CZ9827767 INVESTIGATIONS ON THE CAESIUM-137 HOUSEHOLD OF LAKE LUGANO, SWITZERLAND J. Drissner1», T. Klenk", E. Klemt", R. Miller", G. Zibold", M. Burger2», A. Jakob2' 1) Fachhochschule Ravensburg-Weingarten, 1AFStrahlungsmeßtechnik, Postfach 1261, D-SH241 Weingarten, Deutschland 2) OK, AC-Laboralorium Spiez, Sektion Sicherheitsfragen, Zentrale Analytik und Radiochemie, CH-3700 Spiez, Schweiz Sediment samples were taken from different basins of Lake Lugano and the vertical distribution of caesium-137 in the sediment was measured In all samples, a distinct maximum in the distribution can be attributed to the 1986 Chernobyl fallout. From the position of the maxima, we deduce sedimentation rates of the order of 5 to 10 cm within 10 years. Chernobyl caesium-137 inventories in the sediment vary between 10 and 110 kBq/m2, whereas the direct input through the lake surface caused by the Chernobyl fallout amounted to 22 kBq/m2. Close to tributaries, typical specific activities of about 1 kBq/kg can still be found in the top layer of the sediment. This relatively high specific activity can be explained by a continuous input through the tributaries. At all locations, 5 step caesmm-137 extraction experiments on sediment samples result in percentages of extracted caesium which are at least a factor of 2 up to a factor of 8 higher than those of Lake Constance sediments where the caesium is strongly bound to illites. Consequently, an enhanced potential for caesium-137 to redissolve into the pore water and ultimately into the water of the lake has to be assumed. Our findings are consistent with the 1996 measurements of the specific activity of the pore water of Lake Lugano sediments of about 400 mBq/1 and that of the lake water of 8 to 43 mBq/l close to the ground and 3 to 8 mBq/l close to the surface. The specific activity of the water of the main tributaries Vedeggio and Cassarate is of the order of 10 mBq/l. Thus, including the measured cation exchange capacity (CEC) of about 400 to 500 mval/kg and a total caesium K<r value of the order of 103 I/kg, Lake Lugano takes a middle position between deep mesotrophic lakes with a high self-cleaning power like Lake Constance, and glacially formed shallow eutrophic lakes like the Vorsee, 30 km north of Lake Constance. -54-.