JP0150450 JAERI-Conf 2000-016
4. Invited presentation
4.1 STUDY OF MIGRATION BEHAVIOR OF TECHNOGENIC RADIONUCLIDES IN THE YENISEY RIVER-KARA SEA AQUATIC SYSTEM
Yu. Kuznetsov1, E. Legin1, V.Legin \ A. Shishlov2, Yu. Savitskii2, A. Novikov3, and T. Goryachenkova3
1Khlopin Radium Institute, St. Petersburg 2Krasnoyarsk Mining and Chemical Combine, Krasnoyarsk-26 institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, Moscow
ABSTRACT
For 35 years Krasnoyarsk Mining-Chemical Combine (MCC) manufactures weapon plutonium in single-pass production reactors cooled with water of the Yenisey River. Water discharge from these reactors is the major source of radioactive contamination of the Yenisey River. We have demonstrated that after putting the reactors out of operation (in late 1992) the contamination level of the Yenisey River with short-lived radionuclides considerably decreased, and now the radioactive contamination is caused essentially by Cs-137, Eu-152, Pu-239,240, Sr-90, and Am-241, whose concentration in the aqueous phase is lower than in bottom sediments and, particularly, flood-land deposits by several orders of magnitude (except for Sr-90). The flood-land deposits are classified with the most contaminated environmental objects in the territories under the impact of MCC: their radioactivity is comparable with that of low-level waste. Taking into account the considerable depth and area of the flood-land deposits, this allows their classification as a great technogenic radiation anomaly. Comparison of the maximal Cs-137 and Pu-239,240 levels in flood-land soils and bottom sediments of the Yenisey River with those in bottom sediments of the Pripyat' River and the Kiev reservoir shows that these values are close each to other. A direct correlation is found between the spatial distribution of Cs-137 on the one hand and Pu-239,240, Eu-152, and Am-241 on the other hand in the aqueous phase and bottom sediments, which is not the case for Sr-90. Data on the distribution coefficients of the indicated radionuclides between the deposits and aqueous phase (obtained with actual and model systems) and also on the radionuclide distribution throughout geochemical mobility forms suggest that the essential part of Cs, Pu, Eu, and Am migrates with fine-disperse suspended material, the transport and distribution of which is controlled by the hydrological regime of the Yenisey River. By contrast, strontium migrates as soluble species weakly sorbed by the solid phase, causing the observed low content of Sr-90 in flood-land deposits and bottom sediments of the Yenisey River. The indicated migration behavior of radionuclides is characteristic of the Yenisey Gulf and the adjacent part of the Kara Sea also. We made similar conclusions when studying the migration behavior of Cs-137, Pu-239,240, and Sr-90 in the Kiev reservoir (1987). The formation of radioactive flood-land deposits is provided by rapid deposition of suspended material in stagnant zones during periodical flood. Humus compounds contribute significantly to accumulation of radionuclides in. the flood-land deposits and bottom sediments, which is supported by the observed
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correlation' between the radionuclide (Pu, Am, Eu) and total organic carbon distributions in them. Radiochemical analysis of separate fractions showed that about 20% of Pu and Am are associated with the organic fraction: Pu is nearly equally distributed between humic and fulvic acid fractions, whereas Am is preferentially associated with the fulvic acid fraction (the most mobile fraction of humus matter). It was demonstrated in model experiments that the calcium- hydrocarbonate type of water of the Yenisey River causes suppression of formation of mobile fulvate complexes of hydrolyzable radionuclides and, therefore, their transfer into the aqueous phase. In combination with the observed very high distribution coefficients of the radionuclides and low content of their mobile geochemical forms in flood-land deposits of the Yenisey River this suggest that they cannot contribute somewhat significantly to the secondary radioactive contamination of the river water by all mechanisms except migration by mechanical transfer.
The major source of radioactive contamination of the Yenisey River is the Krasnoyarsk Mining-Chemical Combine (MCC), known as Krasnoyarsk-26, situated at the right bank of the Yenisey River, 60 km to the northeast from the Big Krasnoyarsk. The region is characterized by a complex lay of the ground, including rolling and plain parts. MCC lies partly in the rolling part belonging to the joint zone of the West Siberian platform and the Sayan-Altai-Yenisey folded area. The hydrological regime of the Yenisey River is controlled by the Krasnoyarsk Hydroelectric Power Plant (HPP) put into service in 1967, which is situated approximately 85 km upstream from MCC. It reduces the annual fluctuations in the river flow in the areas affected by water discharge from MCC. Near MCC the river is not frozen throughout the year. The average annual temperature of water is 7°C, the current velocity is 1.7 m/sec, the average width is 1000 m, and the average annual discharge is 2760 m3/sec. The section of the Yenisey River from MCC (discharge of contaminated water) to the Kara Sea is about 2500 km long. Mean annual run-off of the Yenisey River (603 km3/a) is the highest among all the rivers flowing into the Arctic Ocean. For 35 years MCC manufactures weapon plutonium in 3 single-pass production reactors cooled with water of the Yenisey River. It is water discharge from these reactors that the major source of radioactive contamination of the Yenisey River. Two single-pass production reactors were put out of operation in late 1992. The third, close-cycle power reactor provides heat supply of the town; only control rod cooling water is discharged to the river from it. The radioactive contamination of the Yenisey River before putting the reactors out of service was determined by short-lived activated and fission products (Na-24,P-32,Sc-46,Cr- 51,Mn-54,56,Co-58,Fe-59,Cu-64,Zn-64,Nb-95,Ru-103,106,Sb-124,Cel41,144,Np-239). Contribution of long- and medium-lived radionuclides (Cs-137, Sr-90, Pu-239,240, Co-60, Eu-152) to the total specific radioactivity of water was small [1.2]. Major part of these radionuclides (except Sr-90) was accumulated in flood-land deposits and bottom sediments. When the production reactors were in service, considerable supply of short-lived radionuclides to water, biota, bottom sediments and flood-land deposits was observed [1, 2]. Although at all times the contamination level of most radionuclides in the Yenisey River water was substantially lower than the maximal permissible values for drinking water (in correspondence with NRB-96 regulations), the Na-24 and P-32 concentrations in some species of industrial fish were close to the maximal permissible radiation dose accepted for the critical group of population eating river fish [2]. After putting the reactors out of service
-139- JAERI-Conf 2000-016 the radioactive • contamination of river water and fish abruptly decreased, and the concentration of long-lived radionuclides in it were close to the global level. Somewhat different pattern is observed in the flood-land soils and bottom sediments, which, in contrast to water bulk, are characterized by certain inertia with respect to changing concentration of long- and medium-lived radionuclides in them: thus, even after putting the single-pass reactors out of operation, these sediments preserve high contamination levels with Cs-137, Pu-239,240, Co-60, Eu-152, and Eu-154. In contrast to other studies, where the major attention was paid to short-lived radionuclides, in this work, aimed at long-term prognosis of the radiation situation in the territories under the impact of MCC, we focused, naturally, on the migration behavior oif i i yi ddu 6 au 0 a of long- and medium-lived radionuclides (Cs-137, Sr-90, Pu-239,240 Am-241, Eu-152, Co-60). In the most thoroughly studied section of the riverbed (up to 600 km downstream the discharge point of contaminated water from MCC) unconsolidated bottom sediments are not typical, since the river bottom is largely rocky, covered by coarse pebble. The bottom sediments are found in stagnant parts of the river (tail parts of island, inlets, etc.), where suspended material is actively deposited. The sediments are composed essentially of various fractions of silted sand. It is these sediments that are most strongly contaminated with radionuclides (except Sr-90). Thus, the maximal radionuclide contents (Bq/kg dry sample) found in them are presented in Table 1:
Table 1. Maximal radionuclide contents (Bq/kg dry sample) in bottom sediments and flood-land soils of the Yenisey River.
Bottom sediments
Content Cs-137 Sr-90 Pu-239,240 Am-241 Eu-152 Co-60 Bq/kg 950 20 6.4 1.5 265 310 dry sample
Flood-land soils
Content Cs-137 Sr-90 Pu-239,240 r Am-241 Eu-152 Co-60 Bq/kg 4000 25 30 5 2000 1500 dry sample
The most contaminated objects in the Yenisey River basin are flood-land deposits (soils). The comparable contamination levels with Cs-137 and Pu-239,240 were observed in bottom sediments of the estuarine parts of the Pripyat' River after Chernobyl accident [4-6]. Let us compare these data with those obtained for flood-land soils of the Yenisey River (Table 2). It should be noted, that, in contrast to the territories contaminated with Chernobyl radionuclides, flood-land areas of the Yenisey River are not permanently populated; the residence time of people in them is limited, resulting in considerably lower radiation doses taken by them as compared to the case of Chernobyl.
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Table. 2. Comparison of maximal level of radioactive contamination in aquatic systems under impact of Chernobyl and MCC
Environmental Chernobyl MCC (Yenisey River) object Cs-137 Sr-90 Pu-239,240 Cs-137 [ Sr-90 Pu-239,240 Bottom Up to Up to Up to Up to 20 6.4 Sediments, 2xlO4 lxlO4 8xlO2 lxlO3 Bq/kg Flood-land - - - Up to 25 30 soils, Bq/kg 4xlO3 Water, Bq/nT3 Up to 48 36 13 0.018 4xlO3 Distribution 2xlOJ 105 - 106 104 101- 102 105 coefficient, Kd
The radionuclide spatial distribution in bottom sediments of the Yenisey River is spot-like (Fig.l, for all the figures see Attachment 1): high contamination level is found not only near the contamination source, but even at distances up to 1000 kilometers downstream (note that upstream from MCC the contamination levels with Cs-137, Pu-239,240, and Sr-90 correspond to their global levels). At distances of longer than 1000 km from MCC the radionuclide concentrations in bottom sediments correspond practically to the global level, and only at the outlet from the Yenisey Gulf (Fig.l) (in the mixing zone of river- and seawaters, 2500 km from MCC) the contents of Cs-137 and Pu-239,240 exceed the global level by several times [1, 3], which can be attributed to the specific features of estuarial processes in this zone. Thus, it is well known that in the mixing zone of river water and seawater finely dispersed suspensions are effectively deposited. But it is these suspensions that are enriched with Pu-239,240 and Cs- 137 to the maximal extent. The factors controlling deposition of finely dispersed suspended material are as follows: 1. Increase in the ionic strength in the mixing zone, promoting a simultaneous coagulation of oxyhydrate collectors and the colloid fraction of suspended material. 2. Decrease in the river flow velocity due to sea backwater, which also favors the deposition of finely dispersed fractions of suspended material contaminated with Pu and Cs. In other investigated parts of the Kara Sea the radioactivity is nearly at the global level. The vertical distribution of radionuclides in the bottom sediments is characterized by occurrence of a series of clearly pronounced extrema (Fig.2), which can be attributed to higher rates of radionuclide supply to the Yenisey River in the past. The contaminated flood-land deposits of the Yenisey River includes islands and the coastal line confined by the maximal level of high water, extended (but not continuously) up to 1000 km downstream from MCC. Flood-land soils (or deposits) are formed during seasonal flooding, when the river carries great amounts of suspended matter which are settled in stagnated areas. As an example, in Fig. 3 is presented a schematic diagram of formation of flood-land deposits in the region of the Mamotov and Iskupskii Islands. The river flow, which rate increases in the flooding periods, carries an increased amount of suspended material. This flow strikes the front of the Mamotov Isl. (colored dark in the figure); the flow rate abruptly decreases, which promotes deposition of suspended material, i.e., formation of flood-land deposits in the coastal parts of islands and at the coastal line (colored dark). Similar pattern is observed also for the Iskupskii Island: the
-141- JAERI-Conf 2000-016 river flows round the island at a lowered rate, and flood-land deposits are formed both in the front and tail parts of it. Data on the radionuclide distribution in the surface layer of flood-land deposits and in several tens of core samples, obtained in this work, reveal that, as in the case of the bottom sediments, no clearly pronounced tendency to decreasing contamination level of flood-land soils with distance from MCC is observed: high radionuclide contents were found both just near MCC and at distances of several hundreds kilometers. The flood-land deposits of the Mamotov Isl. (176 km downstream from MCC) and Iskupskii Isl. (1025 km downstream) can serve as an example: Cs-137 content in flood-land soil of the Iskupskii Isl. Is nearly twice that in the Mamotov Isl. (these data are presented in tables, Fig.3). Possibly, the observed spot-like distribution of radionuclides in the bottom sediments and flood-land deposits can be interpreted in terms of processes responsible for secondary redistribution and transport of sedimentary material downstream from MCC. The maximal contamination levels with radionuclides in flood-land soils (found on frequent occasions in horizons 20-30 cm) were shown in Table 1. For the first time we performed an extensive study of the radionuclide vertical distribution in flood-land soils. Such a work is necessary for estimating the total inventory of radionuclides in the flood-land deposits. It was demonstrated that the radionuclide vertical distribution (Cs-137, Pu-239,240, Am-241, Eu-152, Sr-90) in these soils is strongly nonuniform (Figures 4-7) (varying by several times from one horizon to another), which, as in the case of bottom sediments, can be regarded as an evidence in favor of the correspondingly irregular supply rates of radionuclides to the Yenisey River in the past. We attempted to reconstruct the radionuclide supply to the Yenisey River-Kara Sea aquatic system on the basis of experimental data on the vertical distribution of Cs-137 and Pu- 239,240 in the bottom sediments, on the one hand, and the absolute age data (by Pb-210) for the investigated horizons, on the other hand. Location of two cores of bottom sediments (stations 31 and 34 are shown in Fig 8.). The results are shown in Fig.9. Fig. 10 illustrates the vertical distribution of unsupported Pb-210 in these bottom sediments. Using these exponential curves we were succeeded in estimating the absolute age of the deposit layers studied as well as their deposition rates. After making correction for radioactive decay of Cs- 137 (Fig.ll), we were able to estimate the dynamics of radionuclide supply in the period of MCC operation. It follows from Fig.ll (station 31) that about 30 years ago the Cs-137 supply rate to bottom sediments was considerably higher than today. It is hard to say whether this is associated with MCC operation or with carrying out nuclear weapon tests at that time [7,8]. Certain conclusions concerning the possible migration mechanisms of radionuclides in the Yenisey River can be drawn taking into account the following experimental data: 1. The volume concentration of Sr-90 in water of the Yenisey River gradually decreases (Fig.l) with the distance from MCC as a result of dilution. That of Cs-137 and Pu-239,240 decreases stepwise, which can be interpreted in terms of the dynamics of supply and distribution of suspended material in the Yenisey River. 2. The distribution of Cs-137 and Pu-239,240 between the aqueous phase and bottom sediments of the Yenisey River is characterized by high distribution coefficients (104-105), which is reflected in rather high concentrations of these radionuclides in bottom sediments and flood-land soils. The distribution coefficient of Sr-90 (about 102 )is lower by several orders of magnitude; correspondingly, its content in the bottom sediments and flood-land deposits is quite low (1). 3. Cs-137 and Pu-239,240 are tightly fixed (Table 3 ) in the structure of bottom sediments and flood-land deposits, so that the mobile fraction of these radionuclides is small. On the
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contrast, Sr-90 is characterized by high mobility . Fig. 12 schematically illustrates the method for determination of radionuclide mobile forms in bottom and flood-land deposits.
Table 3. Distribution of 239>240Pu, l37Cs, 152Eu, and 60Co throughout geochemical mobility forms and fractions of organic matter in sample no 572.
152 60,-, 137 m-24upu Eu Cs Co % % % % H2O 0,4 0,4 0,4 2,6 1M CH3COONH4 pH=7 1,2 0,4 0,4 3,6 1M CH3COONH4 pH=4,5 2,2 0,4 0,4 0,6 Free fulvic acids 3,6 0,4 1,6 0,7 Free humus acids 13,6 1,2 6,1 0,7 The insoluble residue 79 97,2 91,1 91,8
It is seen from the Table 3 that the water-soluble and exchangeable fractions of hydrolysable radionuclides (Pu, Eu, Co) are rather low. As pH of 1 M ammonium acetate solution decreases, the exchangeable fraction of 239'240Pu increases, and those of 60Co and Eu remain unchanged. 4. A positive correlation is observed between the distributions of Cs-137 and Pu-239,240 in the surface layers ( Fig. 13 ) and vertical profiles (Fig. 9) of the bottom sediments of the Yenisey Gulf and Kara Sea, but not between Sr-90 and these radionuclides(Fig.l3) [7]. These observations (in combination with published data on the behavior of these radionuclides in natural waters) allows conclusion that Cs-137 and Pu-239,240 migrate essentially in suspended form, whereas Sr-90, in water-soluble form. The observed similarity in the behavior of Cs and Pu in the Yenisey River-Kara Sea aquatic system is due to the occurrence of efficient mechanisms of their absorption by suspended and sediment material, although the nature of these mechanisms is substantially different. The common thought ( 1 ) is that the absorption mechanism of Cs is based on irreversible replacement of K+ in the structure of clay minerals of the solid phase by Cs +[1]. This is supported, in particular, by the fact that 6 M HC1 extracts from flood-land soils and bottom sediments up to 60% of Pu and only 12% of Cs (Table 4) which, in our opinion, is practically irreversibly fixed in the structure of clay minerals.
Table 4. Distribution of Fe, Al, Mg, Ca, Mn and radionuclides throughout geochemical mobility forms in flood-land deposits of the Yenisey River (Predivinsky Isl.)
Leaching H2O 1 M CH3COONH4 6MHC1 Residual fraction reagent
Chemical Fe Al Mg Ca Mn Fe Al Mg Ca Mn Fe Al Mg Ca Mn Fe Al Mg Ca Mn elements % of extraction 2 1 26 9 0,2 62 15 93 51 110 8,4 3,5 1,9 2,1 0,4 91,6 96,5 98,1 97,9 99,6 3 10"' io-' io-' io-' 10"' 10-' 10- io-' io-3 10° Radionuclide Pu Co Eu Cs Pu Co Eu Cs Pu Co Eu Cs Pu Co Eu Cs % of extraction 1.1 0.9 1.0 7.3 1.3 0.9 1.1 3.9 62 22 65 12 35 76 63 76 Table 1 also includes the data on the speciation of Co-60 and Eu-152 and the speciation procedure. The suspended fraction of Cs-137 increases in periods of high water (such a conclusion is made in [9] for the Kuji river in Japan). In our opinion [10,14], Pu forms variously associated hydrolyzed species in the aqueous phase, which are effectively sorbed by fine-.disperse fractions of suspended and sediment material. There are all reasons to belief that the
-143- JAERI-Conf 2000-016 migration behavior of other hydrolyzable radionuclides such as Eu and Am is controlled by the transport mechanisms of suspended material also. Similar conclusions were made by us when studying the behavior of Cs-137 and Pu- 239,240 in the Kiev water storage basin [6]: in spite of significant differences in the physicochemical conditions in this basin and the Yenisey River (features of sedimentary material supply, hydrological regime, etc.) and also in the contamination sources (Chernobyl accident and impact from MCC), the forms of migration of Cs-137 and Pu-239,240 in these aquatic systems are similar, in principle. So, for Kiev Reservoir we showed, that: • About 95 % of 239>240Pu and 60% 137Cs are associated with suspended matter. 239 240 137 • Bottom sediments are main depot for - pu and Cs. 239 240 137 • ' pu ancj Cs are tightly fixed in the structure of bottom sediments, so that the mobile fraction of these radionuclides is small (6). The distinction is a higher degree of Cs-137 association with the solid phase in the Yenisey River. It is well known that organic compounds play an important role in radionuclide migration from the soil profile to river basins. Thus, Amano,H.,Matsunaga,T.,Nagab,S., et al. (12 ) have demonstrated that Pu anld Am migrate from the soil profile of the Chernobyl region to the Pripyat' River essentially with humic and fulvic acid fractions. In this work we have established that flood-land soils of the Yenisey River contain organic carbon in substantial amounts (0.5-4%) (Fig.l4B). Since we also found a direct correlation between the Pu and organic carbon distributions in the flood-land soil (Fig. 14A), it appeared advisable to gain a better insight into the Pu and Am association with particular components of the organic fraction of this soil. Therefore, we determined the group and fraction composition of humus compounds in flood-land soil of the Yenisey River (Fig. 14 C). This figure shows the variations in both the total contents of fulvic and humic acids and their particular fractions throughout the profile. In the legend we use the commonly accepted designations for these fractions: I A I, HA II, and HA III are three fractions of humic acid and FA la, FA I, FA II e FA III are four successively extracted fractions of fulvic acid. It has been shown also that cation-exchange capacity of flood-land soils of the Yenisey River ranges from 10 to 30 mg-equiv./lOO g. The contribution of carboxylic protons of humic acids to the total cation-exchange capacity is as large as 30-60%. The results show that in the flood-land soils humic acids dominate over fulvic acids, and in the group of humic acids fraction HA(II) (preferentially associated with Ca) prevails. This suggests that one of the major sources of formation of the flood-land soil is chernozem and soddy-carbonate soils, typical of the Krasnoyarsk-Irkutsk soil province. To study the Pu and Am association with humic (HA) and fulvic acids (FA) we used the procedure commonly used in Russia (13) and abroad [11,12]. Sequential extraction with various reagents allows separation of particular fractions of humic and fulvic acids from the soil. The distribution of radionuclides throughout organic fractions of flood-land deposits is presented in Tables 5,6. Table 5. Pu and Am distribution throughout organic fractions in flood-land deposits of Atamanovski Island (downstream spit, % of the total).
Radionuclide 24Upu 241Am* Horizon, cm 4-6 12-14 16-18 26-28 4-6 12-14 16-18 26-28 Fulvic acid la 13,5 11,5 12,6 8,4 83,8 79,4 86,3 80,9 Fulvic acids 8,6 5,3 7,4 5,4 3,4 4,3 3,5 3,6 Humic acids 15,4 13.1 12.6 15.2 1.5 1.8 0.8 1.0 The sum of humus acids 37,5 29.9 32.6 29.0 88.7 85.5 90.6 85.5 Residual fraction (humins) 62,5 70,1 67,4 71,0 11,3 14,5 9,4 14,5 *) Model experiments
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Table 6. Pu and Am distribution throughout organic fractions of flood-land deposits of the tail of the Atamanovski Island [% of the total (numerator) and % of the content in a fraction of the appropriate acids (denominator)].
Ml Radionuclide m240pu Am Components Humic acids Fulvic acids Humic acids Fulvic acids of humus Fraction / horizon 1+2 3 la 1+2 3 1+2 3 la 1+2 3
4-6 12.5/81.0 2.9/19.0 13.5/61.1 6.7/30.3 1.9/8.6 1.3/86.7 0.2/13.3 83.8/96.1 2.6/3.0 0.8/0.9 12-14 11.5/87.8 1.6/12.2 11.5/68.4 4.9/29.2 0.4/2.4 1.6/88.9 0.2/11.1 79.4/94.9 3.4/4.1 0.9/1.0 16-18 10.3/81.7 2.3/18.3 12.6/63.0 6.9/34.5 0.5/2.5 0.7/87.5 0.1/12.5 86.3/96.1 3.0/3.3 0.5/0.6 26-28 8.4/55.3 6.8/44.7 8.4/60.9 4.6/33.3 0.8/5.8 0.9/81.8 0.2/18.2 80.9/95.7 3.0/3.6 0.6/0.7
In the humus acid fraction 29-37% of Pu and 85-90% of Am are found, 13-15 %) of Pu is contained in the humic acids, from 20 to 22% of Pu is contained in fulvic acids. Am association with fulvic acids is more pronounced ( 89.1- 94.1%) The main part of Pu (62- 71%) is found in poorly soluble residual fraction, while for Am this quantity is much lesser (9-14 %). In the humic acid fraction the major parts of Pu and Am is found in free humic acids, their compounds with mobile hydroxides, and calcium humates (fraction 1+2). This fraction contains 55-88% of Pu and 82-89% of Am from their total contents in the humic acid fraction. In humic acids associated with immobile Fe and Al hydroxides (fraction 3) the content of Pu ranges within 12-45%, and Am, 11-18 %. In the fulvic acid fraction the main parts of Pu (61-68 %) and Am (up to 96%) are found in fraction la. Pu in a larger degree than Am is found in fraction (1+2) consisted of both free fulvic acids and Ca, Fe, and Al fulvates. These results can be useful in interpretation of the migration behaviour of Pu and Am in flood-land deposits. So, we found that more than half of Pu is associated with humin fraction of the soil. On the contrary, the content of Am in this fraction is rather low (11-14%). Plutonium is nearly equally distributed between the humic and fulvic acid fractions. Americium is preferentially associated with the fulvic acid fraction (up to 90%). Generally, both Pu and Am are associated with the most mobile fractions of humic and fulvic acids. Of a particular importance is the problem of secondary contamination of the Yenisey River with radionuclides from flood-land soils. Flood-land soils and bottom sediments, being a natural trap of sort for radionuclides, now contain the major part of long-lived radionuclides supplied from MCC in longer than a 30-year period of its operation. It is important to predict the behavior of this "radiation legacy" in the future, i.e., to determine whether the flood-land soil is the final or intermediate point on the way of radionuclide migration from the contamination source to the Kara Sea. Therefore, it is also necessary to find out what processes in flood-land soils are potential factors of secondary contamination of the investigated aquatic system with radionuclides. So, today we are emphasizing on the flood-land soils. Since metal fulvates are the most potentially mobile component of the absorbing complex of flood-land soils( 15,16), we studied the migration behavior of Am and Eu in the model system Ca,Fe,Al-fulvate-Yenisey River water. Composition of the model system was selected taking into account the results of analysis of actual samples of flood-land soils. It was demonstrated by infrared spectroscopy and potentiometric titration method that in the investigated system at pH 8.0-8.5, typical of the Yenisey River, carboxylic protons of humus acids are totally substituted by Ca ions, which are typomorphous cations in water of the Yenisey River.
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Model systems were prepared as in [17] with the use of fulvic acids both separated from actual samples of flood-land deposits (Atamanovski Isl., horizon 15-20 cm) by alkaline extraction and "native" preparations separated from soddy carbonate soil . To determine the stoichiometry of the fulvate component of the organomineral complex of flood land (sample no. 123, Ust'-Tunguska Isl., horizon 10-15 cm, Table 8) we used the alkaline extract obtained in the course of group and fractional analysis in the stage of separating the fraction of total humic and fulvic acids. pH in the system was adjusted with 0.02 M Ca(OH)2 at vigorous stirring to a constant value of 8.2. The resulting solution was allowed to stand for 3 days in air with intermittent stirring and pH was corrected. In each experiment the liquid phase was refreshed tree times with Yenisey river model water, the phases were separated and analysed for the radionuclides contents.
Table 8. Distribution of Eu-152 and Am-241 between model gel phase and aqueous solution at pH 8
Fraction of Eu(Am) in liquid phase, a, % After refreshing of liquid phase PH Fe(IH)/RCOO A1(IH)/RCOO Initial system %-equiv %-equiv. 1 2 3 Eu Am Eu Am Eu Am Eu Am 8.2 0 0 <0.02 <0.02 <0.02 <0.02 <0.02 <0.02 <0.02 <0.02 8.1 10 5 <0.02 <0.02 <0.02 <0.02 <0.02 <0.02 <0.02 <0.02 8.0 20 10 <0.02 <0.02 <0.02 <0.02 <0.02 <0.02 <0.02 <0.02 8.2" 30 0 <0.02 <0.02 <0.02 <0.02 <0.02 <0.02 <0.02 <0.02 "native" fulvic acid
The data show (Table 8) that regardless of the Fe and Al contents in the model system, Am and Eu are practically totally concentrated in the gel phase; the fulvate concentration in the solution does not exceed 0.005% of the total fulvate in the system. After the liquid phase was triply refreshed the fraction of the mobile forms of Am and Eu (a) approaches the background level (under the experimental conditions this corresponds to about 0.02% of the initial radioactivity). Thus, comparison of the results obtained with those reported in [ 17 ] shows that changing Na for Ca as a pH-determining (typomorphous) cation(18) practically completely suppresses transfer of Fe,Al(Am,Eu) fulvates into mobile state. The data on a (Table 8) allow estimation of the degree of transfer of the indicated radionuclides from flood- land soils to ground solution. It was demonstrated with an example of flood-land soil of the Atamanovski Island that under limiting moistening of the soil in flood, the ratio of the amount of mobile forms of the fulvate fraction to the volume of the liquid phase is of the same order of magnitude as the ratio of the amount of fulvate in the gel phase to the volume of solution in the model experiments. Therefore, in this respect the model is adequate At a solid (metal fulvate) to aqueous phase ratio in the model system, close to that under actual conditions, only 0.02% of Am (and Eu) occur in the mobile form. Therefore, transition of Am and Eu to the aqueous phase (secondary contamination) with mobile metal fulvates is insignificant under current conditions of the flood land of the Yenisey River. With the proposed model system we can study the effects of various natural factors capable to enhance the migration activity of radionuclides. Among such factors are those resulting in acidification of flood-land soil (biochemical transformation of plant residues) and anaerobic processes occurring under conditions of long-time damping [reduction of Fe(III) to Fe(II) producing mobile Fe(II) fulvates].
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Modeling only one of the indicated factors (pH) in laboratory experiments, we have demonstrated that with decreasing pH from 8.2 to 6.0 the mobile fraction of Am increases by 1-2 orders of magnitude (to 0.2-2.0%), i.e., the conditions are realized in this case for a substantially increased transfer of the radionuclide to the aqueous phase.
Table 9. Distribution of Am-241 between gel phase of Ca,Fe,Al fulvate and aqueous solution at 61 6.0 Fraction a (%) of Am in liquid phase Fe(III)/RCOO A1(III)/RCOO Initial After refreshing of liquid % - equiv % - equiv System phase I II III 0 0 18.0 10.1 8.3 8.5 10 5 5.0 3.3 2.3 2.1 20 10 2.1 1.1 0.6 0.5 30* 0 1.9 0.8 0.8 0.4 50 0 1.1 0.4 0.2 0.2
* - "native" fulvic acid.
At present time we are carrying out experiments to study the effect of anaerobic processes, occurring in a long-time contacting of flood-land soil with a great excess of the aqueous phase (flooding conditions), on Am migration with metal fulvates.
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References
1. Kuznetsov, Yu.V, Revenko, Yu.A., Legin,V.K, et al., Evaluation of Contribution of the Yenisey River to Total Radioactive Contamination of the Kara Sea, Radiokhimiya, 1994, vol. 36, no 6. 2. Nosov, A.V. et al., Radioactive Contamination of the Yenisey River by Discharges of the Krasnoyarsk Mining-Chemical Combine, At. Energ., 1993, vol. 74, no 2, pp. 144-150. 3. Kuznetsov, Yu.V. et al., Assessment of Contribution of the Yenisey River and Ob' to Total Radioactive Contamination of the Kara Sea, Proc. Int. Conf. "Radioactivity and Environmental Security in the Oceans: New Research and Policy Priorities in the Arctic and North Atlantic", Woods Hole, Massachusetts, USA, 7-9 June 1993. 4. Pisarev, V.V., Nosov, A.V., Kuznetsova, V.M., et al., Study of Kiev Water Storage Contamination in 1986-1987. In "Chernobyl-88", 1989, vol. 5, no. 2, Moscow: Energoatomizdat, pp.159-175. 5. Voizechovitch, O.V., Radioactive Contamination of the Dnieper Basin, Priroda, 1991, no. 5, pp. 52-56. 6. Kuznetsov, Yu.V., Radioactive Contamination of the Dnieper River and Baltic Sea Basins after Chernobyl Accident, Proc. Seminar on Comparative Assessment of Environmental Impact of Radionuclides Released during Three Major Nuclear Accidents: Kyshtym, Windscale, and Chernobyl, Luxembourg, 1-5 October 1990, vol. 1, pp. 577-595. 7. Kuznetsov , Yu.V. and Legin, V.K., >240Pu, 137Cs, and 90Sr in Bottom Sediments of the Kara Sea: Attempt to Reconstruct Rate of Radionuclide Supply to Bottom Sediments in the Past, Proc. Int. Symp. on Marine Pollution, Monaco, 5-9 October 1998, pp.239-240. 8. Kuznetsov Yu.V.,Legin V.K.,Shishlov A.E et al. The study of Pu-239,240 and Cs-137 Behaviour in the Yenisei River-Kara Sea system. Radioochemistry,vol.41,N 2,1999,p.l81-186. 9. Matsunaga, T., Amano, H., and Yanase, N., Discharge of Dissolved Particulate 137Cs in the Kuji River, Japan, Appl. Geochem., 1991, vol. 6, pp. 159-167. 10. Kuznetsov, Yu.V., Legin, V.K., and Pospelov, Yu.N., Possibility for Prognosis of Plutonium Behavior in the Sea Environment Using Data on Its Natural Radioactivity, Radiokhimiya, 1982, no. 1, p. 136. 11. Watanabe, M., Amano, H., Onuma, Y., Ueno, T., Matsunaga, T., and Yanase, N., Speciation of Radionuclides in Soils and Surface Organic Matters Sampled around the Chernobyl Nuclear Power Plant, Extend. Abstracts, 4-th Int. Conf. on Nuclear and Radiochemistry, vol. II, St.-Malo, France, 8-13 Sept., 1996. 12. H.Amano, T.Matsunaga, S.Nagao, Y.Hanzawa, M.Watanabe, T.Ueno, and Y.Onuma. The transfer capability of long-lived Chernobyl radionuclides from surface soil to river water in dissolved forms, Organic Geochemistry , 1999, vol. 30, p.437-442. 13. Pavlotskaya F.I. Migration of Radioactive Products of Global Fallout in Soils. Moscow,Atomizdat, 1974 (In Russ.) 14. Kuznetsov, Yu. and Legin, V., Forms of Technogenic Radionuclides Migration in System the Yenisey River-Kara Sea. Humanity and the World Ocean: Interdependence at the Dawn of the New Millenium, Symp. of the Russian Acad. Sci., Moscow, Russia, June 23- 25,1999, p.340. 15. Nelson, S.D., Penrose, W.R., et al., Effect of Dissolved Organic Carbon on Adsorption Properties of Plutonium in Natural Waters, Environ. Sci. Tech., 1985, vol.19, pp. 127-137. 16. Choppin, G.R., Humics and Radionuclide Migration, Radiochim. Acta, 1988, nos. 44-45, pp. 23-28.
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17. Legin, E.K., Trifonov, Yu.I., Khokhlov, M.L., and Suglobov, D.N., Distribution of Eu between Gels and Solution of Iron(III) Fulvate at pH 1-9, Radiokhimiya, 1998, vol. 40, no. 2, pp. 188-193. 18. Sorokovikova, L.M., Transformation of Major Ions and Mineralization of the Yenisey Water under Conditions of Regulated Discharge, Water Resources, 1993, 1993, vol. 20, no. 3, pp. 320-325.
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16 15 I01 10
•10 1 8 15 104 187 248 300 510 803 1360 -10 1 > IS 104 187 248 300 5tO 803 1380 -10 f 8 IS 104 187 248 300 510 803 1380 Distance from MCC, km Distance from MCC, to Distance from MCC, ton Radionuclide concentrations in bottom sediments of the Yemisey Mtver (B) I soo -, , s to 8 o 1 o
Distance from MCC, km Distance from MCC, km
Fig.1. RadioraicMde concentriitians in the Yenisey waters (A) JAERI-Conf 2000-016
Pu-239,240 Bq/kg C©-60, Bq/kg 5 10 0,5
Eu-152, Bq/kfl Cs-137s Bq/kg 100 200 300 500 1000
Eu-152 / CO-6Q 50 100
Pig.2, Vertical distribution of Pii~239,24f)9 Co-6©9 Ew-1529 Cs-137 and £u-152 /Co-6§ ratio in bottom sediments of the Yeiiisey River
-153- CuAepxaHtie p*w) MiyjumooB % noms o. MoMonm.-. S1OTB „ iwuaw- rRWra 130*20 . o-s 130 SIC II.S -A ifr-JS V i 95-20 <3 '*?• o aiptm»t>-M •(•K4? 1 ao«u•^^•t ] Fig.3. Scheme of flood-land deposits forming. JAERI-Conf 2000-016 Fig.4 are given the results of Pu-239,240 and Cs-137 determinations in vertical profiles of the Yenisey flood-land deposits sampled at distances from 2 to 1130 km from the outlet of discharge waters from MCC. In Fig.5 the Am-241 vertical profiles are given. Figures 6 and 7 illustrate the vertical distribution of Eu-152 and Sr-90, respectively. Cs-137, Bq/kg Cs-137, Bq/kg Cs-137, Bqfkg Cs-137, Bq/kg 500 1000 15002000 500 1000 1500 2000 500 1000 1500 2000 500 1000 1500 2000 "S. - *• Ct-137 '" •^X^ ^ -t-PMHW 10 - : * ".20- ; A §30- 140 Y * -o-Cs-137 • *• • Pu-239,240 40 - 50 40 - Atamanovski, 2 km Beriozovi, 18 km Tarygin, 24 km Predivinski, 100 km 60 60 SO 50 5 10 15 2 4 1 2 3 5 10 15 20 Pu-239,249, Bq/kg Pu-239,240, Bq/kg Pu-239,240, Bq/kg Pu-239,240, Bq/kg Cs-137, Bq/kg Cs-137, Bq/kg Pu-239,240, Bq/kg Cs-137, Bq/kg 0 500 1000 1500 20 0 1000 2000 3000 4000 5000 S 10 15 500 1000 1500 2000 A&. A §2S: c o N 4-' Jir^ • * Cs-137 A aQ -«-Pu^39,240 40 • Cheremukhov, 250 km /. Usf Tuhgusski, 255 km Gorodskoi, 331 km 75 - 50 2 4 10 5 10 Pu-239,240, Bq/kg Pu-239,240, Bq/kg Pu-239,240, Bq/kg Fig.4. Vertical distribution of Pu-239 and Cs-137 in flood-land deposits of the Yenisey River. Am-241, Bq/kg 0,5 1 Atamanovski - upper part, 2 km Atamanovski - tailing, 7 km Tarygin, 24 km Fig.5. Vertical distribution of Am-241 in flood-land deposits of the Yenisey River. -157- JAERI-Conf 2000-016 Eu-152, Bq/kg ' Eu-152, Bq/kg Eu-152, Bq/kg Eu-152, Bq/kg 1000 2000 3000 10 20 30 500 1000 1500 2000 200 400 i i i t 4 Fig.6. Vertical distribution of Eu-152 in flood-land deposits of the Yenisey River: (1) Atamanovskii Isl., (2) Tarygin Isl., (3) Predivinskii Isl., and (4) Cheremukhov Isl. Sr-90, Bq/kg Sr-SO, Bq/kg Sr-90, Bq/kg Sr-90, Bq/kg 10 20 1 2 5 10 12 3 4 5 4 Fig.7. Vertical distribution of Sr-90 in flood-land deposits of the Yenisey River: (1) Atamanovskii Isl., (2) Tarygin Isl., (3) Predivinskii Isl., and (4) Cheremukhov Isl. -158- Pi8»239924® and Cs-i37 in surface sediments of the Kara Sea |Bq/kg] and their ratio. s (fc-U? .SN 10 ().l 3,(1 11.03 11 a.i 15.0 9X17 £ 17 3.5 .22.6 0.(17 to 19 S.lt aiis Jtt ().» Hl.R &1B 26 9.2 2S.IS ftflS 2* M.S ass M 7.9 tt(M it 2^f Jl.fi air? (I.' HJI (;,«<• J7 1.6 25.4 (Ui6 is tl.S j'.S £» 0.8 1J.I 8.07 41 (1.2 J.0 SWI" 42 (|,7 H.-I O.(W •B (P.5 5.5 tl.Ojj •M SU 5.6 (M(J 45 U.S aw. 51 (U ii.S 0.(16 r< tl.fi -62.61 67.61 -72.61 -77.61 -82.61 Fig. 8 JAERI- Conf 2000-016 (8 3 o H e o 1 Q s 9 5 c« Z 5ft Cft U § CM .S O -*•* es o vE- • o •c a m e .2 ON fe m CO -161- 100 •Station 31 • Station 34 3 m i 2 o I o I—* i to § o I o Oi 0,1 6 8 10 12 14 Depth, cm Fig. 10. Vertical distribution of unsupported Pb- . sediments (stations NN 31 and 34). Station 31. Station 34, Sedimentation rate -1,1 mmly (Pf>-210). Sedimentation rat® -1,8 mm/y (Pb-210). Years Years 0 10 20 30 40 5 SO I 8! I to § o I o (33 12 0 2 14 16 18 Fig. 11. Vertical distribution of Cs-137 and Pn-239f24S in bottom sediments (stations NN 31 and 34) with decay correction for Cs-137. JAERI-Conf 2000-016 Soil - H2O Solution (water-soluble) Residue Solution 1 M CH3COONH4 (exchangeable) Res>idue 6MHCI Solution (acid-sohibie) Residue (insoluble) Fig. 12. Schematic diagram for determination of mobile forms of radionwclides. -167- 2,5 A o y1 1,5 O y / o of o s I y / I—* 11 a. I o O A o O o 0,5 O Pu/Cs 1 —Linear trend (Pu/Cs) | /o o • yo •yo 0 5 10 1S 20 25 30 35 40 10 20 30 40 50 60 70 Cs-137. Bq/kg Fig. 13. Correlations between Cs-137 and Pii-239,24C (A) and Cs-137 and Sr-90-(B) contents Ii surface JAERI-Conf 2000-016 20 30 40 50 c 20 30 40 50 Horizon, cm «#> Total of humic acids Total of fulvic acids HA II Fig. 14. (A) Correlation between distribution of total organic carbon and Pu; (B) total organic carbon distribution and (C) group and fraction composition of humus matter in flood-land soil of the Yenisey River (Atamanovskii Isl.) -171-