Cs-137 and Sr-90 in the southern part of Lake Paijanne and its catchments

In the near-surface water of Asikkalanselka basin, Lake Paijanne, a slight increase in the Sr-90 content due to river discharges was observed in the y ears following the Chernobyl accident; later on, the concentration of Sr-90 remained almost the same, while that of Cs-137 decreased. In bottom sedime nts Sr-90 is detected deeper than Cs-137. In 1995-1997 the transfer of radi onuclides with small rivers from the catchments to Asikkalanselka was studi ed. Cs-137 is leached from bog area into surface water in greater amounts t han from other types of catchment. Most of Cs-137 leached to river water wa s, however, removed with suspended matter to bottom sediment of the river d uring the river flow before reaching Asikkalanselka. Sr-90 is also leached in greater amounts from the bog area than from other types of land. Removal of Sr-90 from water during the river flow is less significant than that of Cs-137. The role of surface runoff of the two radionuclides from the catch ment with small rivers to the lake seems to be of minor importance after te n years elapsed from deposition, as compared to radionuclide exchange with the other parts of the lake, where much greater volumes of water are involv ed. A satisfactory positive correlation was obtained between the amounts of peat bogs and activity concentrations of Cs-137 in river waters per unit d eposition for the five small catchments.