23 
Year 
137 Cs (TBq) 
10 more years, until 2030, to reach the target 
level in the Baltic Proper. These estimates 
are only valid if the effective half-lives remain 
constant. If they increase, as often happens 
with time, the target levels will only be reached 
at a later date. 
3A.4 Inventories of 137 Cs in seawater 
Inventories of 137 Cs in the Baltic seawater 
are given in Table 2. These estimates show 
that the inventory of 137 Cs in the Baltic water 
mass was approximately 1,540 TBq in 1999, 
and had decreased to 870 TBq in 2006. The 
inventories were estimated by calculating 
137 Cs inventories for various seawater regions 
of Baltic Sea (Bothnian Bay, Bothnian Sea, 
Gulf of Finland, Gulf of Riga, Baltic Proper, 
Kattegat, Belt Sea) and then combining these 
estimates. Inventories for different basins were 
calculated using their volumes (HELCOM, 
1996), and their average 137 Cs concentrations, 
which were calculated from the observation 
data. The temporal evolution of 137 Cs inventory 
in the Baltic seawater is presented in Figure 
6. Previous estimates for 137 Cs inventories 
(HELCOM 1995, 1998; Dahlgaard 1989) 
are shown together with the new estimates. 
The HELCOM 1995 and 1998 estimates 
were calculated with the assumption that the 
estimated mean concentration of 137 Cs was 
the same for the entire Baltic Sea. The new 
estimates appear to be more consistent as 
1999 
1540 
2000 
1380 
2001 
1320 
2002 
1230 
2003 
1090 
2004 
1030 
2005 
950 
2006 
870 
Table 2: 
Estimated ,37 Cs inventories in 
Baltic seawater. 
compared to the earlier ones (HELCOM 1995, 
1998). Based on the inventory estimates, the 
effective half-life of 137 Cs in Baltic seawater 
during the period 1993-2006 has been 9.6 
years. With this decay rate, the 137 Cs inventory 
in the Baltic Sea would reach pre-Chernobyl 
levels (250 TBq) by the year 2020, presuming 
that the effective half-life will stay constant, 
and no substantial remobilization of 137 Cs from 
sediments will occur. 
3A.5 Other radionuclides 
90 Sr 
90 Sr concentrations in Baltic seawater varied 
in general from 5 to 15 Bq/m 3 in surface water 
during the period 1999-2006. Similar levels 
were detected in near-bottom water. The 
lowest 90 Sr concentrations were detected in 
the Kattegat, where only 2 Bq/m 3 of 90 Sr was 
detected in 2006 in near-bottom water. The 
90 Sr inventory in Baltic Sea amounted to about 
» 
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Figure 6: 
,37 Cs inventories in the Baltic 
seawater during 1984-2006. 
Baltic Sea Environment Proceedings No. 117