REVIEW OF ACCUMULATION FEATURES STUDY OF HEAVY METAL IN SEDIMENT OF TIDAL FLAT

Abstract

The achievements and current knowledge of heavy metal study in tidal flat sediments were reviewed based on the data from national and international literatures. It is shown that the heavy metals in tidal flat are mainly derived from the products of disintegration of minerals and rocks, human activities, atmosphere and ocean. Various factors, including water dynamics, sediment components, grain size, biological activities, river discharge and human activities, govern the accumulation features of heavy metals in tidal flat. Heavy metal concentrations decrease from high tidal flat to the middle and low tidal flats. Along the tidal flat, the stable and accumulating banks are usually enrich in heavy metals, while the eroded bank is poor in heavy metal contents. The discharges of municipal sewerage or industrial wastes have an important impact on the distribution and accumulation of heavy metals over tidal blanks. For example in Venice lagoon and Jinzhou Bay, heavy metal contents decrease exponentially with the distance from sewage sources. But in other areas the metals is modified by dynamic conditions. In Shanghai tidal flat, metals contents peak in about 1 mile from the entrance of sewerage. Although the vertical profile of heavy metals in different tidal flats vary due to the different backgrounds and dynamics, the vertical profiles reflect the history of pollution and the peak content of heavy metals is consistent with the maximum of metal discharge from human activities. Sometimes the same vertical profiles of the various elements exist in different cores. For example, lead has come from long-range atmospheric deposition of anti-knock compounds in gasoline, copper and zinc are derived either from pollution discharges, natural resources in soils and are transported to the flat via surface waters, rather than atmosphere. Their close relation suggests that they are mixed in some local or regional environmental compartments before delivered to the area. Biological activities are important to the accumulation and migration of heavy metals in tidal flat because of its biodiversity and toxicity. The morphology of biological bodies, biological availability governs the heavy metal accumulation in tidal flat and the accumulation in biological bodies. Biological activity changes the micro-environment of the tidal flat. Although recent publications contain heavy metal accumulation in biological body's transportation and accumulation of heavy metals caused by biological bodies, the quantitative study is still rare. Examples of quantitative work, can be found for mangroves. At Itacurussa experimental mangrove of Brazil, the stem, root and leaves are the pathways of the transportation of heavy metals. The average transfer rates of heavy metals from canopy to sediment through leaf fall were: Mn=0.097kg/hm 2·a, Fe=0.049 kg/hm 2·a, Zn=0.002 kg/hm 2·a. In fact mangrove ecosystems are probably efficient biogeochemical barriers to the transport of metal contaminants across tidal flat. The concentration of heavy metal in tidal flat illustrates the time cycles due to the seasonal changes of plant. For example, heavy metal increased during the growing season with Cu peaking in july and Pb and Ni peaking in september. Methods of heavy metals in sediments also appropriate for the heavy metal study in tidal flat, such as acid and alkali digest methods, Tessler's sequential extract method for metals from sediments, and other methods, like laboratory micro-universal system approach, mesocosm and numerical model, regressive equations and magnetic measurements can be applied to different studies at ecological cycles and resources of heavy metal in tidal flat. Great progresses have been achieved in the past 30 years in the study of heavy metals of tidal flat , but comparing with the study of estuaries, the heavy metal study of tidal flat is few. (1)The horizontal and vertical distribution of heavy metals in tidal flat need further study, especially the quantitative assessment. (2) Transportation pathways of different resource of heavy metals,