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

Abstract

Modern contamination of lake–river systems bottom sediments with global and “Chernobyl” ¹³⁷Cs is estimated. Drainage lakes of North-Western Russia were investigated. Lake Kopanskoe, located south of the Gulf of Finland, is on the “Chernobyl” ¹³⁷Cs fallout plume, whereas Ladoga, Sukhodolskoe, Vuoksa, Imandra lakes are located at its periphery, in Karelia and Kola Peninsula. Following parameters are distinguished: lakes bottom ¹³⁷Cs contamination density (kBq/m²), distribution of ¹³⁷Cs in the profile of bottom sediments, ¹³⁷Cs diffusion coefficients (D) in bottom sediments and content of the exchange chemical form of the radionuclide. ¹³⁷Cs contamination of the lakes was formed due to suspended matter sedimentation with ¹³⁷Cs, ¹³⁷Cs sorption and diffusion in bottom sediments. With sedimentation ≥3 mm/year, the concentration of ¹³⁷Cs increased from the top to the bottom of the core (lakes Vuoksa, Ekostrovskaya Imandra), reflecting the gradual process of ¹³⁷Cs migration into the sediments. The opposite trend of ¹³⁷Cs concentration was observed in the bottom sediments of lakes Ladoga and Sukhodolskoe with sedimentation ≤0.5 mm/year. Here ¹³⁷Cs diffusion with D = (0.5–6.2) × 10^–8 cm²/s caused slow radionuclide transfer in the bottom sediments. The main inventory of ¹³⁷Cs was contained in the top layer 0–5 cm. In Lake Sukhodolskoye bottom sediments an absorbed ¹³⁷Cs in an exchange chemical form, extracted into 1 М NH₄Aс solution, amount only 14.4–20%.