HIGH-LEVELS OF CHLORINATED AROMATIC-COMPOUNDS IN DEEP RHINE SEDIMENTSWITH SPECIAL REFERENCE TO MICROBIAL TRANSFORMATIONS

Sediment cores were taken from the Ketelmeer, a sedimentation area of the River Rhine located in the central part of The Netherlands. Chlori nated benzenes, biphenyls, dibenzo-p-dioxins and furans were determine d in all or in a selected number of core samples. Levels of these comp ounds since the late 1930s were established using radionuclide tracers and area-specific geological time markers. Unexpectedly high concentr ations of chlorinated dioxins and furans were found in layers that wer e deposited in the 1960s and 1970s. The highly toxic 2,3,7,8-TCDD reac hed concentrations up to 400 ng kg-1 in the mid 1960s. Post-deposition al redistribution of pollutants was verified by analysing deeper layer s that were unpolluted at the time of deposition. Downward transport w as only found for some di- and trichlorobenzenes. Possible transformat ions in the anaerobic sediment were evaluated by analysing sediment to p-layer samples taken in 1972. Supported by laboratory experiments, it became clear that some polychlorinated biphenyls and chlorobenzenes h ad been microbially dechlorinated in the anaerobic sediments. Up to 80 per cent of the input in the early 1970s has disappeared. On the othe r hand, the accumulation of a dechlorination product, 1,3,5-trichlorob enzene, from hexachlorobenzene transformation was found. This dechlori nation product is less hydrophobic and therefore more mobile than the parent compound. This phenomenon is of particular concern with respect to groundwater quality.