"Push-pull" Tests for Monitoring Bioaugmention with Reductive Dechlorinating Cultures

The “source zones” of aquifer contaminant plumes are extremely difficult to cleanup up. Bioaugmentation is one promising remediation approach – the aquifer is injected with a microbial community that can degrade high concentrations of the contaminants in situ. Successful biaugmentation requires careful monitoring. Practitioners need a way to quantitatively track changes in the microbial community, the extent of bioaugmentation, and the treatment's effectiveness. WRHSRC researchers Jack Istok, Jennifer Field, and Mark Dolan are working on this problem. The team from Oregon State University is developing the single well “push-pull” test to monitor the effectiveness of bioaugmentation in anaerobic “source zones”.

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Anaerobic Reductive Dechlorination and the Evanite Culture

The researchers’ focus is on clean up of high concentrations of chlorinated solvents such as Tetrachloroethylene (PCE) and trichloroethylene (TCE) by anaerobic reductive dechlorination. In this process, microbes replace chloride molecules in the chemicals structure with hydrogen molecules. Replacment of one chloride molecule transforms PCE to TCE. Replacment of another transforms TCE to cis-dichloroethylene (cis-DCE), then cis-DCE to vinyl chloride, and finally vinyl chloride to the harmless substances ethene and chloride (Figure 1).

The team is working with a microbial culture from the Evanite site in Corvallis, Oregon. WRHSRC researcher Lewis Semprini discovered the culture and has studied its dechlorination ability (see Research Brief 4). It has potential for bioaugmentation because it successfully completes all steps in the transformation reaction sequence (Yu, 2003). Many other reductive dechlorinating cultures slow or stop before transforming the toxic intermediate product vinyl chloride to the harmless substance ethene.

The Physical Aquifer Model

The researchers test environment is a physical aquifer model (PAM) (Figure 2). It is a wedge-shaped structure that is about 10 inches high and 3 feet long. Jack Istok, an environmental engineering professor, designed this type of PAM to simulate a wedge of the radial flow that is typical around a well. The narrow end of the model simulates a test well and has ports where substances can be injected and extracted. They flow toward the broad end of the model, interacting with test sediments and the model “groundwater” along the way. For their bioaugmentation experiments, the researchers filled the PAM with homogenous sediments taken from the U.S. Department of Energy’s Hanford Site.

The model is designed to maintain anaerobic conditions. Its lid is tightly clamped, all seams are sealed, and the system is flushed with nitrogen purged groundwater. The team is monitoring redox conditions in the model using a sensor developed by WRHSRC researcher Jim Ingle (see Research Brief 1). The sensor has been extremely useful in detecting slight changes in redox conditions that can signal a leak in the system and entry of oxygen.

"Push-pull" Tests

The researchers will use the PAM to develop a “push-pull” test for monitoring bioaugmentation with the dechlorinating culture. Jack Istok has developed “push-pull” tests for monitoring many microbial processes including aerobic cometabolism of chlorinated solvents and anaeobic transformations of chlorinated solvents, petroleum hydrocarbons, heavy metals, and radionuclides (link to the "push-pull" website). The tests involve injecting or “pushing” a solution that contains tracers into an aquifer. After specific time intervals, samples are “pulled” from the well and the amount of the initial additives plus the amount of reaction products are measured. The extracted quantities are used to calculate mass balances and reaction rates. In the PAM, samples are taken along the length of the model; those taken further from the injection site are analogous to samples that would be withdrawn later in the “pull” phase of a "push-pull" test.

In a previous study, Istok and his team tested the potential of trichlorofluoroethene (TCFE) to serve as the tracer in "push-pull" tests for monitoring intrinsic anaerobic reductive dechlorination (Hageman et al, 2001). TCFE is an uregulated compound and degrades by an analagous sequence of reductive dechlorination reactions. If the relationship between the degradation rate for TCFE and TCE is known, then data from the "push-pull" test can be used to infer a transformation rate for the target compound, TCE.

Current Project Status

The team is now midway through their experiments. They have set up the PAM and achieved anaerobic conditions. They have developed an optimal way to transport the culture and determined a substrate that can be injected with the culture and serve as an electron donor. In separate experiments, they have studied the potential surrogate contaminant, TCFE, and determined the correlation between its transformation rate and that of the target contaminant, TCE.

The team’s next step is to add TCE to the system and monitoring its degradation in the PAM. They hope to be able to monitor production of all of the daughter products in the reaction sequence and monitor changes in the microbial culture using DNA techniques. The final step will be to introduce TCFE and model the “push-pull” test.