Abstract:

The Napahai wetland is a typical karst wetland on the southeastern edge of the Qinghai-Tibet Plateau, characterized by a fragile ecological environment and frequent human activities. Enhancing research on watershed hydrochemistry is crucial for the protection and restoration of such plateau wetlands. Based on a systematic investigation of the hydrogeological conditions of Napahai, water samples were collected from the surface water body, its main tributaries (recharge areas), and sinkholes (discharge outlets) within the Napahai Lake Basin. To investigate the chemical composition, spatial distribution, recharge sources, and controlling factors of the surface water, we comprehensively utilized ion ratio analysis, multivariate statistical analysis, and geographical detectors. In the Napahai Basin, the surface water pH ranged from 7.08 to 8.70, and TDS values ranged from 103.00 to 620.50 mg·L^-1. Cations were dominated by Ca²⁺ and Na⁺, with abundances descending in the order of Ca²⁺ > Na⁺ > Mg²⁺ > K⁺. Anions were dominated by $\mathrm{HCO}_{3}^{-}$ and $\mathrm{SO}_{4}^{2-}$, showing the order of $\mathrm{HCO}_{3}^{-}$ > $\mathrm{SO}_{4}^{2-}$ > Cl^- > $\mathrm{NO}_{3}^{-}$. The hydrochemical type was primarily HCO₃-Ca, with some samples shifting towards the SO₄-Ca end-member. The coefficients of variation for $\mathrm{NH}_{4}^{+}$, $\mathrm{PO}_{4}^{3-}$, Cl^-, $\mathrm{SO}_{4}^{2-}$, and As were high, indicating strong spatial variability. Notably, 40% of the wetland water samples exhibited elevated As content (>10 μg·L^-1). The δD and δ¹⁸O values ranged from -132‰ to -83‰ and -17.5‰ to -8.1‰, respectively, indicating that the surface water was mainly recharged by atmospheric precipitation derived from oceanic water vapor. The primary water transformation pathway was precipitation → tributary → wetland → sinkhole. Natural controlling factors of surface water chemistry included rock weathering (dominant), followed by evaporation concentration, seasonal dry-wet alternation, and aquatic plant purification. Surface water has also been affected by human activities such as agriculture, domestic sewage discharge, engineering excavation, and artificial dry-wet alternation. Specifically, K⁺, Na⁺, $\mathrm{HCO}_{3}^{-}$, $\mathrm{PO}_{4}^{3-}$, $\mathrm{NH}_{4}^{+}$, and TDS were primarily influenced by mixed non-point sources (e.g., carbonate rock dissolution, agriculture, domestic sewage). Ca²⁺ and Mg²⁺ were mainly affected by mixed non-point sources and sulfide oxidation. $\mathrm{SO}_{4}^{2-}$ was predominantly controlled by sulfide oxidation, while $\mathrm{NO}_{3}^{-}$ was mainly influenced by aquatic plant purification. As was primarily affected by mixed non-point sources, aquatic plant purification, and dry-wet alternation, and pH was mainly influenced by dry-wet alternation. The contribution rates of mixed non-point sources, sulfide oxidation, aquatic plant purification, and dry-wet alternation to solute variation in the watershed were 51.84%, 22.66%, 14.20%, and 11.30%, respectively. This study demonstrates that human activities have significantly impacted solute sources in the Napahai Basin.

Key words: karst wetlands, plateau seasonal swamp wetlands, hydrochemical characteristics, chemical weathering, control factors, surface water