Radioactivity of Surface Water and Freshwater Fish in Finland in 1988-1990
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STUK-A94 FEBRUARY 1992 Radioactivity of surface water and freshwater fish in Finland in 1988-1990 Supplement 6 to Annual Report STUK-A89 R. Saxen and U. Koskelainen STUK-A94 FEBRUARY 1992 Radioactivity of surface water and freshwater fish in Finland in 1988-1990 Supplement 6 to Annual Report STUK-A89 R. Saxen and U. Koskelainen FINNISH CENTRE FOR RADIATION AND NUCLEAR SAFETY P.O. Box 268, SF-00101 HELSINKI Finland Tel. +358 0 70821 ISBN 951-47-5936-2 ISSN 0781-1705 The Finnish Government Printing Centre Helsinki 1992 Sold by: The Finnish Government Printing Centre P.O. Box 516 SF-00101 HELSINKI Tel.+358 0 56601 FINNISH CENTRE FOR RADIATION STUK-A94 AND NUCLEAR SAFETY SAXEN R, KOSKEL-UNEN U. Radioactivity of surface water and freshwater fish in Finland in 19SS-1990. Supplement 6 to Annual Report STUK-AS9, Helsinki 1991. ISSN 0781-1705 ISBN 951-47-5936-2 Key words Radiocesium, ^Sr, surface water, freshwater fish ABSTRACT Changes over time in the activity concentrations of radionuclides in surface water and in the five largest rivers and some smaller rivers discharging into the Baltic Sea were monitored in 1988-1990. The dominant gamma-emitting radionuclides were 137Cs and 134Cs. The effect of the uneven distribution of Chernobyl deposition is still seen in the results. The activity concentrations of 137Cs in surface water have de creased significantly: In the drainage area where the activity concentrations were highest after the Chernobyl accident, the concentrations in October 1990 were only about 1-4% of the maximum values in May 1986. The decrease in the activity concentrations of ^Sr was much slighter. Different changes overtime of the activity concentrations of 137Cs and ^Sr can be attributed to the different behaviour of these two radionuclides in the aquatic environment. The study of areal and temporal changes in the activity concentrations of 137Cs in fish, started in 1986, continued in 1988 - 1990. In all, about 2400 fish samples from southern and middle Finland (excluding Lapland) were analysed gammaspec- trometrically during these three years. Seventeen different fish species were in cluded in the study. The highest activity concentrations of 137Cs were detected in small oligotrophic lakes in the area of highest deposition. Predatory fishes reached maximum values of 137Cs the latest, in 1988, while non-predators had maximum values in the year of the accident, 1986, and part-predators (intermediate) in 1987. The nationwide mean concentrations of 137Cs, weighted for catches, in the three fish groups (predatory, non-predatory and intermediate) were: 1988 1989 1990 Predatory fishes 1300 980 740 Non-predatory fishes 410 220 180 Intermediate 860 650 520 3 FINNISH CENTRE FOR RADIATION AND NUCLEAR SAFETY STUK-A94 The average intake of "7Cs via freshwater fish countrywide was about 3400, 2500 and 1900 Bq a"1 in 1988, 1989 and 1990, respectively. During the fresh fallout situation in 1986, transfer factors from water to predatory, non-predatory and intermediate fish varied from about 200 to 1900 in different drainage areas. In 1990, when the situation had stabilized, these factors varied from about 2 300 to 30 000. Maximum transfer from deposition to fish was 0.2-0.3 m2/kg, which means that 20-30% of the amount of "7Cs deposited in one nr transferred to one kg of fish. For the whole country, transfer factors from deposition to edible fish were 0.01-0.1 m2/kg in 1986-1990. 4 FINNISH CENTRE FOR RADIATION STUK-A94 AND NUCLEAR SAFETY CONTENTS page A RETRACT -5 1 INTRODUCTION 6 1.1 Lakes in Finland 6 1.2 Monitoring of surface water 6 1.3 Monitoring of fish 7 2 MATERIAL AND METHODS 8 2.1 Surface water 8 2.1.1 Samples 8 2.1.2 Pre-treatment and analyses 8 2.2 Fish 9 2.2.1 Sampling 9 2.2.2 Sample treatment 11 2.2.3 Gamma-spectrometric analyses 12 2.2.4 Data treatment 12 2.2.5 Areal averages of 137Cs in fish and intake via fish 13 2.2.6 Transfer of 137Cs from water to fish 13 2.2.7 Transfer of 137Cs from Chernobyl deposition to fish 14 3 RESULTS 15 3.1 Surface water 15 3.2 Fish 16 3.2.1 Radionuclide contents 16 3.2.2 Areal averages of 137Cs in fish and intake via i fish 17 3.2.3 Transfer of 137Cs from water to fish 18 3.2.4 Transfer of 137Cs from Chernobyl deposition to fish 18 4 DISCUSSSION 19 4.1 Surface water 19 4.2 Fish 19 ACKNOWLEDGEMENTS 22 REFERENCES 23 FIGURES 25 TABLES 37 FINNISH CENTRE FOR RADIATION AND NUCLEAR SAFETY STUK-A94 1 INTRODUCTION 1.1 Lakes in Finland In Finland, freshwater lakes and rivers cover an area of 33 600 km2, which is 9.9% of the total area of the country. The number of lakes (surface area > 1 ha) is about 56 000. They are most abundant in central and eastern Finland, where about 25% of the total area is water in some places. Finnish lakes are shallow, their average depth being about 7 m. The total volume of the lakes is about 230 km3. The following figures give the size distribution of Finnish lakes: Surface area Number of lakes > 200 km2 17 > 100 km2 37 > 50 km2 81 > 1km2 2 589 > lha 56 000 Besides the above-mentioned lakes, there are almost 132 000 very small lakes or ponds with an average area of 5 a - 1 ha.3,4 The 37 lakes with surface areas of > 100 km2 account for nearly 50% of the total surface area of lakes in Finland and for over 60% of the total volume of the lakes. 1.2 Monitoring of surface water Monitoring of radioactivity in fresh surface water in Finland was continued with an extensive sampling programme. The results for 1986 and 1987 have been given in earlier reports.12,14 The purpose of the monitoring was to acquire areal data for estimations of intake from drinking water and to gather radioecological infor mation on environmental factors affecting the behaviour of radionuclides in different drainage areas. By analysing the contents of radionuclides in samples from the mouths of the five largest rivers in Finland, the abundances of radionuclides transported by the rivers to the Baltic Sea, can be estimated. 6 FINNISH CENTRE FOR RADIATION STUK-A94 AND NUCLEAR SAFETY 1.3 Monitoring of fish Because the unevenly distributed deposition and environmental factors cause considerable variation in the activity concentrations of 137Cs in fish, the study on the radioactivity of freshwater fish was continued with a rather extensive monitoring programme in 1988 -1990. The results for 1986 and 1987 have been given in earlier reports.13-14 The study was undertaken to monitor fish as a food, that is, to evaluate the importance of freshwater fish as a source of radiocesium to consumers in different parts of the country. Attention was also paid to the differences in radioactivity between fish species and fish groups with different feeding habits. The characteristics of the lakes, such as area, water volume, flow rate and limnological type, and the characteristics of the catchment area also affect the contamination of fish by radioactive substances. However, not all these points are discussed at any length in this paper. The results reported here present figures for the average intake of 137Cs via freshwater fish and thus form a basis for estimating radiation doses to man via freshwater fish in various drainage areas in Finland and in the country as a whole. The doses are assessed in the annual reports for 1988 - 1990. Estimations of transfer factors from water to fish and from deposition to fish are made for different drainage areas. 7 FINNISH CENTRE FOR RADIATION AND NUCLEAR SAFETY STUK-A94 2 MATERIAL AND METHODS 2.1 Surface water 2.1.1 Samples The sampling programme, comprising some 180 sampling stations in different parts of Finland (Fig. 1)WM\ was continued to determine the distribution of radioactive substances deposited in the surface waters of drainage basins. The seasonal hydrological variations were considered by taking the samples four times a year in 1988: in March, May, August and October. In 1989 the samples were taken only in May, August and October, and in 1990 only in May and October. The sample volume from each station was 2 litres. Before analyses, the samples were grouped together to represent different drainage basins or their subregions or watercourses (Fig. 2). Thus the volumes of the samples analysed ranged from 6 to 60 1, depending on how many samples were bulked together. The artificial radioactive substances transported in rivers from Finland to the Baltic Sea were monitored as before. Samples were taken from the mouths of the five largest rivers (the Kymijoki, Kokemäenjoki, Oulujoki, Kemijoki and Tornionjoki) four times a year. In 1990 water samples were also taken from three smaller rivers Porvoonjoki, Vantaanjoki and Karjaanjoki, discharging into the Gulf of Finland, (Fig.l). The sample volumes were about 30 litres. Samples from two lakes (Inari and Päijänne) and one river (Kemijoki) were taken monthly for tritium analyses (Fig. 1). 2.1.2 Pre-treatment and analyses To preserve the surface water samples until further treatment, 2 g of solid citric acid were added in the field to each 2—litre sample. The samples were analysed after evaporation to dryness and ashing of the dried samples at 450°C. All samples were analysed gammaspectrometrically. The germanium detectors used for the analyses in our laboratory are either lithium drifted or high purity crystals with relative efficiencies of between 15% and 39%.