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[Display omitted] • The spatial distribution of heavy metals in surface sediments of the Yellow River mainstream are investigated. • The contamination degree, ecological risk, and source of heavy metals are explored. • The applicability of risk assessment indices of heavy metals in sediments is elucidated. • The reliability of multivariate statistical analyses for source identification of heavy metals is confirmed. Heavy metals in river sediments pose major threats to aquatic ecological environments and public health. This study investigated the characteristics, sources, and risks of heavy metals in surface sediments from the mainstream of the Yellow River. The grain size, loss on ignition (LOI), and content of Mn, Zn, V, Ni, Pb, and Cr in 61 samples were measured. The contamination level was evaluated using six indices, namely sediment quality guidelines (SQGs), contamination factor (C f), geo-accumulation index (I geo), modified degree of contamination (mC d), potential ecological risk index (RI), and pollution load index (PLI), and their spatial distribution was determined through Kriging interpolation. The sources of heavy metals were defined using multivariate statistical analyses, namely Pearson's correlation analysis (P'CA), principal component analysis (PCA), and cluster analysis (CA). The results show that Pb has the highest ecological risk, followed by Cr, Ni, and V, whereas Mn and Zn pose no risk. Mn and Zn mainly originate from natural processes, while Cr from anthropogenic process. V, Pb, and Ni are most likely from mixed sources. The ecological risk of heavy metals increases significantly when the Yellow River passes through the Loess Plateau. Furthermore, compared with SQGs and C f , the assessments of heavy metal contamination by I geo , RI, mC d and PLI are more objective and accurate. This study provides a powerful reference for contamination control and ecological security in the Yellow River Basin. [ABSTRACT FROM AUTHOR]

• The K sat of Zn-contaminated loess increases with time, while a decreasing trend for Cu-contaminated loess. • The hydrolysis of Zn2+ and the DDL effect are beneficial to the development of water passage. • The Cu 2 O precipitation in the Cu-contaminated loess is the main controlling factor for the weakening of pore connectivity. • The relationship among geochemical action, microstructure evolution, and seepage behavior of heavy-metal contaminated loess is revealed. Heavy metal contamination in soils can pose severe challenges to the safety of geotechnical engineering projects. Loess, which is widely distributed in Northwest China, is a preferred engineering construction material for anti-fouling barriers. Therefore, research on the influence of heavy metal ions on its seepage performance is urgently required. To obtain new insights into the seepage behavior of heavy metal-contaminated loess and its underlying geochemical mechanism, laboratory investigations were performed on the saturated hydraulic conductivity (K sat), leaching, and microstructural characteristics of loess contaminated with Cu2+ and Zn2+. The results indicate that the hydrolysis of Zn2+ creates an acidic environment, which promotes the dissolution of carbonate minerals in loess, enhances the leaching capacity, and leads to the quantitative transformation of small pores (2–8 μm) to mesopores (8–32 μm). Meanwhile, the alternating adsorption of Zn2+ and its diffuse double-layer effect compresses the diffusion layer, increasing the abundance of free water channels. Thus, the K sat of Zn-contaminated loess increases by 81.2% during the seepage period. As for Cu-contaminated loess, its seepage behavior is the opposite of that of Zn-contaminated loess, with a K sat decrease of nearly 50%. The primary factor controlling this phenomenon is the formation and enrichment of Cu 2 O in the lower part of the soil, which inhibits the enlargement of pores and reduces the effective connectivity of pores. The findings of this work provide insight into the seepage behavior of saturated loess under erosion by heavy metals and the underlying geochemical mechanism thereof. [ABSTRACT FROM AUTHOR]