Codeployment of Passive Samplers and Mussels Reveals Major Source of Ongoing PCB Inputs to the Anacostia River in Washington, DC.

Remedial investigations of sites contaminated with legacy pollutants like polychlorinated biphenyls (PCBs) have traditionally focused on mapping sediment contamination to develop a site conceptual model and select remedy options. Ignoring dissolved concentrations that drive transport and bioaccumulation often leads to an incomplete assessment of ongoing inputs to the water column and overestimation of potential effectiveness of sediment remediation. Here, we demonstrate the utility of codeployment of passive equilibrium samplers and freshwater mussels as dual lines of evidence to identify ongoing sources of PCBs from eight main tributaries of the Anacostia River in Washington, DC, that has been historically polluted from industrial and other human activities. The freely dissolved PCB concentrations measured using passive samplers tracked well with the accumulation in mussels and allowed predictions of biouptake within a factor of 2 for total PCBs and a factor of 4 for most congeners. One tributary was identified as the primary source of PCBs to the water column and became a focus of additional ongoing investigations. Codeployment of passive samplers and mussels provides strong lines of evidence to refine site conceptual models and identify ongoing sources critical to control to achieve river water quality standards and reduce bioaccumulation in the aquatic food web.

Kinetics and threshold level of 2,3,4,5-tetrachlorobiphenyl dechlorination by an organohalide respiring bacterium.

The time required for a PCB-contaminated site to recover cannot yet be predicted due in part to lack of quantitative information on rates of PCB dechlorination in the porewater phase. We developed a method to measure rate of dechlorination in the aqueous phase at very low PCB concentrations. This approach utilizes a polymer functioning concurrently as a passive dosing system for maintaining a steady-state PCB substrate concentration in the water phase and as a passive equilibrium sampler to monitor the dechlorination product. Rates of dechlorination of 2,3,4,5-tetrachlorobiphenyl (PCB 61) to 2,3,5-trichlorobiphenyl (PCB 23) by an organohalide respiring bacterium, Dehalobium chlorocoercia DF-1, were measured over an environmentally relevant range of 1 to 500 ng L(-1) in sediment-free medium using a high concentration of cells (>10(6) cells mL(-1)). The results indicate that rate of dechlorination is a linear function of PCB substrate concentration below the maximum aqueous solubility of PCB 61 and occurs at concentrations as low as 1 ng L(-1). Demonstration of PCB 61 dechlorination at environmentally relevant concentrations suggests that low numbers of organohalide respiring bacteria rather than bioavailability accounts for low rates of dechlorination typically observed in sediments. Using passive samplers to measure the concentration of dissolved PCBs in the porewater combined with knowledge of congener-specific rates for organohalide respirer(s), it will be possible to project the in situ rate and final concentration of PCBs for a specific site after treatment by bioaugmentation.