Radioactive pollution

Surface waters are a powerful factor that causes migration of radionuclides across the territory of Belarus. For this reason it is essential to take into due account the transit role of rivers in the transportation of radionuclides, including transboundary transfer. In watercourses and flowing water bodies concentration of radionuclides are reducing every year, but they tend to accumulate in sttatic water bodies (lakes, ponds, reservoirs, especially in bottom sediments).

Due to the accident at the Chernobyl nuclear power station (CNPS) one quarter of the country's territory was contaminated by caesium-137 (23%), strontium-90 (10%) and plutonium (about 2 %). Concentrations of caesium-137 by the year 2001 are presented in the (fig.).

According to the monitoring data the radiation situation in the Dnieper-Sozh and Pripyat basins is stable. The annual average concentration of caesium-137 has decreased significantly in large and small rivers for the observed 1987-2000 period. Exceeding of the National permissible levels (NPL-96, 99) for caesium-137 and strontium-90 were not observed. However, caesium-137 concentration in surface waters are closely connected with the annual volume of river flow, as can be seen from the increase in concentrations of caesium-137 in some rivers, where water supply was lower than the perennial average values.

The data analysis of concentration of caesium-137 during the spring flood in the Pripyat basin in 1999 shows that concentration of caesium-137 in a dissolved form in the Pripyat basin (the city of Mozyr) remains at the level of the average indices for the prior period (1996-1998). Yet concentration of this radionuclide has considerably increased in dredges. This means that caesium-137 is washed out and transported by flood flow with sediments.

According to the measurements of the total radioactivity of caesium-137, transferred by rivers during 12 years (1987-1999), 85% were transferred by the Ipyt during 2 years, 81% by the Sozh during 3 years, the Besed during 4 years, the Pripyat during 6 years, and 71% by the Dnieper during 9 years. The comparison between the indices of caesium-137 transfer for the period of 1987-1993 at the end-point monitoring checkpoints on the Dnieper and the Pripyat in the Ukraine and at the monitoring checkpoints in Belarus shows that only 26% of the total transfer of radioactive caesium-137 were generated within the 30-kilometer Chernobyl zone. Thus, in the river runoff formation large watersheds had greater impact on secondary river pollution within the first few years (3-4 years) after the Chernobyl accident than surface river runoff from highly contaminated territories with relatively small watersheds.

Since the radiation situation has stabilized, transboundary transport of radioactive elements through river flow has significantly decreased. Mainly it is the Pripyat that transports radionuclides, in particular strontium-90, as they are washed out from the 30-kilometer Chernobyl zone.

Due attention is devoted to studies of the radiation state of small rivers, which are tributaries of the Pripyat (the Braginka, Nesvich and Slovechna Rivers) and the Sozh (the Lipa and the Senna) in the most contaminated areas of the Gomel and Mogilev regions. Over the years radioactivity in water tends to decrease. The exception is the dissolved strontium -90, which is a specific feature for the CNPS zone.

Annual data on the content of caesium-137 and strontium-90 (in soluble and suspended forms) in the bottom sediments and water biota suggest that bottom sediments and water biota are significant contributors to the total radioactivity of surface water systems. A tendency towards a reduction in radioactivity of bottom sediments and water biota is minor.

During spring high water and summer-autumn flood, migration of radionuclides into open water systems occurs both in soluble and in absorbed forms on organic and mineral carrieres.

The ratio between concentrations of caesium-137 and strontium-90 at the end checkpoints of the Braginka and Senna Rivers suggests that starting from1992-1993, the concentration of strontium-90 has begun to exceed that of caesium-137. That phenomenon is characteristic for surface watercourses close to the CNPS zone and is explained by an increase in migration ability of strontium-90 due to its release from active particles.