Radioactivity of Surface Water and Freshwater Fish in Finland in 1991 - 1994

STUK-A129 JANUARY 1996 Radioactivity of surface water and freshwater fish in Finland in 1991 - 1994 R. Saxen and U. Koskelainen TTT*'* šs^" 'f<^" ^r^""'* i tkBtkMäj^i STUK-A129 JANUARY 1996 Radioactivity of surface water and freshwater fish in Finland in 1991 - 1994 R. Saxen and U. Koskelainen FINNISH CENTRE FOR RADIATION AND NUCLEAR SAFETY P.O. Box 14 00881 Helsinki Tel. +358 0 759 881 ISBN 951-712-091-5 ISSN 0781-1705 Oy Edita Ab Helsinki 1996 Sold by: Finnish Centre for Radiation and Nuclear Safety P.O. Box 14, FIN-00881 Helsinki Tel. +358 0 759 881 FINNISH CENTRE FOR RADIATION STUK-A129 AND NUCLEAR SAFETY SAXEN R, KOSKELAINEN U. Radioactivity of surface water and freshwater fish in Finland in 1991-1994. Helsinki 1996, 62 p. ISBN 951-712-091-5 ISSN 0781-1705 Key words: 137Cs, ^Sr, freshwater fish, surface water ABSTRACT Changes over time in the activity concentrations of 3H, ^Sr 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 MSr was much slighter, and 30-90% of the values for ""Sr in May 1986 still remained in various drainage areas in October 1992. The difference in changes over time in the activity concentrations of 137Cs and ^Sr refer to the different behaviour of these two radionuclides in the aquatic environment. Amounts of 137Cs and *°Sr 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. The countrywide averages of 137Cs in the three fish groups (predatory, non-predatory and intermediate) were: HNNISH CENTRE FOR RADIATION AND NUCLEAR SAFETY STUK-A129 Year Predatory Nonpredatory Intermediate 1991 660 Bq/kg 170 Bq/kg 410 Bq/kg 1992 550 " 150 " 380 " 1993 520 " 140 " 310 " 1994 380 " 120 " 310 " In the catch of freshwater fish in the three most important fisheries, 137Cs decreased with a half-life of 3.5-6 years during 1991-1994; the figure for the catch of the whole country was about 5 years. FINNISH CENTRE FOR RADIATION STUK-A129 AND NUCLEAR SAFETY CONTENTS ABSTRACT 3 CONTENTS 5 1 INTRODUCTION 6 1.1 Lakes in Finland 6 1.2 Monitoring of surface water 6 1.3 Monitoring of fish 6 2 MATERIAL AND METHODS 8 2.1 Surface water 8 2.1.1 Samples, their treatment and analyses 8 2.1.2 Calculations 9 2.2 Fish 10 2.2.1 Samples, their treatment and analyses 10 2.2.2 Data treatment and calculations 13 3 RESULTS 15 3.1 Surface water 15 3.1.1 Radionuclide contents 15 3.1.2 Transfer of 137Cs from deposition to surface water 16 3.1.3 Amounts of 137Cs and ^Sr transported by rivers 16 3.2 Fish 17 3.2.1 Radionuclide contents 17 3.2.2 137Cs in fish catch, in various areas and countrywide 18 3.2.3 Transfer coefficients 19 3.2.4 Half-life of 137Cs 19 4 DISCUSSSION 20 4.1 Surface water 20 4.2 Fish 21 ACKNOWLEDGEMENTS 23 REFERENCES 24 FIGURES 27 TABLES 43 FINNISH CENTRE FOR RADIATION AND NUCLEAR SAFETY STUK-A129 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 occupied by water in some places. Finnish lakes are shal- low, 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 > lkm2 2 589 > lha 56 000 In addition to the above, 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 The purpose of the monitoring was to acquire data on the long-term behaviour of radionuclides deposited in surface water in different drainage basins. The results can also be used for intake estimations via drinking water. The abundances of radionuclides transported by rivers to the Baltic Sea were estimated. The estimation was based on analyses of water samples from the mouths of the five largest rivers in Finland. 13 Monitoring of fish Eating freshwater fish after deposition of radionuclides is one of the most important exposure pathways to man in the long term. To be able to estimate radionuclide intakes and radiation doses via fish, we need information on long- FINNISH CENTRE FOR RADIATION STUK-A129 AND NUCLEAR SAFETY term radionuclide transfer to freshwater fish. The purpose of this study was to acquire a true countrywide picture of the radioactivity situation in freshwater fish, not merely to look for the highest possible activity concentrations. The uneven distribution of deposition and certain environmental factors cause large variations in the activity concentrations of 137Cs in fish; therefore the study was still rather extensive in 1991-1994. Figures are presented for estimating intakes of 137Cs and radiation doses to man via freshwater fish in different parts of Finland and in the country as a whole. The study also gives data on factors affecting long-term radionuclide transfer from deposition to different types of fish in different types of environment. Attention was 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, as well as the characteristics of the catchment area also affect the contamination of fish by radioactive substances. Not all of these points are discussed at any length in this paper, but were still taken into account in sampling. FINNISH CENTRE FOR RADIATION AND NUCLEAR SAFETY STUK-A129 2 MATERIAL AND METHODS 2.1 Surface water 2.1.1 Samples, their treatment and analyses The distribution of radioactive substances deposited in the surface waters of various drainage basins was determined in 1991-1992 with the aid of a sampling programme of the National Board of Waters and Environment that involved about 180 sampling stations all over Finland (Fig. I)1-3. The samples were taken in May and October 1991, and in October 1992. 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. Samples were taken from the mouths of the five largest rivers, the Kymijoki, Kokemäenjoki, Oulujoki, Kemijoki and Tornionjoki, usually four times a year, in March, May, August and October (Fig. 1). In 1993 and 1994, the samples were taken only three times. Samples from three smaller rivers, the Porvoonjoki, Vantaanjoki and Karjaanjoki, which discharge into the Gulf of Finland, were also taken (Fig. 1). The sample volumes were about 30 1. Samples from two lakes (Inari and Päijänne) and one river (Kemijoki) were taken monthly for tritium analyses (Fig. 1). To preserve the surface water samples until further treatment, 2 g of solid citric acid was added to each 2-litre sample in the field. In the laboratory 10 ml of concentrated nitric acid, 50 mg of Sr-Cs and 20 mg of Ba-Ce carriers were added to the samples. The samples were analysed after evaporation to dryness, and then ashed at 450°C. All samples were analysed gammaspectrometrically.13 The eight germanium detec- tors used for the analyses in our laboratory are either lithium drifted or high purity crystals with relative efficiencies of between 30 and 100%. The measurements were performed in background shields of 12-cm lead, which are lined with copper (2 mm) or cadmium (1 mm) and copper (0.5 mm) to reduce X-rays from the shields. The measuring times were between 6 and 24 hours. The measurements were made in beakers with cylindrical geometry (volume 30 ml). The activity FINNISH CENTRE FOR RADIATION STUK-A129 AND NUCLEAR SAFETY concentrations of the samples were calculated using the computer program GAMMA-83.20-21 After gammaspectrometric measurement, the bulked surface water samples were analysed for s9Sr and ^r using the method described by Osmond et al.8 or the extraction method of Lapid et al.6, both of which were modified to some extent.

Load more