Radioactivity of Surface Water and Freshwater Fish in Finland in 1987
Total Page：16
File Type：pdf, Size：1020Kb
STUK-A77 May 1990 RADIOACTIVITY OF SURFACE WATER AND FRESHWATER FISH IN FINLAND IN 1987 Supplement 3 to Annual Report STUK-A74 Ritva Saxen , ; v, r * • r- RADIOACTIVITY OF SURFACE WATER AND FRESHWATER FISH IN FINLAND IN 1987 Supplement 3 to Annual Report STUK-A74 Ritva Sax6n Finnish Centre for Radiation and Nuclear Safety P.O.Box 268, SF-00101 HELSINKI FINLAND ISBN 951-47-1468-7 ISSN 0781-1705 Helsinki 1990 The Finnish Government Printing Centre 3 ABSTRACT Changes over time in the activity concentrations of radio nuclides 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 radio nuclides were 137Cs and 134Cs. The uneven distribution of Cherno byl deposition is still seen in the results. The activity con centrations of * 37 Cs have decreased significantly: In the draina ge 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. A decrease of by about 20% from the highest values in May 1987 in the activity concentrations of 90Sr was noticed in October 1987. Different changes over time of the activity concentrations of 137 Cs and *°Sr refer to the different behaviour of these two radionuclides in the aquatic environment. The study on areal and temporal changes in the activity concen trations of 137Cs in fish was expanded in 1987. In all about 1550 samples were analysed gammaspectrometrically, and a few samples were also analysed radiochemlcally for 903r. Fifteen different fish species were Included in the study. The highest activity concentrations of I37Cs were detected in small oligotrophic lakes in the area of highest deposition. The level of 137C8 in non-predatory fish had clearly decreased, while that in predatory fish was almost double what it had been in 1986. Activity concentrations of i37Cs in semi-predatory fish were also slightly higher than in 1986. 4 intake estimates were based on the average activity concen trations, weighted for catches, in each of the eight statistical fishery areas and in the whole country, and on the average consumption of freshwater fish. The average intake of 137Cs via freshwater fish countrywide was about 4200 Bq a1 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. i6 CONTENTS ABSTRACT 3 CONTENTS 5 1 INTRODUCTION 7 1.1 Surface water 7 1.2 Fish 7 2 MATERIAL AND METHODS 9 2.1 Surface water 9 2.1.1 Samples 9 2.1.2 Pre-treatment and analyses 9 2.2 Fish 10 2.2.1 Sampling 10 2.2.2 Sample treatment 12 2.2.3 Analyses 13 Gammaspectrometric analyses Radiochemical analyses 2.2.4 Data treatment 14 2.2.5 Intake via fish 15 RESULTS 16 3.1 Surface water 16 3.2 Fish 17 3.2.1 Radionuclide contents 17 3.2.2 Intake via fish 19 4 DISCUSSSION 20 4.1 Surface water 20 4.2 Fish 20 ACKNOWLEDGEMENTS 23 REFERENCES 24 FIGURES 27 TABLES 37 7 1 INTRODUCTION 1.1 Surface water In Finland, freshwater lakes and rivars 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 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 > 1 km2 2 589 > 1 ha 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. Monitoring of radioactivity in fresh surface water in Finland was continued with an extensive sampling programme. The purpose of the monitoring was to acquire areal data for Intake esti mations from drinking water and to gather radioecological Infor mation on environmental factors affecting the behaviour of ra dionuclides in different drainage areas. To be able to estimate the abundances of radionuclides trans ported by rivers to the Baltic Sea, analysis of samples from the mouths of the five largest rivers in Finland was continued. 8 1.2 Fish Because the unevenly distributed deposition caused large vari ation in the activity concentrations of 137Cs in fish, the study on the radioactivity of freshwater fish, started in 1986, was extended in 1987. 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 areas. The sizes of Finnish lakes were considered to find out if the radioactivity of fish depended on the size of the lakes. 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 surface area, water volume and abundance of nutrients, and the characteristics of the catchment area also affect the contamination of fish by radioactive substances. However, not all of these are dis cussed at any length in this paper. The results reported here present figures for the average Intake of * 3 7 C3 via freshwater fish and thus form a basis for estimating radiation doses to man via freshwater fish in various deposition areas in Finland and in the country as a whole. The doses will be assessed in the Annual Report for 1987.15 9 2 MATERIAL AND METHODS 2.1 Surface water 2.1.1 Samples The sampling programme, comprising about 180 sampling stations all over Finland (Fig. I)110 , was continued to determine the discribution 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 March, May, August and October. The sample volumes were 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 as above (Fig. 1). The sample volumes were about 30 litres. Samples from two lakes (Inari and Päijänne) and one river (the Kemijoki) were taken monthly for tritium analyses (Fig. 1). 2.1.2 Pre-treatment and analyses To preserve the surface water subsamples until further treatment, 2 g of solid citric acid were added to samples. The samples were analysed after evaporation to dryness and ashing of the dried samples at 450°C. All the samples were analysed gammaspectrometrlcally. The eight germanium detectors 10 used for the analyses in our laboratory are either lithium drifted or high purity crystals with relative efficiencies of between 15% and 39%. The measurements were performed in back ground shields of 12 cm lead, which are lined with copper (2 sun) 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 concentrations of the samples were calculated using the computer program GAMMA-83.12-13 After gammaspectrometric measurement the bulked surface water samples were analysed for s 9 Sr and '°Sr using a method described by Osmond et al.7 , which was modified to some extent for surface water. After radiochemical separation, 89Sr and 90Sr were measured together as strontium carbonate. The measurements were made using a low-background gasflow beta-counter, Berthold LB 770-1. After a two-week ingrowth period, 90Y, the daughter of 90Sr, was precipitated as Y-oxalate and measured twice to check the purity of the 90 Y samples. The samples for tritium analyses were distilled to dryness to remove impurities. The tritium level was then determined by the electrolytic enrichment method presented by östlund et al.17 , with some modifications. 2.2 Fish 2.2.1 Sampling The monitoring programme included fish samples taken from five of the largest lakes in Finland (Fig. 3) and from other large lakes of great importance for freshwater fishing. In order to locate the highest concentrations of 137Cs in fish, samples were also taken from small lakes (surface area less than 1 km3 ), mainly in the area of high deposition (fallout categories 4 and 5 in Fig. 5) 11 The total numbers of fish samples analysed and the number of lakes from which fish samples were taken in the various prov inces (cf.