltic Sea, the North-East Atlantic,
or the European Polar Seas (Norwegian Sea, Greenland Sea, Barents Sea).
Radioactivity monitoring of the marine environment is based on a continuously
operating radioactivity monitoring network and, additionally, on analyses
of specific nuclides in sea water, suspended particulate matter and sediment
on board research vessels. Cs-137 occupies a special position among artificial
radionuclides because, being a long-lived radionuclide with a half-life of
30 years, it is often used as a radionuclide tracer in sea water and constitutes
the radionuclide of greatest radiological significance in the marine environment.
It is transported over long distances by ocean currents and contributes
substantially towards radioactive contamination of the marine food chain.
The atmospheric nuclear weapons tests carried out in the 1950s and 1960s
led to a global contamination with fallout nuclides, especially in the northern
hemisphere. In the marine environment, the long-lived radionuclides tritium
(H-3), strontium-90 (Sr-90), caesium-137 (Cs-137), the plutonium isotopes
Pu-239, Pu-240, Pu-238, Pu-241, and americium-241 (Am-241) have been traced
back to this source. After the great powers had stopped their atmospheric
tests in the early sixties, concentrations of artificial radionuclides in
sea water have decreased continuously. Another increase in the activity
concentration of Cs-137 in the North Sea observed in 1970/71 was due to
radioactive wastes discharged into the Irish Sea by the nuclear reprocessing
plant at Sellafield (called Windscale at the time), and into the Channel
by the La Hague reprocessing plant. The discharges from Sellafield showed
a much higher level of contamination, in almost all radionuclides investigated,
than those from the La Hague plant. Since 1980, and particularly since 1985,
inputs of Cs-137 and other radionuclides have decreased due to a substantial
reduction of radioactive discharges from the Sellafield plant which, with
a certain time lag, has led to lower Cs-137 levels in the North Sea. Owing
to the long-term monitoring of the activity distribution, it has also been
possible to estimate the transport times for contaminated water masses from
the Irish Sea into the North Sea, Baltic Sea, Barents Sea, and Greenland
Sea. A simulation of Cs-137 drift was carried out within the framework of
the Kara Sea Project
Exchange processes between the North Sea and Baltic Sea transported contaminated
saline water into the Baltic, and Cs-137 was found to have spread as far
as the Gulfs of Bothnia and Finland, its levels decreasing with decreasing
salinity. Until 1986, the inventory of artificial radioactivity in the Baltic
Sea was determined by fallout and by the Sellafield discharges.
The reactor accident at Chernobyl in May 1986 changed the inventory of artificial
radionuclides in the North Sea and Baltic Sea as well as in the European
Polar Seas. Fallout from Chernobyl could be distinguished from the other
sources by its characteristic activity ratio of Cs-134/Cs-137 (on 1 May 1986:
Cs-134/Cs-137 = 0.54). By contrast, inputs of Sr-90 from Chernobyl played
only a minor role. In the longer term, only Cs-137 will be relevant in studies
of radiation exposure. The highest input of Chernobyl fallout was recorded
in the Bay of Bothnia. Today, Cs-137 levels in the entire Baltic Sea are
still higher than before the accident.
In sediment analyses, surface sediments in the Baltic Sea are generally found
to have higher specific activities than in the North Sea. This is due, on
the one hand, to lower water turbulence than in the North Sea and, consequently,
a higher proportion of fine-grained sediment, and, on the other hand, to
originally higher concentrations in Baltic Sea water. The specific activity
of Cs-137 in sediments from the North Sea coastal waters reaches values up
to 20 Bq/kg dry matter. Sediments from the western Baltic have activities
ranging between 2 (Arkona) and 190 Bq/kg (Eckernförde Bight). The inventory
up to a sediment depth of 16 cm is 40 Bq/m² to 1.2 KBq/m² in North Sea coastal areas, and 50
Bq/m² to 5.6 KBq/m² at the Baltic coasts.
Fig. 1: Temporal development of Cs-137 concentrations in German Bight water
at the positions of the former light-vessels "Elbe 1" and "Borkumriff".
Fig. 2: Temporal development of Cs-137 and Sr-90 activity concentrations
in Baltic Sea water at the position "Schleimündung" (Schlei estuary)