22 
Figure 5: 
Time series of ,37 Cs mean 
concentrations from stations 
characterising the inflow of 
bottom water into the Western 
Baltic. 
0 □ 
reprocessing plants (see Chapter 2) such as 
239 Pu, "Tc and 129 l, whereas the outflowing 
surface waters from the Baltic Sea represent 
a significant source of 137 Cs for the North Sea. 
The consequent impacts are detectable along 
the entire south coast of Norway at least until 
60°N. Today the Baltic Sea can be regarded 
as the most significant source of 137 Cs for 
the North Atlantic, only comparable with the 
sediments of the Irish Sea. The quantification 
of this source is still unclear, because of 
the high variability of outflow rates, but it is 
estimated at tens of TBqs per year. 
Table 1 : 
Effective half-lives of ,37 Cs in 
surface water in different basins 
of the Baltic Sea (1993-2006) 
Baltic Sea Region 
Time period 
1986-1988 
(years) 
1993-2006 
(years) 
Bay of Bothnia 
- 
10 
Bothnian Sea 
2.5 
9 
Gulf of Finland 
0.8 
13 
Baltic Proper 
- 
15 
3A.3 Effective half-life 
and target levels of 137 Cs 
The effective half-life of a radioactive 
contaminant is the time required for its 
concentrations to decrease by 50% as a 
result of physical, chemical and biological 
processes. Half-lives are specific to each 
radionuclide and each environment where 
they may occur. Effective half-lives have been 
calculated for 137 Cs in various parts of the 
Baltic Sea, as shown in Table 1. Currently, 
the effective half-lives of 137 Cs in surface 
water vary from 9 years in the Bothnian 
Bay to 15 years in the Baltic Proper. The 
longer residence time of 137 Cs in the Baltic 
Proper is most likely due to inflows of more 
contaminated water from the northern part 
of the Baltic Sea. In the time period following 
Chernobyl, 1986-1988, the effective half- 
lives of 137 Cs were much shorter in most 
contaminated regions: 0.8 years in the Gulf 
of Finland and 2.5 years in the Bothnian 
Sea. The shorter effective half-life of 137 Cs in 
Gulf of Finland as compared to the Bothnian 
Sea during 1986-1988 was probably due to 
different water exchange and sedimentation 
processes in these two regions (llus et al. 
1993). Overtime the effective half-lives have 
increased in both regions, and currently the 
residence times of 137 Cs are 13 and 9 years 
in the Gulf of Finland and in the Bothnian 
Sea respectively. The target level for 137 Cs 
concentrations in Baltic seawater is defined 
as 15 Bq/m 3 , equivalent to average pre- 
Chernobyl concentrations. Based on the 
calculated effective half-lives, this level will be 
reached by the year 2020 in both the Gulf of 
Finland and the Gulf of Bothnia. But it will take