18 Results for bed sediments are presented in Figure 9. In this case measurement results have a much larger spread than for water. Thus, model results generally lie within error bars. Nevertheless, the trends produced by all models are very similar. In this case there is an increase in 137Cs activity concentrations due to the input of radionuclides from the water column. Moreover, even di?erences between predicted values are relatively small. Model results shown in Figures 8 and 9 are mean values over a number of grid cells or boxes, thus di?erences between models could be reduced because of this averaging process. Nevertheless, di?erences between models in specific points (time series shown in Figures 6 and 7) also remain relatively small. Consequently, it can be concluded that e?ectively there is a considerable agreement between all model predictions. Maps of calculated 137Cs activity concentrations in water and bed sediments for January 1991 using the hydrodynamic models (THREETOX and USEV) are presented in Figure 10. It is evident that the calculated distributions are very similar, both for water and sediments. The USEV model produces slightly higher activity concentrations than THREETOX in the Bothnian Sea. The high activity concentrations produced by THREETOX in the eastern Gulf of Finland may be attributed to the high SPM concentrations in this area (see Section 2.3). The USEV model uses uniform SPM distribution, but THREETOX includes a SPM transport model. Thus, these high concentrations are reproduced by THREETOX but not by the USEV model. A high SPM concentration increases scavenging of radionuclides from the water column to bed sediments. As described above, models with very di?erent structures (box and hydrodynamic models) have been applied to simulate the dispersion of 137Cs in the Baltic, including interactions of radionuclides with suspended matter particles and bed sediments. These interactions are also described in di?erent ways: using an equilibrium distribution coe?cient or using a dynamic approach. In spite of the di?erences between the applied models, results are consistent. Similar levels of 137Cs in water and sediments are predicted for the selected locations and basins, as well as for inventories in the water column and in the seabed. A major component of uncertainty associated with model results is due to the di?culties of representing interactions of dissolved contaminants with the solid phases [36–37]. However, in a previous intercomparison exercise for the Dnieper-Bug estuary [37], it is noted that these di?culties did not a?ect model performances. Such an estuary is a relatively energetic environment, with significant currents. Thus, it was suggested that the relatively fast water dynamics meant that water–sediment interactions did not significantly a?ect radionuclide transport and dispersion [37]. However, currents in the Baltic Sea are not significantly larger than in the Dnieper-Bug estuary. Moreover, the Baltic Sea is an almost closed and shallow water system with several deeper basins. Consequently, interactions of dissolved radionuclides with the solid phases may be significant. Nevertheless, in this environment, water–sediment interactions also do not appear to be a significant source of discrepancy between model results.