TRANSMER hydrodynamic model simulates dispersion of dissolved substances in seawater at the scale of the English Channel and the south of the North Sea with a mesh size of one kilometre and a time step of 30 minutes. This model evaluates the medium- and long-term consequences of releases of soluble pollutants in the marine ecosystem under normal or accidental conditions, at a very low computation cost (it takes less than one hour to simulate one year). It has been qualitatively and quantitatively validated by 1400 measurements of 125Sb in seawater in the English Channel and the North Sea. Budgets of measured and simulated radionuclide quantities are balanced for theses seas. The correlation factor between individual measurements in seawater and calculation results is 0.88 with an average error of ± 54%, the error attributable to the measurement process being 15% on average. TRANSMER appears as a robust tool for modelling medium- and long-term consequences of releases of soluble pollutants in the marine ecosystem under normal or accidental conditions.