Persistent organic pollutants in boreal and montane soil profiles: distribution, evidence of processes and implications for global cycling.

The distribution of persistent organic pollutants (POPs) within background soil profiles was investigated in boreal (Norway) and montane (Italy) areas. The typical build-up of slowly mineralizing humus layers, containing high amounts of soil organic matter (SOM) makes soils of such ecosystems an important global sink for POPs released to the environment. The study focused on evidence and implications of processes influencing the fate of POPs in soil. POP deposition, interaction with SOM, volatilization, leaching, degradation, and bioturbation are discussed. Results indicate that the less volatile POPs such as hexa- and higher chlorinated biphenyls (CBs) or polybrominated diphenyl ethers are very stable in soil profiles, undergoing little translocation or (re)transfer to other environmental compartments. In contrast, more volatile compounds (e.g., tri- and tetra-CBs) were found in soil layers below those formed from vegetation ever directly exposed to airborne POPs. This suggests the occurrence of downward transport and hence limited surface-air exchange of more volatile POPs as they are removed from the top layers. Such soils may therefore be able to retain higher amounts of these compounds than just addressed by the capacity of their surface layers.