Halogenated, hydrophobic contaminants of aquatic sediments pose a serious challenge. These toxic, bioaccumulating pollutants include legacy industrial chemicals, such as polychlorinated biphenyls (PCBs), ubiquitous polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), as well as current commercial manufacturing chemicals including brominated flame retardants. To date, remediation of contaminated sediments has mainly involved sediment removal with destruction or sequestration of the compounds. New methods are needed for in situ containment and degradation of halogenated contaminant mixtures. Stimulation of natural communities of dehalogenating bacteria and bioaugmentation using specialized dehalogenating bacterial strains holds promise for development of new approaches for sediment remediation.
The objectives of this project were to extend techniques and amendments that enhance microbial dehalogenation for placement in sediments contaminated with organohalide mixtures and to develop methods and tools to monitor the effectiveness of the biostimulation process.
The project focused on developing stimulatory amendment mixtures (e.g., bioaugmented dechlorinating bacteria, organic electron donors and halogenated co-amendments) to sediments and their placement methods in conjunction with capping. Molecular tools to monitor dehalogenating bacteria were developed and refined for use to assess the effectiveness of remediation treatments.
A suite of molecular methodologies was optimized for rapid high-throughput detection, enumeration and diversity characterization of bacterial communities that reductively dehalogenate organohalides. To monitor the activity of dehalogenating bacteria and their response to different bioremediation treatments, phylogenetic analysis targeting dechlorinating Chloroflexi including Dehalococcoides species and functional analysis targeting putative reductive dehalogenase (rdh) genes was developed. Polymerase chain reaction (PCR) assays were developed to quantify and monitor biostimulated and bioaugmented dehalogenation in microcosms and mesocosms, and eventually in the field. Complementary molecular approaches—denaturing high-performance liquid chromatography (DHPLC), terminal restriction fragment length polymorphism (TRFLP), and nested PCR denaturing gradient gel electrophoresis (DGGE)—were combined.
The protocols for assaying dehalogenating potential and the effects of different treatments and amendments were tested in different PCB and PCDD/F-contaminated sediments (Anacostia River, Kearny Marsh, Kymijoki River). Active dehalogenating bacteria are indigenous to these sediments and these communities have PCB and PCDD/F dechlorinating potential. Biostimulation may enhance the activity of both native Dehalococcoides spp. and the bioaugmented dehalogenating bacteria, such as D. ethenogenes strain 195. Microorganism(s) harboring various rdh genes play a key role in dechlorination. This project’s findings on the identity of species and genes involved in anaerobic PCB dechlorination may be used to evaluate environmental PCB and PCDD/F dechlorination potential and to monitor the dechlorination process under various treatments.
Organohalide-contaminated sediments contain diverse communities of dehalogenating microorganisms and addition of appropriate amendments can enhance microbial dehalogenation of historic organohalide contaminant mixtures, including PCBs and PCDD/Fs. The enhanced dechlorination correlates with increased numbers of dehalorespirer populations and rdh genes, supporting the hypothesis that the halogenated co-substrates enhance dechlorination of historic pollutants by supporting growth and activity of dehalogenating bacteria. A combined bioaugmentation/biostimulation approach can be effective in bioremediation of sediments contaminated with organohalide mixtures. Identification of the specific microbial members associated with PCB- and PCDD/F-dechlorinating activity should allow for better strategies to enhance dehalogenation in contaminated environments.