Scavenging of Chernobyl 137Cs and natural 210Pb in Lake Sempach, Switzerland

Radioactive fallout from the burning Chernobyl nuclear reactor provided a pulsed release of 137Cs to Lake Sempach at the beginning of May 1986. The time-dependent removal of 137Cs from the water column into the sediments has been investigated by analyzing water samples, settling particles, and sediment cores. A flux balance has been established to determine and to quantify the removal processes in the epilimnion and hypolimnion of Lake Sempach. Between May 1986 and March 1988, removal residence times of 137Cs averaged 150 days in the epilimnion and 280 days in the hypolimnion. 137Cs accumulated in the hypolimnion during stratification, and its scavenging from the hypolimnion into the sediments was the rate-limiting step of the overall process of removing 137Cs from the water column. Resuspension of sedimentary 137Cs occurred during circulation periods in winter. Scavenging trap fluxes of 137Cs and 210Pb linearly correlated with increasing particle fluxes up to a maximum of about 2.5 g m-2 d-1. Higher particle fluxes, likely due to calcite precipitation, did not enhance removal fluxes and scavenging efficiency for these nuclides. The distribution coefficient for the partition of 137Cs between lake water and settling particles, Kd, was determined to 7.0 ± 1.0 x 104 cm3 g-1 for epilimnetic particles and to 5.3 ± 1.1 x 104 cm3 g-1 for hypolimnetic particles. The distribution coefficient for 210Pb was estimated to 1.1 x 106 cm3 g-1. The possibility of postdepositional mobility of Chernobyl and nuclear weapons 137Cs and natural 210Pb in sediments was proved. 72 refs., 9 figs., 9 tabs