3A Radionuclides in Seawater
Jürgen Herrmann 1 , lisa Outola 2 , Tarja K.lkäheimonen 2
1> BSH, Federal Maritime and Hydrographic Agency Germany
2> STUK, Radiation and Nuclear Safety Authority, Finland
3A.1 Introduction
This chapter describes the distribution of
artificial radionuclides in seawater in the
Baltic Sea over the years 1999-2006. During
this period nine countries contributed their
results for a total of almost 2,000 seawater
samples from all sub-regions of the Baltic
Sea to the common database. The monitoring
programme covered all sub-basins during
the report period, with some gaps in the
Archipelago Sea (see Figure 1). As presented
in earlier reports (HELCOM 1995a, 2003) the
predominant radionuclide in the Baltic Sea
is 137 Cs, as this radionuclide was released in
great amounts by the Chernobyl accident in
1986. The other main contaminant released
in the Chernobyl event, 134 Cs, has practically
vanished to concentrations below the
detection limit because of its relatively short
physical half-life of 2.07 years.
Other artificial radionuclides of relevance
in the seawater of the Baltic Sea are
90 Sr, 239 Pu and "Tc. The sources of these
radionuclides are described in Chapter
2. The concentrations mentioned in this
chapter are generally understood as activity
concentrations. A detailed description
of methods was given in the earlier joint
report (HELCOM 1995b) and updated in
the Appendix of this report. The collecting
of monitoring data was accompanied by a
thorough programme of quality assurance,
covering both 137 Cs and 90 Sr in seawater in
annual exercises, also shown in the Appendix.
3A.2 Distribution and
temporal evolution of 137 Cs
The fate of any pollutant introduced into the
sea is determined by both its own chemical
properties and hydrographical conditions
of the sea itself. As a relatively small, semi-
enclosed, brackish sea, which is connected
to the North Sea and thereby to the North
Atlantic only by the narrow Danish Straits,
the Baltic Sea suffers possibly more than any
other part of the World Ocean from any form
of pollution. The Chernobyl accident made this
situation most clear, as its sorry legacy is still
abundant 20 years after the event.
The Chernobyl accident resulted in the very
uneven 137 Cs deposition in the Baltic Sea
region. The Bothnian Sea and the Gulf of
Finland were the two most contaminated
sea areas. Since 1986, the spatial and
vertical distribution of Chernobyl-derived
137 Cs has changed as a consequence of river
discharges, the mixing of water masses, sea
currents, and sedimentation processes (Mus
2007). In the early phase after Chernobyl,
137 Cs concentrations decreased rapidly in
the Gulf of Finland and in the Bothnian Sea,
while at the same time increasing in the Baltic
Proper (Figures 1 and 2).
During the period 1999-2006 concentrations
of 137 Cs have continued to decrease in all
regions of the Baltic Sea. In the beginning of
this period, the highest 137 Cs concentrations
were reported in the Bothnian Sea, where
concentrations decreased from 82 to 49 Bq/
m 3 during the monitoring period. In the Baltic
Proper, concentrations decreased from 69 to
47 Bq/m 3 , and by 2006 concentrations were at
the same level in both the Bothnian Sea and
the Baltic Proper. In both the Gulf of Finland
and in the Bothnian Bay, concentrations
were lower, at around 37 Bq/m 3 in 2006.
Concentrations in the Western Baltic have
been lower overall, decreasing from 53
to 31 Bq/m 3 during the monitoring period.
Variations in 137 Cs concentrations in surface
water between different seawater regions
are becoming less pronounced. In 2006
137 Cs concentrations varied by a factor of 1.5
between the Baltic’s most contaminated area,
the Bothnian Sea, and the least contaminated
waters in the Western Baltic.
The vertical distribution of radionuclides in
the water column is influenced by physical
and biological processes as described above.
The distribution of 137 Cs between surface and
near-bottom water in different basins is shown
in Figures 3 and 4. The average ratio of 137 Cs
