Nonequilibrium state of uranium isotopes in the ground waters of the Ob-Zaysan folded area

Introduction. The migration ability of uranium in solution is determined by the form of its location, which is controlled primarily by the geochemical situation, redox conditions of the environment and the presence of water-dissolved organic substances. In a closed system, when the secular equilibrium is reached, the ratio of uranium isotopes 234U/238U (γ) tends to unity. However, in some cases, this balance shifts in a positive direction. Purpose of the research. The purpose of this work is to determine the ratios of 234U/238U activities in groundwater of contrasting hydrogeological environments of the Ob-Zaisan folded area (OZSO), as well as to study the relationship between the concentration of uranium and the ratio of 234U/238U activities in groundwater. Materials and methods. The rapidly changing parameters of groundwater samples were measured at the sampling site, the radon content was determined at the Laboratory of Hydrogeology of Siberian sedimentary basins of INGG SB RAS, the total beta activity of waters and the activity of uranium were determined at the CCP IGM SB RAS. Results. The waters under consideration can be divided into two groups: groundwater and drainage waters of quarries. HCO3 Mg–Ca waters with a mineralization of 198–1568 mg/dm3 and a pH of 6.8–8.3 predominate (average 7.6). Eh from –331.6 to +304.1 mV, dissolved oxygen content 0.93–10.60 mg/dm3 (average 4.65 mg/dm3). Discussion. Studies have shown that the concentration of uranium in groundwater varies over a wide range and reaches 1.2 mg/dm3. The Lower Devonian-Carboniferous stratum (⩾10 mg/dm3) is characterized by the highest concentrations of uranium among sedimentary complexes, while Permo–Triassic granitoids of the Priobskoye and Barlak complexes (n•102 micrograms/dm3) are among the intrusive ones. The highest value of the ratio (γ = 18) was found in the waters of the Cambrian rocks. Elevated ratios are recorded in the waters of rocks of Devonian age. Waters with 234U/238U > 7 are most common in rocks of the Upper Devonian age. Elevated values exceeding γ = 8 were found in the field of distribution of carboniferous rocks. Resume. Thus, the high values of the γ ratio in the study area can be explained by the fractionation of these isotopes during the formation of soluble uranyl ion (UO2), and a more active transition to a solution of a light uranium isotope. Suggestions for practical applications and directions for future research. The subject of further research will be the implementation of studies that make it possible to assess the radiological hazard of groundwater for the purposes of drinking water supply to the population and use these materials to justify exploration sites for uranium ores in the region.