High levels of chlorinated aromatic compounds in deep rhine sediments with special reference to microbial transformations

Abstract

AbstractSediment cores were taken from the Ketelmeer, a sedimentation area of the River Rhine located in the central part of the Netherlands. Chlorinated benzenes, biphenyls, dibenzo‐p‐dioxins and furans were determined in all or in a selected number of core samples. Levels of these compounds since the late 1930s were established using radionuclide tracers and area‐specific geological time markers. Unexpectedly high concentrations of chlorinated dioxins and furans were found in layers that were deposited in the 1960s and 1970s. the highly toxic 2,3,7,8‐TCDD reached concentrations up to 400 ng kg−1 in the mid‐1960s. Post‐depositional redistribution of pollutants was verified by analysing deeper layers that were unpolluted at the time of deposition. Downward transport was only found for some di‐ and trichlorobenzenes. Possible transformations in the anaerobic sediment were evaluated by analysing sediment top‐layer samples taken in 1972. Supported by laboratory experiments, it became clear that some polychlorinated biphenyls and chlorobenzenes had been microbially dechlorinated in the anaerobic sediments. Up to 80 per cent of the input in the early 1970s has disappeared. on the other hand, The accumulation of a dechlorination product, 1,3,5‐trichlorobenzene, from hexachlorobenzene transformation was found. This dechlorination product is less hydrophobic and therefore more mobile than the parent compound. This phenomenon is of particular concern with respect to groundwater quality.