Abstract

CEJUDO-RUIZ, Rubén et al. Correlation between potentially toxic elements and magnetic properties in Mexico City soils for the identification of contaminated sites: definition of threshold values. Rev. mex. cienc. geol [online]. 2015, vol.32, n.1, pp.50-61. ISSN 2007-2902.

Although its large territorial extension, Mexico City metropolitan area does not has a reliable environmental monitoring system for potentially toxic elements (PTE) in soils. Previous systematic studies of urban soils worldwide have shown a reasonably good correlation between the content of PTE and magnetic parameters. However few studies have proposed threshold values for the magnetic parameters to identify when a soil is contaminated. The principal aim of this work was to identify rock-magnetic parameters that statistically correlate with the concentration of PTE in soils. The use of magnetic parameters is being tested as alternative method of environmental monitoring for the evaluation of the soil quality. Eighty eight topsoil samples were collected in Mexico City considering the geological context, since the city is built on two types of geological environments (volcanic and lacustrine). Thirty seven samples belong to the volcanic zone and 51 come from the lacustrine area. The correlations between the magnetic parameters and the content of PTE were determined for the two groups separately. The magnetic parameters used in this work are the mass magnetic susceptibility (χ), the percentage frequency-dependent susceptibility (χ_fd%), the saturation isothermal remanent magnetization at 0.7 T (SIRM), and the SIRM/χ ratio. The magnetic mineralogy, concentration and magnetic coercivity were determined from the change in the volume magnetic susceptibility (κ) with temperature (T), curves of isothermal remanent magnetization (IRM) and the ratio S_-200. The concentration of potentially toxic elements (Cr, Cu, Ni, Pb, V y Zn) and oxides (Fe₂O₃ y TiO₂ ) in the soil samples was determined by energy dispersion X-ray fluorescence (ED-XRF) spectroscopy. In addition, the calculated pollution load index (PLI) was used to compare the levels of pollution at each site. The only magnetic parameter that showed a statistically significant correlation with the content of some PTE in both geological zones was the SIRM: Cr (0.54), V (0.36), TiO₂ (0.41) y Fe₂O₃ (0.50). A mean PLI value of 1.85 was obtained, which suggests that pollution by potentially toxic elements in many soils of Mexico City is moderate. The statistically determined threshold value of SIRM for polluted sites is 46 mAm² kg^-1, which is valid for soils of both volcanic and lacustrine zones. This study confirms the feasibility of using magnetic parameters as an alternative monitoring method to estimate the concentration of potentially toxic elements in soils of Mexico City.

Keywords : magnetic susceptibility; isothermal remanent magnetization; potentially toxic elements; threshold values.

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