| Impacts of Co-Solvent Flushing on Microbial Populations Capable of Degrading Trichloroethylene Vijayalakshmi Ramakrishnan,1 Andrew V. Ogram,1 and Angela S. Lindner2 1Department of Soil and Water Sciences, and 2Department of Environmental Engineering Sciences, University of Florida, Gainesville, Florida, USA Abstract
With increased application of co-solvent flushing technologies for removal of nonaqueous phase liquids from groundwater aquifers, concern over the effects of the solvent on native microorganisms and their ability to degrade residual contaminant has also arisen. This study assessed the impact of ethanol flushing on the numbers and activity potentials of trichloroethylene (TCE) -degrading microbial populations present in aquifer soils taken immediately after and 2 years after ethanol flushing of a former dry cleaners site. Polymerase chain reaction analysis revealed soluble methane monooxygenase genes in methanotrophic enrichments, and 16S rRNA analysis identified Methylocystis parvus with 98% similarity, further indicating the presence of a type II methanotroph. Dissimilatory sulfite reductase genes in sulfate-reducing enrichments prepared were also observed. Ethanol flushing was simulated in columns packed with uncontaminated soils from the dry cleaners site that were dosed with TCE at concentrations observed in the field ; after flushing, the columns were subjected to a continuous flow of 500 pore volumes of groundwater per week. Total acridine orange direct cell counts of the flushed and nonflushed soils decreased over the 15-week testing period, but after 5 weeks, the flushed soils maintained higher cell counts than the nonflushed soils. Inhibition of methanogenesis by sulfate reduction was observed in all column soils, as was increasing removal of total methane by soils incubated under methanotrophic conditions. These results showed that impacts of ethanol were not as severe as anticipated and imply that ethanol may mitigate the toxicity of TCE to the microorganisms. Key words: co-solvent flushing, methanotrophs, particulate methane monooxygenase, perchloroethylene, 16S rDNA, soluble methane monooxygenase, trichloroethylene. Environ Health Perspect 113: 55-61 (2005) . doi:10.1289/ehp.6937 available via http://dx.doi.org/ [Online 8 December 2004]
This article is based on a presentation at the conference "Bioremediation and Biodegradation: Current Advances in Reducing Toxicity, Exposure and Environmental Consequences" (http://www-apps.niehs.nih.gov/sbrp/bioremediation.html) held 9-12 June 2002 in Pacific Grove, California, and sponsored by the National Institute of Environmental Health Sciences (NIEHS) Superfund Basic Research Program. The overall focus of this conference was on exploring the research interfaces of toxicity reduction, exposure assessment, and evaluation of environmental consequences in the context of using state-of-the-art approaches to bioremediation and biodegradation. The Superfund Basic Research Program has a legacy of supporting research conferences designed to integrate the broad spectrum of disciplines related to hazardous substances. Address correspondence to A.S. Lindner, Department of Environmental Engineering Sciences, University of Florida, Center Dr., A.P. Black Hall, P.O. Box 116450, Gainesville, FL 32611 USA. Telephone: (352) 846-3033. Fax: (352) 392-3076. E-mail: alind@eng.ufl.edu We thank M. James and S. Roberts, directors of the University of Florida Superfund Basic Research Program, for their support and encouragement of this work. We acknowledge P. Nkedi-Kizza of the Department of Soil and Water Sciences at the University of Florida for his kind generosity and guidance during the column experiments. This work was funded by the U.S. Environmental Protection Agency and the NIEHS Superfund Basic Research Program. The authors declare they have no competing financial interests. Received 23 December 2003 ; accepted 7 April 2004.
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