2.4. Transport of 137Cs in Large Finnish Drainage Basins

Abstract

Area-specific transfer parameters are useful in predicting internal radiation doses incurred via the consumption of fish or the drinking of fresh water, as well as for the estimation of counter-measures which might be needed for an accidental release of radionuclides. The results obtained from the countrywide studies of the Finnish Centre for Radiation and Nuclear Safety, carried out in the aquatic environment after 1986, were used to compare the transfer of 137 Cs in seven large drainage areas in Finland. During the first six months after the accident at Chernobyl, the correlation between area concentrations of 137 Cs in water and area deposition of 137 Cs was linear, as also the correlation between the median contents of 137 Cs in fish in the drainage basins concerned and the area deposition of 137 Cs. In subsequent years, temporal changes of 137 Cs in water were more profoundly affected by the character of the catchment area. Though differences in the amount of runoff were taken into account, differences in transfer of 137 Cs from deposition to water still existed between the drainage areas. Annual transfer factors from deposition to water were highest in the drainage basin characterized by large bog areas and lowest in the drainage basin characterized by large clay areas. In coastal drainage areas with a low percentage of lakes, transfer to water was somewhat lower than in inland drainage basins with higher lake percentages. The decrease of 137 Cs in water had at least two components in all the drainage areas studied. At first the concentrations decreased rapidly with an effective ecological halflife of 0.15 - 0.36 years, except in the drainage area with abundant bogs in the catchment where the decrease was remarkably slower with a halflife of 0.94 years. The slower components of the decrease began at different times in different areas. The halflives of the next phase varied from 1.2 to 2.8 years. The transfer of 137 Cs from deposition to fish was highest in the second or third year after the accident in the different drainage areas. The annual maximum transfer from deposition to predatory fish in the area where it was highest was about 0.4 m 2 /kg, whereas in the other areas it was about 0.2 m 2 /kg. The observed ecological half-life of 137 Cs in predatory fish after the beginning of the decline was 0.7 years in the drainage area differing most from the others, whereas in the other areas it varied from 3 to 5 years. The 137 Cs deposited in the drainage areas in 1986 was almost totally retained in the areas and only a little, 1 - 2%, was removed with the rivers flowing from the drainage areas to the sea for five years after the deposition.