Hydrogeochemical Signature of Cretaceous Geothermal Waters of the Zharkunak Zone, Eastern Ili Depression

Abstract

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.

English abstract from the publisher’s page of the article. The authors’ abstract, reproduced under CC BY 4.0.

Keywords

• geothermal water
• hydrochemistry
• Upper Cretaceous aquifer
• fluoride
• radon
• Ili Depression
• Kazakhstan

In plain English

The Zharkunak zone lies in the eastern Ili Depression in south-eastern Kazakhstan. Its geothermal water comes from a confined, artesian Upper Cretaceous sandstone aquifer about 2.3–2.9 km deep, above Paleozoic basement. The authors used certified analyses from five deep wells (5539, 1-RT, 3-T, 1-TP and 2-TP), sampled in 2016 under stable discharge and taken from archived project reports.

The water is very hot, 89–103 °C, alkaline (pH 8.1–9.0) and only weakly mineralised, at 0.3–1.0 g/L. Sodium dominates; the water types range from Na–HCO3–SO4 to Na–SO4–Cl. The authors conclude that reactions between water and rock, and cation exchange, control the chemistry, with little effect from evaporation or outside sources of salt.

Fluoride reaches up to about 10 mg/L in the compiled records, and dissolved silica 22.5–60.3 mg/L, more than 50 mg/L in well 5539. The authors link both to long contact between hot water and silicate-rich rocks with little calcium. Radon reaches 0.32 nCi/L in well 5539. Together with trace elements, this points, in their view, to deep circulation along faults.

Modelling with PHREEQC shows the water close to equilibrium or slightly supersaturated with silica, so silica may form scale as the water cools; carbonate scale should be limited. The authors conclude that the Zharkunak system has strong potential for geothermal use, provided fluoride, radon and silica scaling are managed. They call for isotope analyses and reactive transport modelling next.

How to cite

Balnur Kismelyeva, Aisulu Kalitova, Dulat Kalitov, Vyachaslav Zavaley, Yergali Auyelkhan, Rinat Akpanbayev, Raushan Koizhaiganova, Murat Kalitov and Zaure Atabekova (2026). Hydrogeochemical Signature of Cretaceous Geothermal Waters of the Zharkunak Zone, Eastern Ili Depression. Water, vol. 18, no. 7, 870. https://doi.org/10.3390/w18070870