Sediment characteristics, floods, and heavy metal pollution recorded in an overbank core from the lower reaches of the Yangtze River

Overbank sediments contain abundant information about floods, paleoenvironmental changes, and recent pollution history of a river basin. For the purpose of understanding modern overbank sediments and their environmental significance, this multidisciplinary study sampled a core from the overbank and a short core from the overbank outcrop at Nanjing in the lower reaches of the Yangtze River. The overbank deposit comprises fine-grained discontinuous strata, including silt, sandy silt, silty sand, and sand layers. Field and laboratory observations indicated that the overbank sediment contained a series of layers of sandy silt coupled with thin silty sand layers, each a few millimeters thick, which formed muddy layers that were dozens of centimeters thick. Between the muddy layers were coarser sand layers. Grain size analyses, Cs-137 dating, and heavy metal measurements contained information on flood events and pollution history over the past 60 years. The coarser sand layers corresponded with severe floods in 1954 and 1983. Between the two severe floods, the study area experienced an overbank building stage. From the 1990s to the early 2000s, the overbank sedimentation rate declined, reflecting human activities-especially the dams in the Yangtze River Basin that trapped more sediment. The average heavy metal concentrations in the overbank sediments are higher than the background levels for suspended sediment in the Yangtze River. Heavy metals in overbank sediments were assessed by enrichment factors and sediment quality assessment values for freshwater sediment. Cr and Ni values were high, especially from the early 1980s to the early 2000s. The impacts of floods, economic development, and environmental management on these heavy metal variations were discussed.