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The paper presents the results of radiation surveying and determination of radionuclide and elemental composition in water objects (soil, bottom sediments, water) of the transboundary rivers Shu, Shor-Koo, Aksu, Karabalta, Toktas as well as of Tasotkel water-storage in Kazakhstan in vicinity of the border with Kyrgyzstan. Increased contents of natural radionuclides of 238 U and 232 Th series, As, Co, Cs, Cu, Hf, Mo, Pb, Sb, Sc, Zn, Zr and rare earth elements were observed in soil and bottom sediment samples. Increased contents of the following toxic elements were revealed in water samples: B, As, Mo, Ba, U. Negative influence of radiation-hazardous sites in Kyrgyzstan on the contamination of the transboundary rivers with natural radionuclides and toxic elements was revealed.

The paper reports on the study of radionuclide and elemental composition of water, bottom sediment and soil samples collected at the border areas of the following transboundary rivers in Kazakhstan: Chagan, Ural, Ilek, Tobol, Ayat, Irtysh, Emel, Ili, Tekes, Shu, Karabalta, Talas and Syrdarya. The employed analyses include the following methods: instrumental gamma-ray spectrometry, radiochemical analysis, neutron activation analysis, XRF and the inductively coupled plasma mass spectrometry (ICP-MS). Evidence of water environment contamination with radionuclides and toxic elements has been revealed in many of the studied rivers both in Kazakhstan and in adjacent countries. Transboundary transfer of the contaminants is most likely related to local industry (uranium mining and processing) and the presence of radioactive substances in the river basins.

The article describes the monitoring results of the content of chemical elements (As, Cr, B, Ba, Li, Mo, Pb, Sb, Sr, U, and Cr) in the water of transboundary rivers of Kazakhstan (Shagan, Ural (kaz. Zhayik), Ilek, Tobol, Ayat, Irtysh (kaz. Yertys), Emel, Ili (kaz. Ile), Tekes, Shu, Kara-Balta, Talas, and Syr Darya) conducted in 2020. The toxic element concentrations underwent comparison with background levels (Clark[e] numbers) and maximum permissible concentrations (MPC), with sub-sequent calculation of the total toxicity index (KHL). The study showed that practically all the inves-tigated rivers were subject to contamination, with uranium and lithium as the greatest contributors to surface water toxicity. The rivers in Southern and South-Eastern Kazakhstan – namely, the Kara-Balta, Syr Darya and Shu – were found to be most exposed. For instance, the KHL of the Kara-Balta River water exceeded the permissible threshold by over 5.9 times. The lowest KHL value (<1) corre-sponded to the Yertys and Ile Rivers. The research made it possible to identify the toxicity of trans-boundary waters flowing into the territory of Kazakhstan, as well as provided basis for further in-vestigation to identify pollution sources.

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