Distribution, sources, and ecological risks of heavy metal contamination at the sediment-water interface in the Dongjiang Basin based on in situ high-resolution measurements

As a critical drinking water source for over 40 million people in southern China, the Dongjiang River faces growing ecological threats from sediment-derived heavy metals (HMs: As, Cd, Co, Cr, Cu, Mn, Ni, Pb, and Zn). This pioneering study is the first to integrate DGT, HR-Peeper, and BCR techniques, achieving three breakthroughs: (1) millimeter-scale resolution mapping of SWI dynamics, (2) a mechanistic assessment of mobility and bioavailability through DIFS modeling and flux calculations, and (3) a quantitative source apportionment that combines lead isotopes with machine learning–enhanced multivariate statistics (PCA-PMF-RF). Key findings demonstrate the following: (i) the accumulation of HMs downstream, (ii) Cd as the predominant risk factor despite moderate pollution levels, and (iii) Mn-Cd release as a previously underestimated threat, evidenced by positive diffusion fluxes and kinetic parameters. Using Pb isotopes and multivariate statistics (PCA-PMF-RF), we identified three dominant sources: natural/agricultural (42.6 %), industrial (18.9 %), and composite anthropogenic (38.5 %). This multi-methodological framework sets new standards for SWI studies and provides actionable data for watershed management.