Low concentration of heavy metals in the Yangtze estuarine sediments, China: A diluting setting

An examination of the spatial and temporal distribution of 17 major heavy metals, i.e., Fe, Zn, Ni, Mg, Co, Mn, etc., was made in the Yangtze estuarine sediments revealed by a number of vibrocores taken from the different sediment zones, including delta front, prodelta, and delta - shelf transition. The results obtained from the numerous core samples, which were also measured for Pb-210 and Cs-137, show that: (1) the silty clay comprising the prodelta facies serves as a depositional sink attracting high concentrations of heavy metals delivered from the river mouth; (2) after being normalized to aluminum (as a proxy for grain size), most heavy metals presented in the prodelta facies have lower concentrations than in the other adjacent sediment zones; (3) also after normalization, concentrations of most heavy metals in the vibrocore sediments tend to increase up-core; and (4) sedimentation rates in the study area range from ∼2.0 to 6.0 cm/a; hence, the vibrocores contain a sedimentary record of metal deposition covering more than 50 years. It has previously been assumed that sediments off the river mouth were heavily polluted due to industrialization of the Shanghai metropolitan area, which peaked about 50 years ago. However, the low concentrations of heavy metals in the study area before and after normalization do not support this assumption. The Yangtze estuary is characterized largely by the tremendous runoff and 'unpolluted' sediments derived from the upper drainage basin to constitute a unique diluted setting, in which the dispersal behavior of heavy metals from the adjacent industrialized coast is influenced substantially. As a result, the heavy metals in the study area are obviously lower than those previously determined along the coast.