Lake Balkhash is one of the largest lakes in the world. It is located in arid Central Asia and receives major water from the Ili River, which is an international river flowing across China and Kazakhstan. Hydrochemical and isotopic measurements of waters can provide an improved understanding of hydrogeochemical processes and environmental characteristics, which is useful for water resource management in arid regions. In this study, δD, δ18 O, and major ions in water samples from the Lake Balkhash catchment were analyzed using an integration of mathematical statistics, Piper diagrams, Gibbs model, and principal component analysis (PCA). Water types and main mechanisms controlling the hydrochemistry presented a clear spatial heterogeneity. The chemical composition of lake waters was dominated by SO4-Na and Cl-Na type, whereas river waters were classified as a HCO3-Ca type. The chemical composition downstream of the Ili River evolved from HCO3-Ca to SO4-Na-Cl type. Gibbs model suggested that the main mechanisms controlling the lake water chemistry were evaporation-crystallization processes, and that major ions in the river water were affected by rock-weathering and evaporation processes. The main controlling factors of the water chemistry changed from the upstream to the downstream, and may have related to spatial differences whereby the upstream area experienced higher rainfall and snow melt, and the downstream area experienced relatively higher evaporation. PCA analysis showed that human activities also played an important role in the chemical composition of water sampled from the lake, middle and lower reaches of the Ili River, and other rivers. The isotopic compositions of the lake and river waters varied spatially. In the lake waters, positive isotopic ratios and negative deuterium-excess values indicated that evaporitic enrichment dominated the changes in the isotopic signature of the lake water. In river waters, isotope values located near the global meteoric water line (GMWL) and lower slopes of the regression lines (r=0.91, P<0.001 for Ili River, and r=0.63, P<0.001 for other rivers), were associated with a stronger influence of rainfall and weaker evaporation. Correlation analysis showed that there were significant relationships between isotope values and chemical parameters in the lake water, especially in the eastern area, thus suggesting that extensive evaporation led to simultaneous enrichment of isotopes and ions in the lake water.
Keywords: Ili River; Lake Balkhash basin; hydrochemistry; hydrogen and oxygen isotopes; water samples.