of the Problem
The "push-pull" test is an innovative way to monitor
groundwater remediation. It involves injecting a
test solution into an aquifer and then withdrawing
the test solution and groundwater mixture. Comparisons
of the “pushed” and “pulled” solutions
provide information about the physical, chemical,
and biological conditions within the aquifer.
WRHSRC researcher Jack Istok has studied "push-pull"
tests and their applications extensively (link
to the "push-pull" website).
This research brief describes his collaboration with
researchers to develop a "push-pull" test for monitoring
bioaugmentation of aquifers contaminated with high
concentrations of contaminants
such as tetrachloroethylene
(PCE) and trichloroethylene (TCE).
"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
of the contaminants in situ. Successful biaugmentation requires
Practitioners need a way to quantitatively track changes in the
microbial community, the extent of bioaugmentation, and the treatment's
WRHSRC researchers Jack
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
on highlighted words and definitions and illustrations
will pop up.
Anaerobic Reductive Dechlorination and the Evanite Culture
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
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
is working with a microbial culture from the Evanite site
in Corvallis, Oregon. WRHSRC researcher Lewis
the culture and has studied its dechlorination ability (see
Research Brief 4). It has potential for bioaugmentation because
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
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
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
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.
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
(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.
For More Information
Mark Dolan, link to the "push-pull"
website or read more in the following references:
Hageman, K.J., Istok, J.D., Field, J.A., Buscheck, T.E.,
and Semprini, L., 2001, In Situ Anaerobic Transformation
of Trichlorofluoroethene in Trichloroethene-Contaminated
Groundwater, Environmental Science and Technology , 35(9),
article as a pdf.
Yu, S., 2003, Kinetic and modeling investigations of the
anaerobic reductive dechlorination of chlorinated ethylenes
using single and binary mixed cultures and silicon-based
organic compounds as slow-release substrates, Ph.D. Dissertation,
Department of Civil, Construction, and Environmental Engineering,
Oregon State University.
Yu, S. and Semprini, L., 2004, Kinetics and modeling of
reductive dechlorination at high PCE and TCE concentrations,
Biotechnology and Bioengineering, in press.
Yu, S. and Semprini, L., 2002, Comparison of trichloroethylene
reductive dehalogenation by microbial communities stimulated
on silicon-based organic compounds as slow-release anaerobic
substrates, Water Research, vol. 36, p.4985-4996.
ATSDR, 1997, Public Health Statement for Tetrachloroethylene,
CAS# 127-18-4, http://www.atsdr.cdc.gov/toxprofiles/phs18.html .