137Cs Migration and Profile in Bottom Sediments of Deep Drainage Lakes, North-Western Russia

Modern contamination of lake-river systems bottom sediments with global and "Chernobyl" Cs-137 is estimated. Drainage lakes of North-Western Russia were investigated. Lake Kopanskoe, located south of the Gulf of Finland, is on the "Chernobyl" Cs-137 fallout plume, whereas Ladoga, Sukhodolskoe, Vuoksa, Imandra lakes are located at its periphery, in Karelia and Kola Peninsula. Following parameters are distinguished: lakes bottom Cs-137 contamination density (kBq/m(2)), distribution of Cs-137 in the profile of bottom sediments, Cs-137 diffusion coefficients (D) in bottom sediments and content of the exchange chemical form of the radionuclide. Cs-137 contamination of the lakes was formed due to suspended matter sedimentation with Cs-137, Cs-137 sorption and diffusion in bottom sediments. With sedimentation >= 3 mm/year, the concentration of Cs-137 increased from the top to the bottom of the core (lakes Vuoksa, Ekostrovskaya Imandra), reflecting the gradual process of Cs-137 migration into the sediments. The opposite trend of Cs-137 concentration was observed in the bottom sediments of lakes Ladoga and Sukhodolskoe with sedimentation <= 0.5 mm/year. Here Cs-137 diffusion with D = (0.5-6.2) x 10(-8) cm(2)/s caused slow radionuclide transfer in the bottom sediments. The main inventory of Cs-137 was contained in the top layer 0-5 cm. In Lake Sukhodolskoye bottom sediments an absorbed Cs-137 in an exchange chemical form, extracted into 1 \M NH4Ac solution, amount only 14.4-20%.