122 FIG.X-1. Scheme representing 1-step and 2-step kinetic models. The equation which gives the time evolution of radionuclide concentrations in suspended matter is: ?(???) ?? + ?(????) ?? + ?(????) ?? + ?((? ? ??)???) ?? = ?? ??(???) ??? + ??(???) ??? ? + ? ?? ?? ?(???) ?? ? ? ????? ? ????? ? ????? (X-3) where ws is the particle settling velocity and SED is the deposition of radionuclides from the deepest water layer to the sediment evaluated according to: ??? = ??(?)? ??(?) ? (X-4) It should be noted that (b) means that the corresponding magnitude is evaluated at the deepest water layer. The equation for the temporal evolution of radionuclide concentration in the bottom sediment mixed layer is: ??? ?? = ??? ??(?)? ???? ? ????? + ??? (X-5) where the deposition is now calculated as: ??? = ??(?)? ??(?) ???? (X-6)