8 
Because Cs-137 and Cs-134 were the most 
abundant constituents in the Chernobyl fallout, 
the main emphasis in the monitoring has 
focused on these nuclides. In addition, the 
amounts of Ru-103, Ru-106, Ag-110, and Sb- 
125 clearly increased in Baltic Sea sediments 
as a consequence of the Chernobyl accident. 
The occurrence of these nuclides followed the 
distribution pattern of the caesium isotopes 
rather well, generally being most abundant in 
1987; but due to their relatively short half-lives, 
they already started to decrease by 1988/1989. 
Observations of Co-60 (half-life 5.3 years) in 
sediments also increased in various sub-regions 
of the Baltic Sea in the late 1980s. 
The studies have focused on the occurrence and 
behaviour in the sediments of the fallout nuclides 
originating from the above-mentioned events. 
In this sense, the sedimentation processes and 
sedimentation rates of the radionuclides, as well 
as the accumulation of the fallout nuclides into 
sediments, have been monitored. 
Figure 1. 
Terrestrial deposition 
of Cs-137 in the Baltic 
Sea drainage area 
(kBq nr 2 ); compiled 
by STUK.