Application of the migration models implemented in the decision system MOIRA-PLUS to assess the long term behaviour of 137Cs in water and fish of the Baltic Sea

• The MOIRA-PLUS models were applied to 137Cs in water and fish of Baltic Sea.
• The model results were compared with empirical values from the HELCOM dataset.
• Values of some model parameters were adjusted to improve the model performances.
• The parameter adjustment helps to evaluate radionuclide fluxes through the Baltic.

This work presents and discusses the results of an application of the contaminant migration models implemented in the decision support system MOIRA-PLUS to simulate the time behaviour of the concentrations of 137Cs of Chernobyl origin in water and fish of the Baltic Sea. The results of the models were compared with the extensive sets of highly reliable empirical data of radionuclide contamination available from international databases and covering a period of, approximately, twenty years. The model application involved three main phases: a) the customisation performed by using hydrological, morphometric and water circulation data obtained from the literature; b) a blind test of the model results, in the sense that the models made use of default values of the migration parameters to predict the dynamics of the contaminant in the environmental components; and c) the adjustment of the model parameter values to improve the agreement of the predictions with the empirical data. The results of the blind test showed that the models successfully predicted the empirical contamination values within the expected range of uncertainty of the predictions (confidence level at 68% of approximately a factor 2). The parameter adjustment can be helpful for the assessment of the fluxes of water circulating among the main sub-basins of the Baltic Sea, substantiating the usefulness of radionuclides to trace the movement of masses of water in seas.