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

Abstract

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 finegrained 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, 137 Cs 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 activitiesespecially 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.

Effects of Tailing on Surrounding Water, Soil, Plants and Sediment

“…The tailing slurry is discharged into river, the tailing sinks and then impacts on sediment directly; on the other hand, tailing wastewater is discharged into river, the toxic and harmful substances release into river and partition between water and sediment, and then accumulate in sediments in the end. Zhang et al [26] investigated the stream sediments of Tonglin mining area and found that Xiangsi River sediment was uniform particle size tailing sands, while the particle size of Shengchong River sediment was coarser because of mining activity, and its pH value was high, reaching 10.6. The toxic and harmful substances such as heavy metal, sulfide and cyanide precipitate, diffuse and migrate between water and sediment.…”

Mixed response of trace element concentrations in fluvial sediments to a flash flood in a former mining area

“…However, they are mostly washed out of the catchment, as floodplain deposition occurs during receding waters [14]. During the following stages, the situation depends on the sediments eroded: while some studies report high discharges leading to intensified input of pollutants into the fluvial system (e.g., by erosion of pollution sinks) and, in turn, to enhanced concentrations of contaminants in flood sediments [4,14,15,[31][32][33][34], others provide evidence for the limited influence of flooding on pollutant levels [35,36] or even decreasing of pollutant concentration due to flushing of the catchment [37,38] or suggest dilution with 'clean' sediments [26,30,[39][40][41][42]. Depending on the pollution intensity and distribution within the catchment, there is a variable discharge threshold above which additional pollutant sources outweigh the dilution effect [43].…”