Characterization and Implications of Water Chemistry and Heavy Metal Pollution in the Sixi River, Hunan, China

Situated within the metallogenically critical Nanling metallogenic belt of Hunan Province, the Sixi River basin exemplifies subtropical watersheds experiencing compounded anthropogenic pressures from historic tin mining and intensive agriculture. This hydrogeochemical investigation examines heavy metal contamination dynamics across aquatic matrices in this Pearl River tributary. Field analyses reveal severe Hg (20× WHO guidelines) and As exceedances with distinct spatial stratification: contamination frequencies follow tailings dams (87.61%) > ponds (81.86%) > rivers (67.64%) > wells (71.76%), posing significant neurotoxic and carcinogenic risks. Dominant HCO 3 –Ca·Mg hydrochemical facies reflect carbonate‐granite weathering regimes, with ionic concentrations declining from tailings (12.01 mg/L) to wells (7.40 mg/L). Pollution indices demonstrate pH‐dependent metal mobility, where alkaline conditions (pH > 8.5) exacerbate Hg/As dissolution in lotic systems. Principal component analysis delineates dual pollution pathways: PC1 (33.3% variance, As–Hg–Cu) traces agricultural inputs in alluvial plains, whereas PC2 (19.9%, Tl–Pb–Sn–Mn) aligns with fault‐controlled sulfide mineralization in the Bailashui tin belt. Critically, anthropogenic loading from fertilizer‐enriched runoff exerts greater influence on basin‐wide degradation than mining effluents, underscoring the lithogenic–anthropogenic interface in subtropical mining watersheds.