Contamination of sediments with persistent, bioaccumulative toxins such as polychlorinated biphenyls (PCBs) can cause significant risk to humans and wildlife and commonly requires expensive and disruptive remediation efforts. The traditional approach to addressing contaminated sediments is to assess risk by comparing the total contaminant concentration in the sediment to risk-based benchmarks and to remediate sites by removing sediments for burial or treatment elsewhere. Recent studies have shown that strong-sorbing materials such as soot and activated carbon present in sediment significantly reduce bioavailability of hydrophobic pollutants, such that the total measured concentration may overestimate exposure and risk. These "supersorbents" may result from natural processes (fires), inadvertent anthropogenic additions (coal gas manufacturing plants), or purposeful additions for in situ remediation. This study centered around how these particles influence microbially mediated dehalogenation reactions and dissolved-sorbed partitioning of PCBs.

The overall hypothesis of this study was that the extent of biogeochemical reactivity and mobility of sedimentary contaminants is controlled by desorption to support dissolved
concentrations. In this project, researchers sought to address the following three questions: (1) What are the relationships between PCB dechlorination rates and fundamental sorption characteristics of PCBs in sediments? (2) What are the relationships between PCB flux into water and fundamental sorption characteristics of PCBs in sediment particle types? and (3) How can the enhanced understanding of PCB fate processes improve long-term risk assessments and comparison of in situ treatment options?

This study employed two new assessment tools to quantify the bioavailability of sediment-bound contaminants to evaluate the efficacy of in situ activated (AC) carbon treatments. First, a most probable number-polymerase chain reaction (MPN-PCR)-based assay was utilized that detects and quantifies indigenous PCB dechlorinating species in soils and sediments. Microbial dehalogenation is not only an important sink for PCBs in sediments but also may be used as a probe of PCB speciation and bioavailability. Second, a solid phase microextraction (SPME) technique was utilized that rapidly measures truly dissolved PCB congeners in sediment suspensions, allowing desorption kinetics and bioavailability to be determined. These two assessment tools were used to compare PCB availability to total solid phase concentrations to directly examine PCB mobility in historically contaminated sediments. This approach has both fundamental and applied aspects: the AC additions are tools to study PCB speciation in sediments and pilot-scale evaluations of a promising in situ remediation technique.

Seven separate studies were conducted to address the project objectives:

• Study 1. Development and Application of Molecular Technique to Characterize in situ Dechlorination of PCB Congeners
• Study 2. Intrinsic Bioavailability of PCB Congeners in Historically-Contaminated Sediments
• Study 3. Impact of Activated Carbon on Volatilization and Microbial Bioavailability of PCBs in an Aerobic Sediment Slurry
• Study 4. Modeling the Impact of Flocculation on the Fate of Organic and Inorganic Particles during Resuspension Events in an Urban Estuary
• Study 5. Erosion of Activated Carbon-Amended River Sediments Under Controlled Experimental Conditions
• Study 6. Modeling the Impact of Flocculation on the Fate of PCBs during the Resuspension event in an Urban Estuary
• Study 7. Predicting the Behavior of PCBs in Activated Carbon-Amended Sediments
  A summary of each study is provided in this report.

Many Department of Defense facilities face challenges from contaminated sediments, and existing remediation options are slow and expensive. The broad goal of this project was to assist in the development of the next generation of contaminated sediment management tools that will be applied in situ, resulting in more efficient and cost-effective reduction of risk at these sites. (Project Completion - 2011)