A MULTITRACER STUDY OF RADIONUCLIDES IN LAKE ZURICH, SWITZERLAND .2. RESIDENCE TIMES, REMOVAL PROCESSES, AND SEDIMENT FOCUSING

Residence and settling times of particles and particle-reactive nuclides evaluated from in situ tracer studies can be used as diagnostic indicators of trace element pathways in lakes. Natural (e.g., Pb-210, Be-7, and Be-10) and artificial (i.e., Chernobyl Cs-137) radionuclide fluxes through Lake Zurich (at 50 and 130 m depth) from 1983 to 1987, atmospheric fluxes of the same nuclides and fluxes of Be-10 during 1987 allowed the calculation of nuclide residence times as well as particle settling and transit velocities in Lake Zurich. The residence time of Pb-210 in the lake is approximately 1 month and, hence, of the same order as the residence times of particles and stable Pb. Steady state residence times for Be-7 were calculated as 150-170 days, while non steady state, instantaneous removal residence times ranged from 50 to 800 days. Polonium 210 was removed from Lake Zurich with removal times of 10-26, months indicating slow removal processes or efficient recycling in the lake water. Transit velocities of particles, calculated from the attenuation of Chernobyl Cs-137 fluxes at 50 m and 130 m depth during early May 1986, were 17 m d-1, while 2-4.5 m d-1 was calculated as an effective settling velocity from the attenuation of Be-7 fluxes at 130 m depth as compared to 50 m depth. Model calculations reveal that the overall removal process of atmospherically deposited Be-7 from the water column to the sediments is mainly controlled by the adsorption/coagulation step and not by the rate of particle settling. Extra inputs of Be-10, Pb-210, or Cs-137 into the lower trap (i.e., "rebound flux"), originating from episodic lateral inputs of fine particles with a high content of Pb-210, Be-10, and Cs-137, were observed during the lake stratification period (i.e., summer). This novel observation of summertime radionuclide and sediment focusing is different from previously described wind-generated resuspension and focusing effects caused by river plumes and lake sediments during the stagnation period. Boundary and focusing effects for radionuclide-bearing particles are proposed to explain observed summertime enhancements of longer-lived nuclide fluxes at 130 m depth as well as the decrease in calculated effective settling velocities.