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Chernobyl & the marine environment: The radiological impact in context

Scientists at the IAEA's Marine Environment Laboratory in Monaco have played an integral role in post-Chernobyl studies

by I he Chernobyl nuclear accident in April 1986 phers around the world. The accidental release of Pavel Povinec, had a significant impact on both the terrestrial substantial amounts of radioactivity to the at- Scott Fowler, and marine environments. The total activity of mosphere essentially initiated a worldwide tran- and Murdoch the nuclear debris released was so high (1-2.1018 sient tracer experiment on a scale that would Baxter ) that the radioactive fallout distributed never have been planned deliberately. Shortly widely after the accident actually dominated an- after the accident, fission and activation products thropogenic environmental levels in various released by the fire entered marine parts of the world. throughout . They became involved in Concentrations of anthropogenic radionu- many of the elemental cycles that oceanogra- clides generally vary from to region, ac- phers have for decades been trying to charac- cording to the location and magnitude of the terize using a wide variety of conventional tech- different sources of contamination. The main niques. Suddenly, immediately after the Cher- global contribution to marine radioactivity, as in nobyl accident, a suite of radioactive tracers be- the terrestrial environment, is still from fallout came available as a pulse to trace, rather like a from nuclear tests in the atmosphere, particularly coloured dye, the movement of elements through during the 1950s and 1960s. the . IAEA-MEL scientists took part in However, in some , like the Irish and this exciting and serendipitous experiment North , the concentrations of anthropogenic through temporal monitoring of (e.g. -137 and - both the coastal and open ecosystems. 239) in the marine environment have been sig- For open work, one of the most important nificantly influenced by discharges (e.g. from innovations in marine monitoring over the past European reprocessing plants). On the other 15 years has been the development of hand, the Baltic and Black Seas have been the traps to directly measure fluxes of materials as- seas most affected by the Chernobyl accident. In sociated with sinking particles. Moored sediment all these latter regions the spatial and temporal traps can be left unattended at any depth in the trends in the concentrations of anthropogenic ocean to collect discrete, -series samples at radionuclides have been quite dynamic. They are pre-determined intervals. a result of changing source terms and marine As part of a joint French-IAEA study of open processes, including horizontal and vertical Mediterranean particle flux, in mid-April 1986, transport in seawater, marine sedimentation, re- IAEA-MEL scientists had, by a happy coincidence suspension from sediment and biological uptake, of timing, moored their automated time-series sedi- and food-chain transfer. ment trap at a depth of 200 meters in the between Monaco and the of Corsica. In order to study timescale changes in particle flux, IAEA-MEL tracer studies each of the six trap collection cups was set to sample sinking particles for consecutive periods of The Chernobyl accident, perhaps surpris- 6.25 days. Following the accident on 26 April, ingly, was of considerable interest to oceanogra- atmospheric measurements made at Monaco by IAEA-MEL indicated that most of the peak Cher- nobyl fallout entered the Ligurian Sea essentially as Mr Baxter is the Director, Mr. Fowler, Head of the Radio- a single pulse during the 4-5 May. ecology Section; and Mr Povinec, Head of the Radiomet- rics Section of the IAEA's Marine Environment Laboratory The sediment trap was retrieved on 22 May in Monaco. and the paniculate material, along with other

18 IAEA BULLETIN, 1/1996 FEATURES

Top: As part of an IAEA-MEL training course in Istanbul in November 1994, scientists collect sediment samples in the Marmara Sea using the University of Istanbul's ship. Left A team of IAEA-MEL scientists deploy a sediment trap such as the ones used during their post-Chernobyl marine studies. Above: Typical particles collected in sediment traps include oval, rectangular, and cylindrical fecal pellets produced by that are actively feeding on micro-organisms. (Credits: IAEA-MEL).

IAEA BULLETIN, 1/1996 19 FEATURES

marine samples, was analyzed for radioactivity depth by one month after the accident when the by gamma spectrometry. The radioanalyses sediment trap stopped sampling. In sharp con- showed that the primary pulse of paniculate ra- trast, only 0.2% of the corresponding caesium- dionuclides arrived at 200 meters depth between 137 deposition had passed through 200 meters 8 and 15 May, that is only about seven days after by that time, an observation which is consistent peak radioactivity was delivered to the sea sur- with the generally non-reactive behaviour of this face. The time lag implied an average sinking long-lived nuclide in seawater. For this reason, speed of the radioactive particles of approxi- Chernobyl-derived caesium-137 has proved to mately 30 meters per . This pulse of radioac- be very useful as a mass movement tracer tivity sinking through 200 meters was particu- in the Mediterranean and other seas for several larly evident for the particle-reactive fission years after the accident. products (e.g. , niobium, and IAEA-MEL was not the only of ma- radionuclides) which were either not detectable rine scientists deploying sediment traps in Euro- or present in very low amounts in the last sedi- pean waters following the Chernobyl accident; ment trap sample collected after 15 May. time-series traps were collecting particles at The rapid descent of these radionuclides on a nearly the same time in the , the Black timescale of a few days indicated that they were Sea, and Lake Zurich. Where comparisons could not sinking as fallout particles according to be made, concentrations of Chernobyl-derived Stokesian settling models. Rather, they were in- radionuclides in the different sinking particles corporated into large aggregates which are were surprisingly similar, and in most cases bio- known to sink at speeds of tens to hundreds of logical activity in the upper water column was meters per day. considered to be the driving force in transporting From earlier laboratory research at IAEA- the radioactivity downward. However, large dif- MEL, it was suspected at the time of the accident ferences in radionuclide flux were evident due to that biological activity in surface waters might be variations in particle mass flux which is normally responsible for absorbing radionuclides like a site and depth specific. The extreme case was sponge and removing them to depth. Primary observed in Lake Zurich where roughly 20% of production of minute plant-like the fallout was removed from the water column cells creates surfaces onto which contami- in two months due to the sinking of a massive nants like radionuclides are adsorbed. The bloom of calcareous algae. zooplankton which feed on these radioactive Viewed collectively, the temporal flux data cells subsequently defecate large aggregates (fe- for the Chernobyl radionuclides collected cal pellets) which can further scavenge radioac- throughout Europe after the accident have tivity as they sink. proven extremely important for refining general Therefore, to test this hypothesis, on 6 May models of contaminant removal and transport in 1986 live zooplankton were netted from the wa- aquatic systems. ters over the sediment trap and allowed to defe- cate in special aquaria on board ship. When ana- lyzed, these freshly produced fecal pellets were Environmental & radiological aspects found to contain radionuclides with similar con- centrations and relative distributions as were pre- Of more than 20 radionuclides which were sent in the sinking particles trapped at 200 me- released in significant quantities during the ters. Microscopic examination of the trap sam- Chernobyl accident, only a few have been stud- ples confirmed that they were rich (70% by ied extensively in the marine environment. weight) in the same type of fecal pellets as the Among the most important have been strontium- netted zooplankton had produced. Thus, this ac- 90, caesium-134, caesium-137, and plutonium- cidental " field experiment" was actually the first 239+240. Other radionuclides, such as iodine- direct and convincing demonstration of the bio- 131, have half-lives that are too short to be harm- logical processes by which ocean waters are ful or relevant to understanding marine proc- cleansed of contaminants like radionuclides. esses, or had very low concentrations (for exam- Following the Chernobyl accident, the differ- ple, iodine-129). ent radionuclides which entered the surrounding As mentioned previously, considerable dif- seas were removed to different degrees depend- ferences in marine behaviour were observed. ing upon their chemical reactivities. For exam- Strontium-90 and caesium-137 are typical repre- ple, in the case of the particle-reactive radionu- sentatives of elements which are soluble in clides, cerium-141, cerium-144, and plutonium- seawater and can be used for studies of water 239+240, from 50% to 75% of the total radionu- dynamics. Their particle reactivity is very low in clide inventories deposited in this region of the comparison to, for example, plutonium , Mediterranean had transited through 200 meter which lie at the other extreme, in a group of

20 IAEA BULLETIN, 1/1996 FEATURES

Bq/m3

Above left: The average caesium-137 activity of surface seawater (Bq m'3) estimated for the reference year 1990. The Mediterranean regional sea values were extracted from the Commission of the European Community's 45° MARINA-MED report. Above right: Caesium-137 contours in the surface seawater of the for the period 1986-88 as extracted from the IAEA-MEL Global Marine Radioactivity Database. The data were mostly supplied by the Bundesamt fur Seeschiffahrt und Hydrographie, Hamburg, . Right: Caesium-137 activity isolines (Bq m"3) in surface waters of the western as extracted from the measurements in 1988. Adapted from G. G. Polikarpov etal.. Journal of Environmental Radioactivty (Vol.13,1 40°NL (1991)). elements having low and high particle ent caesium-134/caesium-137 activity ratio of reactivity. Plutonium isotopes do not travel long about 1:2. distances from the source because they are de- Radiologically, the sea most affected by the posited into sediment, which therefore contains Chernobyl accident was the Baltic, since the first their main inventory in the ocean. radioactive clouds from Chernobyl travelled to As the plutonium input to the oceans follow- the north and caused high deposition over the ing the Chernobyl accident was small and local- Scandinavian region. Atmospheric deposition ized, this article will concentrate on discussing played a dominant role in determining the radio- the impact of Chernobyl radiocaesium on the activity of this sea. The mean caesium-137 con- marine environment. The caesium isotopes were centration in surface waters estimated for the both the most widespread and most abundant of reference year 1990 was highest in the Baltic the ones released. Sea. (See maps.) Because of the closed of The behaviour of radiocaesium in the oceans this sea and its small exchange of water with the has been studied over a long period with refer- North Sea, the levels of caesium-137 of this sea ence to its fallout from nuclear bomb tests and in have remained the highest in Europe. discharges from plants. In Caesium-137 contours shown on the map of particular, discharges from in the the Baltic Sea for the period 1986-88 illustrate have been extensively used to the enhancement of caesium-137 concentrations study water and sediment dynamics in the Irish, in seawater, with clear evidence of the effect of North, and Norwegian Seas. The radiocaesium run-off from land, particularly from Sweden. from weapons test fallout was exclusively cae- The measured range in 1986 was from a few sium-137. Caesium-134 has, however, been pre- becquerel (Bq) to 2400 Bq per cubic meter, i.e. sent in Sellafield discharges as well as in Cher- two to three orders of magnitude higher than in nobyl debris. The ratios of caesium-134 to cae- other European seas. sium-137 have been different, however. Cae- The next most perturbed sea following the sium from Chernobyl was thus readily distin- Chernobyl accident was the Black Sea, where the guishable from other sources by having a differ- mean caesium-137 concentration in seawater in

IAEA BULLETIN, 1/1996 21 FEATURES

1990 was 52 Bq per cubic meter, comparable to nuclear techniques to understand water move- that in the Irish Sea. The highest deposited activ- ment and element cycling in the heavily polluted ity was observed in 1986 in its northernmost marine system. area, about 500 Bq per cubic meter, i.e. 30 In the Baltic Sea, the Helsinki Commission higher than the pre-accident values. Strontium- MORS programme (Monitoring of Radioactive 90 activity measured in 1988 in surface waters of Substances) has received significant support the western Black Sea was mostly between 10 from IAEA-MEL in terms of organization of and 50 Bq per cubic meter. Generally, a similar specially designed analytical quality assurance distribution was observed in 1988 for caesium- exercises for marine radioactivity assay. This 137 as well, but the levels were higher for this support will be further increased by the organiza- radionuclide by a factor of two. Strontium-90 tion, in Finland, of an IAEA training course on and caesium-137 surface concentrations in marine radioactivity studies in September 1996. waters were much lower, between 5 Again, the work will feature particular reference and 11 Bq per cubic meter. The distribution pat- to the Chernobyl impact and to the needs of the terns of strontium-90 and caesium-137 observed new Baltic States (Estonia, Latvia, and Lithu- in the surface waters of the Black Sea can be ania) which have joined the IAEA. explained in terms of two main source functions — namely by a short-term atmospheric deposi- tion which dominated immediately after the ac- Overview of marine radioactivity cident, and then by a long-term transfer from the Kiev Reservoir and the catchment area of the An overview of marine radioactivity per- Dnieper, Dniester, and Danube rivers. spectives has been provided by the IAEA's re- For the , the main Cher- cently completed CRP, " Sources of Radioactiv- nobyl contribution then arrived by exchange of ity in the Marine Environment and their Relative waters with the Black Sea, which has essentially Contributions to Overall Dose Assessment from acted as a radioactive source. Atmospheric depo- Marine Radioactivity (MARDOS)". This study sition and river inputs were estimated to have provided new up-to-date estimates of doses to played minor roles. The mean caesium-137 con- the public from anthropogenic caesium-137 centration in surface water estimated for 1990 (originating from global fallout, the Chernobyl was 5.7 Bq per cubic meter. The caesium-137 accident, and authorized discharges) and from levels in regional seas as estimated by the Com- natural -210 through consumption of mission of the European Community's MA- marine food. RINA-MED project ranged from 2.9 to 9 Bq per It included a study of fishing areas as defined cubic meter, clearly showing a west-east trend by the United Nations Food and Agriculture Or- towards the highest values in the Aegean Sea. ganization (FAO). Analysis of fishing area No.37 (the Mediterranean and Black Seas) has shown that the collective effective dose commit- Technical assistance and training ment for caesium-137 in marine food (fish and shellfish) in 1990 was 6 man (Sv) — The IAEA has been active in assisting Mem- much smaller than the 700 man Sv derived from ber States both to monitor and understand the polonium-210 ingestion. The highest doses (86 effects of the Chernobyl accident on their marine man Sv) in world oceans due to caesium-137 systems. Firstly, in the Black Sea region where were found in the North Atlantic area (FAO Chernobyl radioactivity is among the foremost fishing area No. 27, which also includes the Irish, public concerns in relation to health, the IAEA North, Baltic, Norwegian, and Barents Seas). has initiated a major programme of technical However, they are still negligible in comparison co-operation. Activities are directed at strength- with 2900 man Sv derived from polonium-210 ening the capabilities of the regional Member ingestion. States to measure and monitor marine radioactiv- Generally, it can be concluded that the ity, especially alpha-emitters, and involve care- Chernobyl accident has had a measurable im- ful evaluation and selection of equipment and pact on the marine environment. Radionuclide training approaches. levels (mainly caesium-137) were two to three In November 1994, a 2-week regional train- orders of magnitude higher than the pre-Cher- ing course on marine radioactivity monitoring nobyl levels. However, the doses to the public was held in Istanbul. In addition, a complemen- from ingestion of caesium-137 in marine food tary Co-ordinated Research Programme (CRP) have been estimated to be at least an order of on isotopic tracers in the Black Sea has been magnitude lower than those due to natural po- carried out. It is being done with a view to using lonium-210. O the Chernobyl pulse of radionuclides and related

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