Tracing of heavy metal sources and mobility in a soil depth profile via isotopic variation of Pb and Sr

This study aimed to investigate soil profiles from four localities in Quanzhou, southeast China, namely an industrial zone (P51 profile), an agricultural zone (paddy field; P105 profile), a commercial residential area (P115 profile), and a traffic zone (P16 profile). Fe, Mn, Zn, Pb, Ni, Cr, and Cu concentrations were determined using inductively coupled plasma-mass spectrometry. Community Bureau of Reference sequential extraction was used for heavy metals speciation. The pollution sources in the soil profiles were examined using a Pb and Sr isotope tracing technique. The results showed that Pb and Mn were present throughout the P105 soil profile, while Zn and Cu in the surface soils exceeded background values. Pb pollution occurred deep in the soil profile, while Cu and Zn were enriched in surface soils, but not in the lower soils. In the P16 profile, Mn and Ni occurred throughout the entire soil profile, Pb occurred in the lower 100 cm of the profile, and Zn occurred in the topsoil. These values exceeded background levels, which was possibly due to contamination by traffic. In the P115 profile, only Mn in surface soils exceeded background levels, and there was only slight overall contamination by other metals. Heavy metals speciation analyses demonstrated that Mn, Cr, and Pb largely occurred in the form of non-residual fractions. Pb isotope tracing showed that sludge and bedrock parent materials were the main sources of Pb in the industrial and agricultural samples. Combining Sr and Pb isotope tracing confirmed the Pb isotope analysis.