Transfer and behaviour of 137Cs in two Finnish lakes and their catchments.

The long-term behaviour of 137Cs was studied in two freshwater ecosystems in southern Finland in an area most loaded by the Chernobyl fallout in 1986. Samples were taken from water, sediments, aquatic plants and fish in the lakes and from soil, mushrooms and seed plants in the catchments. The activity concentrations of 137Cs in fish have remained at a relatively high level and decreased much more slowly in these two lakes than in other lakes studied by us. One reason for the continuously high concentrations in fish is evidently the prolonged stay of caesium at a relatively high level in the water of these lakes, which is associated with a slow sedimentation rate. The hydrographical properties of the lakes, i.e. the oligotrophic character associated with a deficiency of potassium in water and a low pH are other reasons for the effective uptake and long retention time of 137Cs in fish. The effect of humic substances on the uptake and delay of caesium in fish could not be proved clearly in this study. The swampy soil type of the catchment associated with a more oligotrophic status and lower pH of the water in Lake Siikajärvi explain at least partly the difference in activity concentrations and transfer of 137Cs between the two lakes studied. This refers to the higher transfer from the catchment to the lake and the higher uptake of 137Cs by fish and other biota in Lake Siikajärvi than in Lake Vehkajärvi. Perch and pike were more efficient accumulators of caesium than the best indicators among the aquatic plants. In the terrestrial environment, caesium was most effectively accumulated by mushrooms.