Accumulation and distribution of PAHs in winter wheat from areas influenced by coal combustion in China

In order to investigate level and potential sources of polycyclic aromatic hydrocarbons (PAHs) in wheat fields affected by coal combustion in Henan and Shaanxi Provinces and to investigate distribution and transfer of PAHs in winter wheat grown in the areas, various tissues of the crop and the corresponding rhizosphere soils were collected during the harvest season of winter wheat. The mean concentrations of USEPA 15 priority PAHs (sum of the three- to six-ring PAHs) ranged from 486 to 1117 μg kg⁻¹ in the rhizosphere soils, indicating serious PAH contamination. Based on both the isomeric ratios of PAHs and a principal component analysis (PCA), the main sources of PAHs in the agricultural soils were from combustion of biomass, coal and petroleum, and petroleum. ∑₁₅PAHs were significantly (p < 0.001) higher in the roots (287–432 μg kg⁻¹) than those in aerial tissues (221–310 μg kg⁻¹). There were two decreasing gradients of PAH concentrations, one from roots, stems to leaves, and the other from glumes to grains. Regardless of sampling sites, most PAHs detected in the roots and in the aerial tissues were three-ring PAHs (acenaphthene, acenaphthylene, fluorene, phenanthrene, and anthracene) and the percentages of three-ring PAHs were much higher in the aerial tissues (72.5–82.7%) than in the roots (49.5–74.0%) and in the rhizosphere soils (36.3–65.7%). The distribution of PAHs with different ring numbers in the stems, leaves, and glumes was quite similar to each other but was significantly different from that of the roots and rhizosphere soils. Combined with significant results from partial correlation and linear regression models, the present study suggested that partial three- to four-ring PAHs in the aerial tissues are derived from root-soil uptake and that six-ring PAHs may come from the air-to-leaf pathway, although the quantity contribution of foliar uptake and root uptake was yet to be further studied.