The influence of the ammonium acetate concentration on the degree of cesium extraction from bottom deposits in Lakes Svyatoe and Kozhanovskoe is studied. It is shown that degree of cesium desorption is independent of the concentration of the displacing solution in the range 10 –3 –1 M, provided that the amount of ammonium acetate per unit mass of sorbent remains constant. This result shows that the collapse of interpacket spaces of clay minerals either does not happen at all or it has the same effect on the degree of extraction of cesium by solutions with different concentrations. The biological accessibility and migration rate of radionuclides in ground and water ecosystems are largely deter- mined by the forms in which they occur in soil and bottom deposits. A classical method for determining these forms is the method of successive extraction in which the exchange form is determined by extraction with a unimolar solution of ammo- nium acetate [1, 2]. However, it is thought that the real fraction of the exchange form of 137 Cs, based on the dose-forming radionuclide today, is much higher than that determined by the method in [3, 4]. It is believed that a concentrated solution of ammonium acetate can cause the expanded edges of the regions of interpacket spaces of clay minerals to collapse. As a result, the exchange 137 Cs bound with them is transformed into a nonexchange form and is not extractable. To avoid underestimat- ing the real fraction of the exchange form of 137 Cs, a method which uses as the extracting agent a dilute solution of ammo- nium acetate (10 –3 M) has been proposed. In this method, a special sorbent absorbs 137 Cs from the solution (infinite-dilution method) [4]. This method makes it possible to extract from soil and deposits more 137 Cs than the method of successive extrac- tions [3]. It is believed that this is due to the use of a diluted displacing solution, but this assumption has never been checked experimentally. The purpose of the present work is to study the influence of the ammonium acetate concentration on the degree of 137 Cs desorption from bottom deposits in fresh water lakes. Description of Experiment. Bottom deposits from the lakes Svyatoe and Kozhanovskoe in Bryansk oblast were used in the experiment. The bottom deposits were obtained in August 1998 from a 5 cm thick top layer and stored in closed plastic containers in a refrigerator. Their degree of contamination was 25–28 and 17–19 Bq/g, respectively. The fixing power of the bottom deposits of the lakes with respect to 137 Cs was relatively weak, since they contained mainly organic matter and the fraction of clay minerals which fix cesium well was small. Specifically, the humus content in the top 5-cm layer in the bottom deposits was 45–50% in Lake Kozhanovskoe and about 40% in Lake Svyatoe. This explained the high content of mobile (water-soluble and exchange) forms of 137 Cs: from 10 to 17% in bottom deposits in Lake Kozhanovskoe and from 35 to 50% in the bottom deposits in Lake Svyatoe. Atomic Energy, Vol. 93, No. 2, 2002 EFFECT OF THE AMMONIUM ACETATE CONCENTRATION ON THE EXTRACTED FRACTION OF THE EXCHANGE FORM OF 137 Cs IN BOTTOM DEPOSITS E. V. Samokhvalova, A. A. Bulgakov, and A. V. Konoplev UDC 631.43.539.16 Scientific and Industrial Association Taifun. Translated from Atomnaya Énergiya, Vol. 93, No. 2, pp. 144–146, August, 2002. Original article submitted December 5, 2001. 1063-4258/02/9302-0689$27.00 © 2002 Plenum Publishing Corporation 689