Research Feature: "Push-pull" tests - an innovative way to study underground microbes

Posted: April 3, 2006
: Lewis Semprini, 541-737-6895,

Engineers at OSU are developing an inexpensive way to test for and monitor the capabilities of underground microbial communities. Called “push-pull” tests, the technique involves injecting a test solution into a groundwater well and then withdrawing the test solution and groundwater mixture. Comparisons of the “pushed” and “pulled” solutions provide information about the microorganisms living underground and the functions they carry out.

“With this type of test, researchers can study a subsurface microbial community in situ,” says Lewis Semprini, OSU professor of Environmental Engineering. “It has potential to be both less expensive than current study methods and to present a more comprehensive picture of underground processes.” Traditional methods usually require drilling to remove samples of underground soil and rock and then culturing the material’s microbial community in the laboratory.

A) Injection ("Push") Phase
(B) Extraction ("Pull") Phase
Injection and extraction phases of a "push-pull" test.

Semprini’s research focuses on the utility of “push-pull” tests for monitoring bioremediation – a clean-up method for polluted soil and groundwater. He recently co-authored a paper on this topic with colleagues Young Kim from Korea University and Jack Istok, a "push-pull" expert from OSU. Their paper appears in the January 2006 edition of the Journal of Contaminant Hydrology and focuses on a type of bioremediation called cometabolism.

In cometabolism microbes degrade contaminants by producing enzymes that can convert them into harmless substances. The prefix co- indicates that the breakdown is part of a secondary reaction, it occurs fortuitously as the microbes feed off another food source. At many contaminated sites, the microbes are present in situ – bioremediation then involves adding a food source and oxygen to stimulate their growth and exploit their beneficial effects.

The team’s test site was the former McClellan Air Force Base, California. Sixty-five years of aircraft maintenance operations contaminated the site’s groundwater with a variety of toxic compounds. Earlier studies using traditional coring and culturing methods also suggested that the site hosted a community of microbes that could degrade many of the contaminants by cometabolism and that they could be stimulated by adding propane and inhibited by adding another chemical, acetylene.

With their field study, the team hoped to demonstrate that these same diagnostic tests could be carried out in situ with “push-pull” tests. “We hoped to create a little reactor under Earth,” says Semprini. The “push-pull” tests would provide information about the reactions taking place out of sight underground and avoid the step of culturing and testing the organisms in the laboratory.

The tests involved pumping groundwater out of the aquifer, mixing in different additives such as dissolved propane and a non-toxic tracer, and “pushing” the solution back underground. The team then monitored for microbial activity by “pulling” samples from the well at specific time intervals and measuring the concentrations of additives, tracers, and reaction products.

The tests were successful – lower concentrations of additives and tracers and higher concentrations of reaction products showed that the microbial community was present and could be stimulated to cometabolize the contaminants. Later “push-pull” tests showed that additions of acetylene inhibited the microbes’ activity, just as it had in laboratory studies.

Semprini believes that this study represents just one of the potential applications of “push-pull” tests for investigating the subsurface biosphere. He hopes that other OSU faculty and students will consider their application to other studies of underground ecosystems.

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