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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.

We have developed a new concept on the formation of diverse geochemical types of salt lakes, in particular for those in the Transbaikal region. This concept explains the nature of their different chemical composition under the close landscape-climatic and geological-geochemical conditions. The extensive hydrogeochemical field data and thermodynamic calculations demonstrate that in addition to evaporation, interactions with rocks play a substantial role in the formation of the composition of these salt lakes. Such processes are most widespread in soda lakes, which is confirmed by the highest pH values (9.0–10.7). These lakes provide evidence of the interaction of the lake waters with aluminosilicates; their hydrolysis provides a pH increase and consequently a high concentration of carbonate ions. These high pH and HCO3− and CO32− contents in turn lead to the maximum precipitation of Ca, Mg and Fe carbonates. Soda lakes also contain high chloride ion concentrations, since a small portion of those ions evaporate under the lake conditions. If evaporation continues and the amount of carbonate ions in the lake does not increase, or increases slowly due to binding with carbonate minerals, the chlorine content becomes higher, the pH lowers to less than 9.0, and the lakes become chloride. Another fundamental process in these lake environments is sulphate reduction, which results in the production of hydrogen sulphide, fundamentally changing the geochemical environment from oxidizing to reducing. The presence of an oxidizing environment and sulphides in rocks gives additional sources for sulphates, which ensures the formation of sulphate-type lakes.

The reduced sulfur species, sulfide, elemental and thiosulfate were considered in the thermal waters of Jiangxi Province for the first time. It is shown that the sulfur speciation content significantly varies and depends on the pH values. The major part of reduced sulfur refers to sulfide species in the nitric thermal waters, to elemental—in the carbon dioxide thermal waters. The presence of both reduced and oxidized sulfur speciation indicates the possibility of sulfide minerals hydrolysis and disproportionation of the product of this reaction (SO2) with the participation of hot water with the formation of elemental and sulfate sulfur. The isotopic composition of dissolved sulfate and sulfide sulfur speciation has shown that the process of bacterial reduction proceeds in the thermal waters, accompanied by accumulation of relatively heavy sulfur isotope in sulfates. Simultaneously with reduction, the oxidation of both sulfide minerals and newly formed hydrosulfide proceeds with formation of elemental, thiosulfates and also sulfates in the discharge zone was proceeded. It is shown that the process of sulfide oxidation mostly occurs in carbon dioxide thermal waters. Therefore, the elemental sulfur is predominant in carbon dioxide waters. The oxidation process is less significant in the nitric thermal waters, whereby the concentrations of sulfide ion are higher than sulfates. The ambiguous effect of sulfate reduction on the hydrogeochemical environment of the thermal waters is confirmed by the differing value of the carbon isotope ratio of HCO3− in the considered waters. The obtained isotopic composition data 34δS(SO42−) indicate host rocks as a source of sulfates in the thermal waters of Jiangxi Province.

The aim of the present research is to identify the main mechanisms of sulfur behavior in saline lakes in the course of time and followed transformations in their chemical composition. The influence of water on chemical composition of biochemical processes involved in decomposition of organic matter was determined by the study of behavior of reduced forms of sulfur in lakes. The determination of reduced forms of sulfur was carried out by successive transfer of each form of sulfur to hydrogen sulfide followed by photometric measurements. The other chemical components were determined by standard methods (atomic absorption, potentiometric method, titration method and others). The salt lakes of the Altai steppe were studied in summer season 2013–2015. Analysis of the chemical composition of the saline lakes of Altai Krai has shown that carbonate-, hydrocarbonate- and chloride ions dominate among anions; sodium is main cation; sulfates are found in subordinate amounts. Reduced forms of sulfur occur everywhere: hydrogen and hydrosulfide sulfur S2- prevail in the bottom sediments; its derivative—elemental S0 —prevails in the lakes water. The second important species in water of soda lakes is hydrosulfide sulfur S2-, and in chloride lakes is thiosulfate sulfur S2 O32-. The lag in the accumulation of sulfates in soda lakes in comparison to chloride lakes can be explained by their bacterial reduction, followed by the formation and deposition of iron sulfides in sediments. In chloride lakes gypsum is a predominantly barrier for sulfates.

The article presents the comparative characteristic of lake water and pore water of bottom sediments of saline lakes located in the South of Western Siberia. Studied lakes are mostly Cl-Na composition and rarely Cl-HCO3-Na. The values of pH vary from 7.6 to 9.9, TDS from 5 to 300 g/L. It is shown that redox conditions change from oxic in the upper water layer to anoxic in pore water and the bottom layer. As a result, there is dissimilarity in interaction between water layer, bottom sediment and secondary minerals caused by differences in biological and geochemical processes in these two environments. According to thermodynamic calculations, lake water is in equilibrium with calcite, quartz, kaolinite, illite, Mg-montmorillonite, barely with respect to gypsum, thenardite and halite. Simultaneous formation of evaporite minerals and bacterial mats, CH4 and H2S accumulation lead to microbial sediment development.

Data are presented on detailed sampling of the surface and pore waters of soda Lake Doroninskoye (Eastern Transbaikalia, Russia). Particular attention has been paid to the possible REE precipitation species from both water types. Thermodynamic modeling at 25 °C and 1 bar total pressure was performed with the “HCh” code using a free-energy minimization algorithm. We modeled the heterophase 43-component system; both individual minerals and ideal solid solutions of the REE and numerous dissolved species were incorporated into the model. According to the calculations, redox conditions in the lake waters are established by the sulfur species. The main result is that the surface water is slightly supersaturated with respect to CaCO 3 -REE (65%) and F-apatite-REE (35%) solid solutions, as well as bastnasite Ce 0,5 La 0,25 Nd 0,2 Pr 0,05 CO 3 F. A trend of the HREE predominance in the solution during the process of early diagenesis has been confirmed. Nevertheless, the thermodynamic model does not entirely explain the REE fractionation in pore waters (pore solutions are enriched in the LREEs in situ but are drastically depleted according to the thermodynamic model). The results of this study suggest that further considerations about solid solutions should be taken into account to model and understand exact REE fractionation during water-rock interaction in salt lakes.

This paper presents our analysis of the distribution of REE in mineral chloride and soda lakes of Southeastern Transbaikalia. With increasing water salinity, it was determined that concentrations of dissolved HREE increased. The prevalence in absolute concentrations of MREE and HREE was observed in lakes with the development of bacterial processes (sulfate-reduction). A geochemical barrier for accumulating the LREE in the waters can be fluoro-carbonates. The saturation degree of fluorcarbonates in soda lakes can be several orders higher than in chloride lakes. Under oxidizing conditions Ce(OH)4 can be formed and, in a reducing environment – Ce(OH)3.

The chemical composition of the supergene zone groundwater in the Eastern Transbaikalia is considered. The main geochemical processes for steppe landscapes are identified and determined the causes of some component’s concentrations in groundwater. To understand the formation conditions of NaHCO3 water, were calculated equilibria of this water with the main minerals of the region’s rocks.

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.

Chemical and isotopic groundwater composition analyses in the south-eastern Transbaikal case study region allowed three geochemical types of waters and fifteen subtypes to be identified: bisiallite (Ca–HCO3), soda (Na–HCO3) and salt (Na–HCO3–Cl–SO4). All three types differed significantly with respect to TDS content, pH, composition of chemical elements and heavy water isotopes. The transition from the first to the subsequent types was accompanied by CO32−, HCO3−, SO42−, Cl− and Na+ accumulation, whereas a weaker dependence was exhibited for Mg2+ and K+, and no relationship with Ca2+ was found. Water was found to be gradually enriched in heavy isotopes and some trace elements (e.g. F, Mo, Br, B and Li). Findings showed that the groundwater–rock system is one of an equilibrium-disequilibrium. From the extent of the deeper horizons to the surface water, groundwater in the study region continuously dissolves minerals that are in disequilibrium with the groundwater and simultaneously forms new minerals. Moreover, over time, not only does the composition of the solution change, but the type of secondary minerals and the ratio between chemical elements also changes because some elements precipitate out of solution, while others continue to accumulate. The stage of soda-type groundwater formation was found to be at an intermediate level between the formation of fresh bisiallite (Ca–HCO3) water of different cationic composition and salt (Na–HCO3–Cl–SO4) water with mixed anionic composition.

The article presents the studies on understanding the chemical evolution of Maloye Yarovoe lake that is located in the semi-arid landscape of and represent the most widespread chemical type of saline lake (Cl-Na) on the territory of Kulunda steppe (south of Western Siberia). The lake has gone through several changes in its state which are well represented in its sediments: section 1 (4.52&ndash, 4.97 m) represents the subbase, section 2 (3.0&ndash, 4.57 m) corresponds to a &ldquo, shallow lake&rdquo, stage and contains two sublayers, section 3 (0&ndash, 2.25 m) illustrates an increase in erosion of the shoreline and corresponds to a &ldquo, deep lake&rdquo, stage. The extrapolation of radiocarbon dates suggests that the most significant changes in the state of the lake occurred at about 6.5&ndash, 7.1 ka BP (transition from a small lake to a large one with intensified coastal erosion) and at about 5.69 and 3.5 ka BP (increase in the organic component). The results of sequential extraction of elements correspond to chemical changes occurred in a certain time of the lake state by changing in their distribution level and fraction type along the core. The results of lake water evaporation and organic matter degradation modelling performed in Phreeqc for oxic and anoxic condition showed the following stages of chemical divides could be expected in case of further lakes desiccation: Halite&mdash, Epsomite&mdash, Bischofite/Carnalite with possibility of Nahcolite precipitation in carbonate enriched periods (summer and autumn periods of organic matter degradation). Water&ndash, rock interaction processes in combination with biogeochemical reactions affect drastically not only the migration features of elements in water and sediments, but also the type of mineral crystallization in lake sediments.

We have developed a new concept for the formation of geochemical types of salt lakes which explains the nature of their different chemical composition under close landscape-climatic and geological-geochemical conditions. The extensive field hydrogeochemical material and thermodynamic calculations demonstrate that in addition to evaporation, the interaction with rocks plays a great role in the formation of the composition of lakes. This interaction begins in the catchment area of a particular lake and continues directly in the lake. Such processes are most widespread in soda lakes, which is confirmed by the presence of the highest pH values (9.0-10.7). Besides, one of the most important processes is sulfate reduction, which results in hydrogen sulphide in lakes fundamentally changes the geochemical environment from oxidizing to reducing one and leads to a change in the oxidation state of many elements, changing their migration ability and, therefore, the ability for concentration or binding by secondary minerals. The presence of an oxidizing environment and sulphides in rocks gives additional sources for sulfates, which ensures the formation of sulfate-type lakes.