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higher than the target of the HELCOM ecological 
objective “radioactivity at pre-Chernobyl level”. 
This is particularly true for the Bothnian Sea and 
the Gulf of Finland, which received the largest 
amounts of the fallout in the Baltic Sea from the 
Chernobyl accident in 1986. Nonetheless, in 
2000-2005 the concentrations of man-made 
radionuclides in sediments were generally at or 
below the concentrations of naturally occurring 
radionuclides and are not expected to cause 
harmful effects to the Baltic Sea wildlife. 
In the course of time, the long-lived fallout 
nuclides, such as Cs-137 originating from the 
nuclear weapons tests and the Chernobyl acci 
dent, will continue to sink from the water phase to 
the seabed and then become buried into deeper 
sediment layers. Simultaneously, they slowly lose 
their availability for consumption by pelagic organ 
isms and become available for consumption by 
benthic organisms; ultimately overtime, they are 
buried deep enough in the sediment that they are 
no longer available to the biota. At the same time, 
the amounts of radionuclides such as Cs-137 with 
a half-life of 30 years continue to decay according 
to their radiological half-life. 
After the Chernobyl accident, a large amount of 
data has been collected concerning the quantities 
of Cs-137 (and other gamma-emitting radionu 
clides) in the Baltic Sea sediments. Consequently, 
the present estimation of the total inventory of 
Cs-137 in the seabed of the Baltic Sea was based 
on an exceptionally large number of observations. 
To achieve a more comprehensive view of the 
total amount of all radioactive substances bound 
in the Baltic Sea sediments, however, it is recom 
mended to continue the acquisition of additional 
data also on the more difficult to analyse radionu 
clides, such as Sr-90 and transuranic elements, 
and the naturally occurring radionuclides, many of 
which were not included in this consideration. 
Long-lived radionuclides in the seabed of the Baltic Sea