SCAVENGING OF CHERNOBYL CS-137 AND NATURAL PB 210 IN LAKE SEMPACH, SWITZ.

Radioactive fallout from the burning Chernobyl nuclear reactor provided a pulsed release of Cs-137 to Lake Sempach at the beginning of May 1986. The time-dependent removal of Cs-137 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 Cs-137 averaged 150 days in the epilimnion and 280 days in the hypolimnion. Cs-137 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 Cs-137 from the water column. Resuspension of sedimentary Cs-137 occurred during circulation periods in winter. Scavenging by settling particles was the dominant removal process of Cs-137 in epilimnion and hypolimnion. Sediment trap fluxes of Cs-137 and Pb-210 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 Cs-137 between lake water and settling particles, K(d), was determined to 7.0 +/- 1.0 x 10(4) cm3 g-1 for epilimnetic particles and to 5.3 +/-1.1 x 10(4) cm3 g-1 for hypolimnetic particles. The distribution coefficient for Pb-210 was estimated to 1.1 X 10(6) cm3 g-1. The possibility of postdepositional mobility of Chernobyl and nuclear weapons Cs-137 and natural Pb-210 in sediments was proved.