Revealing hydrochemical characteristics and evolution process of river and groundwater in the Nalenggele Basin, northwest China: insights from major ions, multi-isotopes (H、O 、Sr、B and Li) tracers, and positive matrix factorization

The Qaidam Basin, located on the northern margin of the Qinghai-Tibet Plateau in China, is the second-largest inland basin in the country. Hosting numerous salt lakes rich in mineral resources, with particularly prominent lithium reserves. The Nalenggele River, the largest inland river within the Qaidam Basin, hosts the most extensive brine-type lithium deposit in China in its terminal salt lake region. With lithium resources reaching 2.3 million tons, this deposit accounts for approximately 87% of Chinese total lithium reserves. The genetic mechanism of brine in this terminal salt lake area is closely linked to hydrogeochemical processes. Previous studies have primarily focused on the qualitative analysis of hydrogeochemical processes, while relatively few have quantitatively assessed the impact of different hydrogeochemical processes. In this study, river water and groundwater from the mountainous areas to the basin within the watershed are selected as the research objects. Multiple isotopic tracers (δ²H, δ¹⁸O, ⁸⁷Sr/⁸⁶Sr, δ¹¹B and δ⁷Li) are employed to trace the material sources and evolutionary processes of elements. The material sources, controlling factors and evolutionary mechanisms of hydrochemistry in the Nalenggele River Basin are clarified. The Positive Matrix Factorization model is applied to quantitatively identify the recharge sources of different water bodies, trace the material sources of major ions, and elucidate the evolutionary processes of the watershed hydrological cycle. The results show that: (1) The hydrochemical compositions of both river water and groundwater are dominated by Na⁺ and Cl⁻. The hydrochemical types evolve from mixed Cl·HCO₃·SO₄-Na·Ca type in the upstream rivers to Cl-Na type in the downstream waters. Analyses of ionic ratios and strontium isotope data confirm that water-rock interaction is the primary controlling factor of hydrochemical compositions, which is characterized by silicate weathering as the dominant process, supplemented by carbonate weathering and evaporite (halite, gypsum, mirabilite) dissolution. Cation exchange exhibits spatial heterogeneity: forward exchange (Ca²⁺/Mg²⁺ vs. Na⁺/K⁺) occurs in the upstream and downstream areas, while reverse exchange takes place in the midstream area. (2) Evidence from δ²H and δ¹⁸O indicates that river water is mainly recharged by atmospheric precipitation from the southern mountainous areas with the elevation of 4700m. Groundwater has close hydraulic connectivity with river water, showing bidirectional recharge-discharge interactions. (3) The observed B and Li isotopic footprints in the Nalenggele River Catchment are significantly depleted in heavy isotopes compared with those in other geological systems dominated by natural weathering processes. In the upper reaches of the Nalenggele River, the concentrations of B and Li increase sharply, while the δ¹¹B and δ⁷Li values decrease gradually. The mechanism responsible for the B and Li enrichment is mainly associated with the Li-B supply potential of material sources, favorable tectonic conduits for water circulation, and high evaporation rates. (4) The Positive Matrix Factorization model quantitatively reveals the contribution rates of different hydrogeochemical processes during the hydrological cycle, specifically: evaporite mineral dissolution (28%), mixed evaporite dissolution (25%), agricultural activity (17%), and silicate weathering (30%). This study provides a comprehensive framework for integrating multi-isotope tracers and statistical models to quantify hydrochemical processes in arid inland basins.