| 其他摘要 | The heavy metal contamination related to mining is increasingly serious. The mineral resource of lead (Pb) and zinc (Zn) is abundant in China, and the consequent cadmium (Cd) pollution during mining and smelting is quite serious and presents a high environmental concern. This thesis aims to understand the geochemical distribution and dispersion of Cd and other associated heavy metals to assess their ecological environment effects in the Jinding Lead and Zinc Mine area in Yunnan Province, China as a case study. The cope of work of this thesis focuses on the study of distribution and dispersion of Cd in rocks, ores, soils, waters, river sediments and vegetation and their sequent environment impacts, and the following findings are obtained.
1. The ore leaching experiment shows that the oxidized ores of Pb and Zn are susceptibly to be oxidized or dissolved and readily release Cd and other deleterious heavy metals into the ambient environment. These leached elements in the leachate may deposit rapidly with the order of Zn>Pb>Cd. The mineral of smithsonite in the oxidized ore is the key factor to drive Cd loss in leaching processes. In certain open system under rock and water interaction, the natural weathering of Cd-rich rocks and minerals shows potential environmental risk to the aquatic ecosystem of the local catchment.
2. The concentrations of Cd vary in different rocks and ores in the Jinding Pb and Zn mine area. In host rocks, Cd concentrations range from 50 to 650 ppm with an average at 310 ppm. In primary ores, Cd concentrations range from 14 to 2800 ppm with an average at 767 ppm. However, in the oxidized ores, Cd concentrations show the highest enrichment with a range of 110 to 8200 ppm and an average at 1661 ppm. The ratio of Zn/Cd in primary ores is higher than that of oxidized ores, which applies that Cd is rich in oxidized ore but Zn is released to the environment. The concentrations of Cd and Ca in oxidized ores present negative correlation and it suggests the Cd enrichment and Ca loss or isomorphic substitution of Cd for Ca in the weathering progress.
3. Cd in soil outside the Jinding Pb and Zn mine area presents high concentrations, which is above the background value of China soil and may be attributable for the reiogional anomaly of Cd. However, the high concentrations of Cd causing Cd pollution in soil of the mine area are related to mill factories, tailings piles, waste rock drainage and bedrock of mine. Cd concentrations in soil along the Bijiang River are above the baselines of both the background area and attributable for the agricultural irrigation by Cd-polluted river water. Based on the weighted pollution index assessment, Cd is the main metal pollutant in the local soil rather than Zn and Pb by the order of Cd> Zn> Pb. The metals pollution in soil tends to disperse from the mine area to the downstream of Bijiang River.
4. The mining activities of Jinding Pb and Zn Mine tend to release Cd into the local aquatic ecosystem. The water bodies in mine area present high Cd concentrations from 15 to 30 µg/L, exceeding 50 to 100 times of natural water. The dispersion pattern of Cd in the local water system presents the tendency of shallow groundwater of mining area > stream water of mine area > river water of Bijiang River. The river water of lower reach of Bijiang River is obviously polluted by Cd with an average at 15.7 µg/L in water, 49.3 mg/kg in suspended sediment, and 203.7 mg/kg in river sediment. The enrichment of Cd in the local aquatic system of the mine area is related to the natural weathering process of Cd-rich rocks and minerals, and presents high ecological risk.
5. Cd concentrations also vary in the mining solid wastes. The waste rocks present low concentrations of Cd at Paomaping waste rock dump but higher Cd contents in Beichang and Jiayashan waste rock dumps due to the differences of the types of waste rocks in individual waste rock dump. In respect to Cd concentrations in the profile of the tailing pond, they present a decreasing tendency with the depth in surface layer, irregular distribution in the middle layer, and significant enrichment in down layer. Concentrations of Cd in acid exchangeable and reductive forms in surface layer of tailing pond are higher than in deep layer, and suggest that Cd in surface layer of tailing pond is easily released to the ambient environment and pose a high environmental risk. Total Cd contents in the newly deposited tailing pond are lower than those in the old ones due to improved ore processing technology.
6. The determined δ34S values in polluted river waters are obviously lower than those of unpolluted river upstream waters of Bijiang. The δ34S value in the water bodies in mine area are similar to those in minerals from the mine area, and suggests for contribution of sulfur and associated metals from the mine area. Based on the theory of dilution of SO42- in water, about 85% S in water of Bijiang River is estimated to originate from the mine area.
7. The metals dispersed from the mining activity tend to enrich in certain local vegetation. Four species of herbaceous plants are identified as hyperaccumulators for Cd, Zn and Pb. Among them, the species of Potentilla lancinata Card In Lecomte and Galinsoga parviflora Cav. are hyperaccumulators for Cd, Equisetum ramosissmum Desf. for Zn, and Picris hieracioides L. for Pb. These four identified hyperaccumulators are significant for ecological remediation on local polluted soil by Cd, Zn and Pb.
8. The geological environment model of Jinding Pb-Zn mine was proposed. This model effectively demonstrates the processes of Cd release and transportation in the ambient environmental of the Jinding Mine area. The Cd-rich acid mine drainage produced from rock and ore weathering tends to be dispersed and transported to the ambient soil and water bodies. The Cd released into soil causes soil pollution. The Cd transported into water was adsorbed by goethite and calcite to be deposited in river sediments. The Cd in both river water and river sediment tends to deteriorate the local aquatic ecosystem quality. The enrichment of Cd released from the mining activity may finenally enter the food chain to cause human health problem. |
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