Effects of total organic carbon content and leaching water volume on migration behavior of polycyclic aromatic hydrocarbons in soils by column leaching tests

The risk of soils transferring polycyclic aromatic hydrocarbons (PAHs) into groundwater has caused widespread concern. Research on the leaching behavior of PAHs in soil profiles is very important for assessing this risk. Column leaching tests were carried out to provide insight into the effect of TOC and leaching water volume on leaching behavior of PAHs. Four groups were leached intermittently by deionized water under the same leaching rate for 10 d, 30 d, 90 d and 120 d. These four leaching periods are equivalent to 1 yr, 3 yr, 9 yr and 12 yr of rainfall time under natural conditions, respectively. The results showed that residual concentrations of PAHs on the surface of soil (0-5 cm) in three columns after 30 d of leaching were 37.9 μg/g, 18.5 μg/g and 3.7 μg/g, respectively, which was consistent with their TOC contents. According to the correlation analysis, both residual concentrations of ∑16PAHs and PAHs with different ring numbers were significantly correlated with the TOC content at depths of 5-100 cm after 30 d of leaching. With increased leaching water volume, PAH migration rates significantly decreased (from 3.13 μg/g/d to 0.005 μg/g/d) from 10 d to 120 d, which indicates that the initial period of the leaching process has a stronger effect on PAH vertical migration than the later stages of the process. Under long-term leaching, PAHs that were not leached previously were capable of migrating deeper into the soil profile. Therefore, it has the risk of PAH-contaminated soils transferring PAHs into groundwater.