# Transmission of Heavy Metals in River Water and Self-Purification Capacity of Ile River
## 1 Introduction  
Many of the major problems facing humanity in the XXI century are related to water quality issues [1]. These problems will be aggravated in the future by climate change due to rising water temperatures, melting glaciers, etc., and will further cause an inverse relationship with the deterioration of the water environment worldwide [2, 3]. Thus, the most acute problem of the Ile River basin has become water quality change and, as a consequence, the state of the aquatic ecosystem under the influence of economic activities and transboundary influences. Transboundary problems for Kazakhstan become acute not only in the field of preserving the optimal volume of river water inflow into the water basins of the country but also in the fact that transboundary rivers carry the inflow of various toxic compounds since the territory of Kazakhstan occupies the lower reaches of all transboundary watercourses. A large volume of river flow comes from China. In China, industrialization, urban development, and intensive agriculture are the main factors of water quality degradation [4]. Environmental security is an important part of Kazakhstan's security. In this regard, particular concern is caused by the continuing anthropogenic pollution of the main water basins of Kazakhstan, both by transboundary runoff and as a result of industrial, agricultural, and domestic wastewater discharged into them. In this regard, there is no exception to the Chinese-Kazakh transboundary Ile River, which is the main water artery of the Lake Balkash basin. According to the World Health Organization (WHO) in 2019, one third of the world's population does not have access to safe drinking water [5]. That is why understanding water chemistry and river quality is important for both society and ecosystems [6]. The retreat of glaciers due to global warming has had a marked impact on the water chemistry and heavy metal pollution levels in rivers [7]. Pollution by various toxic and biogenic compounds of transboundary waters flowing from the territory of China along the Ile River has been considered in a number of scientific publications for many years [4; 5; 6; 7; 8; 9; 10; 11; 12]. The primary mechanism for the development of river water chemistry is based on natural processes including the weathering of rocks, soil erosion, and the input of dissolved substances from groundwater, which are also characterized by the geological background and regional water cycle [13]. In particular, soluble substances from soil have been shown to be critical factors dominating stream water chemistry, especially in small watersheds [14; 15; 16]. In addition, detailed information on hydrochemical processes would be useful in order to study and understand the formation, migration, transformation, and enrichment of heavy metals in river water. Among pollutants, heavy metals pose a significant threat to aquatic biota because in contrast to organic pollutants, metals do not suffer degradation but can only be redistributed among the components of the aquatic ecosystem. Studies [17; 18] have shown that heavy metal concentrations in rivers in the Tibetan Plateau are not only affected by rock weathering and water-rock interaction but also by artificial activities such as inputs of geothermal resources [19], domestic wastewater discharge [13], vehicle discharges [20], and mining activities [21; 22]. There are a growing number of studies showing that natural geologic processes must be considered as the sources of pollution in some water bodies [23; 24; 25]. Rock minerals (e.g., serpentine, spinel, pyroxene, and olivine minerals) contain large amounts of heavy metals [23]. Heavy metals in minerals are released on the exposure of rocks to water, which, in turn, causes the contamination of the aquatic environment [24; 25]. In this regard, it is crucial to identify various sources of heavy metal pollution and quantify their contribution to the pollution of the Ile River.