Geophysical techniques help monitor microbial remediation
Electric current and acoustic waves could shed some light on what’s happening in contaminated aquifers.
Understanding how microbes clean up contaminants is difficult without a direct way to observe what the “bugs” are actually doing. In a paper posted to ES&T’s Research ASAP website (es0504035), researchers at Lawrence Berkeley National Laboratory and the University of California, Berkeley, demonstrate how researchers can use two complementary geophysical techniques to monitor microbes that remediate metals in contaminated aquifers—without putting equipment into the ground.
Historically, miners used the same techniques—complex resistivity and acoustic waves—to search for buried oil and mineral sources as deep as 10,000 meters (m). In the past 20 years, environmental scientists have used these methods at the surface to locate underground pollutant plumes or characterize source points, explains Kenneth Hurst Williams, lead author on the paper. “This is the first effort to actively use [both] the techniques to monitor changes brought about by microbial processes.” Specifically, the researchers examine how the geophysical signatures change as sulfate-reducing microbes process metals and form mineral precipitates. These techniques can be run at the surface, cover areas similar to any football field, and have been modified for a high degree of resolution in field tests to a depth of 10 m, he adds.
“The [resulting] ‘images’ do not look like a mineral-encrusted microbe or a sand grain, but one can take the data and generate all sorts of [pictures such as] a color-coded representation of the acoustic wave amplitude as a function of time and depth,” explains Williams.
The “holy grail in subsurface microbiology” is a technique that shows scientists what is happening underground without spending a lot of time and money or disturbing contaminated sediments by drilling boreholes, says Derek Lovley, a microbiologist at the University of Massachusetts, Amherst. Lovley collaborated with Williams on very preliminary field tests of the geophysical techniques at an iron- and uranium-contaminated aquifer in Old Rifle, Colo., last summer. Lovley says the work “looks promising,” and he plans more field tests at the end of August.
Complex resistivity, also known as induced polarization, measures how currents change as they pass through sediments. The acoustic wave technique measures changes in velocity and amplitude of sound waves. Mineral products of microbial reactions alter the way both current and mechanical waves move through sediments.
For example, in the published work, Williams stimulated the sulfate-reducing bacterium, Desulfovibrio vulgaris, contained in a sand column, to degrade iron and zinc compounds. The researchers monitored changes in the column with the nondestructive techniques and identified the mineral products by chemical testing. By analyzing the current and wave data collected over time from the test column, they could create visual representations of the microbes’ progress.
“Biogeophysics is a very new area,” says Estella Atekwana, a geophysicist at the University of Missouri, Rolla, who praises the work. “There’s really good potential to be able to use biogeophysics to investigate microbial processes.”
