Natalie L. Cápiro and Kurt D. Pennell (Tufts University; Medford, MA)
Recent studies have demonstrated that microorganisms are capable of biodegradation within close proximity to chlorinated solvent source zones. This biological activity can enhance (accelerate) aqueous dissolution (removal) of chlorinated solvents, and has the potential to provide more effective source zone treatment, thereby reducing potential exposure and remediation costs. However, the performance of bioremediation, and more specifically, biologically-enhanced dissolution, faces two challenges; sustained release of electron donor (food source) and delivery of electron donor to the intended target. To overcome these limitations, a potential alternative are partitioning electron donors (PEDs), organic compounds (e.g., n-butyl acetate) that are relatively water soluble, but also partition into (directly mix with) chlorinated solvents. When a PED is delivered to the subsurface contaminant source zone, it preferentially partitions into the organic separate phase chlorinated solvent, and then slowly dissolves back into the passing groundwater along with the contaminant. This strategy of electron donor delivery is intended to promote the growth of chlorinated solvent degrading bacteria in close proximity to the contaminant source zone, while minimizing consumption of electron donor in microbial processes not associated with chlorinated solvent bioremediation (e.g., methane production). The specific objectives of this research are designed to assess the physical, chemical and biological processes that govern PED delivery, mass transfer, and consumption to support sustained microbial biodegradation in chlorinated solvent source zones. A combination of laboratory-scale experiments and mathematical modeling will be conducted using trichloroethene (TCE) as a representative contaminant. The research program is structured around four tasks that will: (1) evaluate and select PEDs for detailed study based on abiotic and biotic batch reactor studies, (2) quantify PED delivery and release, and rates of bioenhanced dissolution and degradation in columns containing residual TCE in comparison to current electron donor delivery approaches, (3) measure the spatial distribution and temporal evolution of PED delivery/release, TCE dissolution and degradation, and microbial communities in heterogeneous aquifer cells to assess the potential for improved bioremediation under more realistic conditions, and (4) implement mathematical models to obtain mass transfer and utilization rate parameters from experimental data, and predict responses to alternative PED delivery strategies and subsurface aquifer conditions to support potential scale up to field-application.
The U.S. EPA estimates that more than $209 billion dollars (in constant 2004 dollars) will be needed over the next 30 years to mitigate hazards at 235,000 to 355,000 chlorinated solvent contaminated sites, impacting nearly 30% of U.S. drinking water supplies. Additionally, these cost estimates do not include many sites susceptible to vapor intrusion, which is now recognized as a key exposure pathway in urban areas. The knowledge gained from the testing and validation of this novel remediation technique will provide a sustainable approach to reduce chlorinated solvent source zone longevity and remediation costs through an improved understanding of enhanced biological treatment. Integration of experimental studies with mathematical modeling will yield guidelines and rate parameters necessary for practitioners and researchers to assess PED performance for a range of potential remediation scenarios. Furthermore, the project will incorporate educational initiatives into the research and instructional activities of the investigators, with the goal of extending the impact of acquired knowledge beyond the traditional framework of journal publications and conference presentations. This goal will be achieved through the following initiatives: (a) the inclusion of undergraduate students conducting laboratory research, and (b) recruitment of female and underrepresented minority science and engineering undergraduate and graduate students, (c) dissemination of instructional tools to regulatory agencies and impacted communities.
