12 FIG. 4. 137Cs (Bq/m3) in surface water of the Baltic Sea in October 1986 interpolated from measurements. 2.3. MODELLING EXERCISE 2.3.1. Previous modelling studies Despite the large amount of radionuclide data generated for the Baltic Sea, mainly in the period since the Chornobyl accident on 26 April 1986, relatively few modelling studies on radionuclide transport have been carried out for this environment. A one dimensional (1-D) vertical dispersion model was used to explain the distribution of Cs isotopes in the water column of the central Bothnian Sea for the first 6 months after the accident [26] and a box model was later applied to 137Cs and 90Sr [27]. Very local applications within the Baltic Sea have been described for 14C [28] and for isotopes of Ni, Cs and Th [29]. A Lagrangian model has been described and then applied to 137Cs deposition over the whole Baltic but on a very limited time window [30]. More recently, 137Cs transport has been studied with the box model implemented within the MOIRA-PLUS decision system [31]. A blind application was first carried out (using model default parameters) and, later, model output was improved by tuning parameters.