Antimony (Sb) is a major environmental pollutant that is widely present in nature and possesses toxicity and potential carcinogenicity. The mining of antimony ore can affect the quality of the water environment and the safety of drinking water for residents in the area. Taking the Gaolinggou antimony mining area in the Danjiangkou water source for the Middle Route of South-to-North Water Diversion Project (MRSNWDP) in China as the research object, systematic hydrochemical samples and hydrogen and oxygen isotope samples were collected for comprehensive hydrogeochemical analysis. The results show that: (1) The pH range of surface water in the study area is 3.09-8.30, with a mean of 7.21, indicating an overall neutral condition, while some local areas are acidic. The range of TH is 243.00-439.00 mg/L, with a mean of 314.69 mg/L. The TDS ranges from 262.00 to 598.00 mg/L, averaging at 384.38 mg/L. The dominant cations are Ca2+ and Mg2+, while the dominant anions are HCO3- and SO42-. The hydrochemical types vary frequently in space, including HCO3·SO4-Ca·Mg, SO4·HCO3-Ca·Mg, HCO3-Ca·Mg, and SO4-Ca·Mg. For groundwater, the mean pH is 7.52, with TH and TDS ranging from 269.00-542.00 mg/L and 291.00-652.00 mg/L, respectively, averaging at 426.67 mg/L and 504.67 mg/L, both slightly higher than those of surface water. The dominant cations and anions are Ca2+ and HCO3-, respectively, with hydrochemical types mainly HCO3-Ca·Mg and HCO3·SO4-Ca·Mg. The mine water has a pH value of 8.03, indicating a weakly alkaline condition with the highest TDS content. It is rich in SO42-, Ca2+, and Mg2+, and the hydrochemical type is SO4·HCO3-Ca·Mg. (2) The hydrochemical characteristics in the study area are primarily controlled by rock weathering, with carbonate and silicate weathering and dissolution as the main processes. Positive cation exchange occurs in the water bodies. Surface water is significantly affected by mining activities, while groundwater is influenced by a combination of agricultural activities, domestic sewage, and mining activities. (3) In addition to atmospheric precipitation and dissolution of salt rocks, the excess Na+ in the water bodies mainly originates from the dissolution of sodium-bearing silicates and the contribution of cation exchange processes. Ca2+ and Mg2+ mainly originate from the weathering of carbonate rocks dominated by calcite and the dissolution of gypsum. Carbonic acid and sulfuric acid both participate in the weathering and dissolution of carbonate rocks. So42- and Cl- are significantly influenced by mining activities. The antimony in surface water is primarily influenced by the mining of antimony ore, with the average concentration of antimony in the mining area being 6.4 times that of the mainstream of the Luohe River. Meanwhile, the antimony in groundwater mainly originates from the lateral recharge of surface water. The results can provide a reference for the water resource security assurance and rational development and utilization of the MRSNWDP.

As a critical ecological barrier and core water conservation zone in China, the southern slope of the Qinling Mountains plays a pivotal role in safeguarding regional ecological security and ensuring a safe drinking water supply, largely mediated by groundwater quality. However, systematic investigations into nitrate contamination in this region remain limited, and the dominant sources as well as hydrogeochemical migration pathways of nitrate are still poorly constrained. To address these knowledge gaps, this study targets representative watersheds on the southern slope of the Qinling Mountains, with an emphasis on characterizing the spatial – temporal distribution patterns and identifying the primary sources of groundwater nitrate. A multi-gradient monitoring network was established along the Han River and its tributaries, supporting seasonal sampling across both dry and wet periods. Coupled with δ15N-δ18O dual-isotope tracing and comprehensive regional environmental background surveys, we quantitatively assessed the spatial heterogeneity, temporal dynamics, and source apportionment of nitrate in groundwater. Results reveal pronounced spatial variability: mean nitrate concentrations decrease downstream, and levels adjacent to the main stem are consistently lower than those in tributary zones. Furthermore, the spatial distribution of nitrate closely parallels the regional groundwater flow direction. Vertically, soil nitrate concentrations decline with depth overall, yet exhibit anomalous enrichment at a depth of ~1 m ‒ indicating that nitrate leaching has penetrated to at least this depth. Source apportionment indicates that soil nitrogen, livestock and poultry manure, and domestic sewage collectively account for the majority of nitrate inputs to groundwater. This study provides foundational observational data on groundwater nitrogen cycling in mountain – basin transitional zones, thereby strengthening the scientific basis for groundwater protection and ecosystem management in the region. It also advances theoretical understanding of nitrogen biogeochemistry in complex terrain and supports the development of targeted, evidence-based groundwater pollution mitigation strategies.