Radioactivity of surface water and freshwater fish in Finland in 1991-1994

Changes over time in the activity concentrations of 3H, 90Sr and gamma-emitting radionuclides in lake and river water were studied. The uneven distribution of Chernobyl deposition is still seen in areal concentrations. 137CS in surface water has decreased significantly: In the drainage area where the activity concentrations were highest after the Chernobyl accident the concentrations in October 1992 were only about 1-4 % of the maximum values in May 1986. The decrease in the activity concentrations of 90Sr was much slighter, and 30-90 % of the values for 90Sr in May 1986 still remained in various drainage areas in October 1992. Amounts of 137Cs and 90Sr transported by the five largest rivers to the Baltic Sea after the Chernobyl accident were estimated to be 38 and 5.5 TBq, respectively, during 1986-1994. Areal and temporal changes in the activity concentrations of 137Cs in fish were also studied in 1991-1994. In all, about 1800 fish samples from southern and central Finland were analysed gammaspectrometrically during this period. Nineteen different fish species were included in the study. The highest activity concentrations of 137Cs were detected in small oligotrophic lakes in the area of highest deposition. The average activity concentrations, weighted by catches, in each of the eight statistical fisheries and in the whole country were calculated. (25 refs., 19 figs., 8 tabs.)