Historical changes in heavy metals in the Yangtze Estuary
The Yangtze Estuary, China, is being confronted with serious environmental problems related to industrial development and population growth. Heavy metals were used as the environmental indictor of the 'health' of the Estuary in this study. The main purpose of this thesis is to determine historical variations of heavy metals and possible anthropogenic impacts in the Yangtze Estuary cores taken from tidal flats. Two 6-m cores were taken from tidal flats on Chongming Island and Nanhui on the mainland. An attempt to use 20Pb chronology in the cores was unsuccessful because of high sedimentation rates and small sample sizes. Factor analysis indicated that grain size, local biological input, and drainage basin erosion are the main controls on heavy metals in the Estuary. The likely anthropogenic impact in heavy metals was examined by using normalized data (by % <20 [mu]m) to remove the grain-size effect Only Cu in the core taken from Nanhui shows an increasing trend. Copper is the only normalized metal that is normalized metal that is strongly correlated with organic carbon, both of which may reflect anthropogenic discharges. The environmental assessment was performed by comparing values at the top of the cores with standards determined by the Shanghai Drainage Administration Division. Zinc, Pb and As exceed the standard and Cr is much lower than the standard. This, however, does not reflect anthropogenic contamination because their normalized data stay constant or show a slight decrease toward the surface. Concentration of Cu exceeds the standard slightly at Nanhui. A comparison of cores in the Yangtze Estuary with other estuaries shows that metal concentrations in the Estuary are generally lower than other contaminated areas.