¹³⁷Cs in Bottom Sediments of Oligotrophic Lakes: Migration Mechanism

Abstract

The 5–17-year migration of ¹³⁷Cs in bottom sediments (BSs) of five oligotrophic lakes located on the “Chernobyl” ¹³⁷Cs fallout plume was studied. The layer-by-layer analysis of the ¹³⁷Cs levels in BS cores allowed determination of the ¹³⁷Cs migration mechanism and of the numerical characteristics of its transfer into the thickness of BSs of low-trophy water bodies. The exponential decrease in the ¹³⁷Cs concentration from the core surface to the BS depth was evaluated by the half-loss layer h. The value of h for “Chernobyl” ¹³⁷Cs for BSs of oligotropic lakes was 1.2–2.3 cm. The ¹³⁷Cs transfer in BSs of oligotrophic lakes occurred by the diffusion mechanism. The diffusion coefficients (D) of “Chernobyl” ¹³⁷Cs in the lake BS columns were (2.0–4.7) × 10⁻⁸ cm² s⁻¹. The diffusion coefficients in ¹³⁷Cs migration increased from the near-surface layer of bottom sediments (∼0–2 cm) to the depth of the bottom soil in the range n × (10⁻⁹–10⁻⁸) cm² s⁻¹. The numerical characteristics of the ¹³⁷Cs migration (h and D) refer to BSs of oligotrophic lakes in which the flow of sediments to the lake bottom did not exceed 1.8 mm year⁻¹. In Lake Sukhodol’skoe at a sedimentation rate of 6 mm year⁻¹ and radionuclide exposure of 31 years, the bulk of BSs was contaminated with ¹³⁷Cs by the mixed mechanism: diffusion from the primary accumulation site and sedimentation of suspended matter with ¹³⁷Cs.