The purpose of this study is to identify the major formation processes of the ionic and isotopic composition of chloride brines in the sedimentary basins of the Siberian platform. The object and subject of research are deep-seated strong, very strong and extremely saturated brines as well as their strontium content and the ratio of Sr stable isotopes. The groundwater ionic-salt composition was determined by traditional methods (titrimetric, gravimetric, flame photometry), the ⁸⁷Sr/⁸⁶Sr isotopic ratio was measured using mass-spectrometers (Irkutsk, Russia and Canada). The brines lying at the depth of 1500–3000 m were tapped and classified into saline and subsalt hydrogeological formations. Their feature is high salinity (385–530 g/L) and high content of strontium (2.3–7 g/L). The ⁸⁷Sr/⁸⁶Sr isotopic ratio ranges from 0.708 to 0.713062. By isotopic composition most of the brine samples are close to the waters of the Vendian-Cambrian paleoocean. However, some brine samples from the subsalt part of the sedimentary section of the basin are significantly enriched in the isotope ⁸⁷Sr compared to the paleoocean waters and other samples. It could be due to the substantial input of ⁸⁷Sr into the brines during the long-term interaction of groundwater with the host Lower Cambrian sandstones.

The purpose of this study is to determine the main hydrochemical parameters of Lake Kotokel deep waters, to identify the role of groundwater feeding it, as well as to establish the features of spatial distribution of macro- and microelements in the lake. Field work was carried out during the ice and ice-free seasons. A special sampler was used to take water samples from the bottom of the lake. Water samples were filtered through the filters with a pore size of 0.45 μm at the sampling site. Plastic bottles were used for the water samples for analysis. Polypropylene containers (15 ml) pretreated with 0.1 N nitric acid were used for the water samples for trace elements. The analysis of the macrocomponent composition of water was carried out in a certified Laboratory of Hydrogeology and Geoecology of the Geological Institute of the Siberian Branch of the Russian Academy of Sciences (Ulan-Ude) according to the standard methods intended for fresh and saline waters. Cations (Ca²⁺, Mg²⁺, Na⁺, K⁺) were determined by atomic absorption, F^-, SiO₂ – by the colorimetric method, HCO_3^- , CO₃^2- and Cl^- – by the titrimetric method, SO₄^2- – by the turbidimetric method. The analysis of the trace element content was carried out in the Laboratory of Aquatic Microbiology at the Limnological Institute of the Siberian Branch of the Russian Academy of Sciences (Irkutsk) by the method of inductively coupled plasma on Agilent 7500ce quadrupole mass spectrometer. Conducted research made it possible to determine an inhomogeneous chemical composition of lake water associated with the discharge of fissure-vein waters along the faults that bound the depression from the southeast and northeast and intersect the lake water area from the island to the Istok river. The highest content of dissolved substances was recorded in the strait between Monastyrsky island and the western shore of the lake; the maximum values of hydrocarbonate ion and total mineralization were found here. The maximum content of sulfate ion was found in the southern and southeastern parts of the lake. The dispersion in microelement distribution reaches several mathematical orders. The most variable concentration is characteristic of iron, manganese, copper, zinc, lead, phosphorus, molybdenum, tungsten, strontium. Their high contents were found in the lake water within the location of faults of northeast strike. Therefore, the chemical composition of the water of Lake Kotokel is largely formed by fissure-vein waters. This water is discharged along the tectonic faults of the northeastern strike. The research revealed two centers of subaqueous discharge, which are characterized by the formation of two different associations of microelements in the lake water. The composition of microelements in fissure-vein waters is determined by their interaction degrees with rocks.

The purpose of the study is optimization of geological exploration at the groundwater deposits of the upper hydrodynamic zone formed in the fault-block structure conditions in Eastern Siberia. The authors analyze the structural and tectonic features of the areas under investigation, the results of areal geophysics studies, exploration drilling and the use of the author's methodology in carrying out of the ground inflow testing. The objects of research are groundwater deposits located in various tectonic structures of Eastern Siberia. The analysis of field study results involving researches of groundwater deposits located in various tectonic structures of Eastern Siberia made it possible to identify the double porosity in the aquifers of free water exchange in the region, which is due to the stresses arising during the inland Baikal rift system formation. As a result, the optimization of geological exploration work, methodology and processing methods of ground inflow testing in the areas of fault-block structures development were introduced on the basis of the features of structural-tectonic and hydrogeological conditions.

The paper deals with the methodological features of drilling and completion of wells in the fractured natural reservoirs containing oil and gas accumulations with different reservoir pressures of fluid-pressure systems from abnormally high to abnormally low. The authors had studied the fluid-pressure systems of industrial lithium-bromine brines, oil and gas fields and accumulations in the south of the Siberian platform for the period from 1983 to 2019. The article summarizes the main results, including new technical solutions protected by the Russian Federation patents. The authors proposed and patented a series of new technical solutions for the immediate consolidation of natural permeable fractures during the primary opening of the reservoir by drilling, as applied to a fractured reservoir. The main task of the study is to preserve the permeability of the fractured system in the bottomhole formation zone under the action of compressive stresses (rock mass) that increase with the formation of a drawdown cone, primarily in the bottomhole formation zone with the increase in the drawdown (ΔP) above critical values. Such an area is the bottomhole formation zone within a radius of the first meters around the well that penetrated the fractured reservoir. Practice has proved that the use of innovative solutions through the advanced consolidation of permeable fractures in the bottomhole formation zone (of fluid-producing oil- and gas-bearing, water-bearing reservoir) in the open (initial natural) state ensures the preservation of natural permeability of natural filtering fractures of the reservoir with the fluid system reservoir pressure from anomalously low to abnormally high. The solution ensures constant permeability of the fractured filtration system throughout the cleaning cycles of the bottomhole formation zone rocks from drilling mud, obtaining of the true calculated hydrodynamic parameters based on the results of well testing in the modes of the “steady-state production method” and well flow rate (productivity) stabilization under further well operation.

The article presents and analyzes the data on ground waters of active (suprapermafrost) and hindered (subpermafrost) water exchange of geodynamically different terrains in order to prove the hydrogeological importance of their historical and tectonic characteristics. On the example of Trans-Polar Chukotka it is shown that, under suprapermafrost conditions, the ubiquitous eluvial-deluvial nappes are the most water-abundant on the terrane – a fragment of the passive continental margin, whereas they are the least water-abundant on the terrains of the active margin. Hydrogeological situation changes under subpermafrost conditions: more permeable and water-retaining rocks compose the terranes of the active margin. These differences are associated with the level of rock tectonic decompaction and, accordingly, with different intensity of weathering processes in the terrane rocks of different geodynamic origin in suprapermafrost and subpermafrost conditions. The hypergenesis zone on the terranes of the passive continental margin features coarse-grained rock weathering products accumulated in relatively calm geological and historical environments, the aggregate is sandy. The terranes of the active margin, which underwent long-term subvertical and subhorizontal displacements contain more fine-grained weathering products; the aggregate includes sandy loam and clay sand. Since the permafrost strata in both Trans-Polar Chukotka and Eastern Siberia is greater than the depth of hypergene transformations, the terranes of the active continental margin, the rocks of which were impacted by tectonic decompaction processes, mainly of a strike-slip and thrust nature, feature greater water abundance in subpermafrost conditions.

The purpose of the study is generalization of data on the manifestations of hydrogeological earthquake precursors and volcanic activations based on long-term observations in the wells of the eastern part of the Kamchatka Peninsula. The main problem under consideration is the connection between the manifestations of hydrogeological precursors in several wells with the values of the magnitude M_w and epicentral distance of earthquakes to the wells d_e as well as with the parameters of seismic action in the observation area including specific density of seismic wave energy e and macroseismic intensity of shaking I_MSK-64. The study results revealed that hydrogeological precursors in two-four wells had been manifesting for the period from 1 to 9 months before the strongest earthquakes with M_w = 6.6–7.8 at the epicentral distances d_e = 90–300 km. Such earthquakes were accompanied by the shakings of the intensity of I_MSK-64 = 4–6 points. The specific density of seismic energy under such earthquakes was minimum 0.1 J/m³. The hydrogeological precursors were confined to the area for which the ratios of the earthquake epicentral distance d_e to the maximum linear size of the earthquake source L, km ranged from 1 to 3.7. Using the established relationships between the manifestations of hydrogeological precursors and earthquake parameters, weekly prognostic conclusions were made for expert earthquake prediction councils based on the data of current observations in wells. The well located at the distance of 15 and 20 km from the Koryaksky and Avacha active volcanoes featured the anomalous rise of groundwater pressure before the eruptions in 1991 and 2008– 2009. Therefore, a conclusion can be drawn that observation equipment operating in wells, the study results of hydrogeological precursors of earthquakes and volcanic eruptions as well as their application experience in the work of expert councils can form the scientific and technical basis for the development of geoinformation prediction technology for natural disasters in the Kamchatka Krai.

This research presents the results of long-term monitoring of groundwater levels within the Khabarovsk water node in the Amur and Tunguska interfluve on the area of the Middle Amur artesian basin in the aquifer of Pliocene-Lower Quaternary alluvial deposits. Observations have been carried out on 9 groups of wells of external monitoring and 5 groups of wells of internal monitoring at the Tunguska reservoir, with a depth of 3 observation wells in the group from 15 to 50 m. The interaction parameters of groundwater and the Pemzenskaya channel have been specified for the period from 2012 to 2020. When the channel causes groundwater afflux during the flood, the average value of the equivalent length parameter ∆L is 40 m for the upper level of the aquifer, 87 m – for the middle level, and it is 605 m for the lower level. Vertical water exchange in the productive strata in the shore zone of the channel is characterized by the values of the overflow coefficient of 0.136 days^-1 between the upper and middle observation levels and 0.0116 days^-1 between the middle and lower levels.

The purpose of the work is to study the effect of organic matter on the formation of ion-salt and gas composition of nitrogen-methane and methane thermal water occurring in the sedimentary rocks of deep horizons of artesian basins. The object of research is the Tunka intermountain artesian basin of the Baikal rift zone and the Tungor gas and oil field of the Okhotsk-Sakhalin basin, in the deep horizons of which soda (inversion) low- and high-mineralized groundwater is common. The study combines the results of the traditional study of the composition of natural solutions and the quantitative research of physical and chemical interactions in the “water – rock” system conducted using the Selector software package according to the degree of the hydrogeochemical process, which was set by the value of the rock/water ratio. Chemically pure water and rocks of medium chemical composition were used in interaction. With the use of physicochemical modeling the formation of thermal water composition in sedimentary rocks depending on the interaction degree between water and rock and the amount of organic matter was unravelled. As a result, it was determined that the organic matter present in the rock has the dominant influence on the intensity of the hydrogeochemical process determining the amount of mineralization, the ratio of components, and the amount of methane, nitrogen, and carbon dioxide produced. The correspondent compositions of the model and natural solutions showed the possibility to form low- and high-mineralized sodium bicarbonate groundwater of different gas-saturation degree in the conditions of deep horizons of sedimentary basins due to the internal reserves of the “water – rock” system not involving any components from external sources.

The purpose of the study is to analyze the possibility to identify the pollution genesis of the underground hydrosphere based on the data on polyarenes as geochemical markers. Their marker role is due to their toxicity, persistence and confinement to specific pollution sources and natural and technogenic processes. The main research method is the analysis of indicator ratios of polyarenes. The object of research is groundwater from different regions of the world, which are polluted by natural and anthropogenic polyarenes. The main directions of domestic and foreign researches as well as the problems of their implementation are shown. The use of indicator ratios enabled successful identification of pyro- and petrogenically polluted samples of groundwater also the examples of the study of polyarenes as geochemical markers were demonstrated. The polyarenes are shown to be an effective indicator of the pollution genesis in the underground hydrosphere. However, their analysis requires the use of modern methods of sampling, sample preparation and extraction, which significantly complicates research in practice.

The purpose of the work is to carry out integrated isotope-geochemical studies of the mineral waters of the Belokurikha deposit. The methods of titrimetry, ion chromatography, inductively coupled plasma mass spectrometry (ICPMS) have been used in the laboratory investigation of the chemical composition of waters. The isotope composition of oxygen, hydrogen and carbon in dissolved carbon dioxide has been studied with the help of the Isotope Ratio Mass Spectrometer Finnigan^TM MAT 253 equipped with the attachments for sample preparation H/Device (to analyze the δD ratio) and GasBench II (to analyze δ¹⁸O and δ¹³С_DIC ratios). There are two aquifers at the deposit. The first nonartesian aquifer comprises loose sediments of the Quaternary age. The second artesian aquifer includes the granites of the upper Paleozoic age with the different fracture degree: from monolith to loosened. Three groups of waters are distinguished on the basis of geochemical coefficients: fracture-vein waters bedded in weathered granites; groundwaters of the zone of rare earth mineralization and background composition; surface waters of the Belokurikha river. The isotope data on oxygen and hydrogen provide evidence that the production aquifers of the Belokurikha field are fed through the infiltration of meteoric waters, with the feeding shift to winter precipitation. The paper provides the first data of the integrated isotope-hydrogeochemical studies of nitric-siliceous low-radon thermal waters of the Belokurikha deposit. The composition of these waters is HCO₃-SO₄ Na and SO₄-HCO₃ Na with the total dissolved salts value ranging from 198 to 257 mg/dm³. The waters are characterized by alkaline pH of 8.6–9.6, silicon content ranging from 19.8 to 24.6 mg/dm³, and they are referred to the fracture-vein waters of the Upper Paleozoic granites. ²²²Rn activity is up to 359 Bq/dm³. The ratios of δD (from -126.9 to -102.7 ‰) and δ¹⁸O (from -17.5 to -14.2 ‰) in the studied waters indicate their atmospheric origin. The values of δ¹³С_DIC vary from -9.7 to -25.6 ‰ and point to the biogenic origin of carbon.

The article focuses on the importance of studying the water exchange role of the permafrost, which currently occupies a quarter of the Earth's land and is 1.5 km deep in some regions, as well as assessing the permafrost impact on the formation of surface and ground water resources and regime. First of all, the permafrost water exchange function is associated with the freezing of water-saturated rocks and thawing of ice-saturated ones. The author gives individual consideration to the water exchange role of the active layer and the effect of the permafrost long-term dynamics on water exchange direction and scale. The water exchange function of the active layer appears due to the seasonal phase transitions of groundwater from a liquid to a solid state and back. Thus, the volume of water formed by the thawing of underground ice accumulated in the active layer in winter has been estimated about 4·10¹² m³. In this regard, it is proposed to give cryohydrogenic part associated with the seasonal transitions of groundwater from the liquid to solid state and back in the active layer of the cryolithozone independent consideration within the hydrological (climatic) cycle of the natural water circulation. It is most certain that the permafrost water exchange function is more significant being associated with longterm permafrost development dynamics under the influence of periodic dramatic climate fluctuations. Thus, during the Holocene climatic optimum about 4.5·10¹⁵ m³ of underground ice was converted to the liquid phase. The intensity of water formed from the melting of this amount of ice inflowing its surface and underground reservoirs has been estimated about 820 km³ per year. This fact considered, the author proposes to consider the cryolithogenic part separately in the geological cycle of the natural water cycle.