The Choygan area of southern Siberia, Russia hosts a variety of CO 2-rich thermal mineral and other waters emerging from springs at temperatures between 7 • C and 39 • C. Chemical analyses of the spring waters (n = 33) were carried out to characterise the waters and determine their origin. A continuum of compositions was observed between relatively lower temperature (7 • C) HCO 3-Ca-Na dominated waters with relatively low amount of total dissolved solids (TDS) and high Eh, and higher temperature (39 • C) HCO 3-Na-Ca dominated waters with higher TDS and lower Eh-this reflects largely conservative mixing of these components between near surface low temperature, oxidising groundwaters and higher temperature, more reducing thermal waters derived from a deeper geothermal reservoir. Stable isotopic data are consistent with all the water ultimately being derived from meteoric water that has undergone varying degrees of isotopic fractionation following evaporation. The inferred δ 18 O and δ 2 H isotopic composition of the unfractionationed meteoric waters is lighter than that expected that of mean annual local precipitation, which together with a strong negative correlation between δ 18 O and the elevation of the sampled discharging springs, suggests recharge at higher elevations (1600 m to 3000 m; average 2600 m). Reservoir temperature, calculated using geothermometers and an analysis of saturation indices of plausible reservoir minerals, ranged from 70 • C to 100 • C at an inferred depth of 2 to 3 km. Not all chemical components were found to follow conservative mixing behaviour. In particular, (i) the CO 2 contents of the waters were highly variable, suggesting either varying degrees of degassing and/or near discharge admixture with air, and (ii) SO 4 concentrations in the lower temperature thermal CO 2-rich waters were highly variable, suggesting a role of near surface oxidation processes, for example of pyrite, in modifying the concentration of redox sensitive components. Limited δ 13 C data are consistent with the CO 2 predominately being derived from dissolution of metamorphic/igneous carbonate minerals in the reservoir. Based on geological conditions, isotope and chemical data, a conceptual circulation model of the Choygan hydrothermal system is proposed.

Inductively coupled plasma mass spectrometry (ICP-MS) is a popular method for the analysis of waters with various matrices and salinity. One of the difficulties of routine measurements by ICP-MS is spectral interferences conditioned by polyatomic ion formation in the plasma. The detection of the background concentration of such elements as vanadium, chromium and arsenic in natural waters by ICP-MS is complicated because of the polyatomic interferences, having the same mass-to-charge ratio. Thus, the purpose of this article is to determine the optimal rate of helium flow for the effective correction of polyatomic interferences of vanadium, chromium and arsenic and the reduction of their detection limits in Cl-rich waters. This research has been carried out using an inductively coupled plasma mass spectrometer NexION 300D with a universal cell technology (UCT) (PerkinElmer, USA) and three model solutions. For the detection of vanadium, chromium and arsenic content in chloride matrix water by ICP-MS, a collision mode is preferable for polyatomic interference correction. The optimal helium flow rate for this purpose is 2.5 ml/min. Under these conditions, the detection limit of vanadium, chromium and arsenic decreases by order of two

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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.