[en] Long - term observation results of radionuclides and volume activity (VA) in the Baltic Sea water were analysed. Radiological situations in the coastal zone and the open area of the south-eastern Baltic were observed. The objective analysis method was used to define the radionuclide VA field structure. Thee measured and calculated results of and VA field comparison in the south-eastern Baltic was satisfactory. According to the nuclear hydrophysical model an imitated situation of a 'passive' admixture spreading calculation in the south-eastern Baltic was carried out. In the case of the 'pollution' source concentration 10⁶ times exceeding the 'natural' background at the western wind velocity 10 m/s, the whole coastal area of Lithuania would be contaminated in three days. (author). 7 figs., 2 tabs., 8 refs