Polycyclic aromatic hydrocarbons (PAHs) in soil - a review

PAHs are mainly produced by combustion processes and consist of a number of toxic compounds. While the concentrations of individual PAHs in soil produced by natural processes (e.g., vegetation fires, volcanic exhalations) are estimated to be around 1-10 mu g kg(-1), recently measured lowest concentrations are frequently 10 times higher. Organic horizons of forest soils and urban soils may even reach individual PAH concentrations of several 100 mu g kg(-1). The PAH mixture in temperate soils is often dominated by benzofluoranthenes, chrysene, and fluoranthene. The few existing studies on tropical soils indicate that the PAH concentrations are relatively lower than in temperate soils for most compounds except for naphthalene, phenanthrene, and perylene suggesting the presence of unidentified PAH sources. PAHs accumulate in C-rich topsoils, in the stemfoot area, at aggregate surfaces, and in the fine-textured particle fractions, particularly the silt fraction. PAHs are mainly associated with soil organic matter (SOM) and soot-like C. Although the water-solubility of PAHs is low, they are encountered in the subsoil suggesting that they are transported in association with dissolved organic matter (DOM). The uptake of PAHs by plants is small. Most PAHs detected in plant tissue are from atmospheric deposition. However, earthworms bioaccumulate considerable amounts of PAHs in short periods. The reviewed work illustrates that there is a paucity of data on the global distribution of PAHs, particularly with respect to tropical and southern hemispheric regions. Reliable methods to characterize bioavailable PAH pools in soil still need to be developed.