Mobility Of Heavy Metals In The Mud And Environmental Impacts In Coastal Reclamation Areas

Over the last 20 years, land reclamation has gained much popularity along coastal areas of China due to rapid development of industry and urbanization. While reclamation provides valuable land, it also creates various engineering, environmental and ecological problems in coastal areas. For continuable development, it need to understand more about the relationship between reclamation activities and environments, especially when the reclamation areas locate at estuary where had a thick layer marine mud at the sea bed. Mud is a kind of sediments. As kown, estuarine sediments are recognized as an important sink for a wide range of contaminants and nutrients. They can provide a record of the natural and anthropogenic inputs of contaminants into the aquatic environment. Heavy metals have a high affinity for fine-grained marine mud and are the key contaminants in estuarine sediments. The accumulation of heavy metals in mud can cause significant environmental risks in the surrounding areas. When the reclamation actives begin, usually the area to be reclaimed is drained first and some of the mud is air-dried for a few weeks before it is buried by fill. After reclamation, the terrestrial groundwater, which is relatively acidic and with high dissolved oxygen, will gradually displace the seawater, which is alkaline with high salinity. How dose the reclamation actives impact on the coastal environment. How dose the mobility of the heavy metals present? What are the right methods to prevention and cure the contaminated groundwater? Using land reclamation areas in Shenzhen as a case study, this paper aims to investigate how the reclamation actives impact on mobility of heavy metals between the mud and groundwater. Sequential extraction and desorption experiments were used in this study. Concentrations and speciation of heavy metals in the coastal areas were determined to examine the potential mobility of metals within mud-groundwater system inside the reclamation areas. Major creative outcome has been obtained as follows in this paper:(1) Aim at understanding the special environmental geology engineering problems such as that of large-scale reclamation, the interactional processes of fill material, mud and groundwater were studied in this dissertation. The environmental geology problems of reclamation were system analyzed, and test techniques, analysis theories and estimate methods were built up which were related to reclamation. (2)It were discussed that the disturbances to the groundwater and mud brought by reclamation engineering processes. The trend of the main physics and chemistry parameter of groundwater was concluded. Field survey confirms that the pH and EC of the groundwater in the reclamation site are much lower than the seawater. The reasons of the groundwater freshed were well illuminated. At the same time, the redox environments of marine mud were also changed during reclamation. Mineral structures of the marine mud were tested by scan electron microscope, and the concentrations of heavy metals in the mud were analyzed by atomic absorption spectrum. Considering all above factors including the general characteristics of heavy metals mobility, a forecast were deduced that the changes in the burial conditions of mud and the physical and chemical properties of the pore water in the mud may induce the release of some heavy metals in the mud.(3)To find out the ingenerate factor controlling the mobility of heavy metals, sequential extraction method was used to study the operationally determined chemical forms of five heavy metals(Cu, Ni, Pb, Zn and Cd) in the mud s-amples. If the residual fraction can be considered as an inert phase of the metal that cannot be mobilized, it is the other four forms of heavy metal that cause the noticeable changes in the concentration of heavy metals in mud. On the basis of the speciation of heavy metals, the mobility of metals have the following order: Pb＞Cu＞Zn＞Ni＞Cd.(4)In order to find out the key environmental factors that influence the mobility of heavy metal in the mud during reclamation, desorption experiments were carried out in the lab under the simulating conditions. It was found in the experiments that release of Cu, Mn, Ni, Co, Cr and V from the mud increased when changing the redox conditional of mud and levels of pH and EC in the overlying water.(5)Chemical analyses of groundwater samples collected from the reclamation sites reclaimed indicate that most of the heavy metals in groundwater are increased gradually with time. Fuzzy mathematics method and index number methods were used to evaluate the quality of groundwater in the reclamation area, and the result indicated the groundwater quality was worse. These results confirmed the forecast that heavy metals released from mud to the groundwater in the reclamation area, after other pollution sources were foreclosed by field survey in the study area.(6)The disciplinarians of heavy metal mobility were concluded according to the results of lab experiments and field survey. The relevant prevention and cure measures were selected to provide a reference. Botanical restoration was suggested to apply in the Shenzhen-Hongkong West Corridor reclamation area. Some kinds of plant were recommended, such as, PhytolaccaocinosaRoxb, Phytolacca americana L., Sedumalfredii Hance, Arabis Paniculata L. which have special capacity to absorb heavy metals.