Magnetic Response Of Heavy Metals Pollution Of Urban Topsoil In Northwestern China

With development of industry and acceleration of urbanization, the destruction and deterioration of ecological environment have increasingly become severe in urban areas. High quality and high efficient environmental protection, remediation and treatment should be based on the scientific environmental evaluation; therefore, it has been highly regarded by current international communities and governments. Comparing with tradional chemical methods, magnetic measurement is simple, rapid, exact, low-cost and non-destructive and it has been widely applied in environment monitoring and assessment.In this article, monitoring and differentiation of heavy metals pollution has been conducted based on the magnetic properties of different urban topsoil. Rock magnetic properties and heavy metal contents are studied on these samples and integrated data processing in lab. The aim of this work is to estimate and contrast soil pollution among different cities through the application of a fast and inexpensive methodology, characterize the anthropogenic magnetic particles in topsoil, discriminate the contributions of different pollution sources.Firstly, we established environmental magnetic semi-quantitative studies regarding the monitoring of heavy metal （As, Cr, Cu, Ni, Pb, Zn, Fe） pollution in the Lanzhou area, which have shown the following:when χlf> 40×10^-8m³kg-1, and χfd%<3%, χARMm/SIRM<0.21×10-3mA^-1, it is polluted; when 40≤xlf<150×10^-8m³ kg-1, χfd%<5%, and χARM/SIRM< 0.4×10-4mA^-1, it is moderately polluted to unpolluted; when 150≤χlf< 365×10^-8m³kg-1,χfd%<5%, and χARM/SIRM <0.4×10-3mA^-1, it is moderately polluted; when 365<χlf<750×10^-8m³kg-1,χfd%<2.7%, and χARMm/SIRM<0.2×10-3 mA^-1, it is moderately polluted to highly polluted; when χlf≥ 750×10^-8m³kg-1, χfd%<2.7% and χARM/SIRM< 0.2×10-3mA^-1,it is highly polluted, which is under the premise that coarse magnetite particles dominate the samples.Secondly, in order to promote the magnetic diagnostic modeling methods, the stability and sensitive of the relationship between the magnetic parameters and heavy metals need to be evaluated. Firstly, as the complex geographical conditions in northwest China, Gobi deserts, deserts and mountains are widespread in these arid and semi-arid areas, with some oases scattered throughout. Magnetic properties of natural topsoil are significantly different, especially for there ferromagnetic minerals concentrations. These minerals are mainly derived from the parent material of the coarse-grained ferromagnetic minerals, rather than a process of fine-grained magnetic minerals in soil formation. Significant differences of the magnetic component of natural topsoil among the different area, suggesting magnetic diagnosis model may only apply in a small range. Comparied the three slightly polluted cities （Wuhai, Shizuishan and Yinchuan city） suggested that magnetic properties of clean samples are significantly different, such as magnetic mineral concentration, grain size and high coercivity minerals concentration. When PLI>2, exogenous inputs ferromagnetic particles will gradually dominated topsoil magnetic characteristics, however, when PLI<2, exogenous inputs ferromagnetic particles enhanced magnetic topsoil, but not completely dominated their magnetic characteristics. Secondly, through the comparison of the magnetic and chemical results among the three traditional industrial cities, the results showed that non-steel mine dust particles generated during mining and beneficiation does not result in the concentration of magnetic minerals significantly rising, while the dust particles of ferromagnetic minerals emissioned from smelters, power plants, thermal power plants, chemical plants and other industrial processes, etc. is extremely sensitive. The common sources between magnetic minerals and heavy metals are the basis of estabilishing magnetic diagnostic model.Thirdly, the efficiency of magnetic methods for detecting and discriminating contaminates in the arid and semi-arid regions of northwestern China was investigated. Firstly, the voluntary work in single Yinchuan city showed that combined use of principal components and fuzzy cluster analysis of the magnetic and chemical data set indicates that the magnetic and geochemical properties of the particulates emitted from different sources vary significantly. Samples from university campus and residential areas are mainly affected by crustal material, with low concentrations of magnetic minerals and heavy metals, while industrial pollution sources are characterised by high concentrations of coarse magnetite and Cr, Cu, Pb and Zn. Traffic pollution is characterized by Pb and Zn, and magnetite. Magnetic measurements of soils are capable of differentiating sources of magnetic minerals and heavy metals from industrial processes, vehicle fleets and soil parent material. Secondly, the further study results on topsoil samples from six typical cities （i.e.Karamay, Urumqi, Lanzhou, Yinchuan, Shizuishan and Wuhai） indicate that the topsoil samples from the six cities were all dominated by coarse low-coercivity magnetite. In addition, the average magnetite contents in the soils from Urumqi and Lanzhou were shown to be much higher than those from Karamay, Yinchuan, Shizuishan and Wuhai, and they also have relatively higher clf and cfd% when compared with cities in eastern China. Moreover, specific and distinctive soil pollution signals were identified at each sampling site using the combined various magnetic data, reflecting distinct sources. Industrial and traffic-derived pollution was dominant in Urumqi and Lanzhou, in Yinchuan industrial progress was observed to be important with some places affected by vehicle emission, while Karamay, Shizuishan and Wuhai were relatively clean. The magnetic properties of these latter three cities are significantly affected by both anthropogenic pollution and local parent materials from the nearby Gobi desert. The differences in magnetic properties of topsoil samples affected by mixed industrial and simplex traffic emissions are not obvious, but significant differences exist in samples affected by simplex industrial/vehicle emissions and domestic pollution. The combined magnetic analyses thus provide a sensitive and powerful tool for classifying samples according to likely sources, and may even provide a valuable diagnostic tool for discriminating among different cities.In conclusion, the study is thus a contribution to the potential use of magnetic measurements, as well as their application for evaluating and distinguishing large-scale soil pollution caused by anthropogenic influences in an arid and semi-arid urban environment.