A MULTITRACER STUDYOF RADIONUCLIDES IN LAKE ZURICH, SWITZERLAND .1. COMPARISON OF ATMOSPHERIC AND SEDIMENTARY FLUXES OF BE-7, BE-10, PB-210, PO-210, AND CS-137

Atmospheric and sedimentary fluxes of natural (i.e., Pb-210, Po-210, Be-7, and Be-10) and artificial (i.e., Chernobyl Cs-137) radionuclides and particles through Lake Zurich (at 50 m and 130 m depth) between 1983 and 1987 were compared in order to establish trace metal pathways and their kinetics. Atmospheric fluxes averaged 0.83 dpm cm-2 yr-1 for Pb-210 and 16.0 dpm cm-2 yr-1 for Be-7 during 1984 to 1987. Vertical fluxes of Pb-210 and Be-7, associated with settling particles, were measured in sediment traps deployed at 50 m and 130 m depth at a station near the deepest part of Lake Zurich. Average fluxes of Pb-210 and Be-7 at 50 m depth were 0.94 and 4.90 dpm cm-2 yr-1, respectively, and 16.5 x 10(7) atoms m-2 d-1 for Be-10. Sediment traps at 50 m quantitatively collected atmospherically deposited Pb-210 falling through the water column. At 130 m depth, immediately above seasonally anoxic bottoms waters, Pb-210, Be-10, and Cs-137 fluxes were higher than at 50 m by up to 60% at times during the summer stagnation period. Sediment inventories of Pb-210 and Be-10 are in agreement with atmospheric nuclide fluxes, suggesting only moderate recycling of these nuclides in the bottom waters, whereas Cs-137 inventory in the sediments is lower than expected from sediment trap fluxes, indicating remobilization of this nuclide.