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The isotopic composition of the natural waters of Eastern Transbaikal is studied. It is established that the water of the soda lakes is more enriched in oxygen-18 than in deuterium. It is shown that the enrichment of the salt-lake waters in heavy isotopes occurs not only due to the evaporation process, but also due to water–rock interaction. The participation of the aluminosilicate hydrolysis reactions in the fractionation of oxygen is confirmed by the direct dependence of the magnitude of the oxygen shift on pH, the value of which depends on the degree of such interaction.

Data on the physicochemical properties (pH, Eh, salinity) and content of U and other trace elements (As, Li, B, Br, I, Sr) are provided for the mineralized lakes of the Kulundinskaya steppe of Altai Krai. The sodic lakes of the region are characterized by higher U238 contents (up to 4 mg/l). The U contents in the lake waters are positively correlated with the sum of carbonate and hydrocarbonate ions and the pH of waters and are negatively correlated with salinity. These data confirm the presence of an economic U-bearing zone of the ground reservoir oxidation located along the eastern slope of the Platovskoe uplift in the northwestern direction along the boundary with Kazakhstan.

The concentration of elemental sulfur (S0) in the upper oxic layer of the meromictic (stratified by water mineralization) soda Lake Doroninskoe (about 5.5 m depth) varied in the survey periods from the detection limit (0.002 mg/l) to the registered maximum of 0.444 mg/l, with an average value of 0.12 mg/l. In the lower hydrosulfide layer, these concentrations amounted to 0.012–1.88 mg/l. The results obtained point to the processes of sulfide sulfur oxidation under reductive conditions, and of sulfate reduction to form hydrogen sulfide in the oxic medium. The dynamics of the seasonal S0-thiosulfate ratio testifies to the dual (oxidative and reductive) nature of the latter.

Simultaneous occurrence of oxygen and hydrogen sulfide was established by hydrochemical investigations in the waters of the Black and Baltic seas at a depth of more than 50 m (c-layer) [1‐3, 9, and others]. The most probable reason for the appearance of H 2 S and other sulfur species with intermediate oxidation states (thiosulfates and sulfites) is sulfate reduction in situ, i.e., immediately in the c-layer. A great number of publications have been devoted to the problem of the formation of hydrosulfuric, especially underground, mineral waters [6, 11, and others]. However, the presence of H 2 S in the oxic layer was reported only by few authors [8 and others], who believed that hydrogen sulfide can be supplied in part from bottom sediments. Data on other sulfur species (mainly, thiosulfates and sulfites) are rare [4]. The formation of suboxidized sulfur species (thiosulfates and sulfites) in waters during the oxidation of sulfide deposits has been studied in more detail [7]. The distribution of sulfur species in soda waters of Lake Doroninskoe has been studied since 2003. This work has been supplemented by determinations of oxygen since December 2005. The lake (4.5 km 2 in area and up to 6.5 m deep) is located at the bottom of the Mesozoic Chita‐Ingoda intermountain depression in the Ingoda River basin, one of the sources of the Amur River. The lake is underlain by Early Cretaceous sandy‐silty deposits intercalated with intermediate‐ basic effusive rocks. The bottom muddy sediments (up to 10 m thick) contain up to 0.4% iron sulfides [5]. The basin lacks drainage, and its water is characterized by high salinity due to evaporation. Water was sampled from top to bottom using a plexiglas bathometer or peristaltic pump at two stations located in the central and coastal parts of the lake. Dis