Radioactivity of surface water and freshwater fish in Finland in 1987

Changes over time in the activity concentrations of radionuclides in surface water were monitored as in the previous year. Monitoring of the radioactivity transported by the largest rivers to the Baltic Sea was also continued. All samples were analysed gammaspectrometrically and for 90Sr. Some water samples were analysed for tritium. The dominant gamma-emitting radionuclides were 137Cs and 134Cs. The uneven distribution of Chernobyl deposition is still seen in the results. The activity concentrations of 137Cs have decreased significantly: In the drainage area where the activity concentrations were highest after the Chernobyl accident the concentrations in October 1987 were only about 7% of the values in May 1986. The decrease in the activity concentrations of 90Sr was much slighter. The study on areal and temporal changes in the activity concentrations of 137Cs in fish was expanded in 1987. In all about 1550 samples were analysed gammaspectrometrically and a few samples were also analysed radiochemically for 90Sr. Fifteen 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 level of 137Cs in non-predatory fish had clearly decreased, while that in predatory fish was almost double what it had been in 1986. The average intake of 137Cs via freshwater fish countrywide was about 4200 Bq a-1 in 1987. The average intakes in the various drainage basins ranged from about 500 Bq to 6600 Bq in 1987. The mean activity concentrations of 137Cs in the three fish groups (predatory, non-predatory and intermediate groups) countrywide were 1300, 600 and 1300 Bq kg-1 in 1987, respectively