Water Elemental Composition and Toxicity in Kazakhstan’s Transboundary Rivers

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.