RESEARCH ON EFFECTS
OF SEDIMENT BIOGEOCHEMISTRY ON THE ENVIRONMENTAL
FATE AND PERSISTENCE
OF POLYCHLORINATED BIPHENYLS (PCBs)
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OVERVIEW
Polychlorinated biphenyls (PCBs), 209 compounds formerly
manufactured and used in liquid mixtures as insulation in electric equipment,
are toxic pollutants that persist in the environment. PCBs have contaminated a number of major
rivers and coastal waters of the
The aim of our research is to develop improved understanding and models for the physical, chemical, and biological processes affecting release of PCBs from contaminated sediments. As PCB compounds migrate from the deep sediments to the sediment-water interface by diffusive and advective transport, they move first from the anaerobic sediments through the anoxic-oxic transition zone, then through a thin layer of aerobic sediment adjacent to the water column, and finally through the aqueous phase boundary layer and into the water column. The PCB compounds can undergo reductive dechlorination by microbes in the anaerobic sediments, resulting in production of more mobile, lower molecular weight compounds, and decomposition to biphenyl or even inorganic species by microbes in the aerobic sediments. The biochemical reactions and the transport of the species produced have profound effects on the final chemical forms released to the water column and, ultimately, the food chain.
Our research has a special focus on the biological communities and processes involved with PCB fate and transport in the oxic-anoxic transition zone and at the sediment-water interface. While physical and chemical processes can immobilize or sequester PCBs, only biological processes reduce the total mass of contamination. Research over the past 25 years has demonstrated that there are two controlling biological processes - anaerobic reductive dechlorination, which removes chlorines from the biphenyl structure, and aerobic oxidation of the dechlorinated biphenyl compound to carbon dioxide and water. In sediments, reductive dechlorination occurs in deeper, anoxic sediment zones. Much less well understood is what happens to the lightly chlorinated PCB compounds when they are transported to the more shallow, oxidized sediment zones.
The project is addressing five primary hypotheses: (1) aerobic biotransformation in the oxic-anoxic zone determines the specific PCB compounds released to the water column; (2) the microorganisms responsible for the anaerobic and aerobic biotransformation reactions in sediments can be identified with molecular biology techniques; (3) nutrient and substrate conditions determine the activity of the PCB dechlorinating microorganisms; (4) temperature has a dominant role in governing release of PCBs from contaminated sediments to the water column; and (5) the occurrence of PCB biotransformation in contaminated sediments can be assessed by comparing existing PCB compound distributions to those in the commercial PCB mixtures originally used and discharged to surface waters. Four experimental investigations and an integrative modeling effort are underway to investigate these hypotheses. The five coordinated projects are as follows:
Project 1. Evaluation of control of multiple steps in reductive dechlorination of highly chlorinated PCB congeners
Project 2. Characterization of anaerobic dechlorination microorganisms in PCB-contaminated river sediments
Project 3. Investigation of the effect of dissolved oxygen concentration on PCB biodegradation processes in anoxic-oxic transition
Project 4. Statistical methods to evaluate PCB dechlorination processes
Project 5. Investigation of the effect of temperature on PCB fate and transport in anaerobic near-surface sediment
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