Vertical distribution characteristics and transport mechanisms of polycyclic aromatic hydrocarbons (PAHs) in soil profiles (0-5.5 m) of different water irrigation areas in the southeast suburb of Beijing were analyzed and compared. 16 priority PAHs on the United States Environmental Protection Agency (US EPA) list were analyzed using gas chromatography and mass spectrometry (GC-MS). The relationship between the properties of soil and PAHs was also studied by statistical analyses. The results showed that total PAH concentrations in the topsoils of the wastewater irrigation (WWI) area, reclaimed water irrigation (RWI) area, groundwater irrigation (GWI) area were much higher than those in the deep soils, with the concentrations of 726.0, 206.8 and 42.8 μg kg(-1) (dry wt), respectively. The low molecular weight (LMW) PAHs (2-3 ring) including naphthalene (Nap), phenanthrene (Phe), fluorene (Fl) dominated the layers (0.5-5.5 m) underneath the surfaces. The migration of LMW PAHs was faster than that of high molecular weight (HMW) PAHs and LMW PAHs were transported in dissolved matter. The different soil textures of three sites caused the differences in the variation ranges of PAHs in the profiles. The statistical analyses showed a significant linear positive correlation between PAHs and total organic carbon (TOC). The 2-4 ring PAHs were detected in the wastewater and reclaimed waters, which was consistent with those in the soil profiles. The presence of PAHs in the soil profiles was mainly due to the irrigation of wastewater. Wastewater reuse guidelines and standards for irrigation should be established urgently.

Reclaimed water is becoming one of the most important water resources for irrigation, especially in arid and semi-arid areas. However, whether the organic compounds will pollute the soil and groundwater or not under the condition of long-term irrigation, has become a matter of great concern. Soil samples of 3 boreholes at the depths of 0-5.5m and groundwater samples were collected from the reclaimed water irrigation area of Beijing Southeast suburb. 16 EPA priority polycyclic aromatic hydrocarbons (PAHs) were analyzed using gas chromatography and mass spectrometry (GC-MS). The results showed that the total amount of 16 PAHs at whole profile varied from 12.44-206.75μg·kg -1 (dry wt.). The highest total contents of PAHs appeared in the surface layer, and they declined dramatically along the soil profiles. In the topsoil, high weight molecular PAHs (>4 rings) were the main pollutants which were accounting for 70% of the total PAHs. Under the surface layer, Naphthalene, Fluorene, Phenanthrene, Fluoranthene and Pyrene were the main detected PAHs. The PAHs contents ranked as follows: 2 rings>3 rings>4 rings. The total PAHs concentrations in groundwater ranged from 17.62-27.09 ng·L -1 , 179.41-959.77 ng·L -1 and 52.46-1069.52 ng·L -1 in spring, summer and winter, respectively. The detected PAHs in the groundwater were 2 and 3 rings PAHs, which were consistent with those of the soil profiles. It supposed that reclaimed water was the main source of PAHs. Meanwhile, Naphthalene and Phenanthrene were still the most abundant individual PAHs in groundwater, indicating that long-term reclaimed water irrigation can lead to the vertical migration of low weight molecular PAHs (2-3 rings).