Consequences and implications of dredging remediation on sediment metal variation in a highly-industrialized urban river

This study evaluated the dredging effectiveness in the heavily metal-contaminated Salt River, located in an industrial area in southern Taiwan, after sediment remediation in 2019. A total of six sampling sites with nine seasons of sediments were collected to compare the variations in metal contents (Cu, Zn, Pb, Cr, Cd) before and after dredging. Sediment quality indices [geo-accumulation index, modified degree of contamination (mCD), pollution load index (PLI)] and ecological risk indices [mean probable effects concentration quotient (mPECq), potential ecological risk index (PERI)] were used to assess the improvement of sediment pollution levels and the associated ecological risks after dredging remediation. The overall dredging effectiveness showed sediment pollution reduction efficiencies of 8, 26, 38, 26, and 30% for Cu, Zn, Pb, Cr, and Cd, respectively, indicating that dredging remediation can improve the river sediment quality. However, the pollution levels and ecological risks associated with the post-dredging sediments were still classified as high pollution levels (mCD: 8.3; PLI: 6.5) and considerable incidences of ecological toxicity (mPECq: 1.4; PERI: 403). This can be attributed to sediment dredging on the river upstream, which may cause pollutants to be redistributed via the resuspension process and accumulate in the midstream. In addition, one year after dredging, the sediment metal contents increased by 33% for Cu, 88% for Zn, 27% for Pb, 11% for Cr, and 11% for Cd, compared with those within eight months after dredging. It reveals that for the long-term effectiveness of river pollution improvement, surrounding pollution source control, sewer construction, and regular dredging are still required. This study offers valuable insights into river management and dredging strategies to solve the increasing urban river pollution issues.