Superfund Research Program
Enhancing and Monitoring the Rates of Microbial Transformation of Chlorinated Solvents in Anaerobic Groundwater
Trichloroethene (trichloroethylene, TCE) is a colorless, volatile, nonflammable liquid that is slightly soluble in water and soluble in most other 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, poor handling, and storage and disposal practices, TCE has become one of the nation's most prevalent groundwater pollutants.
It is not surprising then that 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. Jennifer Field leads a team of researchers from the Oregon Health & Science University's SRP that is 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. Because vinyl chloride is a potent human carcinogen, its formation and persistence in groundwater are major problems at many TCE-contaminated sites. Therefore, in situ methods to determine both the rate and extent of dechlorination are needed to assess the potential for intrinsic bioremediation, as well as to design and monitor engineered bioremediation projects.
The methods often used to study dechlorination processes do not yield sufficient data to evaluate the rate and extent of degradation. Laboratory microcosms of groundwater and sediment provide information on potential degradation, but it is widely recognized that these rates are not representative of in situ rates. Methods commonly used to evaluate bioremediation processes in situ rely on detection of degradation products (such as DCE or VC) or assessment of temporal or spatial changes in isotopic composition. While these methods provide evidence for degradation, they do not provide the quantitative rate information required to optimize systems for TCE biotransformation.
Dr. Jennifer Field and her colleagues use several approaches to determine the rates of TCE biodegradation.
Much of their work uses "push-pull" field tests, a fundamentally new field method that they developed. 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. Within the aquifer, injected substrates are transformed to known products. 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 extent of transformation provides a quantitative measure of microbial activity. The in situ transformation rate of an injected reactant is then determined by correcting for the effects of transport processes from measured reactant concentrations using a novel data processing technique data known as forced mass balance.
Push-pull tests are cost-effective because they require only one well. They take less time to conduct than well-to-well tests and they leave the subsurface undisturbed so that in situ conditions are maintained. Push-pull tests examine a representative portion of the aquifer unlike laboratory microcosm experiments that utilize only gram quantities of sediment. This method can be used to determine site-scale variability in processes by comparing test results obtained from single wells within a single site.
Flexibility is a key advantage of the push-pull technology:
The composition of the test solution can be designed and optimized to investigate specific processes unique to the groundwater in different locations.
By modifying the rate and timing of extraction phase pumping ("pulling"), the researchers can control the time that the injected test solution is in contact with aquifer sediments (i.e., residence time). Shorter residence times (~hours) can be used to gather transport information on contaminants and mass balances can be determined. Conversely, longer residence times (~months) allow for slow contaminant transformations to be observed.
The injection volume, which corresponds to the volume of aquifer studied, can be increased to study larger and more representative volumes of aquifer.
High background concentrations of the contaminant of interest rule out the simple addition ("push") of the contaminant because it is difficult to differentiate between background and injected chemicals. To overcome this, Dr. Field's team selects chemical analogs or surrogates that undergo similar transformation in the subsurface and that can be unambiguously distinguished from the background contaminants. For TCE groundwater experiments, Dr. Field selected trichlorofluoroethene (TCFE) as a TCE-surrogate based on evidence that TCE and TCFE undergo analogous reductive dechlorination pathways; are transformed at similar rates; and exhibit similar transport behavior. Dr. Field confirmed the validity of TCFE as a TCE-surrogate in heavily contaminated environments in a push-pull test conducted near Richmond, CA.
Dr. Field injects fumarate to screen 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 the same wells where TCFE reductive dechlorination occurred. Because fumarate reacts more rapidly than TCE, this new technique represents a quick in situ method to screen TCE-contaminated groundwater for dechlorinating activity.
In studies focused on selecting and optimizing electron donors for the purpose of enhancing the rates of anaerobic TCE degradation, Dr. Field's team conducted push-pull field tests in which fumarate was added to five wells exhibiting varying rates of microbial reductive chlorination. 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.