Assessing the environmental risk, fractions, and remobilization of copper and zinc in the sediments of the Jialing River—an important tributary of the Yangtze River in China

Copper (Cu) and zinc (Zn) are two heavy metal pollutants that pose a serious risk in the Jialing River. Cu and Zn are transported into the sediment primarily due to the activities of the mining and smelting industries. In this study, we employed the diffusive gradient in thin films (DGT) technique, sequential extraction, and two assessment methods to evaluate the remobilization, fractions, and environmental risk in the downstream section of the Jialing River. The total concentrations of Cu and Zn in the four study areas followed the order S3 > S2 > S4 > S1, and the assessment results indicated that Cu and Zn presented a low environmental risk in the study area. Cu and Zn were primarily bound to the Fe/Mn oxide fraction (F2) and the residual fraction (F4). The results of the DGT probe showed a clear vertical distribution of Cu and Zn in the sediment (from 3 to − 12 cm), and both elements showed obvious increasing trends at the bottom of the probe. The correlation analysis indicated that C_DGT-Cu correlated well with C_DGT-Zn (r = 0.834, p < 0.01). The flux results showed that the sediment in the downstream section of the Jialing River is a major source of Cu and Zn and that there is a potential risk of release to the overlying water. Further analysis found that C_DGT-Fe was negatively correlated with C_DGT-Cu and C_DGT-Zn, indicating that Fe may influence the remobilization of these metals. In addition, a hotspot of C_DGT-Cu and C_DGT-Zn at the bottom of the probe corresponded with a dark area in the AgI gel measuring C_DGT-S. These results indicate that Fe and S are factors that mitigate the release of Cu and Zn from sediments.