Heavy Metal Pollution Characteristics and Ecological Risk Assessment of Sediments in the Hengyang Section of the Xiang River

• • Mean concentrations of As, Se, Cd, Sb, Pb, Tl, and Bi exceeded local background values; Cd, Se, Bi, and As showed pronounced enrichment, with Cd contributing 87.49% to the potential ecological risk index (RI), indicating its dominance as a contaminant. • • Sequential extraction showed that Zn, Cd, Pb, and Mn had high extractable fractions (F1+F2+F3) exceeding 50%, while Sb, Bi, Se, Tl, Cu, As, V, Cr, and Ni were predominantly in the residual fraction (F4), implying higher mobility and bioavailability for the former group, especially downstream. • • Geo-accumulation index (Igeo) revealed no contamination by Co, Zn, V, or Cr, but varying 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; Cd showed moderate to strong contamination at some sites. • • Source apportionment via PCA and PMF identified three sources: industrial and traffic mixed source (57.60%), natural source (16.00%), and industrial-natural mixed source (26.40%), providing quantitative basis for targeted pollution control.