Use of multi-objective dredging for remediation of contaminated sediments: a case study of a typical heavily polluted confluence area in China.

Sediments in confluence areas are typically contaminated by various pollutants that have been transported there by inflowing rivers. In this study, we evaluated the pollution status of a confluence area in Lake Chaohu (China). Both the nutrients and hazardous pollutants were analysed. Most sediment cores showed large variations in nutrient concentrations at depths of 10 to 18 cm. Positive release rates of NH4(+)-N and PO4(3-)-P were detected in sediment cores. Hg and Cd were the most typical problematic metal contaminants encountered, and their contamination levels extended to depths of 20 and 25 cm, respectively. Polycyclic aromatic hydrocarbons (mostly acenaphthene and fluorine) were the primary persistent organic pollutants (POPs) present in sediments, and contamination levels frequently could be detected up to a depth of 16 cm. Simulated dredging operations were implemented in the laboratory, with a dredging depth of 15 cm found to be suitable for nutrient suppression. With the goal of suppressing nutrients release and removing high-risk metals and POPs, a multi-objective dredging plan was developed. This plan subdivides the confluence area into five parts that were treated with different dredging depths. A demonstration area was dredged in the most heavily polluted part, and the observed dredging effects were consistent with those expected on the basis of the plan. Such an approach to dredging might also be useful in other areas in the future.