Sediment focusing in six small lake basins is inferred from sedimentary radionuclide profiles. The activities of (CS)-C-137 and Pu-239+240 do not decrease to zero above the sub-surface maximum but tail off gradually. Similar behavior is observed in the sediments of three lakes for Cs-134 and (CO)-C-60, which were added only to the lake. This upward tailing is not caused by bioturbation or erosion from the watershed. Sediment focusing is the likely cause, further substantiated by inventories in deep sediments of Pu-239+240 and Pb-210 that are twice as high as expected from atmospheric fallout. Inventories in deep sediments of Cs-137 are in several cases lower than expected from atmospheric fallout, and cannot be explained by hydraulic flushing from the lakes, as this would require unreasonable flushing rates. The low Cs-137 inventories and the sedimentary profiles of the Cs-137/Pu-239+240 ratio are consistent with Cs-137 removal from the water column by boundary scavenging to shallow sediments. The profiles acid inventories of all three nuclides could be reproduced with a simple three box model, assuming focusing of shallow sediments (50-60% of total sediment area) to deep sediments with a rate constant of 5-10% yr(-1).
Rp372Times Cited:17Cited References Count:26