EFFECT OF PCB CONCENTRATION ON REDUCTIVE DECHLORINATION AND DECHLORINATION POTENTIAL IN NATURAL SEDIMENTS

The effect of PCB concentration on microbial reductive dechlorination was examined using two individual congeners, 2,3,4- and 2,3,4,2',4',5' -chlorobiphenyl (CBP), at four concentrations ranging from 4 to 35 ppm . Dechlorination potential in PCB-contaminated river sediments was als o determined as the length of the lag period in a single congener dech lorination assay using contaminated sediments as inocula. Dechlorinati on of 2,3,4-CBP was concentration-dependent and occurred at all concen trations by 14 weeks and the residual concentration of the parent cong ener appeared to be the same regardless of the initial concentrations. By contrast, 2,3,4,2',4',5'-CBP showed dechlorination only at the hig hest concentration after 23 weeks. The concentration dependence and th e lag period thus were congener-specific. Dechlorination potential, me asured as the length of the lag before dechlorination of 3,4,5-CBP, se emed to be longer with inocula prepared from sediments with lower ambi ent PCB concentrations. Taken together, these results imply that in si tu dechlorination may not be able to occur in areas with low ambient P CB levels because both dechlorination rate and dechlorination potentia l are concentration-dependent. In sediments which are no longer submer ged, dechlorinating activity was not found.