MICROBIAL DECHLORINATION OF HEXACHLOROBENZENE IN A SEDIMENTATION AREAOF THE RHINE RIVER

In sedimentation areas of polluted rivers. microbial dechlorination of chlorinated aromatics may be of great environmental significance. Thi s reaction may take place in the deeper, anaerobic sediment layers and involves replacement of a chlorine in the pollutant molecule by hydro gen. In this study. the microbial dechlorination of hexachlorobenzene in a sedimentation area of the Rhine River is evaluated by using Rhine water pollution data. concentrations in historical sediment samples a nd in recent sediment cores. and the results of anaerobic laboratory i ncubations with Lake Ketelmeer sediment. The various data support the conclusion that microbial dechlorination of hexachlorobenzene has occu rred in the anaerobic sediment. Up to 80% of the hexachlorobenzene dep osited in the early 1970s has been dechlorinated. The maximum half-lif e of hexachlorobenzene in the sediment is found to be 7 years. Two lim itations of microbially mediated dechlorination in the natural environ ment have become clear. In the first place, a residual concentration o f about 40 mug/kg remains unaltered in the sediment or transformation rates of this fraction are at least extremely low. Secondly, the lower chlorinated benzenes that are produced from hexachlorobenzene appear to accumulate in the anaerobic sediment.