6 2. THE BALTIC SEA RADIOLOGICAL SCENARIO 2.1. INTRODUCTION This section of the report describes the results of simulations with four models and compares the distribution of 137Cs activity concentration within the Baltic Sea. Simulations are run for a period of five years, starting from October 1986. The objective of this benchmark exercise is to compare predictions and to further develop models for dispersion and transfer of radionuclides in the marine environment, which can be used for radiological and environmental impact assessments in support of decision making regarding accidental releases of radionuclides to the marine environment. The Baltic Sea is a marine environment that is very well suited for this type of modelling exercise given the large volume of radiological measurements which have been undertaken in the region, mainly after the Chornobyl accident, within the framework of HELCOM5. The Baltic Sea is a complex marine system, characterized by significant salinity gradients, both horizontally and vertically (stratification), with stratification reduced towards the northern Baltic. The Baltic Sea is partially covered with ice during part of the year and water exchanges with the North Sea mainly occurs in pulses. Thus, the Baltic Sea also presents a challenging marine environment in which to test marine dispersion models. The main features of the Baltic Sea are described in Section 2.2, including physical and oceanographic characteristics and the main sources of radionuclides. The modelling exercise and brief description of the models which have been applied are presented in Section 2.3 with additional information on the models included in the annexes. Finally, results are presented and discussed in Section 2.4. 2.2. DESCRIPTION OF THE BALTIC SEA 2.2.1. Physical description The Baltic Sea is shallow, with a mean depth of around 50 m (bathymetry given in Figure 1), connected to the North Sea through the Danish Straits. A map of the Baltic Sea, indicating locations of interest for the present study, is presented in Figure 2. Tides in the Baltic Sea are very small, with amplitudes smaller than 5 cm in most of the sea, due to its limited connectivity with the North Sea [10]. The mass balance indicates that there is an excess of precipitation and river runo? over evaporation in the Baltic. Thus, there is an out?ow of fresher low salinity water in the surface layer and a deep in?ow of more dense water through the Belt Sea around Denmark. This channel is very shallow (sill depth around 18 m) and significant mixing between both water layers occurs. However, this results in a permanent halocline and thermocline in the Baltic Sea, which extend over the di?erent basins. As a result, the average in?ow of saline water from the North Sea via the Skagerrak and Kattegat into the Baltic is small [11]. There is a high frequency exchange of water going on all of the time, but it has almost no e?ect on the Baltic Sea, as generally the same water going back and forth have similar characteristics. Only during very exceptional conditions do in?ux and salt water intrusion events last long enough (over two weeks) to reach far enough into the Baltic Sea, not receding again. During such significant pulses, the Baltic Sea receives between 200 and 400 5 http://www.helcom.fi/