The disappearance of several polychlorinated biphenyls (PCBs) was observed in the deeper anaerobic sediment layers of Lake Ketelmeer, a sedimentation area of the Rhine River. Laboratory studies with an anaerobic consortium from this area demonstrated the capabilities for transforming PCBs. The field and laboratory observations indicate that disappearances of PCBs in the sediment might be caused by in situ microbial dechlorination. These processes appear to have lowered the PCB toxicity of four reactive congeners by 75% during the last two decades. In addition, dechlorination reactions are the essential first steps in the only natural pathway that eventually may lead to a complete elimination of PCBs from the aquatic environment. This can only be achieved if the dechlorination products are subsequently mineralized under aerobic conditions. Several implications for environmental policies with regard to contaminated sediments are discussed. Long-term prognoses on sediment quality generated with mathematical models that include these reactions may lead to a distinct, probably less costly, sediment policy.

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