Abstract. This study deals with concentrations of heavy metals and the main uranium series radionuclide in the Syr Darya surface waters (Kazakhstan). Large proportion of studied heavy metals were transported with suspended matter (>0.45 μm), while large proportion of U (98-100 %) and Mo (81-100%) were in dissolved forms and colloidal fraction (<0.45 μm).
New hydrochemical measurements from the Syr Darya provide insights into factors affecting the composition and quality of one of the two major freshwater sources replenishing the Aral Sea. This river is heavily used for power and irrigation and crosses territories of four central Asia republics. It is intensely managed, draining several major tributaries, many reservoirs, and numerous irrigation distribution systems and canals. Analysis of seasonal changes in dissolved ion concentrations using geochemical diagrams, elemental ratios, statistical correlation, and equilibrium modeling allowed the characterization of mineral formation processes that control the dissolved chemical composition. Measured water hydrochemistry and composition type differs substantially from previous reports from the upper reaches of the Syr Darya in Kyrgyzstan. Element ratios, statistical correlation, and the presence of NO3- and NO2- suggest that the downstream trend of increasing total dissolved solids (TDS) from Zhetysay city to the Aral Sea in June is controlled by evaporation–crystallization processes, which contribute to the river dissolved load through soil runoff and return irrigation waters following leaching of secondary salts. Downstream sample composition during the growing season shows significant changes in magnesium-to-calcium ratios in the river water. Elevated magnesium levels in Syr Darya waters may pose a problem to sustainable uses for irrigation.
The use of the method of nonequilibrium uranium in various branches of science led to the necessity to determine its isotopic composition. The content of uranium isotopes in natural waters is often extremely low, therefore, it is necessary to collect significant volumes of samples in order to obtain reliable results of analysis. In this paper, it is proposed to concentrate uranium isotopes from water in-situ. Two alternative methods of uranium coprecipitation in field conditions (on activated carbon and iron (III) hydroxide) are considered. The desorbed uranium isotopes are determined by an alpha-spectrometric method with preliminary radiochemical preparation in laboratory conditions, including extraction with tributyl phosphate and electrodeposition on a steel disc. It was found that when concentrating on activated carbon the chemical yield was from 2 to 32%, and when concentrating on iron (III) hydroxide it is from 15 to 62%. For the second case, the chemical yield is acceptable for radiochemical work, and the proposed method for concentrating of uranium isotopes is recommended for usage in field conditions. Approbation of the method was carried out within the framework of the PEER454 project in 2017 in the valley of the river Ziddy, Pamir-Alay, Republic of Tajikistan. Work in the field conditions showed high efficiency of the method and its full feasibility even in the absence of acceptable laboratory conditions.
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