Long-term box modelling of 137Cs in the Mediterranean Sea

Environmental radiotracers can be used to understand the underlying processes of the environment where they are found. Amongst others, 137Cs, which is a soluble element in sea water, is transported with water masses in the oceans. A numerical model, in which the Mediterranean Sea was divided into 40 boxes (10 regions of 4 layers each), was developed to better understand the distribution and behaviour of 137Cs in the Mediterranean Sea. It was run with a realistic source term, including global fallout, the Chernobyl accident, the nuclear industry and river runoff, and was used to predict 137Cs concentrations during the period 1954–1994. 137Cs surface water predicted concentrations ranged from 0 to 36 Bq•m−3. In most boxes the most prominent feature was the existence of two 137Cs predicted concentration maxima, corresponding to maximum air concentrations due to nuclear weapons fallout (1963) and the Chernobyl accident (1986). Maximum concentrations were predicted in surface waters in all regions, generally decreasing as mean water depth increased. Largest spatial concentration gradients were predicted during, and shortly after, the global fallout maxima. After the global fallout maximum was reached, predicted concentrations decreased, first rapidly and later on more slowly, reaching relatively low levels immediately before the Chernobyl accident. The maximum 137Cs predicted concentrations due to the Chernobyl accident in surface waters of each box showed widely different values because the deposition pattern was highly irregular. Levels reached values similar to those predicted immediately before the Chernobyl accident more rapidly than in the case of the global fallout maxima. In order to perform model validation, results were compared with annual mean values in each box and, in general, were found to be in good agreement. The main achievements of the model and its limitations are discussed.