TABLE 4.23. CAESIUM-137 IN THE BOTTOM SEDIMENTS OF THE DNIEPER RESERVOIRS (JANUARY 1995)

Reservoir	Inventory (TBq)		Averaged density of contamination (kBq/m2)
Kiev	80	± 17	86	± 6
Kanev	18	± 3	31	± 25
Kremenchug	26	± 3	13	± 1
Dniprodzerzhinsk	4.3	± 0.4	7.4	± 0.7
Dnieper	2.2	± 0.7	5.6	± 1.9
Kakhovka	6.8	± 0.7	3.1	± 0.4
Total in cascade	136	± 26		—

Table 4.23 shows the inventory of 137Cs in the bottom sediments of the Dnieper reservoirs and the averaged densities of the bottom contamination of each reservoir as of January 1995.

As noted above, the mapping of the bottom sediment contamination with 90Sr was problematic. Owing to the low sorption of 90Sr on to solid particles, no regularities in its spatial distribution on the bottom of the reservoirs were discovered. However, where there is an accumulation of shells of molluscs and other carbonate deposits on the bottom, there are higher levels of 90Sr. It is not excluded that the strong fixation of 90Sr in the carbonate structure of folds is one of the main factors of irretrievable removal of this radionuclide from the water column on to the bottom.

At present, the contaminated bottom sediments in the reservoirs do not influence significantly the secondary radioactive contamination of the water and aquatic organisms. In all places the deposition of cleaner sediments over the more contaminated ones is observed. Thus the general rule for exploitation of the reservoirs (mainly Kiev) is to minimize scooping and mechanical operations at the places with increased bottom contamination. The natural processes of self-cleaning are working in all the reservoirs. As a consequence, there is a gradual reduction in the radionuclide levels in fish and other aquatic species living in the reservoirs.

Results of modelling of the sediment redistribution following a major flood are given in Section 9.3.6.

4.8. CONCLUSIONS

(a)The levels of 137Cs and 90Sr in flowing rivers are now well below the permissible levels set

by the national authorities and below internationally acceptable levels for drinking water.

(b)Surface water runoff from watersheds and floodplains contaminated with 90Sr remains the major contributor to contamination of the Dnieper reservoirs. The main sources contributing to radionuclide releases into the Dnieper cascade are located within the CEZ. The water remedial actions carried out during recent years (1993–2000) significantly reduced actual and potential fluxes into the river.

(c)Almost all the 137Cs washed out of contaminated areas is immobilized in bottom sediments within the reservoirs of the Dnieper River. The impact of these sediments is low and will decline further with decay and further deposition of sediments on top of the contaminated sediments. The overall strategy should be to leave these sediments as is and avoid processes that will lead to their resuspension.

(d)Identification of the main sources within the Chernobyl zone shows that the heavily contaminated floodplain areas still remain the main source for releases. The waste disposal sites in the flood prone areas are of secondary priority for possible remedial actions.

(e)Monitoring systems have been developed for the river systems and are important inputs to the decision making process on possible further water protection measures in the Chernobyl area.

(f)Monitoring data are collected by various agencies for different purposes; different methodologies are used, some of which are outdated. There needs to be harmonization of results between the various organizations engaged in monitoring.

(g)Mathematical models have been developed to describe the wash-off of radioactivity from

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contaminated land. These models can be used to predict the discharge of radioactivity into rivers and reservoirs from knowledge of the rainfall distribution.

(h)Only a small fraction of the radioactivity deposited on ground via fallout has entered the Dnieper River system, and most of the 137Cs and 90Sr will decay in the ground.

(i)Levels of 137Cs in the lower sections of the

Dnieper River have returned to preChernobyl accident levels. Strontium-90 is more persistent because it does not adsorb as readily on to sediments.

(j)Lakes with no regular inflows and outflows still present a radiological problem that will continue for some time. There is a need for improved understanding of processes occurring within lakes, especially transfer to fish.

(k)Transboundary movement of radionuclides from the Chernobyl accident was greatest prior to 1990. The annual flows of 137Cs across national borders have continued to decline steadily, whereas those of 90Sr continue to fluctuate, depending on the extent of flooding of contaminated land (especially the floodplain area of the CEZ). Currently, the annual flow of 137Cs into the Black Sea is negligible, whereas 1–8 TBq of 90Sr reaches the Black Sea.

(l)There is a need for improved understanding of the 90Sr inventory in sediments and 90Sr fixation processes. Strontium-90 becomes more important with time because of its greater mobility.

(m)The large database developed by SPA Typhoon, the UHMI and the RCREM is very useful in understanding and interpreting data, and should be maintained. Ideally it should be expanded to include aquatic species data and data from the lower parts of the Dnieper River.

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[4.2] DE CORT, M., TSATUROV, Y., Atlas of Caesium Deposition on Europe after the Chernobyl Nuclear Power Plant Accident, Final Report of Joint Study Project No. 6, Rep. 16542, Office for Official Publications of the European Communities, Luxembourg (1996).

[4.3] DE CORT, M., Atlas of Caesium Deposition on Europe after the Chernobyl Accident, Rep. 16733, Office for Official Publications of the European Communities, Luxembourg (1998).

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[4.33] ANTROPOV, V.M., et al., Review and analysis of solid long-lived and high level radioactive waste arising at the Chernobyl nuclear power plant and the restricted zone, DG Environment Project No. B7-5350/99/51983/MAR/C2, Office for Official Publications of the European Communities, Luxembourg (2001) 370.

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