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.