Heavy metals pollution assessment in correlation with magnetic susceptibility in topsoils of Shanghai

This study investigated the heavy metal accumulation, magnetic susceptibility, and their correlation in urban soils of Baoshan District, Shanghai. A total of 127 topsoil samples were collected from industrial (n = 28), roadside (n = 38), residential (n = 21), and agricultural (n = 40) areas. The absolute concentration of heavy metals: lead (Pb), zinc (Zn), chromium (Cr), nickel (Ni), cadmium (Cd), and copper (Cu), was measured using an atomic absorption spectroscopy (AAS). Nemerow pollution index was applied to estimate the pollution level in respective areas. The results showed an excess of heavy metals in the urban topsoil of Baoshan District, among which, Pb, Zn, and Cd were most prominent. The average values of Pb, Zn, and Cd were obtained as 118.5, 228.6, and 0.56 mg kg⁻¹, respectively, which are 5.6, 3.0, and 2.8 times higher than the Shanghai soil background values. Kriging interpolation was employed (according to the pre-evaluated fitted model variogram values of nugget, sill, and range) to assess the spatial distribution of heavy metals. Kriging interpolation depicted that the industrial sites were heavy metal-centric locations. Complimentary to AAS detection method, magnetic susceptibility tool was also applied to evaluate the presence of magnetic particles and their correlation with heavy metals. The magnetic susceptibility of the topsoil in Baoshan District was 148 × 10⁻⁸ m³ kg⁻¹, which is ~5 times higher than the background value of Shanghai soil (29.1 ± 9.8 × 10⁻⁸ m³ kg⁻¹). In most of the samples, the percentage frequency-dependent magnetic susceptibility (χ_fd%) values in the topsoil of Baoshan District were less than 4%, suggesting the absence of pedogenic ultrafine superparamagnetic grains and the presence of anthropogenic multidomain and stable single domain grains. A significantly positive correlation (industrial: r = 0.81, P < 0.01; roadside: r = 0.65, P < 0.01; residential: r = 0.56, P < 0.01; agricultural: r = 0.6, P < 0.01) was found between magnetic susceptibility and heavy metal pollution irrespective of the type of anthropogenic activities at a particular land. The results suggested the reliability of magnetic susceptibility measurement to interpret the heavy metal pollution of the soils.