2.3. The Distribution and Characterization of 137Cs In Lake Sediments

Abstract

Summary The 137 Cs entering a lake is associated with particles and quite rapidly transferred to the sediments because of different mechanisms. The resulting horizontal and vertical distribution of 137 Cs in lake sediment, together with physical and chemical associations, is very important for the bioavailability and circulation of 137 Cs in the ecosystem. In 1987 137 Cs was bound, to a great extent, to chemically labile fractions, but since then the isotope has, in some lakes, been transferred to more stable associations in the sediment. The highest concentration of 137 Cs was found in the smallest particle fraction ( 137 Cs was found in coarser fractions with a lower tendency to resuspension. In concave, flat lakes 137 Cs was uniformly distributed horizontally in the sediment, whereas in convex lakes with steep sides 137 Cs was focussed, for example through resuspension, on the deepest parts of the lake. The content of 137 Cs in the uppermost sediment layer is crucial for determining the flow of Cs from the sediment to other parts of the ecosystem. A deep vertical distribution of deposited 137 Cs was found in lakes or subareas in lakes with high rates of sediment accumulation in combination with strong mixing (bioturbation or resuspension). When mixing was missing as in the deep areas of certain lakes, the pulse of 137 Cs was very quickly covered, at high sedimentation rates, by new material with less or no 137 Cs. The combined effect of diffusion, mechanical mixing and bioturbation gives a stochastic depth distribution of 137 Cs, and the pulse of Cs is mixed with a larger volume of sediment whereby the concentration in the exposed surface layer is lower. The depth of maximum Cs-activity in a sediment core is normally greater in deeper areas of a lake because of the increased sediment accumulation. Processes which can be important for the transport of 137 Cs from sediment to water are diffusion, resuspension, bioturbation and biological uptake. The degree of resuspension was studied, by using sediment traps, in some shallow lakes in central Sweden. A very strong tendency to resuspension was found in these lakes with maximum depths between 4 and 11 m. Resuspension may transport 137 Cs to deeper areas, which probably gives a faster burial of the isotope. On the other hand, the process will increase the availability of Cs to biota, which will delay the recovery of Cs-contaminated shallow lakes.