Saturday, 15 July 2006

Magnetic Properties of Urban Topsoil in Shanghai and Their Environmental Implications.

Xue-feng Hu, Xiao-qing Li, Rong Ye, Yun Pan, and Yu Su. Department of Environmental Science and Engineering, Shanghai University, No. 149 Yanchang Road, Shanghai, 200072, China

Magnetic properties of the topsoils at an urban area, Songnan Town, and a suburban agricultural area, Luojing Town in Baoshan District, Shanghai, were comparatively studied. The results show that χlf in the urban topsoil (Songnan Town) ranges from 127.3 ~ 1959 x10-8m3kg-1, with the average of 450x10-8m3kg-1; whereas χlf in the suburban soil (Luojing Town) from 19.4 ~ 59.0x10-8m3kg-1, with the average of 39.1x10-8m3kg-1. χlf in the urban topsoil is more than one order of magnitude of that in the suburban soil. χfd(%) in the two areas is less than 3%, which implies the absence of pedogenic superparamagnetic grains (SP). The soils in the two areas of Baoshan District are derived from the same parent material and share the same soil type, Entisols. Hence, the extra magnetic minerals present in the urban topsoil are not inherited from the parent material, nor produced by pedogenic processes.

Further studies show that SIRM and χarm in the urban topsoil are also much higher and both are extremely significantly correlated with χlf, implying the dominance of stable single domain (SSD) and multidomain (MD) magnetic grains. Low χarm/SIRM and χarmlf ratios in the urban topsoil also imply its coarse grain size. SOFT(%) in the urban topsoil is 33.5% on average, but HIRM(%) only 2.0%, indicating magnetic soft fraction, ferri(o)magnetic minerals, are the main contributor to the soil magnetism. All the magnetic properties mentioned above imply the extra magnetic minerals accumulated in the urban topsoil originate from anthropogenic activities.

Average values of Cu, Zn, Pb, Cd, Cr, Ni, Mn and Fe in the urban topsoil are 93.4, 794.4, 139.0, 0.30, 243.7, 60.0, 1110.1 and 44.9x103 mg/kg, respectively, compared with 41.7, 238.6, 64.6, 0.080, 111.1, 52.2, 477.8 and 36.1x103 mg/kg, respectively, in the suburban soil. Moreover, Zn, Pb, Cd, Cr, Ni, Mn and Fe in the urban topsoil are significantly correlated with χlf values, indicating that the magnetic enhancement in the urban soil is closely related to heavy metal accumulation.

Affected by industrial activities and heavy traffic, magnetic values and heavy metal contents in the urban topsoil in Shanghai are significantly higher than those in the suburban soil. Magnetic methods reflect not only the content of magnetic minerals, but also their grain size and types, by which, we can distinguish anthropogenic magnetic particles from pedogenic ones. Hence, it is promising to use magnetic techniques for monitoring urban soil pollution.

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