Integration of magnetism and heavy metal chemistry of soils to quantify the environmental pollution in Kathmandu, Nepal

Soil profiles of the Kathmandu urban area exhibit significant variations in magnetic susceptibility (chi) and saturation isothermal remanence ( SIRM), which can be used to discriminate environmental pollution. Magnetic susceptibility can be used to delineate soil intervals by depth into normal (< 10(-7) m(3)/kg), moderately enhanced (10(-7) -< 10(-6) m(3)/kg) and highly enhanced (>= 10-6 m3/kg). Soils far from roads and industrial sites commonly fall into the 'normal' category. Close to a road corridor, soils at depths of several centimeters have the highest chi, which remains high within the upper 20 cm interval, and decreases with depth through 'moderately magnetic' to 'normal' at approximately 30-40 cm. Soils in the upper parts of profiles in urban recreational parks have moderate chi. Soil SIRM has three components of distinct median acquisition fields (B-1/2): soft (30-50 mT, magnetite-like phase), intermediate (120-180 mT, probably maghemite or soft coercivity hematite) and hard (550-600 mT, hematite). Close to the daylight surface, SIRM is dominated by a soft component, implying that urban pollution results in enrichment by a magnetite-like phase. Atomic absorption spectrometry of soils from several profiles for heavy metals reveals remarkable variability ( ratio of maximum to minimum contents) of Cu (16.3), Zn ( 14.8) and Pb (9.3). At Rani Pokhari, several metals are well correlated with chi, as shown by a linear relationship between the logarithmic values. At Ratna Park, however, both chi and SIRM show significant positive correlation with Zn, Pb and Cu, but poor and even negative correlation with Fe (Mn), Cr, Ni and Co. Such differences result from a variety of geogenic, pedogenic, biogenic and man-made factors, which vary in time and space. Nevertheless, for soil profiles affected by pollution ( basically traffic-related), chi exhibits a significant linear relationship with a pollution index based on the contents of some urban elements ( Cu, Pb, Zn), and therefore it serves as an effective parameter for quantifying the urban pollution.