This study characterizes the hydrochemistry and geochemical signature of the Upper Cretaceous geothermal aquifer in the Zharkunak zone (Eastern Ili Depression, SE Kazakhstan) using certified analytical datasets from five deep wells (5539, 1-RT, 3-T, 1-TP, and 2-TP). The waters are hyperthermal (89–103 °C), alkaline (pH 8.1–9.0), and weakly mineralized (TDS 0.3–1.0 g/L), with sodium-dominated facies ranging from Na–HCO₃–SO₄ to Na–SO₄–Cl. Hydrochemical analysis indicates that water–rock interaction and cation exchange are the primary controls on fluid evolution, with limited influence from evaporation or external salinity sources. Elevated fluoride (up to ~10 mg/L) and dissolved silica (H₂SiO₃, often >50 mg/L) reflect prolonged high-temperature interaction with silicate-rich lithologies under low Ca²⁺ conditions. Trace elements and radon activity (up to 0.32 nCi/L) further support deep, fault-controlled circulation pathways. PHREEQC modeling indicates near-equilibrium to slight supersaturation with respect to silica phases, suggesting a potential risk of silica scaling during cooling, while carbonate scaling remains limited. Although the dataset is based on discharge conditions from a limited number of wells, the results demonstrate that the Zharkunak system has strong geothermal utilization potential, with management considerations related to fluoride, radon, and silica scaling. Future work should focus on integrating isotopic analyses and reactive transport modeling to better constrain subsurface processes and long-term system behavior.

The paper examines the geological and structural conditions controlling the formation and distribution of groundwater in the Upper Cretaceous deposits of the Arys artesian basin (southern Kazakhstan), one of the most promising thermal water regions of the country. The aim of the study is to substantiate the hydrogeological subdivision of the Upper Cretaceous aquifer series and to refine the conditions of natural groundwater resource formation using seismic survey data, deep drilling, well logging and hydrometric observations. Correlation of resistivity and spontaneous potential logs of deep wells made it possible to distinguish the Upper Albian–Cenomanian and Upper Turonian– Senonian aquifer systems, each comprising separate aquifers divided by laterally persistent clay layers. Maps of depth, thickness and potentiometric surface of the main aquifers were compiled. The top of the Cenomanian aquifer lies at depths of 280–1800 m; groundwater temperature at the top reaches 40–85 °C with a geothermal gradient of 2.4–4.6 °C/100 m. Wells tapping the Upper Turonian aquifer are free-flowing with heads up to +90.6 m above the wellhead and wellhead temperatures up to 86 °C. In the Karatau foothills the Upper Turonian aquifer is isolated from Palaeozoic fissure-karst waters by Lower Turonian clays up to 100 m thick, and its natural resources are formed mainly by river seepage losses (about 5.4 m³/s). The South Karatau fault is shown to act as a natural boundary controlling the distribution and formation of thermal waters in the basin.