## 3 Results and Discussion
### Contamination by Polychlorinated Biphenyls (PCBs) in River Water  
The Ural River is also contaminated by persistent organic pollutants (POPs), namely polychlorinated biphenyls (PCBs). In accordance with the requirements of the Global Stockholm Convention on POPs, these substances are highly toxic for people and nature, and compounds needs to be studied in the natural environment, their use gradually reduced, and by 2028 they should be completely destroyed on Earth [33]. PCB concentrations between 0.93 and 1.29 \(\upmu\)g/L were mentioned in the water of the lower course of the Ural River in 2012, according to analyses done by the Institute of Geography in Almaty. Still in 2005 the corresponding PCP level of the water of this section of the river was much lower. As can be seen from Figure 6, above Atyrau (Bugorki village), the PCB concentrations in water was 0.93 \(\upmu\)g/L. And at the beginning of the Ural-Caspian Canal, i.e., at the main-stream station along the main riverbed, it increased to 1.29 \(\upmu\)g/L. A similar pattern in the distribution of PCBs along the river was registered in 2005. This increase in the amount of toxicants downstream is obviously due to the influence of waste in the form of sewage and atmospheric emissions from numerous industrial enterprises located in Atyrau and a number of large settlements along the river banks towards the Caspian Sea. Relatively less contaminated water was founded in the less populated Right Yaitskiy Delta arm, which passes through a small part of the river. Removal of PCBs by river runoff led to their accumulation in the water of the pre-estuary Caspian Sea area up to 1.0 \(\upmu\)g/L [34; 35]. According to data [36; 37], the standard of PCBs for potable water is 1.0 \(\upmu\)g/L. The same level of their concentrations (0.93-1.29 \(\upmu\)g/L) is registered in the water of the Ural River, but it does not correspond to the standards for fisheries management water bodies [38]. Our results indicate that 100% of this toxicant is present in the tissues of all fish taken for analysis. These data are an indicator that the river's water ecosystem is contaminated with a highly toxic PCB compound. In the muscles of a representative of peaceful fish--bentophage of _Abramis brama_, the level of accumulation of PCBs was up to 316 \(\upmu\)g/kg, a spike perch and an asp--from 102 to 140 \(\upmu\)g/kg. The main reason for the contamination of the Ural River with PCBs is the waste input into river systems from numerous industrial enterprises, located mainly in the territory of the Russian Federation. There are huge contamination sources of the natural environment. According to research and inventory data [39; 40] in the South Ural region, which tends to the river basin, there are 74.5 thousand large PCB-containing equipment (transformers and capacitors). The total number of PCBs in the Orenburg, Chelyabinsk, Sverdlovsk regions and Bashkortostan is 3354 tons. Due to these huge by PCB contaminated areas the level of load of PCB-containing effluents and emissions to the atmosphere in the upper and middle parts of the Ural River basin becomes clear. However, the possibility of river contamination within the cities of Uralsk, Atyrau, as well as Aktobe and Alga is not excluded. According to the materials of the state monitoring, in 2016-2018, the Ural River water at the territory of Kazakhstan was characterized as <<moderately contaminated>> (Combined Water Pollution Index (CIWP) 1.1-3.0) through the presence of heavy metals, nitrogen compounds, etc. Consequently, the level of river contamination does not decrease [41]. The Elek River is a tributary of the Ural River on the territory of Kazakhstan, which contributes significantly to its contamination. The water of the Elek River is contaminated with sewage and various wastes from ore processing enterprises in the cities of Aktobe and Khromtau. It is one of the most contaminated water courses.