Radiation monitoring of lake ecosystems within the Chernobyl accident exclusion zone

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1 Radiation monitoring of lake ecosystems within the Chernobyl accident exclusion zone NAZAROV Alexander 1, GUDKOV Dmitri 2, 2 State Specialized Scientific and Production Enterprise Chernobyl Radioecological Centre of the Ministry of Ukraine on the Emergency and Affairs of Population Protection Against the Consequences of the Chernobyl Catastrophe, Chernobyl, Ukraine, 727. nazarov42t@rambler.ru 1 Department of Freshwater Radioecology, Institute of Hydrobiology of the National Academy of Sciences of Ukraine, Kiev, Ukraine, 421. digudkov@svitonline.com Abstract: Lake ecosystems are efficient collectors for the wide range of radionuclides, which deposited in abiotic and biotic components after intake by aquatic environments. For the time being, ecosystems of lakes of the Chernobyl nuclear power plant exclusion zone continue to suffer from heavy radioactive contamination, dictating, thus, the need to carry out further research works within the framework of comprehensive regional radioecological monitoring. Particular features of radionuclide accumulation by hydrobionts substantially depend on formation of hydrochemical composition of water, which, in its turn, is a complex process, depending on functioning of aquatic ecosystems and natural conditions of a water catchment basin. Hydrochemical regimes in reservoirs are determined by external factors and internal water body processes. The results of 9 Sr, 137 Cs, 238 Pu, Pu and 241 Am content in water, bottom sediment and hydrobionts of different trophic levels of water objects within the Chernobyl exclusion zone in have been analysed. Keywords: Chernobyl exclusion zone, Lake ecosystems, Radiation monitoring, Radioactive contamination. 1. Introduction The territories of the Chernobyl exclusion zone are characterised by significant heterogeneity of radionuclide contamination, which is significantly reflected by the radioactive substance contents in aquatic ecosystem components. Primarily this is due to the composition and the dynamics of radionuclide emissions into the environment as a result of accident in 1986, as well as to the subsequent processes of radioactive substances transformation and biogeochemical migration in the soils of catchment basin and bottom sediments of reservoirs. Relatively low contents of radioactive substances are found in the river ecosystems. Due to high water change rate the river bottom sediments have undergone decontamination processes (especially during floods and periods of high water) and over the years that passed since the accident have ceased to play the essential role as a secondary source of water contamination. The main sources of radionuclides in rivers are currently the washout from the catchment basin, the inflow from more contaminated water bodies, as well as the groundwater. On the other hand, the closed reservoirs, and in particular the lakes in the inner exclusion zone, have considerably higher levels of radioactive contamination caused by limited water change and by relatively high concentration of radionuclides deposited in the bottom sediments. Therefore, for the majority of standing reservoirs the level of radionuclide content is determined mainly by the rates of mobile radionuclide forms exchange between bottom sediment and water, as well as by the external washout from the catchment basin. Our studies were conducted: (1) to identify dynamics of radionuclide distribution in components of freshwater biocenose of lakes within the Chernobyl NPP exclusion zone; (2) to study dynamic profiles of radioactive contamination levels in hydrobionts of different ecological groups and trophic levels; (3) to assess the major factors, which determine distribution of radionuclides in the freshwater ecosystems; (4) to study the seasonal dynamics of radionuclides content in macrophites and the role of main aquatic plant associations in processes of radionuclides distribution in biotic component of biohydrocenose; (5) to assess a possibility to use hydrobionts of different trophic levels as biological indicators of radioactive contamination of water. 2. Methodology Our researches were carried out during in Azbuchin Lake, Yanovsky (Pripyatsky) Crawl, cooling pond of the Chernobyl nuclear power plant, the lakes of the left-bank flood plain of the Pripyat River - Glubokoye Lake and Dalekoye-1 Lake (Fig. 1). The radionuclide content in biological tissues was measured for 28 higher aquatic plant species, 6 species of bivalve molluscs and gastropod snails as well as 18 species of fish. The tendency of the aquatic organisms to accumulate radionuclides, traditionally expressed as the concentration factor (CF), which is determined by calculating the ratio of the specific activity of radionuclides in tissue to the average annual content (for molluscs and fish) or to the average content in the

2 environment water during the vegetation period (for higher aquatic plants). Fig. 1 Map of the major water bodies within the Chernobyl exclusion zone The 137 Cs content was measured by g-spectrometry complex: PGT IGC-25 detector (France), Nokia LP 49 B analyser ( Nokia, Finland), low-volt feeding source - crate NIM BIN, amplifier NU-821 ( Elektronicus Merokeszulekek Gyara, Hungary) and 1 mm thickness leaden protection. The 9 Sr content was measured on low-background NRR-61 b-radiometer ( Tesla, Czech). Minimal detectable activity was.4 Bq under 1 s sample exposition. 238 Pu and Pu content in electrolytic samples was determined by a-spectrometric tract by NUC-8192 impulse analyser ( Elektronicus Merokeszulekek Gyara, Hungary). The 241 Am content was measured by x-rayspectrometric line including x-ray detector EG&G Ortec LOAX-5137/2 CFG-SU-GMX ( EG&G Ortec, USA) and analyser Nokia LP 49 B. The results of the radionuclide content measurements in hydrobionts are expressed in Bq kg -1 of wet weight at natural humidity and the mistake of estimated radionuclide concentration fell within Results 3.1 Water and bottom sediments The highest radionuclide activity in water among the studied objects was found in the Azbuchin Lake (Tab. 1). During , the average content of 9 Sr and 137 Cs in water of lake reached up to 19 and up to 23 Bq l -1 respectively. The radionuclide contamination density values found in the lake bottom sediments for 9 Sr, 137 Cs, Pu and 241 Am averaged at 6.7, 11.5,.24 and.22 TBq km -2 respectively, with the maximum values of 33.3, 14.4, 1.1 and.29 TBq km -2. The 9 Sr and 137 Cs content in the water of Glubokoye Lake come to and Bq l -1 respectively. The average values of contamination density in the bottom sediments by 9 Sr, 137 Cs, Pu and 241 Am in 1998 were 2.6, 5.6,.7 and.6 TBq km -2, with the maximum values being 1., 13.7,.22 and.23 TBq km -2 respectively. In Dalekoye-1 Lake the average content of 9 Sr and 137 Cs in the research period reached 45 and 2.4 Bq l -1 respectively. The maximum value of radionuclide contamination density in the bottom sediments by 9 Sr in 1999 was 18.9, by 137 Cs , by Pu -.6 and by 241 Am -.4 TBq km -2. The average values were, accordingly, 4., 3.1,.8 and.8 TBq km -2. Tab. 1 Average annual specific activity of 9 Sr and 137 Cs in water of lakes within the Chernobyl exclusion zone in , Bq l -1 Yanovsky Dalekoye Azbuchin Glubokoye Year Crawl Lake Lake Lake Sr Cs Sr Cs Sr Cs Sr Cs * The average specific activity values for 9 Sr and 137 Cs in water of Yanovsky Crawl for the period were 25 and 2.9 Bq l -1 respectively. The radionuclide contamination of the bottom sediments of reservoir is extremely heterogeneous, which is obviously caused by the non-uniform character of the nuclear fallout and by the absence of wind-induced turbulence in deep water. The average content of 9 Sr, 137 Cs, Pu and 241 Am in bottom sediments was, respectively, 16.3, 14.8,.4 and.3 TBq km -2. At the same time, within the bounds of silt sediment deposition, some sites with abnormally high density of contamination by 9 Sr, 137 Cs and Pu (37.1, and 5.3 TBq km -2 respectively, which is 2 times higher than the average values in the crawl), were found. The cooling pond of the Chernobyl NPP has undergone the highest radionuclide contamination in comparison with other reservoirs of exclusion zone. In the course of time, after the cessation of radioactive emissions into the atmosphere and due to disintegration

3 of short-lived isotopes, 9 Sr and 137 Cs have become the main radioactive contaminants of the cooling pond water. During , the specific activity of 9 Sr in the cooling pond water was found to be within the range of , with the range being Bq l -1 for 137 Cs. The heterogeneity of the bottom sediment contamination in the cooling pond is currently determined by the nature of the silt accumulation processes. The height of silt layers at the depth of over 11 m (for up to 35 per cent of the bottom area) reaches up to 1 cm., with the density of contamination by 137 Cs at TBq km -2. The bottom at the depth of 3-11 m consists of primary soils, which are covered, with a 1-6 cm layer of silt, with the contamination density by 137 Cs in the range of TBq km Higher aquatic plants The radionuclide contents in higher aquatic plants (macrophytes) in the studied water bodies were largely determined by the nature of radionuclide contamination of the water objects and nearby territories, as well as by the hydrochemical regime in the reservoirs. The latter affects the forms of radionuclides in the reservoirs, thus affecting the level of their bioavailability to the hydrobionts. The patterns of 9 Sr and 137 Cs accumulation have been shown to be species-specific. Among the species with relatively high 137 Cs content are the helophytes (airwater plants) of genus Carex, Phragmites australis, Glyceria maxima, Typha angustifolia, as well as strictly water plant species Myriophyllum spicatum and Stratiotes aloides. The low values of 137 Cs activity in all reservoirs were found in the representatives of family Nymphaeaceae - Nuphar lutea and Nymphaea candida as well as Hydrocharis morsus-ranae. Relatively high content of 9 Sr was shown by the species of genus Potamogeton. Obviously this is related to this plant s tendency to accumulate large quantities of calcium (which is not washed off during standard sampling) on its surface during photosynthesis. At the same time, calcium carbonate that is removed from the plant could contain 7-2 times more radioactive strontium than the plant tissue [1]. Thus, Potamogeton species makes a good prospective radioecological monitoring object as a specific accumulator of 9 Sr. During the period of studies since 199 till 27, dynamics of the main radionuclide concentrations in tissues of the higher aquatic plants from river ecosystems of the exclusion zone indicated a decrease of specific activity of 9 Sr and 137 Cs (Fig. 2a). So far as concerns closed of low flowage water bodies, the most representative retrievals obtained in the period of have shown that since the late 199-s the higher aquatic plants related to different ecological groups is indicating a frank tendency to 9 Sr content increase in tissues (Fig. 2b). The specific activity of 137 Cs in higher aquatic plants of the lakes under study either decreases or remains practically constant. Hence, in the middle 199s specific activity of 137 Cs in tissues of higher aquatic plants from Krasnensky flood plain was much higher than specific activity of 9 Sr in them, whereas in the late 199-s these values became comparable, and at present specific activity of 9 Sr is much higher than that of 137 Cs. Bq/kg w.w Bq/kg w.w Sr-9 Sr R 2 =,8653 R 2 =, R 2 =,778 R 2 =, Year Fig. 2 Dynamics of radionuclide content in higher aquatic plants of the Chernobyl Exclusion Zone: a - reed-mace (Typha angustifolia L.) from the Pripyat River; b - watersoldier (Stratiotes aloides L.) from Glubokoye Lake It is suggested that 9 Sr specific activity in tissues of macrophytes and fish of Krasnensky flood plain increases due to dynamics of radionuclide transformation in soils of water catchments area and bottom sediments of water bodies. Due to construction a complex of flood control dams, which preventing radioactive substance washing off the soils of contaminated areas not only, but changed the hydrological regime of flood plain flows during floods, this became the reason for intensification of waterlogging and swamping of dam-locked areas. This confirms the increase of a role of water-marsh floristic complex in structure of vegetative cover of Krasnensky flood-lands [2]. As a result of swamping, at the background of general tendencies of 9 Sr mobile form increase in the soils of water catchment areas and bottom sediments of the exclusion zone water bodies; in swamped soils of Krasnensky flood plain fulvic and humic acid concentration increases, that decreases ph Year a 27 b 27

4 value in water and intensifies a denuding of watersoluble forms of 9 Sr forming soluble complexes with fulvic acids. Hence, an increase of concentrations of mobile forms of the radionuclide and their inclusion to biotic turnover of aquatic ecosystems are observed. This also confirms the increase of 9 Sr specific activity, observed in recent years in Krasnensky flood plain lakes at the background of stabilization of this index for 137 Cs. The tendency of the root contamination of plant tissues by 9 Sr was also observed for terrestrial plants in the exclusion zone [3, 4]. Currently, some authors [5] suggest that 9 Sr mobility in soils of the exclusion zone is maximal, and this will last during the nearest decade. Thereafter, the rate of radionuclide decomposition will exceed the rate of its mobilization. determined for Lymnaea stagnalis -.1 and.2 Bq kg -1 respectively in Dalekoye-1 Lake, 2.7 and 6.4 in Glubokoye Lake. The highest content was determined for Stagnicolia palustris from Glubokoye Lake - 14 and 36 Bq kg -1 respectively. The highest activity among gastropods was shown in Planorbarius corneus - 1 and 2 Bq kg -1 respectively from Dalekoye-1 Lake; 25 and 53 Bq kg -1 in Glubokoye Lake. Dreissena polymorpha from the cooling pond of the Chernobyl NPP showed 238 Pu and Pu contents of 3 and 6 Bq kg -1 respectively Sr 3.3 Mollusks Freshwater molluscs are often considered as bioindicators of radionuclide contamination of water objects. These invertebrates accumulate practically all the radionuclides found in water and, due to their high biomass, molluscs play an important role in bioaccumulation processes and radionuclide redistribution in aquatic ecosystems. The 137 Cs and 9 Sr content in molluscs of reservoirs of the Chernobyl exclusion zone are shown in Fig. 3. Activity, Bq/kg Radix ovata Lymnaea stagnalis Viviparus viviparus Planorbarius corneus Stagnicola palustris Unio pictorum Cooling pond Yanovsky Crawl Dalekoye-1 Lake Sr 137 Cs Activity, Bq/kg Cs Azbuchin Lake Glubokoye Lake Activity, Bq/kg Fig. 3 Average specific activity of radionuclides in freshwater mollusks of Chernobyl exclusion zone on , Bq kg -1 The highest CF for both 9 Sr and 137 Cs were found in bivalve mollusc Dreissena polymorpha and Unio pictorum, which are the most active filter molluscs. The highest CF for 9 Sr was noted in Dreissena polymorpha - in excess of 11, while for 137 Cs the highest CF (about 5) was found in the tissues of Unio pictorum. Considerably lower CF was determined for the gastropod species Lymnaea stagnalis, Planorbarius corneus, Viviparus viviparous and Radix ovata (Fig. 4). The average contents of transuranic elements 238 Pu and Pu in mollusc tissues in Glubokoye Lake and Dalekoye-1 Lake were as follows: the lowest value was Fig. 4 Average values and diapasons of specific activity of 9 Sr and 137 Cs in mollusks of Glubokoye Lake Average values of radionuclide CF for gastropod snails and bivalve molluscs of the exclusion zone located in the descending series in the following way: pond snail Am (412) > 238 Pu (59) > Pu (45) > 9 Sr (412) > 137 Cs (31); zebra mussel Am (2925) > 238 Pu (92) > Pu (873) > 9 Sr (11) > 137 Cs (35). Radix ovata Lymnaea stagnalis Planorbarius corneus Viviparus viviparus Stagnicola palustris 3.4 Fish The fish species that are found at the upper levels of the food webs may also constitute a part of human diet Unio pictorum

5 and, therefore, are of a particular interest in radioecological research of water ecosystems. The specific activity of 9 Sr in fish of the lakes was registered in a range Bq kg -1, with highest values in Azbuchin Lake, and 137 Cs Bq kg -1, with highest values in Dalekoye-1 Lake. In comparison with river ecosystems within the exclusion zone and in Kiev reservoir (on the area joining to the exclusion zone) the specific activity of 9 Sr and 137 Cs was registered, accordingly, in a range 1-99 and 3-53 Bq kg -1, with maximal middle indexes for both radionuclides in fish of the Pripyat River. The highest values of specific activity of 137 Cs in all reservoirs are determined for predatory fish. Average values of specific activity of 238 Pu, Pu and 241 Am in fish of Glubokoye Lake were, accordingly,.4,.8 and 6.2 Bq/kg. The analysis of radionuclide distribution on organs and tissues of fish in lakes of the exclusion zone showed that more 9 of 9 Sr is concentrated in a skeleton and scale, up to in muscular tissues and up to in gills. The highest value of specific activity of 9 Sr is registered in scale, the least one - in skin, internal organs and muscles. Basic part of 137 Cs (more 6) is concentrated in muscular tissues, about 3 - in bones and scale, and about 1 is on the other organs and tissues. By the main factors, determining quantitative content of radionuclides in fish and their subsequent dose-forming significance in lakes of the exclusion zone, are the level and composition of radioactive contamination of lakes and adjacent territories, intensity of water exchange rate, and also correlation in water of basic cations - chemical analogues and, accordingly, discriminators of 9 Sr and 137 Cs. In the processes of radionuclide concentration by the fish of lakes within the exclusion zone the primary value of relation of the sum of Ca 2+ and Mg 2+ to the sum of Na + and K + above correlation of radionuclides in water and levels of contamination of adjacent territories is determined. In Glubokoye Lake the steady tendency of increase of specific activity of 9 Sr in fish s tissue of different ecological groups is registered. Due to this the specific activity of 9 Sr in pray fish considerably exceeded such index for 137 Cs, and the radiation dose rate due to incorporated radionuclides grew on comparison with the end of 199-s more than in 6 times. It is possible to suppose, that the main reason of such dynamics is building of complex of water-protection dams on the left-bank flood lands of the Pripyat River, entailing the change of the hydrological mode, and also strengthening of processes of overwetting and swamping of the dammed territories. As a result on a background of general tendencies of increase of mobile forms of 9 Sr in soils of catchment basin and bottom sediments of water bodies within the exclusion zone, there is the increase of specific activity of radionuclides in water of the lakes located within dammed territory and intensity of their concentration by biota. Bq/kg Bq/kg Sr-9 22 Sr R 2 =,114 R 2 =,8733 Fig. 5 Dynamics of radionuclide content in predatory (a) and prey fish (b) from Glubokoye Lake Specific activity of radionulcides in fish of lakes within the exclusion zone during researched period in all cases manyfold exceeded permissible levels, to the norms accordant accepted in Ukraine for fish products [6] - on the average in the hundred one times for 9 Sr and in the ten one times for 137 Cs. 3.4 Radionuclide distribution on the main components of lake ecosystems The studies of radionuclide distributions in basic biotic and abiotic components of lake ecosystems, located at dam-fenced site of Krasnenskaya flood plain, indicated depositing of the main quantity of radionuclides to bottom sediments. Hence, concentrations of radionuclides equalled as follows: by 9 Sr - 89 and 95 for Glubokoye and Dalekoye-1 lakes, respectively; by 137 Cs - 99; by transuranic Pu and 241 Am elements - almost 1 of total content in ecosystems (Tab. 1, 2). The difference in 9 Sr contribution to bottom sediments of the lakes and contents of other radionuclides is stipulated by elevated migration activity of 9 Sr compared with 137 Cs and transuranic elements. This is the determining reason for 24 Year 25 R 2 =,1418 R 2 =, Year a 27 b 27

6 higher concentration of dissolved 9 Sr in waters of both lakes (1.2 and 4.3) compared with 137 Cs (.6 and.5) and transuranic elements (.4 and.3) and, vice versa, lower 137 Cs concentration in seston (.16 and.15) compared with 137 Cs (.25 and.3), respectively. Tab. 2 Total content and ratio of radionuclides in the main components of Dalekoye-1 Lake ecosystem Component 9 Sr, 137 Cs, Pu, 241 Am, Bottom sediments Water Seston Biota * - no measurements were performed *.81.7 The content of transuranic elements in biotic components of ecosystems was minimal.7 and.16, respectively; for 137 Cs it equals.14 and.47, and for 9 Sr -.25 and.61, respectively. Maximal values were determined for Glubokoye Lake, where dissolved 9 Sr concentration in water was also higher. Tab. 3 Total content and ratio of radionuclides in the main components of Glubokoye Lake ecosystem Component 9 Sr 137 Cs Pu, 241 Am Bottom sediments Water Seston Biota * - no measurements were performed * As suggested, such differences in 9 Sr distribution in ecosystems of lakes is, firstly, related to high biomass of the higher aquatic vegetation, intensively overgrowing Glubokoye Lake (covering about a half of water plane). It is known that as growths of macrophytes increase and become denser, the oxygen regime becomes worse, significant quantities of organic substances and biogenic elements are accumulate, and ph of the water medium decreases. As ph decreases, radionuclides are desorbed more actively and transit into the dissolved state, 9 Sr, first of all. This is also confirmed by lower average ph in Glubokoye lake (8.2) compared with Dalekoye-1 lake (8.8) and the ratio between specific activity of 9 Sr and 137 Cs in water equal, on average, 12.5 in Glubokoye lake and 7.3 in Dalekoye-1 lake, respectively. Contamination densities of bottom sediments by different radionuclides are also typical: the average specific activities of transuranic elements in both lakes were practically equal, whereas the ratio between average concentrations of 137 Cs and 9 Sr in Dalekoye-1 and Glubokoye lakes equalled 1.3 and 2.2, respectively. Therefore, affecting hydrochemical regime of water bodies, the intensity of closed lake overgrowth by associations of higher aquatic plants may also change the type of distribution of radionuclides in the components of ecosystems. It is also obvious that a part of radionuclides transit to water from biomass of dead plants and return to the biotic turnover of the water body. In zoobenthos associations of Dalekoye-1 lake the overwhelming majority of 9 Sr (99) is concentrated in bivalve molluscs. Total 9 Sr concentration in these molluscs equals about 89 of its content in the lake biota. About 8.5 is accumulated in higher hydrophytes, in fishes, about 1 - in gastropods, and less than 1 - in representatives of soft zoobenthos (Oligochaeta and Chironomidae). Similar situation is observed for distribution of transuranic elements. However, in this case, quantities accumulated in bivalve molluscs are decreased (down to 7-8), as well as these accumulated in gastropods and fishes (to fractions of percent), whereas the value of macrophytes increases to (Fig. 6) Sr-9 Macrobentos Fish Gastropod snails Macrophytes Pu Fig. 6 The distribution of radionuclides on the main groups of hydrobionts in the biotic component of Dalekoye-1 Lake ecosystem 137 Cs In the bottom invertebrates the part of accumulated in bivalve molluscs is 25 of its total amount or lower. This relates to high CF values for this radionuclide in soft zoobenthos representatives, in which, despite lower biomass compared with bivalve molluscs (1-fold lower), 137 Cs content gives about 65 of its total quantity. About 85 of 137 Cs present in the lake biota is accumulated in high aquatic plants, 7 and 8 - in zoobenthos and fish, respectively, and less than.1 - in gastropods. In Dalekoye-1 Lake biota higher aquatic plants occupying less than 5 of the water plane dominate exclusively in 137 Cs distribution, whereas in Glubokoye Lake comparative biomass of macrophytes is so high Am-241 а

7 that they accumulate about 9 of all radionuclides present in the biotic component of the lake (Fig. 7). The exception is 9 Sr, for which the contribution of aquatic plants is slightly more than 5, and the rest of it is mostly concentrated in gastropod and bivalve mollusc valves Sr-9 Fig. 7 The distribution of radionuclides on the main groups of hydrobionts in the biotic component of Glubokoye Lake ecosystem The main factors determining the contribution of various ecological groups of hydrobionts to total content of radionuclides in the biotic component of lake ecosystems are CF and aquatic organism biomass. Moreover, CF values may be so significant that sometimes the biomass factor is displayed in the only cases, when the difference in this parameter between different groups of hydrobionts reaches two orders of magnitude. 4. Conclusion Macrobentos Fish Gastropod snails Macrophytes Pu Radioactive contamination of the Chernobyl accident exclusion zone has complex distribution structure in different landscapes and characterised by dynamic character of the state of radioactive substance forms, which affect their migration and redistribution by components of ecosystems. The basic questions of radioactive safety of the restriction zone concern radionuclides wash-off with surface drainage water to river network, their export outside the exclusion zone and affection of the water quality in the Dnieper River. Self-purification of closed water bodies in the Chernobyl exclusion zone is extremely slow process. Therefore, ecosystems of the majority of lakes, dead channels and crawls possess high level of radionuclide contamination of all the components. The main quantity of radionuclides in aquatic landscapes of the restriction zone are deposited to bottom sediment of closed water bodies, hence, the distribution of radioactive substances in biotic and abiotic components of hydrobiocoenoses is defined by biogeochemical regularities and Am-241 б transformations of radioactive substances in bottom sediments of water bodies and in soils of adjacent territories. The construction of a complex of flood retarding dams and degradation of existing melioration systems at the site of the left-bank flood plain of the Pripyat River implied a change of hydrological regime and the character of water object overgrowth. The absence of flowage in water bodies, stagnation effects during spring flooding and seasonal runoffs intensified waterlogging and swamping of dam-fenced territories. As a result, at the background of general tendencies of 9 Sr mobile forms increase in the soils of water catchment areas and bottom sediments of the restriction zone water bodies located at dam-fenced sites, as well as increasing intensity of this radionuclide concentrating by higher aquatic plants and, obviously, other autotrophic organisms. For some species of macrophytes, 9 Sr concentration compared with the early 199s has increased by more than an order of magnitude and exceeds the specific activity of 137 Cs. Obviously the specific activity of 9 Sr will also increase at higher trophic levels, however, at present, such dynamics was not reliably detected. It is suggested that for radioecological monitoring of aquatic ecosystems in the restriction zone, higher aquatic plants possessing high CF values of radionuclides are the most sensitive testobjects for recording increasing specific activity of mobile forms of radionuclides in the water of test site water bodies. The results of tests performed confirm the tendency to further deterioration of the radiation situation in aquatic ecosystems of the restriction zone. Swamping of contaminated territories leads to acceleration of 9 Sr deposited form mobilisation processes and their migration and redistribution in closed aquatic landscapes. Hence, an original depot of mobile forms of radioactive substances is formed, which in high-flood periods may become a source of increasing 9 Sr drainage to the Pripyat River and then outside the restriction zone. In this connection, the necessity to implement hydraulic engineering procedures preventing underflooding of territories with high densities of radionuclide contamination; optimisation and enhancement of radioecological monitoring system, and further development of investigations of radionuclide behaviour in aquatic ecosystems of the exclusion zone - the important components in the complex of measures related to forecasting and minimisation of the Chernobyl disaster consequences. Acknowledgement This study was financially supported by the Ukraine Ministry for Emergency Situations and by the National Academy of Sciences of Ukraine (Projects 11U4987, 12U4665 and 12U3541).

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