CATCHMENT BASIN OF THE PRIPYAT RIVER WITHIN CHERNOBYL EXCLUSION ZONE AS A SOURCE OF RADIOACTIVE CONTAMINATION OF SURFACE WATERS
D.I. Gudkov1, V.V. Derevets2, S.I. Kireev2 and A.B. Nazarov2
1Institute of Hydrobiology of the National Academy of Sciences of Ukraine, Geroyev Stalingrada Ave. 12, Kiev, Ukraine
2State Specialized Scientific-Production Enterprise "Chernobyl Radioecological Centre"
Ministry of Ukraine on the Emergency and Affairs of Population Protection Against the Consequences of the Chernobyl Catastrophe, Kirova Str. 17, Chernobyl, Ukraine
After termination of aerosol emissions from destroyed unit of the Chernobyl nuclear power plant (ChNPP) in 1986 the basic migration (80˝95 %) of radionuclides beyond the bounds of the exclusion zone occurs by a surface run-off. During 1986˝2000 about 132 TBq of Cs-137 and 159 ТBq of Sr-90 flow into Kiev reservoir by the Pripyat River ˝ the main waterway of the exclusion zone. Getting into reservoir radionuclides are distributed in all cascade of the Dnieper River reservoirs. The maximal flow of radionuclides by the Pripyat River was registered in 1986 ˝ about 66 TBq of Cs-137 and 28 TBq of Sr-90. About 60 % of Sr-90 flow by the Pripyat River are formed on the part of catchment basin, which is taking place within territory of the exclusion zone. Since 1988 the annual flow of Cs-137 rarely exceeded half of Sr-90 flow, and in separate years did not reach also a quarter. About 85˝90 % of general flow of Cs-137 by the river is formed outside of the exclusion zone. For the afterwreck period was observed some crisis situations connected to surface waters. The greatest radionuclide contamination of Pripyat River waters in the afterwreck period (except for 1986 and 1987) was registered in the end of January˝February 1991, when the powerful ice jams were generated, which have caused abrupt flood within the Pripyat River flood-lands in the exclusion zone. Thus the maximal concentrations of Sr-90 in water of the river have amount 12 kBq/cu.m. The flow of Sr-90 by the Pripyat River for the period of greatest intensity of ice jams has amount about 4 TBq. Three quarters of this amount have flow into the river from the territory of left-bank, so-called, Krasnensky flood-lands. This territory is characterised by the highest levels of radioactive contamination within exclusion zone. The Sr-90 carry-over beyond the bounds of the exclusion zone in 2003 by the main rivers has made: Pripyat River ˝ 1.40 TBq; Uzh River ˝ 0.06 TBq; Braginka River ˝ 0.04 TBq. Altogether in the Dnieper River by these watercourses was carried away 1.5 TBq of Sr-90. It is the lowest value for the afterwreck period. Created till now water-protection structure practically have not prevented the radionuclides distribution beyond the bounds of the exclusion zone, and decrease of radioactive substances flow from the catchment territories occurs due to natural processes. The anthropogenic interference during liquidation of consequences of the accident consisting, in particular, in construction of dams on left-bank flood-lands area of the Pripyat River became the main reasons of additional swamping of the territories, rise of subsoil water level, flooding of trenches with radioactive waste disposal etc. As a result of these processes the probability Sr-90 carry-over by a waterway beyond the bounds of the contaminated catchment basin has increased.