Residual petroleum in sediments reduces the bioavailability and rate of reductive dechlorination of aroclor 1242

Residual petroleum hydrocarbons and polychlorinated biphenyls (PCBs) are fr equently found together at contamination sites. Intrinsic reductive dechlor ination of PCBs at these sites is highly variable. Sediments of Silver Lake (MA), which contain similar to 6.2% petroleum hydrocarbons, did not suppor t PCB dechlorination in laboratory assays. Removal of petroleum components from Silver Lake sediments by solvent extraction did not alter their inabil ity to support dechlorination. When other sediments known to support PCB de chlorination were inoculated with PCB-dechlorinating 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 reducti on in both the rate and extent of PCB dechlorination occurred. The maximal rate of dechlorination observed in these assays appears to depend singularl y on the aqueous-phase PCB concentrations. A single-regression equation rel ating maximal PCB dechlorination rates to the PCB concentrations in water w as developed that accurately described the data presented here and in two p revious studies (R-2 = 0.960). Petroleum components in sediments appear to provide a sorptive phase that lowers the solution concentrations of PCBs,th us diminishing the bioavailability of PCBs and rate of dechlorination.