Multivariate analysis of heavy metal leaching from urban soils following simulated acid rain

Heavy metal pollution in urban soils and its effect on urban ecological environment and human health are of increasing concern. In this study, a leaching experiment was conducted to assess the effect of simulated acid rain (SAR) on the migration of Cd, Pb, Cu, and Zn and their transformation and speciation in nine natural urban soils from Guangzhou, China, combined with two-dimensional hierarchical cluster analysis (2-D HCA) heat map and principal component analysis (PCA). The maximum concentrations of Zn and Cu in leachates were lower than Grade III of Chinese Environmental Quality Standards for Ground and Surface water, although being seriously contaminated with Zn (up to 19,570 mg/kg) and Cu (322.4), thereby suggesting that Zn and Cu leached from urban soils receiving SAR pose little risk of water pollution. However, the maximum concentrations of Pb and Cd in leachates exceeded Grade III and V of the standards, respectively. The proportions of metals in the non-residual fractions were generally decreased after the leaching test for S(oil)5 (with high concentrations of metals in leachate), representing the decreased contamination risk in the soil-water system. A similar trend was observed for S(oil)6 (with low concentrations of metals in leachate) in terms of Zn and Cu, whereas the proportions of Cd and Pb in the non-residual fractions of S6 increased after leaching. Significantly positive relationships were observed between the concentration of heavy metal in leachate and all fractions of heavy metal, as well as the total concentration of soil heavy metals. In terms of Zn, Cd, and Pb, there were negative relationships between the concentration of heavy metal in leachate and clay and silt, respectively, while the inverse relationships were observed for Cu. The release of heavy metals from soil was strongly associated with their fractions and such soil properties as pH, soil texture, and organic matter.