Magnetic response of urban topsoil to land use type in Shanghai and its relationship with city gross domestic product

Due to its effectiveness in terms of time consumption and the high sensitivity to magnetic particles, the magnetic technique has been widely used in the last decades to detect urban soil pollution. As one super metropolis in China, Shanghai is greatly affected by industrial activities and traffic emission. In this study, a total of 192 topsoil samples were collected from four different functional areas, namely, traffic, residential and commercial, unpolluted, and industrial area, covering all administrative districts of Shanghai. Based on the magnetic technique, magnetic properties of urban topsoil were systematically investigated. The results showed that the average values of magnetic susceptibility (χ_lf) and saturation isothermal remanent magnetization (SIRM) were higher than those of the background value in Shanghai, suggesting that enrichment of ferromagnetic mineral in topsoils. Magnetic susceptibility exhibited high values in industrial area and traffic area, but low values in unpolluted, residential and commercial area. Combined the Scanning Electron Microscopy with Energy Dispersive X-Ray Analysis (SEM-EDX), it is indicated that the magnetic mineralogy of topsoil is dominated by iron sheets due to traffic activities and spherules due to fossil fuel combustion. Regarding administrative districts, high concentrations of magnetic materials were from districts having greater industrial activities, while low values were in less-polluted agriculture regions. Additionally, a comparison of topsoil χ_lf value and Gross Domestic Product (GDP) between Shanghai and other cities in China indicated that χ_lf is generally positively correlated with the city GDP. These results suggested that the urban topsoil in Shanghai was impacted by human activities and GDP. The results of this study are furthermore testifying the effectiveness of the magnetic technique in evaluating the pollution of urban soil.