Levels, sources, and spatial distribution of heavy metals in soils from a typical coal industrial city of Tangshan, China

The concentrations, bioavailability, distribution, and sources of heavy metals in soils of a typical region from coal industrial city of Tangshan, North China, were investigated. A total of 240 surface (0-20 cm) and sub-surface (20-40 cm) soil samples were collected and analyzed for the heavy metals of mercury (Hg), lead (Pb), cadmium (Cd), zinc (Zn), copper (Cu), chromium (Cr), arsenic (As), and nickel (Ni). The concentrations of these metals were lower than the national guideline values. However, accumulation of Zn, Pb, Cd, Hg, and Cu in the surface soils, and Zn, Cu, Cd, and Hg in the sub-surface soils was observed compared with their background values. The pollution indices of geo-accumulation (I-geo) and contamination factor (CF) showed that most soil samples were slightly to moderately polluted by Hg and Cd. Cd and Hg had the higher percentages (1.16-46.8%) in the water-soluble and exchangeable fractions and were considered potentially bioavailable and risky to the ecosystem. A combination of spatial analysis, principal component analysis (PCA), and positive matrix factorization (PMF) model was used to assess the sources of heavy metals. Anthropogenic sources contributed to 80.2% of the heavy metal concentrations in soils, indicating the great influence on soil heavy metal accumulation. Coal-related industrial activities, sewage irrigation and the application of agrochemicals, and vehicle emission were the possible anthropogenic sources of heavy metals.