Inflow of Chernobyl 90Sr to the Black Sea from the Dnepr River

Estuarine, Coastal and Shelf Science (1992) 34, 315-320

Inflow o f C h e r n o b y l 9°Sr to the Black Sea f r o m the D n e p r River

G e n n a d y G. P o l i k a r p o v a, H u g h D . L i v i n g s t o n b'c L u d m i l l a G. K u l e b a k i n a a, K e n O. B u e s s e l e r b, ' N i k o l a i A. S t o k o z o v a a n d S u s a n A. C a s s o b ~Department of Radiation and Chemical Biology, A.O. Kovalevsky Institute of Biology of the Southern Seas, Academy of Sciences, Ukrainian SSR, Sevastopol, Crimea 335000, U.S.S.R. and bDepartment of Chemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachussetts 02543, U.S.A. Received 24June 1991 and in revised form 5 August 1991

Keywords: Chernobyl; 9°Sr; Dnepr River; Black Sea Following the Chemobyl reactor accident in April 1986, studies of radionuclides in aquatic systems in general, and in the Black Sea in particular, have focused primarily on the fate and behaviour of direct fallout deposition (Buesseler et al., in press; Livingston et al., 1988; Polikarpov et al., 1991). In this paper we present an evaluation of riverine 9°Sr input and its use as a tracer for circulation studies of Chernobyl labelled shelf waters. We describe how 9°Sr measurements in the Dnepr River in the period 1986-89 can be used to determine the amount and timing of the subsequent 9°Sr inflow to the northwest Black Sea. Comparison of these data with measurements made in the Danube River in 1988 demonstrates that the Dnepr 9°Sr flux to the Black Sea is about one order of magnitude higher than that of the Danube. 9°Sr ( T i n = 2 9 years), while not a major component of direct Chernobyl fallout (Aarkrog, 1988) delivered to the Black Sea in 1986, is mobile in freshwater systems. It is therefore, a major candidate for secondary input to the Black Sea via several major rivers whose freshwater supply to the Northwest Black Sea accounts for > 70% of all riverine inputs to this basin (Tolmazin, 1985). O f these northern rivers, the Danube has by far the largest flow and drains an area substantially impacted by fallout from Chernobyl. T h e D n e p r River has substantially lower flow than the Danube; however its watershed includes the reactor site itself, and hence major transport of 9°Sr from that intensely contaminated area might be expected. T h e behaviour of 9°Sr in lakes and rivers is well known to be primarily in the dissolved phase, presumably as Sr 2+, with neglible amounts associated with particles and sediments. Circulation studies in the Black Sea of °°Sr from these rivers have a bearing on issues related to the dispersal of river-borne agricultural and industrial pollutants, and to the consequences of changes in the Black Sea due to watershed management practices and the reduction of freshwater inputs (Tolmazin, 1985). ~Authorto whom correspondenceshould be addressed. 0272-7714/92/030315 + 06 $03.00/0

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T h e Institute of Biology of the Southern Sea's (IBSS) group has recently described measurements ofg°Sr and 137Cs activities throughout the D n e p r River system in the spring of 1987 and 1988 (Polikarpov et al., 1991). Likewise, data have been reported for both the middle and lower D n e p r during the s u m m e r and a u t u m n in 1986 following the accident (Polikarpov et al., 1991; Izrael et al., 1987). Since the middle of 1986, the I B S S laboratory has been making regular measurements of 9°Sr (and 137Cs) at several points at the m o u t h and in the estuary of the Dnepr. Since 1988, our laboratories at IBSS and the Woods Hole Oceanographic Institution ( W H O I ) have been involved in a joint study of Chernobyl radionuclides in the northwest Black Sea. We use the first set of our joint measurements in the northwest Black Sea in 1989 to demonstrate that there is good agreement between the radiochemical analyses made by each laboratory and, therefore, past and future data sets obtained independently from both groups may be merged and discussed with confidence. During our first joint expedition in the northwest Black Sea in April 1989, water sampling and radiochemical analyses were made in parallel at several locations. Samples for both groups were collected from separate water casts; thus our intercomparison will include some sampling uncertainties. T h e results of our intercomparison for 9°Sr and 137Cs are shown in Figure 1. Only samples found to contain these radionuclides at concentrations above 10 Bq m -3 are included. Below this level, significant differences began to appear due to the higher measurement sensitivities of the W H O I laboratory. I n general the analytical agreement between both labs is rather good. T h e I B S S 9°Sr data average 11% higher than the W H O I data. We suspect that this may be due to a small blank problem in the IBSS 9°Sr data, as their data from samples at concentrations below 10 Bq m -3 showed significant blank effects. All of the data described below for 9°Sr at the

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mouth of the Dnepr are at concentrations 10-40 times the values compared here and are little affected by this. T h e fact that these 9°Sr data in the 15-20 Bq m -3 range agree to within 10% gives confidence in the quality of the IBSS river values at their higher values. T h e W H O I ~37Cs data average 9% systematically and inexplicably higher than those of the IBSS laboratory. Both laboratories reference their results to standards from the IAEA Marine Radioactivity Laboratory and other primary standards. It should be noted, however, that differences in the 9°Sr/137Cs ratio in Black Sea water samples discussed below are much larger than the analytical uncertainties. T h e influx record of 9°Sr from the Dnepr River to the Black Sea has been established from time-series measurements made at the mouth of the Dnepr by the IBSS. Sampling and analyses at regular intervals took place during the 1986-89 time period. Monthly samples were collected at three points in the Lower Dnepr and analysed for 9°Sr. T h e locations ranged from the top of the Dnepr Estuary to a reservoir about 60 kilometers upstream (Figure 2). They correspond to stations 13-15 described by Polikarpov et al. (1991). T h e mean monthly 9°Sr concentration at these three locations is plotted as a function of time in Figure 3. Also shown are the annual fluxes of 9°Sr to the Black Sea, calculated by summing the product of the monthly mean river flow and the 9°St concentrations. T h e monthly Dnepr River flow is shown in Figure 4. Prior to the Chemobyl accident, 9°St concentrations in the Dnepr River from atmospheric nuclear weapons tests are estimated (Vintsukevich & Tomilin, 1987) to have been 7-11 Bq m -3 (for comparison, plotted in January 1986 in Figure 3). During the first year after the accident, 9°St levels at the mouth of the Dnepr rose sharply and peaked in the

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spring of 1987 (Figure 3). Thereafter they followed a pattern of smaller spring peaks in concentration in 1988 and 1989. It is clear that this pattern is in consequence to the downstream flushing of the very high concentrations reported in the water bodies in the immediate vicinity of the reactor site (Polikarpov et al., 1991; Izrael et al., 1987; Polikarpov et al., 1988). In contrast to Sr, 137Cs from the accident site has been shown to have been transferred very efficiently to freshwater sediments in the river and reservoirs south of the Chernobyl region (Polikarpov et al., 1991). T h e fluxes of 9°Sr to the Dnepr Estuary have followed an even greater pattern of seasonal variation than those of 9°Sr concentrations. Annual concentration peaks are coincident with the annual maxima in water flow. 9°Sr input to the Black Sea has therefore shown a strong annually pulsed pattern, with major input each spring. T h e annual input in 1987, 1988, and 1989, respectively was 14.7, 14.0, and 9.9 T B q (398,380, and 268 curies). T o put this in contemporary radioecological perspective, these values may be compared with planned releases of 9°Sr to the Irish Sea by British Nuclear Fuels from its Sellafield nuclear fuel reprocessing plant. T h e y are the same order of magnitude as recent annual Sellafield discharges--18"3 and 15.0 T B q in 1986 and 1987 (Hunt, 1987; Hunt, 1 9 8 8 ) w but more than an order of magnitude less than the higher release rates during the 1970s (Livingston et al., 1982). T h e possibility that other freshwater inputs are a significant source of Chernobyl 9°Sr to the Black Sea may be addressed by comparing radionuclide activities in the Dnepr with those from the Danube River. This comparison will indicate whether the Dnepr 9°Sr fluxes originate primarily from intense contamination around the Chernobyl reactor site, or rather from fallout deposition and subsequent remobilization within the watershed. Sixteen samples of Danube River water collected in March 1988 from Vienna to the Black Sea were analysed for 9°Sr and t3VCs by the IBSS. Concentrations ranged from 7-8 to 16'7 Bq m -3 (mediarl = 13), and 3.7 to 37 Bq m -3 (median = 12) for 9°St and ~37Cs, respectively. Previous 9°Sr levels in the Danube ranged from a maximum of about 80 Bq m -3 in the mid-1960s to around 10 Bq m -3 in 1977-79 (Polikarpov, 1984) as a consequence of atmospheric nuclear weapons testing. As such, the D n e p r 9°Sr levels in 1988 (200-400 Bq m -3) are one-two orders of magnitude higher than the Danube levels. Given that the Danube River flow is four times that of the Dnepr (Tolmazin, 1985), the total input ofg°Sr from the Danube in 1988 may be calculated to be about 15°.o of that from the Dnepr, i.e. 2-2 T B q (58 curies). T h e exact value is subject to the actual annual mean flow and concentration values, but it is clear that the higher Dnepr 9°Sr levels far outweigh the higher Danube flow with respect to annual 9°St input to the Black Sea. Although the watershed of the Danube was significantly impacted by Chernobyl fallout, it does not represent a large source ofg°Sr. Consequently, Dnepr 9°Sr fluxes described here constitute a unique and substantial tracer signal originating from the reactor site. It may therefore be possible to distinguish freshwater derived from the Dnepr from that of the other large rivers in the northwest Black Sea in subsequent studies of circulation and mixing in this basin (Polikarpov et al., 1991). T h e annual pulses of 9°Sr to the Dnepr Estuary described above have entered the northwest Black Sea each year since 1987. Polikarpov et al. (1991) detected their arrival in 1987 and 1988 in the form of elevated 9°St concentrations and 9°Sr/t37Cs ratios in surface water in the northwest. These observations have been corroborated in our joint studies in 1989. Samples with 9°St concentrations greater than 20 Bq m -3 show Dnepr River 9°St influence. T h e highest value found comes from a sample collected just outside the mouth of the Dnepr Estuary. It has a 9°Sr/~3VCs ratio close to one, about four times the ratios

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w h i c h characterize h i g h salinity surface waters i n the central a n d s o u t h e r n part o f the Black Sea basin. T h e D n e p r 9°Sr influx is large e n o u g h to be identifiable as it mixes o n the shelf a n d slope o f the n o r t h w e s t Black Sea a n d offers a tool for e x a m i n i n g the export o f shelfwater into the i n t e r i o r of the basin. O n g o i n g joint studies b e t w e e n W H O I a n d I B S S are directed towards this goal.

Acknowledgements W e t h a n k the officers, crew a n d scientific staff o n the R.V. Professor Vodjanitsky for m a k i n g this j o i n t cooperative U. S . A . / U . S . S . R . research p r o g r a m possible. T h e analytical assistances o f J . E. A n d r e w s a n d the secretarial help o f M . R. Hess are greatly appreciated. T h e U . S . p o r t i o n o f the p r o g r a m was m a d e possible b y grants f r o m the Vetlesen F o u n d a t i o n , the U . S . N a t i o n a l Science F o u n d a t i o n , a n d the U . S . E n v i r o n m e n t a l P r o t e c t i o n Agency. T h i s is C o n t r i b u t i o n No. 7812 f r o m the W o o d s H o l e O c e a n o g r a p h i c Institution.

References Aarkrog, A. 1988The radiological impact of the Chemobyl debris compared with that from nuclear weapons fallout. Journal of Environment Radioactivity 6, 151-162. Buesseler, K. O., Livingston, H. D. & Casso, S. A. (in press) Mixing between oxic and anoxic waters of the Black Sea as traced by Chernobyl cesium isotopes. Deep-Sea Research, Black Sea Oceanography. Hunt, G.J. 1987Aquatic Environmental Monitoring Report 18 (Directorate of Fisheries Research, Lowestoft). Hunt, G. J. 1988 Aquatic Environment Monitoring Report 19 (Directorate of Fisheries Research, Lowestoft). Izrael, Y. A., Petrov, V. N. & Avdjushin S. I. et al., 1987 Radioactive pollution oft_he natural environmentin the accident zone of the Chernobyl Nuclear Power Station. Meteorologija i Gidrologija N2, 5--18. Livingston, H. D., Bowen, V. T. & Kupferman, S. L. 1982 Radionuclides from Windscale discharges I: nonequilibrium tracer experiments in high latitudes oceanography. Journal of Marine Research 40, 253-272. Livingston, H. D., Buesseler, K. O., Izdar, E. & Konuk, T. 1988 Characteristics of Chemobyl fallout in the southern Black Sea. In Radionuclides: A Tool for Oceanography (Guary, J. C., Guegueniat, P. & Pentreath, R. J., eds.). Elsevier, Essex. pp. 204-216. Polikarpov, G. G. I984 Marine Radiochemoecology and Problems of Pollutants (Akad. Nauk. Ukrainskois, SSR, Vol. 34). Polikarpov, G. G., Timoshchuk, V. I. & Kulebakina, L. G. 19889oSrconcentrations in the aquatic environment of the Lower Dnepr River towards the Black Sea. Dokladi Academii Nauk Ukr. SSR, Ser. B NS, 77-79. Polikarpov, G. G., Kulebakina, L. G., Timoshchuk, V. I. & Stokozov, N. A. 1991 ~Sr and ~37Csin surface waters of the Dneper River, the Black Sea and the Aegean Sea in 1987 and 1988.Journal of Environmental Radioactivity 13, 25-38. Tolmazin, D. 1985 Changing coastal oceanography of the Black Sea. I: Northwestern shelf. Progress in Oceanography 15, 217-276. Vintsukevich, N. V. & Tomilin, Yu. A. 1987 Fallout nuclide concentrations in the Dnepr River. Ecologiya (Sverdlovsk) N6, 71-74.