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Research Feature: Bio-Barriers for Groundwater Remediation
Mandy Michalsen, a Subsurface Biosphere IGERT student, just completed her PhD in Civil Engineering. Her dissertation focused on bio-barriers as a potential remediation strategy for groundwater contaminated with radionuclides. Mandy published part of her work in the October 2006 issue of Environmental Science and Technology. In this short web interview, Mandy describes the project and how it fit into her graduate research experience.
How did you get started on this project?
When I started grad school, I was interested in working on reactive barriers as an alternative to “pump and treat” technologies for the clean up of contaminated groundwater. The idea is to create a zone in the aquifer where contaminants are intercepted and their transformation is promoted in situ. They have the potential to be less energy intensive and may be cheaper yet effective for extended time periods. A bio-barrier is a particular type of reactive barrier that uses microbes to transform contaminants. The idea that native microbial communities exist in contaminated aquifers with the potential to clean up contaminants is exciting to me! All we scientists have to do is figure out what the microbes require to promote the desired reaction and then figure out how best to sustain it.
I was looking for a research project when I took Jack Istok’s groundwater class. He mentioned a really complex site he was working on in Oak Ridge, Tennessee, where the groundwater was contaminated with hydrogen ion and metal acidity, extreme nitrate concentrations as well as uranium and technetium. Most bacteria don’t like acid and so this groundwater seemed initially intractable from a bio-barrier perspective. Then we came up with the idea of neutralizing the groundwater with limestone. Not being a geology student, I thought I’d call up a quarry and get some limestone for experiments in the Groundwater Lab here at OSU. Turns out that limestone is hard to come by in the Northwest but there are ample amounts near the contaminated aquifer in Oak Ridge. That’s how the project got started.
What was the goal of the project and how was it set up?
Uranium and technetium are soluble and mobile in groundwater under oxygenated conditions, but under iron- and sulfate-reducing conditions U and Tc form insoluble, immobile minerals. We wanted to see whether native sediment microbes could create reducing conditions and cause the U and Tc to precipitate as immobile minerals – a process called bio-immobilization.
We conducted experiments at Oak Ridge National Laboratory in flow-through columns to simulate a potential bio-barrier configuration above-ground. The columns were 250 cm long, packed with mixtures of native crushed limestone and sediment, and contaminated site groundwater water was continuously pumped through them. Envision groundwater flowing horizontally through the sediment mixture, with a row of sampling and injection ports along the column length. We injected ethanol through the injection ports to serve as a primary carbon and energy source to stimulate the native microbial community to hopefully promote bio-immobilization of U and Tc.
What were your results and what is their significance?
Our results show promise for an in situ bio-barrier at the Oak Ridge site. Ethanol additions sustained U and Tc removal from flowing site groundwater for 20 months. However, hydraulic conductivity of the sediment decreased during the experiment and similar effects could eventually divert groundwater flow from an in situ bio-barrier. There are problems to be worked out and our results certainly do not translate directly to the field. However, this work is an important step forward.
U and Tc are prevalent contaminants at U.S. Department of Energy Cold War legacy waste sites throughout the country. Development of a practical, cheap and effective method of preventing further migration of U and Tc in the environment would be ideal. Our experiments were unique because we were able to use site sediments and groundwater and observe the effects of ethanol additions on U and Tc mobility for a long time.
How did this project fit into your graduate work at OSU in the Subsurface Biosphere IGERT program?
I was comfortable within the realm of engineering design calculations when I started graduate school, where microbial cells are treated as chemical formulas and problems have numerical solutions. The concept of microbial communities and the associated vocabulary was initially intimidating and uncomfortable for me. I hear engineers often make the analogy that learning about microbiology is like learning to speak a foreign language -- that’s exactly how I felt. If only there was a Rosetta Stone: Microbiology for Engineers! That’s where the Subsurface Biosphere IGERT was very relevant and beneficial to my project and my graduate school experience overall. I conducted a month-long internship at the University of Tennessee Center for Biomarker Analysis where I learned signature lipid and nucleic acid-based methods for characterizing sediment microbial communities. The data collected during my internship was central to my research. Through my experience in the IGERT program and through my interdisciplinary research, I have come to fully appreciate the complex coupling of subsurface geochemistry and microbial activity. In the case of in situ bio-immobilization, feedbacks between microbial activity and subsequent changes in aquifer geochemistry may promote or hinder contaminant removal. The trick is understanding these controlling feedbacks and designing accordingly, which is of course tricky.
Now that you have finished grad school what are your plans?
I have recently moved to Seattle and will begin an engineering consulting job in April. My immediate professional goal is to obtain my Professional Engineering license and enjoy contributing to clean up projects that positively impact the environment in the Northwest.
We’re also currently working on a separate paper that describes spatial shifts in the sediment microbial community along the groundwater flow paths (submitted to Applied and Environmental Microbiology), as well as a paper that attempts to link results of the lab, intermediate- and field-scale bio-immobilization experiments.
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