Polycyclic aromatic hydrocarbons and heavy metals in urban environments: Concentrations and joint risks in surface soils with diverse land uses

Although polycyclic aromatic hydrocarbons (PAHs) and heavy metals (HMs) are abundant around cities, few researchers have studied the complex concentrations and joint risks of these pollutants in urban surface soils containing diverse land uses. We collected 69 urban surface soil samples from roadside districts, green belts, public green space, commercial districts, and residential district in urban areas of Nanjing, China to determine the concentrations, distributions, deposition patterns, and joint risks of PAH and HM contaminants. No significant differences were found among the five studied land uses, but a wide range of PAH levels were found in these land uses, as these diverse land uses have similar PAH pollution sources. As these areas persistently generate high PAH levels by gasoline/diesel engine or coal combustion, the PAH distribution characteristics indicated that commercial centers or city centers are the most heavily contaminated sites. The distributions of black carbon and HMs, except for Pb, were found to be significantly correlated. Except for Cd and As, HM levels among diverse land uses also showed no significant differences, possibly stemming from the non-point source pollution of these metals in the study areas. Factor analysis showed that 46.2% of the elevated pollutant concentrations resulted from combustion processes, 24.3% were caused by natural background fluctuations, and 11.9% resulted from non-combustion processes. The studied PAHs and As presented low carcinogenic risks. Cd, Cu, Pb, and Zn depicted acceptable total non-carcinogenic risks in all land uses; however, the risks associated with these HMs possessed detailed differences.