DEPOSITIONAL RECORDS OF HEAVY METALS FOR THE LAST 140 YEARS IN THE ZHEJIANG COASTAL MUDDY AREA OF THE YANGTZE RIVER BASIN AND THEIR RESPONSES TO HUMAN ACTIVITIES

The sediment samples of the vibrocore DZ-28 taking from the Zhejiang coastal muddy deposits are analyzed for grain-size, ²¹⁰Pb and ¹³⁷Cs isotopic activities, concentrations of major elements and heavy metals (Cu, Pb, Zn, Cr, Cd, As, Hg) by the authors. In addition, the comprehensive contamination degree (C_d) and potential ecological risk (RI) are adopted to evaluate heavy metals contamination in the study areas during the past~140 years. The sedimentation rate of the core DZ-28 is 0.85 cm/a, based on ²¹⁰Pb dating. The contents of Al, Fe, K, Mn and Mg increase significantly since early 1980s and the grain-size shows a similar temporal distribution pattern, except the temporal variations of Si and Ca contents in an opposite trend. This may be caused by the construction of water conservation projects such as dam construction in the Yangtze River Basin and the extensive development of water and soil conservation work in the upper reaches of the river. The absolute concentration of heavy metals was nearly constant before early 1980s, and then it was substantially increased from the early 1980s to the middle 2010s, presumably due to the significant increase in heavy metals pollutants from the Yangtze River Basin. After the middle 2010s, the growth rates of absolute concentration of Cu, Cr, and Zn slow down, suggesting that the emission of heavy metals from the Yangtze River Basin has reduced likely due to the effective measures of environmental protection in China, but the Pb content remains significantly increased, possibly owing to the operation of Three George Dam (TGD). The economic risk in the study areas was minor during the last century as evidenced by the far low value of C_d and RI. Our research results demonstrate that the distribution and evolutional characters of heavy metals during the last 140 years in the study area are closely associated with the human activities in the drainage of the Yangtze River.