# Long-Term Dynamics of Persistent Organic Pollutants in Water Bodies of the Aral Sea-Syrdarya Basin
### Water Pollution Problems  
According to the available data from Glazovsky N.F. [10], more than 1,015,000 tons of pesticides were used in the Aral Sea region only from 1975 to 1990, with an annual average of 20-25 kg/ha. Back in the 1970s, 44,000 tons of organochlorine, 6000 tons of organophosphorus, and 15.4 tons of other pesticides were used in the Uzbek Socialist Soviet Republic. During 1970-1990, 118,000 tons of pesticides and herbicides were poured only on the land of Karakalpakstan (an autonomous territory within Uzbekistan), which amounted to 10 kg per capita per year. In Kazakhstan, 8500 and 7500 tones were used in 1990 and 1991, while in 1992 and 1993, 6300 and 5600 tones were used, respectively [39]. About 3% of the applied defoliants' mass, 2-3% of pesticides, and 10-15% of biogenic substances are removed with the drainage water in Central Asia [40]. These data show the enormous number of toxicants carried out annually from irrigated territories into the river network. Persistent organic pollutants (POPs) are industrially produced chemicals. They are also formed as by-products of anthropogenic activities. POPs are highly toxic to living organisms at extremely low concentration levels. These xenobiotics are characterized by their high resistance to physical, chemical, and biological factors; global prevalence by air, water, and migrating species; high ability for accumulation in living organisms; and active migration along trophic chains. At present, UNEP (United Nations Environmental Program) has included PCBs in the group of compounds that should be given priority attention in environmental research [41]. Polychlorinated biphenyls (PCBs) are biologically one of the most dangerous organochlorines. Polychlorinated biphenyls or polychlorinated biphenyls are a group of organic compounds comprising chlorinated biphenyl (diphenyl) derivatives, whose molecule is composed of two benzene rings containing from 1 to 10 chlorine atoms connected to any carbon atom, and corresponding to the general formula C\({}_{12}\)H\({}_{n}\)Cl\({}_{n}\), where n = 1 \(\div\) 10 [42,43]. According to IUPAC (International Union of Pure and Applied Chemistry) recommendations, all of them are assigned numbers from 1 to 209 in ascending order of the degree of chlorination. PCBs are stable against external influences, have a high decomposition temperature and low reactivity and, as a consequence, are difficult to metabolize in natural environments. Hydrolysis and oxidation processes in water do not destroy PCBs. The only real chemical process that destroys PCBs is photolysis. The half-life of mono-, di-, tri-, and tetrachlorinated biphenyls in surface water (depth less than 0.6 m) in summer in bright sunlight is 17 to 210 days [44], but more highly chlorinated PCBs poorly absorb sunlight. The biodegradation of PCBs in water by aerobic and anaerobic microorganisms is very slow, starting with tetra- and pentachlorosubstituted biphenyls, they are almost not biodegradable. The solubility of PCBs in water depends on the number of chlorine atoms in the molecule and varies from 0.00076 mg/L (for dichlorobiphenyl) to 4 mg/L (for monochlorobiphenyl) [45]. Industrial mixtures of PCBs present in water therefore have a variable composition both in terms of the origin and as a result of changes that occur in the environment. According to various literature sources, values of background concentrations of PCBs vary from 0.5 ng/L to 20 ng/L [45,46]. For the analysis of such complex multicomponent mixtures as PCBs, a determination method is used based on several (from four to nine) individual PCBs. Their content is used to identify an industrial mixture of PCBs or the composition of these mixtures and to calculate the total PCB content. The determination method of the sum of PCBs by conversion of all individual PCBs with different content of chlorine atoms toochlorobiphenyl is also known [46].