Magnetic records of heavy metal pollution in urban topsoil in Lanzhou, China

This study is a systematic analysis of the magnetic characteristics and heavy metal elements in soil samples collected from Lanzhou City, China. The 117 surface soil samples were measured to study the feasibility and effectiveness of environmental magnetism for evaluating heavy metal pollution in urban soils. Results of the study indicate that low-coercivity magnetite dominates the magnetic properties in the samples. The high values of the soil magnetic mineral concentration parameters and low values of magnetic particle size parameters are distributed throughout the northern area of Xigu District, the industrial zones of the eastern section of Chengguan District and the districts of the narrow sections connecting Chengguan District and the other three districts; these parameters are fundamentally consistent with the distribution of the high values of the Pollution Load Index and Nemerow Pollution Index. Semi-quantitative studies which use the environmental magnetic method to monitor pollution of heavy metals (such as As, Cr, Cu, Ni, Pb, Zn, and Fe) have shown that when χ_lf?35×10^?8 m³ kg^?1, it is unpolluted; when 35 ? χ_lf<150×10^?8 m³ kg^?1, it is moderately polluted to unpolluted; when 150 ? χ_lf<365×10^?8 m³ kg^?1, it is moderately polluted; when 365 ? χ_lf<750×10^?8 m³ kg^?1, χ_fd%<2.7 and χ_ARM/SIRM<0.2×10^?3 m A^?1, it is moderately to highly polluted; when χ_lf?750×10^?8 m³ kg^?1, χ_fd%<2.7 and χ_ARM/SIRM<0.2×10^?3 m A^?1, it is highly polluted. The region of moderately to highly polluted distribution is caused by industry and vehicles, showing that the change of pollution in Lanzhou City bears the trend of “vehicle emission + industrial”.