The changes in the contents of 137Cs in bottom sediments of some Masurian lakes during 10-15 y observation (Poland).

The measurements of radioactive caesium contents in bottom sediments were carried out in four lakes. First samples (47) were taken in 1992-95. The repeat sampling (109) was performed from the same places in 2005. We examined eight chosen areas in these lakes. In six of them, we observed statistically significant differences in the level of radioactive caesium. It indicates the permanent decrease in the level of 137Cs. The mean annual decrease in the level of radioactive caesium, taking into consideration the radioactive decay, was from 4.2 to 7.8%. In two areas of the profundal zone, we did not observe statistically significant differences in the level of radioactive caesium (lakes Garbas and Rogale Wielkie). Taking into consideration the radioactive decay of caesium, it means about the appearance of the process of accumulation of 137Cs in these areas.