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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 primary scientific goal of studying salt lakes is to better understand the formation of small continental-type hydrogeochemical systems. Many scientists have attributed the metamorphism of the chemical composition of salt lakes to the evaporative concentration of water. However, the formation of soda water is inconsistent with this hypothesis. Thus, analyzing intrabasinal biochemical processes and water—rocks interactions during the evaporative concentration of water allows us to understand the major mechanisms of the formation and evolution of water compositions. Therefore, the aim of this paper is to identify the key processes involved in the formation of the chemical composition of the water in Lake Doroninskoye. An analysis of the distribution of major components shows that Na+, HCO3 −, CO3 2−, and Cl− are dominant in this water. High concentrations of these elements are the result of evaporative water concentration. Calcium, magnesium, and potassium are not accumulated because the water is saturated in minerals containing these elements. The main barrier to the growth of the sulfate content of water is sulfate reduction. This process also contributes to the additional reproduction of carbon dioxide, which reacts with the products of the hydrolysis of aluminosilicates OH− to form HCO3 − and CO3 2−, thus further contributing to the natural processes of soda formation.

Актуальность исследования обусловлена недостаточной изученностью поведения металлов в соленых озерах Восточного Забайкалья в присутствии гуминовых веществ как фактора снижения их токсичности и биодоступности. Цель: выявление основных форм миграции некоторых металлов в присутствии фульвовых и гуминовых кислот в водах содовых озер Восточного Забайкалья с помощью термодинамических расчетов. Объекты: гуминовые вещества, ионы металлов, содовые озера Восточного Забайкалья. Методы: химический анализ водных проб содовых озер - титриметрия, потенциометрия, фотоколориметрия, атомно-абсорбционная спектрометрия, пламенно-эмиссионная спектрометрия, масс-спектрометрия с индуктивно связанной плазмой; термодинамическое моделирование с применением программного комплекса MINTEQ (Generic NICA-Donnan). Результаты. Представлены результаты химических исследований состава природных вод содовых озер Восточного Забайкалья, проведены термодинамические расчеты с применением программного комплекса MINTEQ с учетом присутствия гуминовых и фульвокислот в озерных водах и установлены основные формы нахождения некоторых металлов. Показаны зависимости образования органически и неорганически связанных комплексов металлов от минерализации и pH, селективность сорбции металлов гуминовыми веществами и образование аутигенных минералов в содовых озерах, приведены индексы насыщения озерных вод по отношению к этим минералам. Установлено, что основным барьером на пути концентрирования в рассматриваемых озерах для основных катионов выступают минералы, с которыми вода находится в равновесии и даже пересыщена ими. Количество связанных в комплексы металлов и их положения в рядах определяются содержанием органического вещества, свойствами этих металлов, а также основными геохимическими параметрами среды. Relevance of the research is caused by insufficient information on metals behavior in the presence of humic substances as a factor reducing their toxicity and bioavailability in saline lakes of Eastern Transbaikalia. The aim of the research is to identify basic forms of some metals migration in the presence of fulvic and humic acids in soda lakes of Eastern Transbaikalia using thermodynamic calculations. Objects: humic substances, metal ions, soda lakes of Eastern Transbaikalia. Methods: chemical analysis of soda lakes water samples: titrimetry, potentiometry, photocolorimetry, atomic absorption spectrometry, flame emission spectrometry, inductively coupled plasma mass spectrometry; thermodynamic modeling using the MINTEQ software package (Generic NICA-Donnan). Results. The paper introduces the chemical research results of natural water samples composition taken from soda lakes of Eastern Transbaikalia. The authors have carried out thermodynamic calculations using the MINTEQ software package, taking into account the presence of humic and fulvic acids in lake waters and determined the basic forms of some metals available. The paper demonstrates the dependence of organically and inorganically bound metal complexes formation on mineralization and pH, metals sorption selectivity by humic substances and authigenic minerals formation in soda lakes and introduces the lake waters saturation indices with respect to these minerals. It was determined that those minerals, which lake water is in equilibrium with and even supersaturated by them, are the main barrier against basic cations concentration in lakes considered. The number of metals bound to complexes and their positions in the rows are determined by the level of organic matter, their properties, together with basic geochemical parameters of the environment.

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.

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.

Geochemical specifics of lake water of the Kulunda Steppe territory (Altay region, Russia) are studied. The results show that in the territory mainly chloride and less soda lakes with sodium compound are developed. It is presented that calcite and soda saturation indexes (SI) of lake water increase with growth of pH, but decrease in such minerals as gypsum and barite. The opposite situation is typical for SI depending on the salinity. It is revealed that evaporation, secondary mineral formation and various biological processes have the greatest impact on accumulation of elements in solution.

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.

We studied the distribution of anoxygenic phototrophs in 23 steppe lakes in the Transbaikal region (Russia), in Uzbekistan (Central Asia) and in the Crimean peninsula (Ukraine). The lakes varied in their mineral content and composition (salinities from 0.2 to 300 g L−1). The Transbaikal lakes were alkaline (pH>9), with high amounts of soda. The Uzbek and Crimean lakes were more pH neutral, frequently with high amounts of sulfates. The presence of anoxygenic phototrophs was registered by infrared epifluorescence microscopy, infrared fluorometry and pigment analyses. In mostly shallow, fully oxic lakes, the anoxygenic phototrophs represented 7–65% of the total prokaryotes, with the maxima observed in Transbaikal soda lakes Gorbunka (32%), Khilganta (65%), Zanday (58%) and Zun-Kholvo (46%). Some of the lakes contained over 1 μg bacteriochlorophyll L−1. In contrast, only small amounts of anoxygenic phototrophs were present in highly mineralized lakes (>100 g total salts L−1); Borzinskoe, Tsagan-Nur (Transbaikal), Staroe (Crimea) and in the residual part of the south-west Aral Sea (Uzbekistan). The oxic environment and the specific diurnal changes of bacteriochlorophyll concentration observed suggest that the phototrophic community was mostly composed of aerobic anoxygenic phototrophs. The high abundances and bacteriochlorophyll concentrations point to an important role of aerobic anoxygenic phototrophs in the habitats studied.

1 Introduction Meromictic soda Lake Doroninskoe localized in the intermountain area,filled with continental sediments of the Mesozoic,in the upper basin of the river Amur.Coordinates of its location are N51°14’42'E112°14’40'

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