18 
Figure 6. Average 
activity concentra 
tions of Ra-226 (Bq 
kg -1 d.w.) in surface 
sediments (0-10 cm) 
at different sampling 
stations in the Baltic 
Sea at the beginning 
of the 2000s. 
5.5 Ra-226 
Radium-226 is a naturally occurring radionu 
clide in soils and sediments. Average activity 
concentrations of Ra-226 (Bq kg -1 dry weight) in 
surface sediments (0-10 cm) of the Baltic Sea 
are shown in Figure 6. In general, Ra-226 is quite 
evenly distributed in the seabed; typical activity 
concentrations in soft sediments of the Baltic Sea 
are 30-70 Bq kg -1 dry weight. Since samples 
are usually taken only from the surface layers of 
the sediments and the lengths of the sediment 
cores may vary considerably, it is not relevant to 
evaluate the total inventory of Ra-226 in the Baltic 
Sea sediments based on the results from the 
surficial sediments only. In spite of that, supposing 
that the total amounts of Ra-226 would be equal 
on soft and hard bottoms (a not-proven starting 
hypothesis), a rough estimate results in a total 
inventory of about 420 TBq of Ra-226 in the 0-10 
cm surface layer of the Baltic Sea sediments. 
5.6 Th-232 
Thorium-232 is a long-lived naturally occurring 
radioactive metal in soil, rocks, water, and sedi 
ments. Results on Th-232 were reported only 
from the southern Baltic Proper, the Belt Sea, the 
Kattegat, and the Sound. The activity concentra 
tions varied between 13 Bq kg -1 and 42 Bq kg -1 
dry weight and the total amounts in the surface