Residual Petroleum in Sediments Reduces the Bioavailability and Rate of Reductive Dechlorination of Aroclor 1242

Residual petroleum hydrocarbons and polychlorinated biphenyls (PCBs) are frequently found together at contamination sites. Intrinsic reductive dechlorination of PCBs at these sites is highly variable. Sediments of Silver Lake (MA), which contain ∼6.2% petroleum hydrocarbons, did not support PCB dechlorination in laboratory assays. Removal of petroleum components from Silver Lake sediments by solvent extraction did not alter their inability to support dechlorination. When other sediments known to support PCB dechlorination were inoculated with PCBdechlorinating organisms and amended with incremental increases of pure petroleum hydrocarbons (0-4 wt %/wt) or 6.2% petroleum hydrocarbons extracted from Silver Lake sediments, a reduction in both the rate and extent of PCB dechlorination occurred. The maximal rate of dechlorination observed in these assays appears to depend singularly on the aqueous-phase PCB concentrations. A singleregression equation relating maximal PCB dechlorination rates to the PCB concentrations in water was developed that accurately described the data presented here and in two previous studies (R2 ) 0.960). Petroleum components in sediments appear to provide a sorptive phase that lowers the solution concentrations of PCBs, thus diminishing the bioavailability of PCBs and rate of dechlorination.