Heavy Metal Migration and Sedimentation in Syrdarya River (Tajikistan) and Possibility Accumulation in Kayrakkum Reservoir

This work is devoted to monitoring the migration of heavy metals along the Syrdarya River bed in Tajikistan and determining the rate of their sedimentation. The concentrations of heavy elements As, Cd, Cu, Ni, Pb and Zn in water samples collected at points along the river were used to determine the concentrations of elements, their migration and sedimentation rate depending on the acid-base conditions of the aquatic environment. Water samples were collected at five points along the river bed, starting from the section where the river crosses the border of Kyrgyzstan and Tajikistan (“Akjar”). Atomic absorption spectrometer “A Analyst 800” was used to analyze heavy metals. It was found that the main form of formation of complexes of the studied heavy metals in the aquatic environment is their hydroxides and the sedimentation rate increases with an increase in the alkalinity of the environment. In addition to anthropogenic factors, natural factors such as water erosion of rocks play a special role in the formation and change of the chemical composition of water bodies. The mobility of heavy metals in the aquatic environment and their uptake by aquatic components of the biosphere largely depend on their state. The state of heavy metals is determined by the types of complexes of heavy metals with ligands and is reflected in the degree of their binding to active centers. Movement of chemical elements by river runoff along the water-soil-plant chain depends on the biological properties of the flora of a given territory and individual elements and is characterized by the transfer coefficient. In turn, the transfer coefficient depends on the soil condition and agrochemistry, pH, clay minerals, chemical composition of irrigation water Ca2+, K+, amount of organic matter in the soil, plant species and other environmental conditions. Taking into account that water resources of most rivers in Central Asia are widely used for irrigation of agricultural lands planned and regular monitoring of hydrochemical parameters of rivers is of special importance. The results of monitoring the distribution of heavy metals (Cd, Zn, As, Pb, Cu, Cr) in the Syrdarya River are presented. It is noted that the regularity of changes in the concentration of elements from the upper to the lower reaches of the Syrdarya River on the territory of Tajikistan is not traceable. It is assumed that the occurrence of such a situation is largely due to the formation of complex compounds of metals with inorganic and organic substances.