21 2.5. CONCLUSIONS Four radionuclide dispersion models have been applied to simulate the transport and distribution of 137Cs fallout from the Chornobyl accident into the Baltic Sea. Models correspond to two categories: box models and hydrodynamic models. In all cases, interactions of dissolved radionuclides with suspended matter and bed sediments are included. Thus, models are very di?erent in structure and parameters. Model results have been compared with extensive field data obtained from the HELCOM database. Inventories in the water column and seabed, as well as 137Cs activity concentrations over 5 years in water and sediments of several sub-basins of the Baltic, have been used for model comparisons. Two main aspects are considered when comparing results: (i) the significant conceptual, numerical and parameterization di?erences between models; and (ii) the complexity of the Baltic Sea system. In spite of these two aspects, model results are consistent, even in the results observed in bed sediments which have been recognized as a significant source of model discrepancy. The same temporal trends are predicted by the models for 137Cs inventories in water and bed sediments and for 137Cs activity concentrations in these two phases in a number of sub-basins. Values predicted by the models for the target magnitudes are very similar and close to experimental values. There is an increase in activity concentrations in bed sediments as radionuclides are scavenged from the water column, where activity concentrations slowly decrease. Results from this exercise suggest that some processes are not very relevant for radionuclide transport within the Baltic Sea, for instance the roles of ice cover and, surprisingly, water stratification by the halocline and thermocline. It is also clear that Chornobyl fallout is the dominant 137Cs source into the Baltic Sea. In addition, results confirm previous findings concerning multi-model applications. Because models with very di?erent structures and parameters have been applied to the same environmental problem, no criteria can be found to decide which could be the most appropriate one. The recommended model to be applied, of course, depends on the modelling purpose, for instance, a fast assessment after an acute accidental release or a long term radiological study. The scale of the present exercise, i.e. timescale and spatial resolution of results (5 years and sub-basin level respectively), may be considered as an intermediate one. At this level, there is significant agreement between box and hydrodynamic models for the present scenario. The discrepancy would probably increase when moving towards smaller scales not properly solved by coarse box models. When moving towards longer timescales and larger domains, the situation can hardly be handled by complex hydrodynamic models, due to computational limitations, and box models might therefore be the best choice.