# Long-Term Dynamics of Persistent Organic Pollutants in Water Bodies of the Aral Sea-Syrdarya Basin
## 3 Results and Discussion
### Sydarya River Basin
It is also known that in the basin of the Syrdarya River in the territories of the Central Asian states, several cities, industrial enterprises, and energy facilities serve as pollution sources. Further, continuing the discussion of the organochlorine pesticides (OCPs) dynamics in the Syrdarya River water, we note first of all the presence of DDT and HCCH in all water samples taken in 2022. The analysis of other toxicants of the OCP group showed the presence of heptachlor and aldrin in water samples (Table 1). The contents of registered pesticides in the water of all the water bodies we studied were generally low: DDT metabolites varied from 0.001 to 0.015 and 0.016 \(\upmu\)g/L, with maximum concentrations found in the water of the Syrdarya River mouth. Increased concentrations of HCCH up to 0.002-0.003 \(\upmu\)g/L were observed in the transboundary river flow near the cities of Kyzylorda and Kazakh. The content of aldrin varied within narrow limits of 0.001-0.002 \(\upmu\)g/L; low concentrations of DDT and HCCH were also found in the waters of different small lakes within the catchment. The comparison of the data in Table 1 with the long-term data given above clearly shows a decrease in pesticide concentrations in the water courses and water bodies of this basin by tens and hundreds of times. Apparently, this was primarily the result of the reduction in the volume of pesticide application on agricultural fields of the river basin in the Central Asian countries, including Kazakhstan. However, the fact that pesticides such  
Figure 6: Pesticide content in the water of the Syrdarya River mouth. as DDT and HCCH, the use of which have been prohibited all over the world for a long time, are still used in the agricultural fields of the ASSB was also evident. According to the results of the chromatographic analysis, PCBs were registered in all water samples taken in September 2021 and May 2022 along the Syrdarya River. The contents of the toxicants in the river water in 2021 varied from 0.130 to 0.479 \(\upmu\)g/L, and in 2022, from 0.355 to 1.080 \(\upmu\)g/L (Figure 7). In May 2022, compared with the data from 2021, the concentration of PCBs in the Syrdarya River water was significantly higher, especially in the lower reaches of the river and in the estuary zone. The results of the analysis show that the pollution of the Syrdarya River due to PCBs occurred in the territory of Kazakhstan. Figure 7 clearly shows that the maximum concentrations of the toxicants were registered in the water of the border zone and Shardara Reservoir due to PCB contamination of the transboundary inflow. But along the course of the river in the territory of Kazakhstan, their concentrations in the river water increased near the cities of Kyzylorda and Kazaly. No significant decrease in the level of water pollution in 2021 as far as the mouth of the river was determined. The congener composition of PCBs in river waters was very broad. In 2021, 33 individual PCB isomers were recorded, belonging to homologous groups from tetrachloropiphenyls (PCBs 40 and 49) to heptachloropiphenyls (PCB 171). In 2022, 20 congeners were found that were within these homologous groups. Among the congeners found were strictly controlled \"marker\" congeners, namely, PCBs 52, 101, 138, and 153, and highly toxic dioxin-like congeners, namely, PCBs 105, 114, and 118, belonging to the pentachloropiphenyl homology group. Eleven congeners were registered in the waters of the Shardara and Koksarai reservoirs.