Magnetic Properties And Heavy Metals Of Mainstream Sediments In The Middle And Lower Reaches Of The Yangtze River

There is a variety of environmental information contained in the river sediments. Lots of methods have been introduced to research the information stored in the sediments. It's an important developing direction in the Environmental Magnetism to use river sediments' magnetic property to track material source, extract environmental information and assess environmental pollution. The content of heavy metals is an important index to evaluate the quality of the river. Since the middle and lower reaches of the Yangtze River belongs to one of the regions where the economy developed fast and the urbanization reached a high level, the health of water environment is a hot environmental topic. Because of sampling problems and other limited factors, past investigations of the river sediments in the mainstream of the Yangtze River mainly focused on the Yangtze River Estuary, Three Gorges Reservoir area as well as part cities or areas of the mainstream, and they are lack of the systemic study of the whole drainage area.In this work we investigate the sediments of the mainstream of the middle and lower reaches of the Yangtze River and the Yangtze River Estuary. Samples are from the central mainstream, the beach, the tributary, the surface water and the plant such as Spartina alterniflora, Phragmites australis and Scirpus mariqueter in Yichang, Jingzhou, Chenglingji, Ezhou, Jiujiang, Hukou, Anqing, Tongling, Wuhu, Nanjing, Jiangyin and Zhenjiang etc.. Sampling works have been done before the Three Gorges Reservoir stored water to the altitude of 156m in 2006. Through comprehensive use of Environmental Chemistry, Environmental Magnetism, Ecology, Pedology, Hydrology etc. as well as testing methods, heavy metals geochemistry and environmental magnetic property of sediments in the middle and lower reaches of the Yangtze River are discussed.Following results have been acquired:(1) The magnetic property of sediments in the middle and lower reaches of the Yangtze River is dominated by the ferromagnetic magnetite, with the particle size being pseudo single domain (PSD) - multi-domain (MD). From the middle reaches to the lower reaches, the content of magnetic minerals is decreasing, and their particles are fining. It's the same from the central river to the beach. The content of imperfect anti-ferromagnetic materials in the tributary is higher than that in the mainstream, and their particles are finer. With sand from tributaries inflowing into the Yangtze River, the magnetic property in the mainstream changed accordingly. The material from tributaries is an important factor to the change of the magnetic property in the mainstream.(2) Contents of heavy metals and bio-available heavy metals are low in the central river. They are in the scope of background value of soil elements in this drainage area. So there is almost no pollution due to heavy metals. Sediments in the beach and tributaries have been polluted by heavy metals. The content of elements related with urban contamination draining such as Cu, Pb, As, Ni is high. The content of bio-available heavy metals is higher in the beach and tributaries. So heavy metals in the beach and tributaries are delivered more easily through food link and do greater harm, which should be paid more attention. The distribution of heavy metals is of great difference from the middle reaches to the lower reaches. The particle size is closely correlated with the distribution of heavy metals. The distribution of heavy metals fluctuates much steadier after the particle size correction.(3) The content of mercury is lower than the environment quality standard value for soil- category I in the central mainstream of the Yangtze River, mostly in the scope of the natural geological context. The content of mercury in the beach is obviously higher than it in the central mainstream of the Yangtze River, which shows that waters near the beach are influenced more frequently by human activities. The content of mercury in the beach shows such characteristics, that is, it is lower in both the upper reaches and the lower reaches and higher in the middle reaches, and the highest value appears in the area from Jiujian to Tongling, which is possibly related with the exploitation of mineral resources.(4)The plant absorbs heavy metals by the root mainly. When absorbing heavy metals, all heavy metals are absorbed into the plant root and selective absorption is not found. It shows regional differences when the plant absorbs and accumulates heavy metals. The content of heavy metals in Phragmites australis on the north shore is generally higher than that on the south shore. It's possibly related with bank protection works of the south shore that damaged the natural ecosystem of the tidal flat. The content of heavy metals in Scirpus mariqueter of bao-town is higher than the others, most likely because of the pollution from the thermal power plant near around. The ability of Spartina alterniflora on absorption and accumulation of heavy metals is higher than that of Phragmites australis and Scirpus mariqueter. Distributions of heavy metals in Phragmites australis and Spartina alterniflora are both root>stem>leaf. The content of heavy metals in the part of plants aboveground (total of stem and leaf) is generally more than 50%. Phragmites australis as raw materials of the paper have enormous ecological effects on clearing heavy metals in the tidal flat.(5)The particle size is coarser overall in the central river, which is dominated by sand. The content of sand is 63%, and its average particle size is 112μm. The particle size is finer in the beach, which is dominated by silt. The content of silt is 63%, and its average particle size is 112μm. The particle size is the finest in tributaries, which are dominated by clay and silt. Contents of clay and silt are separately 39% and 52%, and their average particle size is 9.2μm.(6)The particle size is an important factor for the magnetic property. The susceptibility of fine-grained sediments is the highest in the middle and lower reaches of the Yangtze River, because in fine-grained sediments. The magnetite whose particle size is dominated by pseudo single domain and multi-domain (silt) accompany the cease quartz and feldspar (fine sand), and they are enrichment in fine sand. However, when the susceptibility is tested separately in three different particle sizes the susceptibility and the particle size change oppositely, that is, with finer particle the susceptibility is higher. The particle size is also an important factor for the content of heavy mental. Different particle sizes have different abilities to adsorb heavy metals. Heavy metals such as Cu, Mn, Pb, As, Ni and so on are mainly in the fine-grained sediments, and show similar properties. On the contrary, heavy metals like Sr, Cr and Zr are mainly in the coarse-grained sediments.The innovation of this dissertation: (1)Heavy metals geochemistry and environmental magnetic properties of sediments in the middle and lower reaches of the Yangtze River were discussed by comprehensive use of Environmental Chemistry, Environmental Magnetism, Sedimentology, and Ecology etc.. The database was established, which could provide scientific comparing parameters for following studies after the full closure of the Three Gorges Reservoir.(2)It was thoroughly investigated on the content of heavy metals and bio-available heavy metals, the pollution, distributions and affecting factors in the mainstream of the middle and lower reaches of the Yangtze River, which provides a basis for taking measures to prevent and treat heavy metals contamination.(3) The plant's ability to absorb and accumulate heavy metals, regional differences and affecting factors were investigated. Also bio-availability of heavy metals was discussed(4)The particle size's affection to the magnetic propertity and heavy metals' characteristics was discussed in this dissertation.