| To reveal the control process of total organic carbon(TOC)on the vertical migration and residues of Polycyclic Aromatic Hydrocarbons(PAHs)in soils,soil column simulation leaching experiments and distribution experiments were designed.The soil samples used for soil column simulation leaching experiments were collected in the Mentougou district,Yanqing district,Shunyi district and Changping district.The TOC of topsoil varies markedly among the four profile.Fourteen groups of soil column simulation leaching experiments have been conducted with the natural soil profiles and manmade soil profiles of different TOC.PAHs and petroleum were spiked on the top of the soil columns as simulating pollutants and were leached intermittently by deionized water for 15~45 days.Three groups of distribution experiments were conducted with soil samples having different organic carbon contents and the experiment time were 1 hour,24 hours and 360 hours,respectively.The results suggest that:(1)PAHs are mainly accumulated in the surface soils after being leached.The contents of both total and individual PAHs are sharply decreased with the increasing depth.The migration rate of different PAHs is different.The PAHs with low molecular weight are more readily to migrate to the deep horizon of the soil profile than the PAHs with high molecular weight,indicating the different migration ability of individual PAHs.The leaching water flow,TOC in soil and physicochemical property of PAHs all have important influences on the migration of PAHs in soil.(2)Some PAHs molecular biomarker parameters for source apportionment are changed because of the fractionation of PAHs during the leaching procedure.The biomarker values of An/(An+Ph),Ba A/(Ba A+Chr),IP/(IP+B(g,h,i))P decrease and then keep stable with the increasing depth.The biomarker value of Fl/(Fl+Py)increases and then keeps stable with the increasing depth.Overall,the value of biomarkers LMW/HMW increases with the depth.(3)At the early period of leaching(0 ~ 4 days),total content of PAHs in water leachate from soil column is low,and then rapid increases during 4 ~ 8 days,and is followed by decrease to low content after 12 days later,and finally reaches a steady state.Content of anthracene in leachate is higher than other PAHs species.Soil column of lower TOC leads to higher content of PAHs species leached than soil column of higher TOC,suggesting the TOC is the important factor that affects migration of PAHs in soil.Blank experiments showed that the content of PAHs in natural soil does not ruin the accuracy of the results of simulation experiments.(4)The leaching simulation experiments with of oil shows that soil column of lower TOC contains less PAHs in the whole soil profile than soil column of higher TOC.PAHs in crude oil entered into the soil,distributed between the soil organic matter and pore water,and vertically migrated with pore water.PAHs continously experienced the redistribute between the soil organic matter and pore water in the process of leaching through soil profile.The soil profile with less organic matter maintained relatively less PAHs.A positive correlation was observed between 2 ~ 3-ring PAHs concentration and TOC in the soil.(5)With the same distributing time in the distribution experiment,the distribution rate(ratio of PAHs contents in soil to those in water)of the total PAHs and TOC in the soil is significantly positive correlated,but correlation coefficient varies with PAHs species as 3-ring PAHs > 4 ~ 5-ring PAHs > 6-ring PAHs > 2-ring PAHs.Among them,3 ~ 5-ring PAHs distribution rate is significantly correlated with TOC,but there is almost no correlation between these two parameters for naphthalene.With the different distributing time,the correlation coefficients between the distribution rate of PAHs species(except naphthalene)and TOC are first increases then decreases after experiment respectively for 1h,24 h,360h.It shows that PAHs reach a better equilibrium between the soil and water after the distributing time of 24 hours. |
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