Transformation And Environmental Effect Of Mercury And Some Heavy Metals In Yangtze River Estuary And Its Adjacent Area

Heavy metals, as an important pollutant in water and atmosphere, can accumulate in the body of animals, plants and even human beings through the food chain, causing healthy problems in people. Mercury, the only kind of heavy metal element in liquid in nature, is one of the most toxic heavy metals commonly found in the global environments. It can be transported and deposited into lakes, reservoirs, estuaries and oceans by atmosphere and rivers. Methylmercury (MeHg), a sort of organic compound of mercury, is easy to penetrate the biofilm and accumulate in food chain, harming to human central nervous system. The Yangtze River Estuary, with high turbidity, and its adjacent mud area are the typical land-ocean interaction zones in the world. The low content of dissolved oxygen and abundant organic matter in this zone promoted the generation and transformation of mercury methylation. Systematical survey and discussion on the transformation and environment effects of mercury and some other heavy metals in the Yangtze River Estuary and its adjacent area can not only help to find the laws of the transformation and enrichment of heavy metals, but also have important significance to the protection of marine environment, national food safety and the development of our society and economy.This research is supported by the National Basic Research Program (973 program) of China (No.2002CB412409) and the National Natural Science Foundation of China (No.41076022). Two cruises, the cruise of synthetic investigation of Yangtze River Estuary environment and the spring cruise of National Natural Science Foundation, were carried out in June,2003 and 2012 respectively. During these two cruises, not only samples of suspended matters, seawaters and sediments were recovered, but also in situ data such as temperature, salinity and fluorescence of seawater and the speed of ocean current were measured. The concentration and heavy metal content of suspended matters were determined in the turbidity maximum zone around Yangtze River Estuary. Combining with the current speed, the temperature and salinity of seawater, the variation of heavy metal contents during a tidal cycle were analyzed and their fluxes were also estimated, at the same time, the influence factors were discussed.This thesis also focuses on the transformation processes and environment effects of mercury in the Min-Zhe coastal area. The bulk contents of mercury in seawater, suspended matter and sediment samples from three sites were determined and MeHg in surface and core sediment samples were also obtained. The vertical distribution profiles of dissolved and particulate Hg in the water column were demonstrated and the transformation mechanisms between them were discussed. Distributions of bulk mercury and MeHg in three core sediment samples from the Min-Zhe coastal area were also demonstrated and methylation mechanism of mercury and influencing factors were systematically analyzed. Combining with results of the 210Pb dating, the mean sedimentation rates and mean annual deposition fluxes of mercury and MeHg in mud area were estimated and the deposition record of mercury in the coastal mud area was established. Centennial-scale response of mercury to human activities was also explored.The main conclusions of the thesis are as follows:(1) For suspended matters from the turbidity maximum in Yangtze River Estuary, the average contents of heavy metals during a tidal cycle are in the order of Zn>Ni>Pb>Co>>Cd. Average contents in ebb tides are generally higher than those in flood tides. Zn、Pb、Co and Ni, mainly terrigenous, are transported towards east-northeast, settling down with the suspended matters in the alongshore area. Influenced by marine-derived materials, Cd is transported towards northwest. The source of heavy metals, the salinity of seawater and the concentration of suspended matters are the main factors controlling the distribution of heavy metals in suspended matters during a tidal cycle. There is a positive correlation between the content of heavy metals and the salinity of seawater, while the opposite correlation between the content and the concentration of suspended matter.(2) The content of bulk mercury in suspended matter samples from mud zones along the Min-Zhe coastal area is between 1.20×105ng/kg and 16.53×106 ng/kg and the dissolved concentration of mercury is from 0.26 ng/L to 2.69 ng/L. The transformation of mercury between particulate and dissolved phase is mainly controlled by the adsorption-desorption of suspended matters. At the same time, it is affected by the chelation of dissolved organic matters, the complexation of Cl-1 with Hg2+, the oxidation-reduction conditions near the bottom and the releasing of mercury from surface sediments to their overlying water. The chelation of dissolved organic matters and the inrush of the seawater with high salinity can promote the desorption of mercury from suspended particles. The releasing of mercury from surface sediment to its overlying water and the oxidation-reduction conditions near the bottom also have effect on the transformation of mercury between bottom water and surface sediment.(3) Mercury and MeHg are enriched in the sediment samples from the Min-Zhe coastal area (the alongshore area of Zhejiang to the south of Hangzhou Bay). Methylation in core sediments in mud zones mainly occurs in the shallow depth from surface (the depths are respectively 0～15 cm,0～8 cm and 0～13 cm in northern, middle and southern study areas). In Min-Zhe coastal mud area, MeHg mainly comes from in situ methylation. Downcore variations of environmental conditions can lead MeHg to be transferred or reassigned during the diagenetic process. Influencing factors of methylation and downcore distributions of bulk mercury and MeHg in sediments include sources of pollutions, bottom current patterns, grain-sizes of sediments, contents of TOC and activities of sulfate-reducing bacteria. Contaminant and circumfluence primarily control the distribution of mercury in the sediments. Grain-size affects the degree of enrichment of MeHg. The effect of TOC on the methylation is not obvious in surface sediments. In cores, the TOCs can not only promote the methylation, but also inhibit it, mainly depending on the type of organic matter in sediments. The activity of sulfate-reducing bacteria is one of the main controlling factors of mercury methylation.(4) Combining with results of the 137Cs and 210Pb dating, the mean sedimentation rate is estimated to be 0.11cm/a for the core DH7-2 (the southern mud zone in Min-Zhe coastal area). The mean annual deposition fluxes of mercury and methylmercury here vary from 6.63 ng/(cm2·a) to 60.96 ng/(cm2·a) and from 15.86 pg/(cm2·a) to 272.45 pg/(cm2·a) respectively. Most mercury transported by Yangtze River to the Min-Zhe coastal area, settled down in a narrow zone along the shore. In the past one hundred years, the mercury contents and contamination levels had a positive correlation with human activities. With the rapid development of Chinese economy, mercury content in the nearshore sediments gradually increases, and the degree of contamination increased seriously.