Hydrogeochemistry of thermal waters in the intermountain basin of the Tien Shan Region

This study presents new data on the chemical composition, content, and distribution patterns of stable oxygen and hydrogen isotopes in natural waters from the Issyk-Kul intermountain artesian basin. This area has significant balneological potential due to the abundance of mineral waters with diverse temperatures, chemical and gas compositions, and total dissolved solids (TDS). The uniqueness of this region lies in the coexistence of two distinct types of mineral waters: fissure-pore waters confined to intermountain artesian basins and fissure-vein waters associated with tectonic fault zones in rock massifs.The study is based on field research conducted in the Issyk-Kul basin, located in the Tien Shan region. The temperature of mineral waters at the sampling sites varies widely (16.2-52.3°C), as does TDS, which depends on the hydrogeological structure. CO₂-rich waters with low TDS (0.3-0.5 g/L) form within rocks and open fractures, while carbon dioxide-nitrogen or nitrogen-methane waters with TDS ranging from 2.0 to 35.0 g/L are associated with significant sedimentary cover thickness. A common pattern in anion composition is observed, as all mineral waters contain sulfate (SO₄²⁻) and chloride (Cl⁻) ions. Sodium (Na⁺) consistently predominates in the cationic composition.The content of stable isotopes of oxygen (δ18O) and hydrogen (δD) in the studied waters also varies significantly, from -13.9‰ to -8.5‰ for δ18O and from -95.8‰ to -66.0‰ for δD. Most data points on the δ18O-δD binary diagram align with the global meteoric water line, indicating an infiltration origin with a pronounced altitude effect.It was also established that the trace element composition of thermal waters serves as a marker for the hydrogeological conditions of their formation and circulation: waters from the sedimentary cover of intermountain artesian basins are enriched with Sr, Ba, Mn, B, Mo, and U, whereas waters from rocky massifs contain elevated concentrations of F, Rb, W, and Sc. The calculation of the water migration coefficient revealed a dependence of the accumulation rate of trace components on the type of host formation and the hydrogeological conditions of water formation.Ion-salt geothermometers were applied to estimate the deep formation temperatures of the mineral waters, revealing a broad range of values (21.4-144.8°C). These results reflect diverse formation conditions for the studied waters.