The abundance of various chemical elements (including trace, rare-earth, and radioactive) in the carbon dioxide mineral waters of the Choigan complex has been investigated. Three groups of waters are recognized according to the geochemical conditions and chemical composition: groundwaters of regional-jointing zone with oxidizing conditions; CO 2 -enriched groundwaters of regional-jointing zone with oxidizing conditions; and groundwaters of fault zones with reducing conditions. It is shown that water-rock interaction intensified by high temperature and carbon dioxide action is the main process determining the chemical composition of groundwaters.

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Groundwater with high salinity is widespread in different climatic and geologic environments of the world. The formation of its chemical composition, however, is still debatable. The chemical composition of groundwater has been studied in 19 springs of the Tuva depression. In this area, hydrocarbonate, sulfate, and chloride waters with different cation compositions discharge. Their TDS value varies mainly from 1 to 6 g/L, reaching 315 g/L at only one locality. The chemical composition of the studied waters is reflective of the geostructural, hydrogeologic, landscape, and geochemical conditions. The main processes determining the chemical composition of the waters are their interaction with aluminosilicate minerals, dissolution of gypsum and halite, evaporation, and oxidation of sulfide minerals.

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The spa resort of the Ush-Beldir is located in the south-eastern part of the Tuva region, Russia in the zone of influence of the Baikal rift. The Ush-Beldir territory has about 10 thermal springs and 4 exploration wells used for treatment and prevention of diseases. Hydrological and hydrochemical features of the thermal groundwaters are considered. The studied waters are thermal (T=50–83°C), alkaline (pH=9.3–10), low mineralized (TDS<370 mg/L) Na–НСО3 waters with a high content of SiO2 (up to 125 mg/L) and F (up to 14 mg/L). The correlation of F- with SiO2, НСО3–, Cl-, Na+ as well as the calculation of saturation indices of minerals, allows one plausible solute source for these thermal waters, viz. the dissolution of rock-forming aluminosilicates, most likely plagioclase, gabbro and/or amphiboles in shales.