Magnetic responses to heavy metal pollution of the industrial soils in Shanghai: Implying the influences of anthropogenic magnetic dustfall on urban environment

Using the magnetic method to detect soil pollution has attracted increasing interest. To study the influence of magnetic dustfall on urban environment, 105 soil samples (0–5 cm) were collected along three equidistant gradient profiles with respect to a major iron and steel industrial complex (ISIC) in the northern suburb of Shanghai, Southeast China. The soils and atmospheric particulate matter (APM) in the area were examined using magnetic and micromorphological analyses. As a result, magnetic susceptibility (χlf) of study soils is commonly significantly enhanced, with a mean of 284.8 × 10−8 m3 kg−1. 68.6% of the soils are higher in χlf than 100 × 10−8 m3 kg−1, more than three times the magnetic background. Moreover, it shows an increasing trend toward the ISIC. Frequency-dependent susceptibility (χfd%) of the soils is mostly less than 2%, and is negatively significantly correlated with χlf, suggesting that the enhancement of soil magnetism is due mainly to coarse anthropogenic magnetic particles (multi-domain (MD) or stable single domain (SSD)). A number of magnetic spheroids were observed in the soils and APM, which are believed to mainly come from the ISIC. The persistent deposition of anthropogenic Fe-borne magnetic dustfall has significantly influenced the urban environment, causing the synchronous enhancement of magnetic susceptibility and heavy metal contents in the soils. The study soils are moderately polluted by Zn and Pb, and the soils of Profile I, the nearest to the ISIC, are heavily polluted by Pb and moderately polluted by Cd and Zn. χlf can reflect the overall level of heavy metal pollution of the soils effectively. Moreover, it is positively significantly correlated with the content of Cd, Cu, Zn, Ni, Cr, Pb and Mn (p < 0.01) in the soils. However, it is difficult to measuring the content of one or more heavy metals in the soils using the magnetic method as locally quantitative correlations between χlf and heavy metal content cannot be established even within this small industrial area.