Calculation of the environmental dredging depth for removal of river sediments contaminated by heavy metals

Abstract

Sediment contamination by heavy metals can result in significant damage to the ecological water environment. Sediment dredging is a useful way to reduce the adverse effects of heavy metal pollution in freshwater. The dredging depth is a key parameter in environmental dredging engineering. In this paper, we propose an innovative method called the critical-risk-depth method for calculating the environmental dredging depth that has been specifically designed for removal of river sediments contaminated by heavy metals. To determine the critical risk depth for dredging, the heavy metal concentrations at different sediment depths and their potential ecological risks must be tested and evaluated. The first step of the method involves analyzing sediments to determine the lateral and vertical distribution of heavy metals. In the next step, Hakanson’s potential ecological risk index is used to assess the ecological risk of heavy metals at different sediment depths. Finally, the recommended environmental dredging depths are calculated based on the potential risk for change in the vertical distribution and the given threshold level for the potential critical risk from heavy metals. We carried out a case study to determine the dredging depth for river sediment in Pinghu. The sediment analysis results show that the contents of Cd, Zn, and Pb are excessive when compared with the local soil background levels. Because of the accumulation effect of heavy metals in sediments, the heavy metal contents tend to decrease with sediment depth, but this trend may change as a result of human activities and other river dredging events. There is a high potential ecological risk level from heavy metal pollution in sediments in the study area, and the recommended environmental dredging depths of the ten rivers range from 35 to 100 cm.