| The influence of mineral resources exploitation on ecological environment has been becoming increasingly serious. Particularly, a series of severe pollution events triggered by the exploitation of sulfide-rich metal deposit have received extensive attention of the world. The Yunfu pyrite, located in the Yunfu City, Guangdong Province, is one of the numerous sulfide-rich metal mines in the southern China. It is the China’s largest, and the world’s second largest pyrite. Therefore, it is a typical and representative object for study of the influence of sulfide-rich metal deposit exploitation on soil and water environment. On the other hand, the application of environmental magnetism in heavy metal monitoring attracts extensive attention owing to its a series advantages, such as cost-effectiveness, speediness and non-destructiveness. As shown by many previous researches, significant positive correlationships were appeared between soil magnetic properties and heavy metal concentration. However, the mechanism of the occurrence of magnetic minerals and heavy metals within soil are still unclear. Mining is the relatively single pollution source because the Yunfu pyrite is far away from the core industrial areas of the Pearl River Delta and the urban centers. Therefore, the Yunfu pyrite and its surrounding area should be an ideal area for mechanism research of heavy metal magnetic monitoring.Content, spatial distribution and transportation rule of trace elements were analyzed for the surface soil, mining dusts, water and sediments collected from the mining and its surrounding area. Based on the analytical results, heavy metal pollution and ecological risk were assessed systematically. Magnetic properties, including magnetic minerals composition, contents, and particle sizes distribution were determined for surface soil and sediments using environmental magnetism and mineralogy methods. The correlationship between magnetic parameters and heavy metal contents and the coexistence mechanism of magnetic minerals and heavy metal elements were studied intensively. The feasibility of using magnetic properties of soil/sediment to invert heavy metal content was discussed. Using heavy metal content as dependent variable and magnetic parameters as independent variables, the multiple stepwise regression equations were established. The reliability of these regression equations were verified using geochemical data. Through this study, the following understanding and conclusion at least can be deduced:1. Spatial distribution of trace elements within surface soil surrounding the mining area is mainly influenced by the parent material, sedimentary environment of parent rock, mining activities, acid rain, terrain and traffic and industrial emissions. Elements Nb, U, Th, Ga, Ge, Ti, Ta, Zr and Hf have similar spatial distribution with significant low values are appeared at the northeast and south sides, which should be mainly influenced by parent material. Elements Cu, Pb and Zn have similar spatial distribution with concentric distributed concentration contours around the mining area, and the nearer the mining area, the higher contents of elements, which is mainly influenced by mining activities. Spatial distribution of Mn and Co is similar, which is mainly impacted by the distance of parent rock sedimentary environment to their source. Element Rb and Cs have similar spatial distribution with a apparently low concentration area in the position of tailings, which is mainly affected by the leaching of acid rain. Element Sr, Cr and V have similar spatial distribution with a apparently low concentration area in the mining area. Sr and Cr contents are mainly affected by terrain and easy to enrich at low-lying place. Meanwhile, V is mainly affected by nearby traffic and industrial emissions. Ba and Ni have distinct spatial distribution pattern. Ba2+ should be combined with SO42- in Yunfu rainwater and be precipitated as barium sulfate(Ba SO4). As to Ni, it is mainly affected by nearby traffic and industrial emissions. Results of geological accumulation index(Igeo) showed that except Zr, Hf, Th and U, other measured elements were influenced by mine exploitation activities at different degrees. Compared to the national soil quality standard(GB15618-1995), Ni, Zn, Pb pollution and multiple metal combined pollution were only appeared for a few surface soil samples nearby the mining area.2. Contents of Co, Ni, Cu, Ga, Ge, Cd, Pb, U, Ti, Fe, Sr and Tl in downstream water were significantly affected by leaching water from the tailings. However, impact of leaching water on Cr, As, Ba, Zr, Bi and Th contents within downstream water is very limited. The p H value of water is a key influencing factor for trace element contents in stream water. No obvious precipitation is appeared at the acid discharge, indicating that the impact range of acid mine drainage is exceeded the study area. Compared to the national quality standard for agricultural irrigating water(GB5084-92) and integrated standard for wastewater discharge(GB8978-1996), serious As pollution, Cd pollution and As-Cd combined pollution were detected for most of water samples.3. Trace element contents within sediments were mainly influenced by ingredients of sediments, leaching water from the tailings, duck dung in channel and p H value of stream water. The ingredients of sediments are silt, clay and particulate matter in acid mine drainage. Results of geological accumulation index(Igeo) showed that different degree of Mn, Co, Ni, Zn, Ge, Sr and Pb pollution were tested for sediments from the Datai reservoir to upstream of Qingfeng hydropower station. It is mainly effected by the mining activities. From duck pond to Qingfeng village, sediments were appeared different degree of Ti, V, Cr, Cu, Ga, Cs, Ba and U pollution, which is mainly caused by duck dung in the channel. Duck dung have elevated p H value of stream water, and duck dung also has a better adsorption ability for metal cations. Compared to the national soil quality standard(GB15618-1995), most of sediment samples from the Datai reservoir to Qingfeng village were appeared Cr, Ni, Zn and Pb pollution and multiple metal combined pollution at different degrees.4. The numerical simulation results showed that the major area of dust deposition is about 4 km, 2.5 km and no more than 2 km away from the mining area along the southeast, northwest and other directions, respectively.5. Results of ecological risk index showed that ecological risk caused by mining activities has not yet brought to the surrounding environment. However, results of harmful index indicated a low As and Cd ecological risk in stream water.6. Magnetic minerals within surface soil around the Yunfu Pyrite are mainly magnetite, maghemite, hematite, goethite, iron-containing silicate and a new type of ferrimagnetic mineral with Curie temperature 530℃, may be titanomagnetite or iron-manganese spinel. Magnetic domain state of these minerals is mainly pseudo domain(PSD). Within surface soil, Co and Ni are coexisted with magnetic minerals through adsorption or cation substitution mechanism. Meanwhile, elements Ge, Cu, Zn and Pb are co-precipitate at surface soil with iron-bearing magnetic minerals. Mn can be coexisted with magnetic minerals in soil by adsorption, lattice displacement, and co-deposition. Magnetic minerals within sediments are mainly PSD magnetite, lepidocrocite and hematite.7. Magnetic properties of surface soil are mainly dominated by types, content and grain size distribution of magnetic mineral within soil. The spatial distribution of χ and χARM/SIRM showed that magnetism of surface soil surrounding the mining area was significantly elevated. The closer to the mining area, the higher content of magnetic minerals and coarser magnetic particles are appeared, indicating that soil magnetic enhancement was caused by coarse magnetic particles derived from mining activities. The spatial distribution of HIRM and S-ratio showed that relative higher amount of "hard" magnetic components(such as hematite and goethite), which was mainly generated from weathering process of tailings, was appeared at the closer region around the mining area.8. Correlation analysis between heavy metal content and magnetic parameters showed that only contents of Mn, Co, Ni, Ge, Cu, Zn and Pb were significantly positive correlated with content-dependent magnetic parameters for surface soil. Contents of Cu,Zn,Pb,Mn and Ge within surface soil are related to mining activities, and they can be inverted or monitored using parameters χlf, SIRM, and HIRM as proxies. Within sediments, only contents of Mn and Zn were significantly positive correlated with content-dependent magnetic parameters. Mn and Zn contents within sediments can be calculated by χlf and χARM. The verification results of multiple linear regression equation between heavy metal contents and magnetic parameters showed that magnetic parameters could reflect heavy metal element content for all samples. As for a specific sample, certain deviation was appeared between the predicted results and measured data. Therefore, more empirical research is needed to confirm and perfect the empirical formula. |
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