A multitracer study of radionuclides in Lake Zurich, Switzerland: 2. Residence times, removal processes, and sediment focusing

Abstract

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., 210Pb, 7Be, and 10Be ) and artificial (i.e., Chernobyl 137Cs) radionuclide fluxes through Lake Zurich (at 50 and 130 m depth) from 1983 to 1987, atmospheric fluxes of the same nuclides and fluxes of 10Be during 1987 allowed the calculation of nuclide residence times as well as particle settling and transit velocities in Lake Zurich. The residence time of 210Pb 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 7Be 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 137Cs 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 7Be fluxes at 130 m depth as compared to 50 m depth. Model calculations reveal that the overall removal process of atmospherically deposited 7Be 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 10Be, 210Pb, or 137Cs into the lower trap (i.e., “rebound flux”), originating from episodic lateral inputs of fine particles with a high content of 210Pb, 10Be, and 137s, 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.