Remobilization of Metals from Tidal Flats of the Changjiang (the Yangtze River) Estuary during Simulated Sediment Resuspension

In rivers which endured extensive damming, as the Changjiang has, changes in suspended material concentration and transformations of the estuarine environments from depositional to erosional are expected to occur. This can result in serious environmental degradation since it allows reintroduction of sediment bound chemicals (e.g. trace metals, organic pollutants…) into the water column. To assess whether the sediments deposited within the Changjiang estuary can become a significant pollution source, this study investigates remobilization of potentially toxic metals during simulated resuspension of surface sediments from the river mouth tidal flats (Fig. 1). Experiments were carried out with river water as extracting agent. The sedimentological and geochemical characteristics of the deposits (grain size, metals concentrations in labile sediment phases, etc.), and their surface physico-chemical properties (SSA and CEC) were determined. The results revealed that adsorption/desorption behavior of metals during sediment resuspension is somewhat site-dependent. Only for Mn, Cu and Co continuous increase in concentrations was observed regardless of the sampling station, whereas behavior of Fe, Ni, Zn, Cr, Pb and Cd varied. Pronounced release of metals was observed during the resuspension of sediments with high content of fine grained particles and high concentrations of labile phase metals, e.g. MTB station (Fig. 2). Elevated concentrations of dissolved Fe and Mn under oxic conditions indicate their presence in colloidal form. The behavior of Pb, Cr, Zn and Cd follows the behavior of Fe, thus also indicating release of metals in the form of colloid complexes. Continuous release of Cd and Ni into the dissolved phase was observed at HPRM station, indicating possible enrichment of sediments with these metals. The results otherwise indicate that resuspension of the Changjiang estuary tidal flats sediments does not result in significant release of metals into the water column.