Superfund Research Program
Quantifying Enhanced In Situ TCE Biodegradation
Trichloroethene (TCE) is a colorless, volatile, nonflammable liquid that is soluble in water and organic solvents. TCE is used primarily as a metal degreaser but also in products such as dyes, printing ink, and paint. Because of widespread use and poor handling, storage and disposal practices, TCE has become one of the nation's most prevalent groundwater pollutants - TCE is present at one-third to one-half of all Superfund sites. The most common method for removing TCE from groundwater is to pump the water from the aquifer and treat it above ground, which is costly and time-consuming. Researchers from the Oregon Health & Science University's SBRP are working to develop and evaluate technologies for monitoring and enhancing in-situ TCE biodegradation in anaerobic groundwater.
Anaerobic TCE degradation occurs by reductive dechlorination, a reaction in which hydrogen atoms sequentially replace chlorine substituents. In the commonly observed TCE transformation pathway, TCE is sequentially reduced to dichloroethene (DCE), vinyl chloride (VC), and ethene. In situ TCE transformation rates are needed to assess the potential for intrinsic bioremediation and to design and monitor engineered bioremediation projects. Because vinyl chloride is a potent human carcinogen, it is critical to determine both the rate and extent of dechlorination.
Dr. Jennifer Field leads a team of scientists using several approaches to determine the rates of TCE biodegradation. Much of their work uses a fundamentally new field method developed by the investigators, "push-pull" field tests. Push-pull tests are conducted by injecting ("pushing") an aqueous test solution containing a nonsorbing, nonreactive tracer and one or more reactants into the saturated zone of an aquifer via a monitoring well. Samples of the test solution/groundwater mixture are then extracted ("pulled") from the same well over time and analyzed for tracer, reactant and product concentrations. The researchers have used push-pull tests to evaluate strategies for determining rates of TCE biodegradation. These include:
- Injection of trichlorofluoroethene (TCFE) in TCE-contaminated groundwater as a surrogate to determine TCE transformation rates. TCFE is reductively dechlorinated under anaerobic conditions to dichlorofluoroethene, chlorofluoroethene, and fluoroethene by a pathway analogous to that of TCE. Push-pull field tests conducted in California revealed that TCFE and TCE transformations were affected by similar rate-controlling factors and confirmed similarities in the in situ TCFE and TCE transformation pathways, including degradation product predominance. These findings indicate that TCFE can be used to determine the rate and extent of TCE transformation.
- Injection of fumarate to screen TCE-contaminated groundwater for dechlorinating microbial activity. Fumarate is non-toxic, non-volatile, relatively inexpensive and undergoes reduction to succinate under redox conditions similar to those under which TCE is reduced. Dr. Field's team conducted push-pull tests in five wells to determine if a correlation existed between TCFE reductive dechlorination and fumarate reduction to succinate. Rapid fumarate reduction to succinate was observed in wells where TCFE reductive dechlorination had been observed in earlier tests. Fumarate was also removed at wells where TCFE reductive dechlorination was not previously observed, but no succinate was detected, indicating that fumarate was removed by a pathway other than reduction. Because fumarate reacts more rapidly than TCE, this new technique represents a quick in situ method to screen TCE-contaminated groundwater for dechlorinating activity.
- Forced mass balance, a novel data processing technique to estimate in situ transformation rates. This technique removes the effects of transport processes such as advection, dispersion and sorption, allowing for determination of in situ transformation when sorbing reactants and their products formed in situ are not transported identically. Coupling push-pull tests with forced mass balance data analysis advances bioremediation technology by making it possible to estimate in situ transformation rates of sorbing chemicals.
- Addition of acetylene as a reversible inhibitor to probe the biological activities of reductive dechlorination. The researchers have shown that acetylene can be added to inhibit reactions and then removed to permit reactions to proceed. Thus, it can be a powerful tool for investigating the enzymes responsible for reductive dechlorination in TCE-contaminated groundwater.
The researchers are currently applying these techniques to evaluate strategies to enhance rates of the reductive dechlorination of TCE. They are focused on selecting and optimizing electron donors for the purpose of enhancing the rates of anaerobic TCE degradation. The researchers conducted push-pull field tests in which fumarate was added to five wells exhibiting varying reductive chlorination rates. At each location, they observed increased in situ reductive dechlorination rates and increased number of transformation products. These findings indicate that fumarate amendment has the potential to stimulate reductive dechlorination, even in aquifers where no reductive dechlorination activity has been reported previously.
By finding effective methods for increasing the rate and extent of TCE biotransformation, the amount and toxicity of this groundwater contaminant can be effectively reduced, ultimately reducing the potential for human exposures. The accurate measurement and prediction of in situ rates of transformation obtainable through the application of methods developed by this research are critical to the improvement of risk assessment capabilities, the development and validation of bioremediation strategies, and determination of the potential for bioremediation of TCE-contaminated sites.
For More Information Contact:
Jennifer A Field
Oregon State University
Department of Environmental and Molecular Toxicology
1111 Ag and Life Science Bldg
Corvallis, Oregon 97331-3701
To learn more about this research, please refer to the following sources:
- Hageman KJ, Field JA, Istok JD, Schroth MH. 2003. "Forced mass balance" technique for estimating in situ transformation rates of sorbing solutes in groundwater. Environ Sci Technol 37(17):3920-3925. PMID:12967114
- Pon G, Hyman MR, Semprini L. 2003. Acetylene inhibition of trichloroethene and vinyl chloride reductive dechlorination. Environ Sci Technol 37(14):3181-3188. PMID:12901668
- Hageman KJ, Istok JD, Field JA, Buscheck TE, Semprini L. 2001. In situ anaerobic transformation of trichlorofluoroethene in trichloroethene-contaminated groundwater. Environ Sci Technol 35(9):1729-1735. PMID:11355185
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