Heavy Metal Pollution Characteristics and Ecological Risk Assessment of Sediments in the Hengyang Section of the Xiang River
Sediments from the Hengyang reach of the Xiangjiang River were analyzed for concentrations, chemical fractions, and sources of 14 heavy metals (As, Se, Cd, Sb, Pb, Tl, Bi, Co, Ni, Mn, Zn, V, Cr, Cu). Mean concentrations of As, Se, Cd, Sb, Pb, Tl, and Bi exceeded local background values, with Cd, Se, Bi, and As showing pronounced enrichment. Spatial heterogeneity was marked, with higher levels downstream; overall concentrations were lower than previously reported. Sequential extraction revealed that Sb, Bi, Se, Tl, Cu, As, V, Cr, and Ni were predominantly in the residual fraction (F4), while Zn, Cd, Pb, and Mn had higher extractable fractions (F1+F2+F3), with bioavailable fractions generally elevated downstream. Geo-accumulation index (Igeo) indicated no contamination by Co, Zn, V, or Cr, but varying degrees of contamination by Mn, Ni, As, Se, Cd, Sb, Pb, Tl, Bi, and Cu, with pollution severity order: Se>Cd>Bi>As>Sb>Pb>Tl>Ni>Cu>Mn>Zn>Cr>Co>V. Enrichment factors showed significant enrichment for Cd, Sb, and Bi, moderate for Pb, and low for others. Potential ecological risk index (RI) revealed Cd as the primary contributor (87.49% of total risk), with overall moderate risk at downstream sites and slight risk across the entire section. Source apportionment using PCA and PMF identified three sources: industrial and traffic mixed source (57.60%), natural source (16.00%), and industrial-natural mixed source (26.40%). These findings enhance understanding of heavy metal pollution mechanisms in the Hengyang section and recommend priority control of industrial and traffic emissions, with focus on Cd mobility, to support sediment remediation strategies.