Characterization of the polychlorinated biphenyls in the sediments of Woods Pond: Evidence for microbial dechlorination of Aroclor 1260 in situ

PCB-contaminated sediments from the Housatonic River have accumulated in Woods Pond, a shallow impoundment on the river located in Lenox, MA. We used GC/ECD analysis to determine the PCB congener distributions in 181 sediment samples from 94 locations in Woods Pond in order to learn which Aroclors were originally present and whether there was evidence of dechlorination: The congener distributions of all samples showed the hexa-, hepta-, and octachloro-biphenyls (octa-CB) characteristic of Aroclor 1260, but key hexa- and hepta-CBs were decreased by as much as 45% relative to Aroclor 1260, and the tri-, tetra-, and penta-CBs were increased. GC/MS analysis revealed unusual tetra-, penta-, and hexa-CBs, many containing 2,4- and 2,4,6-chlorophenyl rings, which are uncommon in higher Aroclors, and provided strong evidence of dechlorination. A quantitative mother-daughter analysis of key hexa- and hepta-CBs that were decreased relative to Aroclor 1260 and their putative dechlorination products yielded mass balances ranging from 90 to 108%. The data indicate that the PCBs consist of partially dechlorinated Aroclors 1260 and 1254(similar to 95:5). Relative-to Aroclor 1260, the most extensively dechlorinated samples had lost 11-19% of the meta chlorines and 2-7% of the para chlorines. This modest dechlorination caused a 20-28% decrease in the concentrations of the PCB congeners reported to be most persistent in humans.