Six deep sampling boreholes are used for studying the sources and vertical distribution of 16 polycyclic aromatic hydrocarbons (PAHs) in an abandoned coking plant. The maximum depth of soil sampling ranged from 9.5-42 m. The present study analyzes the distribution characteristics and sources of PAHs in addition to the important factors affecting migration of PAHs in the unsaturated zone. Results showed that the maximum values of total PAHs range of 134.79-11266.81 mg·kg-1 at vertical depths ranging from 1-5 m. We found that 2+3 rings dominated total PAHs and that the highest content was Nap. Results also showed that the main source of PAH pollution was coal combustion. Coal tar pitch and different oils from deep processing of tar played an important role in PAH pollution. The gravel layer served as a good infiltration channel for pollutants, and the sand lens below 20 m depth became the main enrichment layer for PAHs through adsorption and interception. Oils and wastewater from discharge and leaching of chemicals contributed to the migration of PAHs through inter-miscibility and competitive adsorption, which led to deep soil pollution. Soil layers above 1 m depth were affected by artificial disturbance, rainfall leaching, and degradation, and the unsaturated zone below 30 m depth was affected by leaching from groundwater. Consequently, the ratio of low to high rings first increased before decreasing with increased depth of the unsaturated zone. The types of pollution sources, rock particle sizes, organic matter content, and hydrogeological conditions all affected the vertical distribution and migration of PAHs.
Keywords: heterogeneous soil; polycyclic aromatic hydrocarbons(PAHs); sources apportionment; unsaturated zone; vertical distribution.