# Long-Term Dynamics of Persistent Organic Pollutants in Water Bodies of the Aral Sea-Syrdarya Basin
## 3 Results and Discussion
### Sydarya River Basin
The first one was in September-November, which was caused by the intensive inflow of CDW to the river network from irrigated areas. The second one was in early spring (March, April, rarely a slight rise in May), which occurred as a result of pesticide washout from the surface of the catchment area and irrigated lands during winter thaws and spring snowmelt. Autumn witnessed a rise in pesticide concentrations in the river mouth water 1 and 2 months later than in the upstream sections, which depended on the time of arrival of more contaminated water, the filling of the Shardara Reservoir, and the water discharge in the river [62]. In the spring and fall of 2004 and 2005, only organochlorine forms were found in the transboundary flow of the Syrdarya River out of the 17 pesticides of various classes that we determined. Out of them, metabolites of DDT were not detected, but isomers of HCCH were constantly present, in particular, their increased concentrations were registered in the spring of 2005. The received data also followed the increased level of HCCH concentration, as well as the occurrence of DDT in rather high concentrations (up to 1.36-1.40 \(\upmu\)g/L) in the water of the central and near-drainage parts of the Shardara Reservoir. Such phenomena, registered both in 2004 and in 2005, were obviously caused by the accumulation of these toxicants in the reservoir in autumn-winter and early spring periods during its filling due to releases from upstream reservoirs and CDW coming to the river network from irrigated areas. During these periods, a large number of suspended solids also accumulated in the reservoir, which was brought by the river from the adjacent territories during high winter and early spring water discharges. It is known that toxic compounds, such as metals and pesticides, are transported in considerably larger amounts with suspended and carried river loads in the adsorbed state [2,63]. The accumulation of these persistent organochlorine pesticides, both in the water column and in the bottom sediments of the reservoir, as a result of these processes is a constant source of pollution to the ecosystem of the reservoir and the river. Despite the fact that various pollutants accumulate in the reservoir itself, the downstream Syrdarya River is also contaminated by these toxic compounds under the influence of sources in the Kazakh part of the river, such as pesticides and heavy metal ions. The obtained results for the mouth of the Syrdarya into the Small Aral Sea (Figure 6) indicate that HCCH isomers were present in the water of this section almost continuously in these years, except for 2000 and 2003. DDT metabolites were recorded inconstantly, and they were not detected in 2002-2005. The highest concentrations of pesticides in the river water of this section were found for HCCH isomers before 2001, and for DDT metabolites in 1994, 1998, and 2001, which was associated with their continued use in the Syrdarya River basin. Thus, besides the transboundary inflow of pesticides, there were local sources of pesticide pollution of the river in Kazakhstan, as evidenced by the results of our studies along the entire river flow within the Kazakh Republic, long-term observations in the estuary zone, and available information from the literature. The last 15 years have been characterized by the absence of reliable data on pesticide pollution of water bodies in this basin. Our studies conducted in 2021 and 2022 allowed us to obtain materials on the current level of POP contamination of the main current watercourses and water bodies of the ASSB within the territory of Kazakhstan (Figure 4). Taking into account the significant reduction in pesticide load on irrigated lands of Central Asian republics in recent decades because of a markable decreasing use of pesticides in agriculture, special attention in the given years was paid to the study of industrial pesticides, namely, PCBs, which are the most toxic compounds from the group of POPs.