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Cleaning Up Groundwater Contaminants

Posted on November 13th, 2013

Magic Microbes

Tim Mattes says there is great interest in new methods to clean up PCEs and TCEs in the groundwater.

Tim Mattes says there is great interest in new methods to clean up PCEs and TCEs in the groundwater.

You might call Tim Mattes a magician of sorts — he may not pull a rabbit out of his hat, but he can transform known human carcinogens into a safer substance, using only naturally-occurring microbes.

With funding from the National Science Foundation, the IIHR associate research engineer is exploring the use of micro-organisms to transform vinyl chlorides in the groundwater into an environmentally benign product. The project is a collaborative effort between Mattes and Alison Cupples, an associate professor in Civil and Environmental Engineering at Michigan State University, and an expert in stable isotope probing.

Groundwater Contamination

For decades, the military used chlorinated solvents such as TCE (trichloroethene), often disposing of it in a way that allowed it to seep into the groundwater. Dry cleaners used a similar solvent, PCE (tetrachloroethene), which also persists for many years. Through natural processes, the TCEs and PCEs are changed into intermediate products in the groundwater such as vinyl chloride (VC) — a known carcinogen.

According to Mattes, there is great interest in new methods to clean up PCEs and TCEs. “You can’t just pump that stuff out,” he explains. His work is exploring in situ bioremediation strategies using naturally-occurring microbes.

Mattes, who is also an associate professor of civil and environmental engineering, says molecular biology tools allow scientists to detect and quantify the presence of the bacteria that consume vinyl chloride in the presence of oxygen, transforming the vinyl chloride into benign substances — carbon dioxide, chloride, and water.

Abracadabra!

There are at least two slightly different variations in the VC biodegradation process. In the first, the bacteria (VC-cometabolizers) oxidize the vinyl chloride fortuitously, forming reactive and potentially harmful metabolites; in the second, a specialized subset of these bacteria (VC-assimilators) actually grow on the vinyl chloride, assimilating the carbon into their cell material. The latter process offers several advantages, but scientists can’t currently distinguish between the two microbes at a contaminated site using molecular biology tools.

Using stable isotopes, Mattes and his team hope to develop new molecular tools that can be used to detect and quantify the abundance and activity of VC-degrading microorganisms in the environment. Specifically, they aim to develop a procedure to differentiate VC-assimilating bacteria from VC-cometabolizing bacteria by using stable isotope probing techniques (SIP), along with existing tools.

Mattes says that the VC-assimilators actually become weightier when fed VC containing the stable isotope Carbon-13, which is slightly heavier than normal carbon molecules. After feeding Carbon-13 VC to a mixed microbial community containing both VC-assimilators and VC-cometabolizers, researchers extract their DNA and use an ultracentrifugation step to see if it has become heavier.  By combining this technique with existing tools, Mattes hopes to discover new VC-assimilating bacteria and to differentiate the VC-assimilators from the VC-cometabolizers.

This might seem like a simple thing, but it has eluded scientists from many years, Mattes says. He became interested in using bioremediation techniques to clean up chlorinated solvents in the groundwater in the mid-1990s as an environmental engineering graduate student at Johns Hopkins University. It’s a subject that has continued to fascinate him through his professional career at IIHR and the University of Iowa.

An Interdisciplinary Effort

Although Mattes is an engineer, much of his work has had a decided microbiological slant. His students reflect the multifaceted nature of the research. “It needs to be interdisciplinary,” Mattes explains. About a third of his students come from an engineering background, a third from environmental science, and a third from biology/microbiology.

It’s a “green” technology in the sense that no genetic engineering has to take place, Mattes explains. Groundwater is an increasingly important source of drinking water for millions, and bioremediation techniques of this sort (which Mattes called the “intelligent use of nature”) hold great promise for the future.

It may not be magic, but clean drinking water makes a big difference to us all.

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