Spatial Patterns and Pollution Indices of Dissolved Metals in the Lhasa River: Associations with Dissolved Organic Matter

Spatial variation in ten dissolved metals and their associations with dissolved organic matter (DOM) were assessed at 32 Lhasa River sites sampled in May 2025, using pollution indices, correlation analysis, principal component analysis (PCA), and dissolved organic carbon (DOC) measurements. Localized maxima of Cd, Co, Cu, Mn, Ni, and Zn occurred at L8, where Mn, Cd, and Zn reached 59.51, 0.230, and 6.58 µg/L, respectively, whereas reach-level differences were supported for As, Pb, and Sb after multiple-testing correction. All single-factor indices were below 1, while the Nemerow index ranged from 0.0689 to 0.6980 under the adopted screening references. DOC averaged 1.067 mg/L and was positively associated with nine metals (R2 = 0.132–0.330; Benjamini–Hochberg (BH)-adjusted q < 0.05), although Cr showed no supported relationship. When L8 was omitted in sensitivity tests, only As and Sb retained significant DOC associations, indicating that the apparent multi-metal relationships were partly hotspot-dependent. These observations support targeted monitoring of localized maxima and downstream As–Sb enrichment, but do not establish DOM-mediated transport mechanisms or quantify source contributions.