11 2.2.2. Radionuclide inputs The occurrence of man-made radionuclides in the Baltic Sea has four main causes: ? Fallout from atmospheric nuclear weapons tests; ? The Chornobyl accident in 1986; ? Discharges from the two European facilities for the reprocessing of spent nuclear fuel, at Sellafield in the UK and La Hague in France; ? Authorized discharges of radioactivity into the Baltic Sea occurring during the routine operation of other nuclear installations. The impact of non-nuclear facilities (e.g. hospitals, industry, etc.) on the radioactivity in the Baltic Sea is negligible and very local [20]. The relative contributions of each source is given in Table 5 [21]. From this table it is obvious that Chornobyl fallout is the main source, accounting for 83% of the total input. It is noted that 134Cs was also deposited by this fallout with a 134Cs/137Cs activity ratio of approximately 0.5, but due to its half-life of only approximately 2 years, this radionuclide was only detectable until the beginning of the 1990s [22]. Deposition from the Chornobyl accident was evenly distributed, and the highest contaminated areas were the Gulf of Finland and Bothnian Sea. A map of 137Cs activities in surface water (sampling depth less than 10 m) in October 1986 is presented in Figure 4. The distribution also re?ects the deposition on land in the drainage area of the Baltic Sea. This map has been constructed from an interpolation of measurements [23–25]. Reported mean values in di?erent basins (see Figure 2 above) are listed in Table 6. TABLE 5. TOTAL AND RELATIVE 137CS INPUTS TO THE BALTIC SEA UP TO 2010. ADAPTED FROM REF. [21] Source Total (TBq) Relative contribution (%) Nuclear weapon tests 800 13 Chornobyl fallout 4700 83 Sellafield and La Hague 250 4 Authorized discharges 2.4 0.04 TABLE 6. 137CS MEAN ACTIVITY CONCENTRATIONS IN SURFACE WATER FOR 1986 [21] Basin 137Cs (Bq/m3) Bothnian Bay 100 Bothnian Sea 470 Aland Sea 800 Gulf of Finland 480 Northern Baltic 375 Gotland 150 Southern Baltic 75