The application of fucus vesiculosus as a bioindicator of 60Co concentrations in the Danish Straits

J. Environ. Radioactivity 2 (1985) 215-227

The Application of Fucus vesiculosus as a Bioindicator of '*Co Concentrations in the Danish Straits

S. B o e l s k i f t e Ris~ National Laboratory, DK-4000 Roskilde, Denmark

ABSTRACT The occurrence of e°Co in the Danish Straits is investigated by applying the seaweed Fucus vesiculosus as a bioindicator. In order to describe different dispersion situations, three areas have been studied separately: the North Sea, where it is possible to measure e°Cofrom sources in France and~or the UK; the Kattegat, where the sources are the Swedish nuclear power plants Ringhals and Barsebiick; and the Sound, where the initial mixing of the release from Barsebiick takes place. A power function can be estimated for the Kattegat and the Sound describing the content o f ° C o in Fucus as a function of distance from Barsebiick. Problems of uncertainty related to differences in environmental parameters are discussed and new investigations to improve the use of Fucus as a bioindicator are suggested.

INTRODUCTION During the summers of 1982 and 1983, the brown alga Fucus vesiculosus was collected at a number of sampling stations in Denmark. Fucus is useful as a bioindicator through its ability to accumulate different radionuclides and here it is used as a tool to derive dispersion models for the Danish Straits. The Swedish p o w e r plant Barseb/ick is located at the Sound (Fig. 1) and its liquid discharges contain 1-20 G B q 6°Co per month (Sydkraft). This 'large-scale tracer experiment' provides the possibility of generating an 215 J. Environ. Radioactivity 0265-931X/85/$03-30 O Elsevier Applied Science Publishers Ltd, England, 1985. Printed in Great Britain

216

S. Boelskifte

empirical model for the transport and dispersion of effluent in the Danish Straits. The concentration of 6°C0 in water is too low for direct detection except close to the power plant, whereas Fucus is convenient to use because it accumulates 6°C0 to approximately 10 ~ times the concentration in water (on a dry weight basis) (UNSCEAR, 1982). The main purpose of the investigation is to describe the transport of radionuclides from Barseb~ick but it can also provide interesting infor-

59~N

--~58:N F,J. v@. F u sO. '30

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w2 7

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55CN

5Z3N

7o30'E

9OE

10o30'E

12~E

13o30'E

15aE

Fig. 1. 6°Co in sea plants in 1982 (units: Bq kg -l, dry wt). Fu. ve.: Fucus vesiculosus, Fu. se. : Fucus serratus.

F. vesiculosus as bioindicator of 6°Co concentrations in the Danish Straits

217

mation on oceanographic parameters such as the mean residence times in given basins.

M A T E R I A L AND M E T H O D S The sampling points are shown in Figs 1 and 2. Each sample consisted of about 5-20 plants depending on their size. The fresh weight of one sample 10°E

12~E

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0.95

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55°N I

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15°E

Ftg. 2. 6°Co in Fucus vesiculosusand Fucus serratus 1983 (units: Bq kg -l, dry wt) (location names, cf. Fig. 1).

218

S. BoeLskifie

was typically 3 kg. The samples were dried at 100°C, thus reducing weight by a factor of about five to what is referred to as 'dry weight'. The samples were then incinerated at 400°C to reduce their volume further and were then ready for Ge(Li) ~/-spectroscopy. The efficiencies of the Ge(Li) detectors are - 20% (relative to 3" × 3" NaI(T1)) and the counting time was usually three days. Depending on the level of activity the counting error was between 5 and 30%. The photopeaks at 1173 and 1332 keV were used for identification. Since 1959, Fucus has been collected in Denmark for monitoring purposes (Aarkrog et al., 1983). The purpose of the present investigation was to provide as detailed a picture as possible for all parts of the Danish coastline. The collection of Fucus in 1982 was repeated in 1983 in Danish waters, first to examine the changes from year to year and second to increase areal coverage. Besides the above programme, another study has been carried out in which Fucus vesiculosus and Fucus serratus were collected each month at a given location. In addition, continuously integrating water sampling was performed near Ringhals (a Swedish power plant situated approximately 30 km south of G6teborg), and thus concentrations of ~°Co in water and Fucus could be compared each month to estimate the concentration factor, CF, i.e. CF=

Activity in Fucus (Bq 6°Co/kg dry wt) Activity in water (Bq 6°Co/litre)

where a steady state is assumed. Details of this experiment will be published later.

RESULTS Study of Figs 1 and 2 makes it obvious that at least three areas required consideration. The Sound can be regarded as the 'near field' where initial mixing of 6°C0 takes place and where there is a strong gradient perpendicular to the main current. In the Kattegat the levels are influenced both by Ringhals and by Barseb~ick and here less pronounced gradients are found. In the North Sea no contribution from Ringhals or Barsebfick can be assumed and, therefore, this area must be considered separately to locate the sources of the 6°C0 levels found there.

F. vesiculosus as bioindicator of WCo concentrations in the Danish Straits

219

The Sound This area resembles a river with the important exception that the flow has two reversed main directions dependent on meteorological conditions, especially wind direction. Transport perpendicular to the current direction can only be due to mixing outside the Sound followed by a change in current direction. From the Fucus 6°Co concentrations shown in Fig. 1 it is seen that the plume from Barseb/~ck closely follows the Swedish coast. Even at the narrowest section of the Sound the concentration of 6°Co is about twice as high at the Swedish side. The Danish coast was very poorly populated by Fucus in 1982 and 1983 but the few samples collected indicate a decrease in 6°Co concentration by a factor of two from Helsing6r to Copenhagen although Copenhagen is nearer to Barseb/ick than is Helsing6r. This narrow form of the plume is also seen in the samples from the little island, Hven, situated in the middle

103 -i Fucus: Bq S°Co kgdw = 1200 X - ] . z ;

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101

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Fig. 3. 6°Co in seaweed related to the distance from Barsebiick (Sweden).

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of the Sound, where ~ e a t e r values are found on the east than on the west side. In a southerly direction from Barseb~ck power plant, too few samples were found to assess the dilution trend but, northerly, a regression line can be drawn to describe the concentration of ~Co in F u c u s as a function of distance, X, from Barsebtick (samples both from the Sound and from Kattegat) (Fig. 3). Thus the relationship is: Concentration in Fucus (Bq ~Co kg -~, dry wt) = 1200X -l4 where the 95% confidence limits are, for the prefix term, 800 and 1800 and, for the exponent, 1.3 and 1-5. This correlation is in good agreement with earlier Swedish (Mattsson et al., 1980) and Danish (Aarkrog et al., 1982) results, the latter producing a very similar relationship: A = k X -t'~=l'I

where A is the activity concentration in F u c u s (Bq kg -~, dry wt), Xis the distance from Barseb/ick in km and k is a factor dependent on the release (i.e. varying from year to year). Because of the uncertainty related to the fresh weights, dry weights are employed here.

Kattegat There are two main sources of the ~°Co concentrations observed in the Kattegat, namely Barsebfick and Ringhals. In Fig. 1 it is seen that levels increase towards northern Jutland indicating that the influence from Ringhals can be seen here. From Aarkrog etal. (1982) it is evident that the decrease in concentration south of Ringhals is different from that in a northerly direction (X -°91 and X -°~-, respectively). Furthermore, it is seen that the S°Co concentration in F u c u s at a given distance north of Ringhals is much smaller than the concentration in F u c u s at the same distance north of Barseb~ick although a correction for difference in releases has been made. The reason for this discrepancy is not yet known but will be examined further by intensive comparison of concentration factors from both places. As in the Sound, there is a tendency for the plume to follow the Swedish coast very closely. Higher values are found at the islands of Anholt and Laeso than at east Juland and in 1982 it is seen that levels at the Swedish

F. vesiculosus as bioindicator of WCo concentrations in the Danish Straits

221

west coast were even higher. The main transport direction in the Sound and the Belts is northwards from the Baltic to the North Sea. Therefore, very low concentrations are expected in the southern part of the Danish Straits. This trend is in agreement with our results, which show low levels in the southern part of The Great Belt. ~°Co has been measured directly in the water at two different locations in the Kattegat in 1983. On a cruise in May 1983 with F / S Gauss from the Deutsche Hydrografische Institut in H a m b u r g it was possible to use a 1700 litre container for analysis. Sea water was collected at 56 ° 15'N, 12° 22'E and 56 ° 30'N, 11° 30'E and, at each, the result was 0-12 Bq ~Co m -3. Assuming a concentration factor between water and Fucus of 2 x 10 ~on a dry weight basis (see discussion) this level is in accordance with expectations. The equal results from the two places indicate that the area is well mixed. The estimate of the regression line shown in Fig. 3 is based on samples both from the Sound and Kattegat and for 1982 and 1983. Various checks have been made to test the sensitivity of the equation. For example, it has been found that it makes no difference whether the points near Barseb/ick are included or not, indicating that this power function is valid over a considerable distance, Furthermore, although the sampling stations were different in each of the two years, the regression lines were quite similar. suggesting a steady-state situation in Kattegat. The S~Co inflow to Kattegat from other sources can also be estimated (the North Sea: ~Co proportional to salinity; fallout: a constant level; Ringhals: a similar distance d e p e n d e n c e ) and subtracted from the measured levels, but this does not change the regression. The p o w e r function can be used to estimate the mean residence time for the water in Kattegat by calculating the total amount of 6°Co in the area and dividing this by the monthly mean discharge from Barsebfick. If we assume a concentration factor of 2 x 104, a mean depth in Kattegat a b o v e the halocline of 12 m (the correction factor between litre and k r n 3 is 10 ~'-) and a fraction 1/10 to indicate the area represented by Kattegat c o m p a r e d with a circle of radius 270 km (the distance between Barsebfick and Skagen), the total amount o f ~ C o in Kattegat, I, can be estimated as:

I = f~7°2rrR x (2 x 10a)-' x 1200R -v4 × 0-012 × 0"1 x 10~:dR

I = 21

x 1 0 9 Bq

~Co

222

S. Boelskifte

Assuming that the level of 6°Co in Fucus depends on the concentration in water integrated over the last six months (Dahlgaard, 1982) and thus employing a 54 x 10~Bq 6°Co figure for the release from Barseb~ick in the first half year of 1982 (Sydkraft), a mean residence time in Kattegat of 21 x 109 Bq (54 x 109 Bq per 6 months) -~ = 2.3 months is calculated. This is in agreement with oceanographic information, e.g. in Nielsen etal. (1981), where 3500 km 3 is postulated to be the water flow through the straits each year, the volume of the Kattegat is estimated at 529 km 3 and the resulting mean residence time is about 2 months with large variations during the year. The North Sea

As stated earlier it is necessary to know if there are any other 6°Co sources producing the concentrations observed in Fucus in the Kattegat. Fucus is found only at a few places along the west coast of Jutland but, nevertheless, some useful information has been obtained. A clear decrease in ~°Co concentration is found from north to south. Radiocaesium measurements show the same trend and the radiocaesium may come from Sellafield (Aarkrog et al., 1983), following the current around Scotland. Part is then mixed in the North Sea and, about four years after release from Sellafield, it reaches the Danish coast. Due to water movements in the North Sea (Fig. 4) a concentration gradient is generated with low values observed towards southern Jutland. Although the concentration factor for 6°Co is about 100 times that for t37Cs, the Sellafield contribution cannot be seen in seaweed in Denmark. This is because, firstly, discharge of 6°Co is 5000 times lower than the discharge of '3VCs (BNFL, 1979) and, secondly, it reflects the affinity of 6°Co for sediments and the effects of radioactive decay during the four years of transit from Sellafield to Denmark. Other possible sources are from La Hague in France and Winfrith on the south coast of England and these also could explain the observed gradient. According to Kautsky (1973), the main current from the south-west reaches the Danish west coast near Hvide Sande. From the northern part of Jutland, only samples other than Fucus vesiculosus could be obtained. Two samples of Fucus spiralis show higher values at the northernmost point of Jutland (1.00 Bq kg -~ and 1-37 Bq kg -~) indicating a dilution northward from the west coast (F. spiralis should not be compared directly with F. vesiculosus).

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DISCUSSION For the purpose of dose calculation, Fucr~ is interesting because it has the highest sensitivity for the most biologically available physicochemical

v--24

S. Boelski.fte

species of a radionuclide. For the purpose of dispersion models, the concentration of a ~ v e n radioisotope in Fucus is interesting only as long as it can provide reliable information about the concentration in water. Earlier it was stated that the concentration factor is based on a steadystate assumption. While this is an idealisation of conditions in nature, it is important to determine the time period for which the concentrations in Fucus and water should be compared to get an optimal estimate of the concentration factor. For this purpose, an experiment has been comm e n c e d near the Ringhals and Barsebfick power plants in which continuous water sampling is being performed. This experiment is planned to run for at least one year, but preliminary results imply a concentration factor of about 2-3 x 104 Bq kg -L (Bq per litre) -I on a dry weight basis. This is a higher value than that used by U N S C E A R (1982). Concentration factors, however, depend on salinity, the less salt the higher being the uptake of ~Co. This effect tends to decrease the S~Co levels found in Fucus northwards from Barseb~ick while the salinity is increasing. It can also explain a part of the difference between our results at Ringhals (salinity -20(3~) and the U N S C E A R result based on data for salinity -35%~. O n e further problem exists in using Fucus vesiculosus and, indeed, this has been mentioned here several times: it does not always grow where the radioecologist wants to find it? Sometimes Fucus vesiculosus, sometimes Fucus serratus, and sometimes nothing is found. Therefore, it is worth assessing whether there is a systematic difference between the ~Co concentration found in Fucus vesiculosus and in Fucus serratus, or whether one can be used for the other. At a low level area in Denmark, Klint (55 ° 58'N, 11 ° 35'E), samples of both species have been collected (Table I). The first set of samples showed a significant difference between the two and so the collection was repeated at a place with higher concentration. The difference was then less pronounced and a repeated collection at Klint showed no significant difference. One reason for these observations may well be that Fucus vesiculosus has the strongest seasonal variation but it is necessary to continue the experiment to reach a fuller understanding of this phenomenon. The calculation of the mean residence time in Kattegat involves highly sensitive estimates of physical parameters. Moreover, only information about sedimentation of ~'Co in general and in the Sound is available, for which reasons sedimentation was not included in the calculation.

F. vesiculosus as bioindicator of°° Co concentrations in the Danish Straits

225

TABLE 1 Comparison of Concentrations of 6°Co in Fucus vesiculosus and Fucus serratus Collected at Klint

Concentration in Bq e°Co kg -~ dry wt Date

14/02/83 24/03/83 25107183 18/08/83 16/09/83 21/10/83 24/11/83 06/01/84 23/01/84 24/02/84 23/03/84

Fucus vesiculosus (A)

Fucus serratus (B)

A/B

1"32 1.27 2-25 2"80 2-66 3-04 2-81 2-53 2'15 2.07 1-90

2-59 2" 11 2"29 3-01 2"61 2.97 3.67 3.98 3.91 3-07 2-86

0-51 0"60 0"98 0-93 1-02__ 1-02_ 0-77 0-64 0"55 0-67 0-66

The rate of loss due to sedimentation, X~, can be found (Clark et al., 1980) via KdCr

h(l + KaS)

where Kd is the marine concentration factor (10 ~ Bq t -~ (Bq m-3) -I for S°Co), cr is the rate of sedimentation (5 x 10 -3 tm -2 y-i for the Baltic), h is the mean depth (55 m for the Baltic), S is the suspended sediment load (10 -6 tm -3 for the Baltic). For 6°Co in the Baltic this gives X~ = 0'9 y-i leading to a half-life clue to sedimentation of 0.77 y. The mean depth in Kattegat is less than 25 m indicating a half-life for 6°Co in that area of about 1.5 y, which is a long period compared to the two months residence time. Until now 6°Co has not been measured in sediments in the Kattegat, but in Swedish (Notter, 1983) and Danish (Aarkrog, 1983) reports, values ranging from 1 to 26 Bq kg -~ and 9 to 200 Bq m--" have been found close to

226

S. Boelskifie

Barseb~ick, the variations indicating the difficulties in extrapolating t h e data to further distances from Barseb~ick. Notter (1983) estimates 6% of the total release of 6°Co from Barseb/ick to be in the sediment within an area of 110 km'- near the plant but, due to the different sedimentation conditions of different areas, nothing can be deduced from this about the 6°Co levels in sediments in the entire Kattegat. The influences of salinity, temperature and light have been studied in the laboratory (Dahlgaard, 1983) and the effect of the age of the plant has been investigated, for example, by Notter (1983). Together with parameters describing uptake and loss of 6°Co these factors are important for an understanding of the behaviour of Fucus but are difficult to estimate quantitatively. They have not been taken into account in this dispersion investigation where the statistical variation between two samples taken at the same place is about 10% (with few exceptions) and thereby is too large for application of a small correction, e.g. for salinity.

CONCLUSION In spite of the uncertainties there is no doubt that Fucus can provide useful information about the marine environment. It enables measurement of low-level concentrations of different radionuclides at levels which are undetectable in water. When, from Fucus analysis, a model for the spread of activity from a power plant is available, it is possible to make calculations of the doses to humans from the intake of marine products. It is only necessary to know the nuclide concentration factor for fish and t h e seafood catch. In locating different nuclide sources, Fucus is convenient because different radionuclides can be seen in the same sample. Better estimates for the concentration factors are needed, including information on their dependence on time of year, salinity, chemical form of the radionuclide, temperature, etc. Also, better estimates are required for the uptake and loss of a given radionuclide as a function of time.

REFERENCES Aarkrog, A., Botter-Jensen, L., Dahlgaard, H., Hansen, H., Lippert, J., Nielsen, S. P. & Nilsson, K. (1982, 1983). Environmental radioactivity in Denmark 1981, 1982, Riso-R-469, Riso-R-487, Denmark, Riso National Laboratory.

F. vesiculosus as bioindicator of WCo concentrations in the Danish Straits

227

BNFL (1979). British Nuclear Fuels Limited, Annual report on radioactive discharges and monitoring of the environment (1979), Risley, Warrin~on, Cheshire, UK, Director of Health and Safety. Clark, M. J., Grimwood, P. D. & Camplin, W, C. (1980). A model to calculate exposure from radioactive discharges into the coastal waters of Northern Europe, NRPB-R109, Harwell, UK, National Radiolo~cal Protection Board. Dahlgaard, H. (1982). Transferfaktorer i akvatisk radiookologi. 3die Nordiske Radiookologiseminar, Hyvinkaa, Finland, 11-13 May 1982 (In Danish). Dahlgaard, H. (1983). Transuranics, rare earths and cobalt, zinc and caesium in Fucus vesiculosus (seaweed): Laboratory exercises and field realities. International symposium on the behaviour of long-lived radionuclides in the marine environment, La Spezia, Italy, 28-30 September 1983. Kautsky, H. (1973). The distribution of the radionuclide caesium-137 as an indicator for North Sea watermass transport. Deutsche Hydrographische Zeitschrift, 26 (6), 241-6. Mattsson, S., Finck, R. & Nilsson, M. (1980). Distribution of activation products from Barseb~ick nuclear power plant (Sweden) in the marine environment. Temporal and spatial variations as established by seaweed. Environ. Pollut. Series B, 1, 105-15. Nielsen, G. B., Jacobsen, J. S., Gargas, E. & Boch, E. (1981). Evaluation of the physical, chemical and biological measurements, The Belt Project, Denmark, The National Agency of Environmental Protection. Notter, M. (1983). Aspekter ph Fucus vesiculosus anvandbarhet sore bioindikator vid normala utstfipp av radioaktivitet from karnkraftverk. Statens naturv~rdsverk snu pm hr. 1646 (In Swedish). Sydkraft. Mhnadsrapport, and Vattenfall: Rapport 6ver luft- och vfitskeburna utsl/ipp . . . . Ringhals. (Monthly reports to the Swedish authorities on discharges from Barsebfick and Ringhals, respectively) (In Swedish). U N S C E A R (1982) Ionizing radiation: Sources and biological effects, 1982 Report, United Nations Scientific Committee on the Effects of Atomic Radiation.