Urban soil contamination in Bangkok: heavy metal and aluminium partitioning in topsoils

The objective of this study was to assess the extent and severity of surface soil contaminations from urban parts of the Bangkok metropolitan region. We sampled 30 soils at 0-5 cm depth along a N-S-bound 'main axis' with 'suburb', 'central', and 'industrial' branches at a right angle. All soils were Eutric or Dystric Gleysols derived from 2- > 30 yr old deposits consisting of clayey loam, quartz sand, and often high waste contents. In bulk soil and along the main axis also in aggregate core and surface fractions Al, Cd, Cr, Cu, Fe, Mn, Ni, Pb, and Zn were sequentially extracted in seven fractions. Average heavy metal concentrations were lower than in Manila, London or Hamburg but comparable to those in Hongkong soils except for Cd. However, at some sites in the southern part of the study region Cd (up to 2.5 mg kg(-1)), Cu (283), Pb (269), and Zn (813) concentrations were high. Based on a principal component analysis the metal groups (i) Al, Cr, Fe, Mn, Ni dominated by the concentrations of the parent materials and (ii) Cd, Cu, Pb, Zn dominated by anthropogenic input may be distinguished. Easily soluble metal concentrations were low because the soil reaction was predominantly controlled by the carbonate buffer. Metal partitioning was similar to that of terrestrial soils of the temperate regions. Metals of the contaminated sites tended to be more soluble than those of the noncontaminated. Most metals except for Mn, Pb, and Zn were depleted in the aggregate surface compared with the core fractions, but heavy metals of the surface fractions were more soluble than those of the core fractions. The Pb and Zn enrichment in the surface fractions was more pronounced in the contaminated south of the study region. (C) 1998 Elsevier Science B.V. All rights reserved.